^ / J/ 'Ji PRACTICAL TREATISE ON THE CONSTRUCTION, HEATING, AND VENTILATION HOT-HOUSES; INCLUDING CONSERVATORIES, GREEN-HOUSES, GRAPERIES, AND OTHER KINDS OF HORTICULTURAL STRUCTURES. WITH PRACTICAL DIRECTIONS FOR THEIR MANAGEMENT, IN REGARD TO LIGHT, HEAT, AND AIR. ILLUSTRATED WITH NUMEROUS ENGRAVINGS. BY ROBERT B. LEUCHARS, GARDEN ARCHITECT. BOSTON: JOHN P. JEWETT AND COMPANY, 17 & 19 Cornhill. 1851. L5 UHSKARY-AGWICUL.TIJWE Entered according to Act of Congress, in the year 1850, BY JOHN P. JEWETT & Co., In the Clerk'a Office of the District Court of the District of Massachusetta, Stereotyped by HOBART & ROBBINS; NEW ENGLAND TYPE AND STEREOTYPE FOUNDERY. BOSTON. TO STreatfse, DESIGNED TO PROMOTE THE ADVANCEMENT OF EXOTIC HORTICULTURE. OP WHICH HE IS A ZEALOUS PATBON AND ADMIRES, I* M^pectMIg Drticatefc, BI HIS OBLIGED AND OBEDIENT SKBTAHT, THE AUTHOK. 66T791 0& is K Oi -OiJlV? PEEFACE. HAVING for many years past devoted my attention to the subjects treated of in this work, and from the general call for appliable information thereon, I have been induced to give it to the public, in the full persuasion that it will be acceptable to horticultur- ists, gardeners, and others engaged in this depart- ment of horticulture. From the numerous inquiries which I have received, there appears to be a great want of practical knowledge on these subjects ; and though much information may be gleaned from vari- ous English works, they are either unobtainable, or the information is inapplicable to the wants of this country. When I commenced this treatise, I intended it as a series of articles for periodical publication ; but the development of the subjects, and the accumula- tion of facts, swelled it to such a size as to render its publication in that form impossible. In prepar- ing it for the press, in its present form, I have been desirous to add nothing but what is necessary to a full understanding of the subject in hand, and have given figures and diagrams where illustration is required. The changes which have occurred, during the last- twenty years, in the method of constructing and PREFACE. managing horticultural structures, render the works of that period of little value to gardeners at the present day. I have here given all the latest im- provements and most approved methods at present in use, with plans and suggestions for their further improvement. From what has been said, I hope no one will sup- pose that this treatise is given as a complete work on Exotic Horticulture. Much has yet to be learned, on many points connected with hot-houses, which futurity will, no doubt, unfold. My warmest expressions of thanks are due to Professor Dana, of Yale College, for the generous manner in which he has favored me with his opinions. The readiness with which that gentleman has replied to my inquiries, on matters of science relating to my subject, even in the midst of his laborious literary pursuits, shows how willing he is to aid the most humble inquirer. This expression of thanks is due from me here, as the only way in which I can suf- ficiently show the high value at which I estimate his kindness and liberality. K. B. L. Boston, Oct. 3, 1850. INTRODUCTION. THE object of the following treatise is chiefly to lay before its readers a series of facts and observations relating to the con- struction and general management of all kinds of horticultural structures, drawn from the developments of science, and an extended experience, with the view of leading those who are interested in this delightful pursuit to a more practical inquiry regarding the comparative cost and economy of the various methods now commonly adopted, as well as to draw the atten- tion of practical gardeners to the utility of studying the theory as well as the practice of those manifold operations on which the success of exotic horticulture depends. In a short treatise, on such comprehensive and varied subjects, it is impossible to be strictly scientific ; but we have endeavored to show the rationale of those methods and operations which we have here recommended, and which have been successfully car- ried out by us in practice. The treatise is avowedly a practical one, and intended chiefly for the use of practical gardeners, and those desirous of obtaining that knowledge which is necessary to enable them to superintend the erection and future management of their own garden structures. In the management of hot- houses, there is a systematic regularity required in all the oper- ations, a neglect of which is generally attended with disorder and confusion. In fact, there is a system, the details of which succeed each other like the links of a chain, each operation being essentially connected with the one immediately following and preceding it ; and here we have a most encouraging truth, that the more scientific our principles of working, the more simple and easily performed are our operations, and the more reliable are the results. 8 INTRODUCTION. It is doubtful if any branch of horticulture has received less aid from science than that which forms the subject of the present work. Science has indeed been brought to bear upon horticul- tural generalities, but, as far as regards its application to exotic jiorticultu,ral .details, it is little better than a sealed book ; and hence it is that w,e', find cultivators clinging to antiquated sys- tems, which the plain demonstrations of science and practice are Jarly f)i?ovin'te 2. Materials of the Frame of the Building, fyc. The most suitable material for the frames of horticultural buildings has lately been made the subject of considerable discussion and ex- periment, which has not been without its use in the elucidation of facts hitherto unknown, or, at least, unnoticed in general practice. The case of wood versus iron has been investigated on various grounds, by practical and scientific men, without, however, coming to a unanimous decision on the superiority of either. In this matter, as in some others like itself, some have adopted extreme views of the various merits and defects of the different materials, and have come to their conclusions by refer- ence to some single or specific property. These views and con- clusions, however, have been of considerable utility in bringing the subject before the bar of unbiased inquiry, which, if it has not already done so, is likely to result in the adoption of modi- fied views, and the recognition of specific principles, that, when fully considered and duly weighed against each other, will ulti- mately lead to a more definite result. The use of iron in the construction of hot-houses, like every other really valuable improvement, has met with much opposi- tion from the still slumbering spirit of prejudice, which is gener- ally slow to believe in the superiority of anything different from that with which it has been long acquainted, even when this superiority cannot, on reasonable grounds, be denied. This 102 MATERIALS OF CONSTRUCTION. spirit, however, which has long held undisputed sovereignty over the minds of gardeners, is fast giving way before the sweep- ing current of mechanical inventions ; and when science comes to the aid of mechanism in the building of hot-houses, as in the erection of factories, steam-engines, and other works of art, then the flimsy barriers reared by prejudice will be swept away, and I think I may fearlessly assert that, in regard to the opposition that has been given to the erection of iron hot-houses, this has nearly taken place. Gardeners ^from^ the early ages of Abercrombie and Nicol, h&ye} been Vjfregiarffced* against metallic hot-houses, and, to our knowledge, n fo\s prejudice, is still entertained by some whose leyrbi k ng;azid^i'iit^fli^tiee-would encourage us to look for more accurate judgment. The objections which have been raised against metallic houses for horticultural purposes, are chiefly the following : Contraction and expansion, oxydation, abduction of heat, at- traction of electricity, and original cost. In regard to the first, and principal cause of opposition, viz., its susceptibility to the influences of heat and cold, a fact which cannot be denied, yet it is proved by experience that if a house be properly constructed of good material, this susceptibility is of no practical importance. In very small houses the incon- venience occasioned by sudden fluctuations of temperature may be more sensibly felt, although, in the management of small iron vineries, in England, we have never seen the slightest incon- venience result from external changes; indeed, all our expe- rience in the management of hot-houses goes to prove the superiority of iron over wood, for every purpose to which timber is generally applied. It has been stated that metallic roofs are more liable to break the glass than wood ; practice has also proved that this statement is without foundation, and if it has ever taken place, can only be in copper or compound metallic roofs. Cast-iron or solid wrought-iron bars have never been known to cause breakage of glass, or displacement of joints, and some have asserted that the breakage of glass is even more, during sudden changes, by wood than by iron roofs. The expansibility of copper being greater than that of iron, MATERIALS OF CONSTRUCTION. 103 in the proportion of 95 to 60, therefore copper is above one third more likely to break glass than iron. But when it is considered that a rod of copper expands only T^uVtftf part of its length with every degree of heat, and that iron only expands T^Vra P art > the practical effects of even the hottest portion of our climate on these metals can never amount to a sum equal to the expan- sion required for the breakage of glass. The second objection which we have mentioned is also unde- niable. All metals are liable to rust; but painting easily rids us of this objection, at least it will so far prevent it as to form hardly any objection. The power of metals to conduct heat is an objection which, like the others, cannot be denied, but may be partially obviated. The abduction of heat, like the expansibility of metallic roofs, is very little felt in using them ; the smaller the bars, the less their power of conduction. The paint, also, and the putty used to retain the glass, obviate this objection. Heat may be supplied by art, but light, the grand advantage gained by metallic bars, cannot, by any human means, be supplied but by transparency of roof. The objection raised on the ground of attraction of electricity, is easily answered. If metallic hot-houses and conservatories attract electricity, they also conduct it to the ground, so that it can do them no harm. What is corroborative of this position is the fact, that no instance has come under our knowledge of iron hot-houses having been injured by the electric fluid. The objection regarding the expense of iron hot-houses, has been sufficiently refuted in England, and we have observed, with pleasure, a refutation of the same objection, by an enter- prising gentleman of Cincinnati, who has lately erected an iron- roofed vinery. Mr. Resorr has given a cut, and description of this house, in the " Horticulturist " for Sept. 1849, p. 117. This is the only substantial account we have seen of the comparative cost of iron and wood roofs. This gentleman, who is in the foundery business, has every opportunity of knowing the accu- rate cost of such a house, and plainly states, " that those wish- ing to build a good, substantial house, can do it, and make the roof of iron, as cheaply as of wood, the other parts costing the 104 MATERIALS OF CONSTRUCTION. same." From inquiries and calculations which we have made, we have come to the same conclusion, although, from a want of the requisite knowledge, and from the expense of having patterns made for the castings, it may, in some localities, cost more than a structure of wood. In small houses, sudden changes of the external temperature are much sooner and more sensibly felt than in large structures, whether they are constructed of wood or iron, which arises from the fact that the smaller volume of air confined within becomes more rapidly heated, and hence the change is the sooner felt. Supposing the circumstance to be more strikingly sensible in the case of small iron houses, then all that is necessary to coun- terbalance it, is just a little more attention to ventilation, during sudden changes of external temperature. For large structures iron is incomparably superior to wood, and even for forcing-houses we would decidedly prefer the same material. The contraction and expansion of metallic hot-houses may be dreaded in the Southern States, if built on a very small scale, and badly managed ; but in structures of moderate size, this evil will be found practically of little importance, unless they are badly constructed, and negligently managed. The finest horticultural structures that have yet been erected in Europe are made of iron, and no houses of any importance are now being erected of wood, which proves its superiority over the latter material. The great conservatory, or Palm-house, at Kew, is wholly of iron, constructed under the auspices of the most scientific men in England. The Botanic Society's conser- vatory, in the Regent's Park, (already spoken of,) is made of iron. The fine plant-houses in the Glasnevin Botanic Garden, near Dublin, are constructed of iron, and the quite unequalled range of forcing-houses at Frogmore, in Windsor Park, are also of iron. In fact, the most extensive horticultural erections in Europe are made of iron, and many others, now in course of erection, are being made of the same material. Admitting that properly constructed iron houses would cost, at the outset, somewhat more than w r ooden ones, their lightness and elegance render them much superior in point of appearance, and, when their durability is taken into consideration, they will, MATERIALS OF CONSTRUCTION. 105 undoubtedly, be found cheaper in the end. But the cost of con- struction will vary, according as the details are understood by the constructors ; for if Mr. Resorr can make a vinery of iron as cheaply as of wood, then other tradesmen, when they have prop- erly understood the nature of the work, will surely be able to do the same. The Palm-house at Kew was constructed by a tradesman from Dublin, while some of the most extensive hot- house builders in England lived within the sound of their ham- mers, and the material and workmen were all brought across the channel, costing nearly as much as if brought to America ; yet the workmanship was superior, and the cost said to be less, proving that practice and knowledge of the details lessen the original cost of construction.* * As instances of comparatively easy transportability of iron hot- houses, we might mention, that the whole of the materials of the immense structure at Kew were manufactured and fitted together at Dublin, and transported from thence to London. The unequalled range of forcing-houses at Windsor, one thousand feet in length, was made at Birmingham, and fitted together in the works, before they were trans- ported to their final destination. Now it would have been just as easy, and perhaps little more expensive, to have shipped them to New York, or Boston, or Philadelphia, or Baltimore. When this is done in England, how long will American enterprise be behind them ? We prophesy, not long. SECTION VI. GLASS. 1. EXPERIMENTS which have hitherto been made, in regard to the physical properties of glass as a transparent medium, have been conducted, generally, on purely chemical principles, and mostly without reference to observed facts, as regards the growth of plants, excepting, perhaps, those of the most common and obvious character. Partly for this reason, and partly from care- less negligence, hot-houses have long been, and still continue to be, glazed with material of a very inferior description. If any one doubts this, let him look at some of the finest hot- houses in the country, and he will easily perceive the truth of this statement ; the sickly and scorched appearance of the plants under its influence, being far more painful than agreeable to the eye of any one who takes an interest in the vegetable kingdom. This evil, alone, renders the very best cultivation of no avail. The most elaborate and practically useful investigations that have yftt been made, in this department, are those lately under- taken, with the view of securing the very best material that science and art could produce, for the glazing of the great Palm- house at Kew. We cannot do better than present our readers with the following extract from Mr. Hunt's report to the com- mittee, which we take from Silliman's Journal of Science and Art, vol. iv., p. 431. " It has been found that plants growing in stove-houses, often suffer from the scorching influence of the solar rays, and great expense is frequently incurred, in fixing blinds, to cut off this destructive calorific influence. From the enormous size of the new Palm-house, at Kew, it would be almost impracticable to adopt any system of shades that would be effective, this building being 363 feet in length, 100 feet wide, and 63 feet high. It GLASS. 107 was, therefore, thought desirable to ascertain if it would be pos- sible to cut off these scorching rays by the use of a tinted glass, which should not be objectionable in its appearance, and the ques- tion was, at the recommendation of Sir William Hooker and Dr. Lindley, submitted, by the commissioners of woods, &c., to Mr. Hunt. The object was to select a glass which should not permit those heat rays, which are most active in scorching the leaves of plants, to permeate it. By a series of experiments, made with the colored juices of the palms themselves, it was ascer- tained that the rays which destroyed their color belonged to a class situated at the end of the prismatic spectrum, which ex- hibited the utmost calorific power, and just beyond the limits of the visible red ray. A great number of specimens of glass, vari- ously manufactured, were submitted to examination, and it was at length ascertained, that glass tinted green appeared most likely to effect the object desired, most readily. Some of the green glasses that were examined, obstructed nearly all the heat rays ; but this was not desired, and, from their dark color, these were objectionable, as stopping the passage of a considerable quantity of light, which was essential to the healthy growth of the plants. Many specimens were manufactured purposely for the experi- ments, by Messrs. Chance, of Birmingham, according to given directions ; and it is mainly due to the interest taken by these gentlemen, that the desideratum has been arrived at. " Every sample of glass was submitted to three distinct sets of experiments. " First. To ascertain, by measuring off the colored rays of the spectrum, its transparency to luminous influence. " Second. To ascertain the amount of obstruction offered to the passage of the chemical rays. " Third. To measure the amount of heat radiation which permeated each specimen. " The chemical changes were tried upon chloride of silver, and on papers, stained with the green coloring matter of the leaves of the palms themselves. The calorific influence was ascer- tained by a method employed by Sir John Herschel, in his ex- periments on solar radiation. Tissue paper was smoked on one side, by holding it over a smoky flame, and then, while the 10 108 GLASS. spectrum was thrown upon it, the other surface was washed with strong sulphuric ether. By the evaporation of the ether, the points of calorific action were most easily obtained, as these dried off in well defined circles, long before the other parts pre- sented any appearance of dryness. By these means it is not difficult, with ease, to ascertain exactly the conditions of the glass, as to its transparency to light, heat, and chemical agency, (actinism.) " The glass thus chosen is of a very pale yellow green color, the color being given by oxide of copper, and is so transparent that scarcely any light is intercepted. In examining the spec- tral rays through it, it is found that the yellow is slightly dimin- ished in intensity, and that the extent of the red ray is diminished in a small degree, the lower edge of the ordinary red ray being cut off by it. It does not appear to act in any way upon the chemical principle, as spectral impressions, obtained upon chlo- ride of silver, are the same in extent and character as those procured by the action of the rays which have passed ordinary white glass. This glass has, however, a very remarkable action upon the non-luminous heat rays, the least refrangible calo- rific rays. It prevents the permeation of all that class of heat rays which exists below, and in the point fixed by Sir William Herschel, Sir H. Englefield, and Sir J. Herschel, as the point of maximum calorific action, and it is to this class of rays that the scorching influence is due. There is every reason to con- clude that the use of this glass will be effectual in preserving the plants, and at the same time that it is unobjectionable in point of color, and transparent to that principle which is necessary for the development of those parts of the plant which depend upon external chemical excitation, it is only partially so to the heat rays, and it is opaque to those only that are injurious. The absence of the oxide of manganese, commonly employed in all sheet glass, is insisted on, it having been found that glass, into the composition of which manganese enters, will, after exposure for some time to intense sun-light, assume a pink hue, and any tint of this character would completely destroy the peculiar properties for which this glass is chosen. Melloni, in his in- vestigations on radiant heat, discovered that a peculiar green GLASS. glass manufactured in Italy, obstructed nearly all the calorific rays. We may, therefore, conclude that the glass chosen is of a similar character to that employed by the Italian philosopher. The tint of color is not very different from that of the old crown glass, and many practical men state, that they find their plants flourish better under this kind of glass, than under the white sheet glass, which is now so commonly employed." We understand the glass employed in the Kew Palm-house has fully answered the intended purpose, viz., of obstructing the most injurious portion of the heat rays; and we have learned, also, that it has answered all expectations as to its influence on the health of the plants, although its perfect utility, in this respect, has been doubted by some practical men. We think, however, that an absolute decision on its merits, in this respect, is rather premature, as we should prefer seeing the plants attain a greater size, so as to fill the structure more completely, and their foliage reach nearer to the glass, before pronouncing defi- nitely upon the calorific effects of the latter. As to the appearance of this glass, it is altogether a matter of taste, which we consider ourselves having no right to ques- tion ; and, upon the whole, we think it in this respect unob- jectionable. When viewed obliquely, from a distance, it is slightly gr&en, but when viewed from within, and at right angles to its surface, it is clear and nearly white. This kind of glass is highly worthy of the attention of glass-makers and horticulturists in this country, and we have no doubt, when its qualities have been fairly tested and made known, it will be extensively employed in horticultural buildings. No kind of economy is more sure to defeat its end than using cheap glass in horticultural structures. Many suppose, if a house is merely covered with glass and made transparent, that all is well. We know this to be a common opinion ; yet we are fully prepared to prove its falsity, not by mere assertion, but by indubitable facts, facts so clear that the most ignorant in these matters will be convinced, from his own observation, and on a scale so extensive, as to justify the conclusions that have been drawn from them. We know of nothing connected with the erection of horticul- 110 GLASS. tural buildings so vexatious as having the roof glazed with, bad glass ; plants of almost every kind are certain to suffer under it. Knotted and wavy glass is the worst of all, as the knots and waves form lenses, and concentrate the sun's rays upon the plants, and that part on which the concentrated ray falls is sure to be burnt. It cannot for one moment be doubted that the glass used in the majority of horticultural buildings is not only inferior, but is of the very worst description ; and, on a recent examination of one hundred houses, we found scarcely one free from the defects here spoken of. Indeed, we are fully aware of the difficulty of procuring really good glass, at reasona- ble prices, for glazing hot-houses. But there cannot be a doubt that the money saved is money lost ; and if the vexation and annoyance subsequently incurred by the use of inferior glass, be taken into consideration, few persons of sound judgment will hesitate in paying an increased price. No doubt many of our readers will suppose that we are unnecessarily particular on this point, but our experience has taught us a severe lesson, and one, too, which no doubt has been strongly impressed upon the mind of every gardener, of lengthened experience in these matters. Against such an evil there is but one resource, and a bad one it is, which is shading, either by means of cloth blinds, or by painting, the worst method of the two; but the one or the other is absolutely necessary. The first is troublesome, the other is unsightly; and, to be done right, both are expensive. We have a large house now under our management, on which the glass is so bad as to render its opacity absolutely necessary to prevent burning, even when the sun's rays have lost their meridian power. In very small houses bad glass may be used with less chance of injury, as they may be easily shaded with blinds during the noonday sun ; but in very large structures this is only accom- plished at very great expense ; and in curvilinear houses, and houses with irregular roofs, covering them with blinds is almost impossible. Painting the glass, then, is the only resource, unless glass be used which does not require it. Little has been said on the effects of glass used in hot-houses, by writers on practical horticulture. Although facts are obvious GLASS. Ill and familiar in regard to it, yet the evils seem to be passed over as results which cannot be prevented. We can at this moment point to houses standing side by side, in one of which it is impossible to grow, and keep in health, any species of vegeta- tion whatever, no matter how hardy the tissue of the foliage may be, without shading the glass almost to opacity ; while, in the other, plants with tender and delicate foliage stand compar- atively uninjured. The cause is obvious : the glass with which the one is glazed is full of waves and blotches, and altogether of the worst description ; while that of the other, though not the best, is yet of better quality. The poorer glass burns vegetation, even when the incidental angle, between the impinging ray and a perpendicular to the roof, is as much as 45. From what has been already said regarding the influence of the different solar rays on vegetation, and, more especially, the experiments made with regard to the Palm-house at Kew Gar- dens, by which it has been found possible to manufacture glass which is opaque to the scorching rays, without at the same time obstructing the light, heat, and chemical rays which are essen- tial to the development of plants, there can be no doubt that the scorching of vegetation in hot-houses, which has long been a serious drawback in exotic horticulture, can be prevented. And when more extended experiments have been made, a good material for glazing can undoubtedly be manufactured at a price that will insure its universal adoption in horticultural structures. It is to be earnestly desired that some of our enterprising manu- facturers, a class so remarkable for their fertility of invention, will take up the matter seriously, and supply us with the material which exotic horticulture so much requires. 2. Glazing. Common sash-glazing is generally performed with a lap of from one to three fourths of an inch, and, by many, with a full inch lap. This is a most objectionable method, as the broader the lap the greater the quantity of water retained in it by capillary attraction, and, consequently, the greater the breakage of the glass ; for when the internal temperature falls, and this water becomes frozen, the glass is certain to crack in the direction of the bars. The lap should never be broader than 10* 112 GLASS. a quarter of an inch, but where the panes or pieces of glass are not above five inches wide, one eighth of an inch is sufficient. Half an inch in roof-sashes, unless they are placed at an angle of not less than 45, is almost sure to produce breakage, except- ing the temperature within be kept sufficiently high to prevent the water retained between the panes from freezing. Broad laps are objectionable, also, on other accounts ; for the broader the lap the sooner it fills with earthy matter, forming an opaque space, and these spaces are so numerous as to have a very considerable effect upon the transparency of the roof, which is injurious by excluding the light, and is also unsightly in appearance. It may be puttied, but its opacity is the same, and its appearance no better than if filled with dirt. Where the lap is not more than one fourth of an inch, it may be puttied without any very disagreeable effect, but if the glass be per- fectly smooth in the edges, puttying is useless, and the glass is better without it. The most approved practice as to the laps, whether in roofs or common sashes, is, to make the breadth of the lap equal to the thickness of the glass, leaving it entirely without putty. But it is extremely difficult to get glaziers to attend to this, and it can only be obtained by employing good workmen, and keep- ing strict supervision over the work. This is not only the most elegant of all modes of glazing, but the safest for the glass, which, as we have observed, is seldom broken by any other nat- ural means but the expansion of frozen water retained between the laps. This mode is also by far the easiest to repair, and is more durable than any method of filling the laps with putty, or with lead. There are various other modes of glazing, as the lead and oopper-lap methods, which, however, are so very objectionable as to be unworthy of occupying space in our description. The methods of shield glazing are equally objectionable, and little used. Curvilinear glazing has been used somewhat extensively, and is, in the opinion of some men of undoubted skill, superior to the other methods already spoken of. Curvilinear lap-glazing appears preferable to the square mode, for various reasons, one of which is, that the curve has a ten- GLASS. 113 dency to conduct the water to the centre of the pane, which is let out by a small opening at the apex of it. If the lap is broad, however, the water is accumulated by attraction precisely in the point where it is calculated to do most injury, acting, in fact, as a power on the end of two levers of the second kind. But when the lap is not more than one sixteenth of an inch in width, no evil of this sort can happen. It ought to be borne in mind that puttying, or otherwise fill- ing up the laps, is in no case necessary if care be taken of the glazing, and smooth glass be used, and if the lap never exceeds one fourth, nor falls short one sixteenth, of an inch. However careful the laps may be puttied, in a very few years the putty begins to decay by absorption of moisture, and, when evapora- tion is great within, it becomes saturated with water, which readily freezes in frosty nights, (unless the temperature of the house is adequate to prevent it,) and breakage of glass is inevi- table. Reversed curvilinear glazing consists in making the lower edges of the panes to curve inwards, in a concave form, instead of curving outwards, in the common way. The effect of this method is the throwing of the condensed moisture down upon the bars, and thus conveying it off at the bottom of the roof, which prevents the moisture from being retained in globules, and dropping down upon the plants. This method is nothing more than reversing the position of the panes in common curvi- linear glazing, and is, according to our opinion, preferable to it. These are the most common and approved modes of glazing, although some others have been used that have not proved worthy of general adoption. Ridge-and-furrow roofs may be glazed in the same way. The size of the panes used makes no difference, large ones only tending to reduce the opaque sur- face. Anomalous surfaces may be glazed with panes according to the figures of the bars. . 3. Color of Walls. The color usually applied to hot-houses is white. As affording the finest contrast with the plants in the interior, and the vegetation around the outside of the house, the general taste is manifestly in favor of this color; and, as it is 114 GLASS. the best reflector of light, it is, also, on that account, preferable to any other. There are some considerations, however, in favor of a dark color, which, as has been already stated, absorbs a larger quantity of heat, and parts with it again on the cooling of the atmosphere. A yellow color we consider the most objec- tionable of all, both on account of its contrasting badly with the glass of the house and the verdure of vegetation, as well as the effects produced by it on the light, which, as will be seen from the preceding investigations, exercises an injurious influence on vegetation. The influence may not be so great in the reflected light, as when permeating yellow or orange-colored media, but the power is, nevertheless, exercised to some extent. The same investigations show the beneficial influence of a blue, or dark color, which perfectly accords with our observations on plants growing against dark bodies, otherwise exposed to abundance of light; and, when it is in accordance with the taste of the proprietor, we think the interior walls of hot-houses should be of a dark color. In England, where the rays of light are less powerful than here, dark-colored walls are now very common. There, light is a more important consideration than heat : the latter can be applied by artificial means ; not so the former. This probably tends to prevent the adoption of a dark color for the interior of their hot-houses. Here, dark walls are more desirable than white, as they absorb the heat-rays, during a powerful sun, and prevent the atmosphere from becoming so rapidly hot. This fact is sensibly felt on standing before walls of the different colors during the mid-day sun. By a white wall, the rays are reflected from the wall back into the air, or on any other body which is near it, by which the temperature of the air and the body is very much increased. A dark-colored wall, on the con- trary, retains the heat which falls on its surface ; and though it may feel colder, it contains more latent heat, which it only parts with when it is abstracted by the reduced temperature of the atmosphere. This, alone, is a good argument in favor of dark- colored walls in lean-to hot-houses. The inner side of the rafters, astragals, and sash-bars, should approach to the color of the glass. As the light-rays do not GLASS. 115 fall on them, nothing is gained by making them dark, and it gives the house a heavy and gloomy effect. The structure is, or should be, transparent. The impression on the mind is that of a house covered with glass; and, as the rafters and astragals are only there as supports to the glass, they should be -deprived, as much as possible, of their opaque character. When they are painted a dark color, the reverse effect is produced. A glaring white color is, also, objectionable ; it is hurtful to the eye, and generally displeasing to a refined taste : some of the different shades of cream, or light stone color, will be more effective and pleasing. The same may be said in regard to the external portions of the roof. It may, by way of contrast, be a shade or two darker than the interior; but a decidedly dark color should be avoided. We have seen various plant-houses painted dark, and even dark red, but have seen very few who admired them. We do not wish to incur censure by finding fault with the taste of those who may fancy these colors, and admit that every one has an undoubted right to gratify his own taste. We give our opinions for the benefit of those who may choose to adopt them. It is a good plan to give the wood-work of the structure a coat of some anti-corrosive paint before the color is put on. The timber is preserved much longer ; and the house requires less painting, as the timber is hardened, and more impervious to moisture. For numerous preservative solutions, see Table XVIII, Appendix. SECTION VII. FORMATION OF GARDENS. 1. Form of the Garden. The form of the garden must be determined by two conditions : first, the natural disposition of the ground chosen for its site ; and, secondly, by the aspect and position of the walls and hot-houses. If there are no hot-houses or walls, the form of the garden will be regulated mainly by the first condition. In most kitchen or culinary gardens, of any importance, if no walls are erected, wooden palings are generally substituted for them, which also regulate the disposition of the ground. The site having been fixed upon, with due regard to the considerations necessary in choosing the site for horticultu- ral structures, (see Sect. I,) these considerations being in both cases equally applicable, the next thing to be done is the dispo- sition and formation of the walks, which also define the size and shape of the borders and principal compartments of the garden. 2. Walks. The principal walks from the house to the garden should be somewhat broader than the garden walks, and should, if possible, enter the garden at the south side. This is more especially desirable if there be hot-houses on the south side. In either case, however, it is desirable, as a more favor- able impression is produced on the mind of the spectator than if entering at either side. The north side is the very worst for the principal entrance, as' the necessary offices connected with the garden, the mould-heaps, rubbish-piles, manure, &c., are generally located in that quarter ; besides, the impression, produced by the best trained trees on the walls or fences, and the general view of the ground, is lost. Next to the south, the east or west sides should be chosen. There are various methods of forming walks, according to the FORMATION OF GARDENS. 117 character of the soil and sub-soil, and the kind of material at hand to form a surface. Where the ground is naturally wet, or where there is a liability of the accumulation of water, the soil should be taken out to the depth of at least twenty inches, the section formed by the excavation forming an obtuse angle towards the centre, or forming the segment of a circle. These excavations should lead into drains, at the lowest points, to carry off the water that percolates through among the stones with which they are filled. They may be filled to within two inches of the intended surface of the walk, the largest in the bottom, and the smaller toward the surface. This forms a durable arid dry walk at all seasons ; and, where the soil contains a consid- erable quantity of stones, which have been thrown out in the process of trenching, or the rubbish of building-materials, this affords a good medium of getting them out of the way. On dry, gravelly ground, however, these excavations are use- less, so far as drainage is concerned ; and, shovelling aside the mere surface-soil, the walk may be laid down on the substratum beneath it. If the walks are on a level, or nearly so, the water generally finds its way off as quickly as it falls, and the cost of excavation is saved. The surface of walks may be formed of grass, gravel, or sand. Good gravel is the best, sand the very worst, and grass can only be introduced with propriety in particular places. Sand, or loose gravel, makes a very uncomfortable walk, and, when of great length, is tiresome and disagreeable to walk upon. A very common error, among those not acquainted with the proper method of making walks, is, to lay on too much surface- material; and, in many places, we have seen trenches taken out for walks, and filled, to the depth of a foot or more, with gravel, which, if laid on a hard surface to the depth of an inch or less, would have made a good walk, but. which, at such a depth, all the walking, rolling, and pressing of years could never make it bind. It requires more skill than is generally supposed to make good walks. Among all the operations of the garden- maker there is scarcely one which we are so much disposed to find fault with, as in the making of walks ; and this is precisely -' 118 FORMATION OF GARDENS. our reason for adverting to a matter which is apparently irrele- vant to the general character of the present work. The durability and comfort of walks consist chiefly in their power of resisting the action of the feet in walking on them, at all seasons of the year. Soft gravel walks, that yield no resist- ance to the motion of the body, are obviously unfit for being in a place where frequent walking is resorted to. Sand, also, makes a pretty walk to look at, but should never be employed where a good hard walk is required, unless it naturally pos- sesses the property of binding. It is quite possible, however, to have a hard solid walk, capa- ble of resisting the action of the feet, and yet appear to have a gravelly or sandy surface, which is frequently admired. This is effected by preparing the lower strata of open material, then a substratum of binding material, and lastly, a thin layer of whatever material is wished for the surface, which should be sifted before being laid on. It is then well watered, if dry; then rolled well in, which has the effect of mixing it with the binding stratum beneath, and leaving a smooth surface, that becomes harder the longer it is used. In making up the sub- strata, it is necessary to tread each layer firmly as it is made up, so that no hollows or inequalities may occur on its subsida- tion, and subsequent use. It must be remembered that the material of which the surface of a walk is composed, will not bind by any mechanical means, unless it contains something of a binding nature within itself. Clean gravel will not bind by any degree of mechanical pres- sure, unless it contains something to induce a general compact- ness and solidity over the w-hole surface. The best material which we have met with in this country, and which is no doubt abundant in many places, is a kind of soft decomposing sandstone rock, containing a large quantity of oxide of iron. It must be laid down where it is finally to remain, when newly taken out of the pit, then subjected to a good shower of rain, or watered, and afterwards rolled or well trodden with the feet ; it makes a solid walk, nearly as compact as the rock itself. It may be objectionable on account of its FORMATION OF GARDENS. 119 color, but this is easily changed by a thin layer of any other material on its surface, which partly mixes and binds with it. Broken, or what is sometimes called rotten rock, containing oxide of iron, is to be preferred to gravel for making a surface, and pit gravel is to be preferred to river or sea gravel, as it con- tains generally more oxide and earthy matter. Clay forms a good under-surface, and when thinly covered with small gravel and well rolled, forms a most excellent and durable walk. Common gravel may also be mixed with coal ashes and lime rubbish, which tends to bind it, and also with common garden soil ; but this is a last resource. Gravel mixed with earth, and more especially vegetable earths, has a great tendency to pro- duce weeds, and is therefore very troublesome to keep clean. It also readily absorbs moisture and becomes soft in wet weather, and especially during winter frosts. Where expense is not spared, a composition may be made, consisting of small shell gravel, or pounded granite, about one tenth part of brick-dust, and cement, mixed together. This, laid down upon a firm, prepared surface, in a wet state, and well rolled, will form a surface as hard as marble. The form of the surface should be nearly flat ; grass walks should be completely so ; gravel walks may rise slightly towards the middle, but not so much as to affect the convenience of as many persons walking abreast as the breadth of the walk will admit. A walk six or eight feet wide should not fall more than an inch towards each side, this being sufficient to throw the water that falls on it towards each side, without being any inconvenience to pedestrians occupying its whole breadth. If the walk be edged with turf, the crown of the walk should, when finished, be on a level with the turf at each side, one inch being quite enough for depth of edging ; besides, the walk gen- erally subsides, while the verges become higher, for which an allowance must be made. The same rule applies to walks edged with box, which are most suitable in a kitchen garden. 3. Borders and Interior Compartments. The width of the borders and size of the compartments must be regulated by the height of the wall or fence, and the extent of the garden. The 120 FORMATION OF GARDENS. best general rule that can be laid down is to make the breadth of the borders equal to the height of the wall or boundary fence, whatever it may be ; they may be made broader, but not nar- rower, for then they produce a bad effect; a narrow border beside a high fence is very displeasing to the eye. The size and number of the compartments are determined by the number and disposition of the walks. It is decidedly a bad plan to have too many walks, as the ground is not only taken up with them, which require a deal of labor to keep them clean, but the effect of the garden is lessened. If less than two acres be enclosed, a walk running parallel with the boundary, say twelve feet distant from it, and another intersecting the garden in the middle, running south and north, will be sufficient ; if more than two acres be enclosed, another intersecting walk, run- ning east and west, may be introduced. If the garden be worked by horse labor, the larger the compartments the better ; if wrought entirely by manual labor, these compartments may be sub-divided for the crops, by rows of fruit-trees, or fruit- bushes, as may be required. It should be observed, that to have a few walks, and those of good width, gives the garden a better appearance, and is in every way preferable to having a large number of contracted ones, and it leaves the compartments to be sub-divided by alleys or other means, as may be most con- venient for access to the crops. In many gardens, trellises or espalier rails are adopted. The proper place for an espalier rail trellis is on the inside of the principal walks, leaving a border of at least six feet. Many gardeners condemn them, arid perhaps justly, in small gardens, as it confines the ground too much ; but in large gardens, espa- liers, if well managed, are both useful and ornamental. The railing should be plain and neat, not more than five or six feet high, with the upright rails, to which the trees are tied, about eight inches apart. It is not our purpose, at present, to dwell on the laying out of gardens. We have merely adverted to the subject, in so far as it is connected with the object of this treatise. FORMATION OF GARDENS. 121 Walls. As garden walls may be regarded as horticultural structures, we will here make a few remarks upon them. In Europe, walls are built around gardens of all kinds, whether the enclosed space be one or twenty acres. Their chief use is for training the more tender kinds of fruit-trees upon their southern aspect. The enclosed space is generally appropriated to the growth of culinary vegetables, and contain- ing also the hot-houses, which occupy a part of their south aspect. These gardens are of various forms, and we have seen them circular, oval, square, and oblong. The latter shape, with the angular corners cut off, is undoubtedly the most desirable shape for a vegetable garden. The oval and polygonal forms are preferred by some, on account of their affording a more equal distribution of sun and shade. But we are at a loss to find out how this can be the case, as, however a wall may be placed, it can only obtain a certain amount of direct sunshine during the day, and the inconvenience resulting from the adoption of these forms is very considerable, both in the management and culture of the interior compartments, and in the training of the trees. Moreover, an equal distribution of sunshine is not so desirable as may appear ; as, while the warmest portion of the wall may be appropriated to the more delicate and early fruits, the coldest, or northern portion, may be as profitably appropri- ated to late sorts, or for retarding earlier kinds, both of which purposes are as useful as an early aspect. In this country, walls have been little employed in the forma- tion of gardens, and only in a few places have they been adopted, as at the fine gardens of Mr. Gushing, at Watertown, and Col. Perkins, at Brookline, in the vicinity of Boston, two of the finest gardens in this country. Some other places have also portions of walls surrounding the garden, but we have seen none where any principles of design have been adopted and car- ried out so much as at the former placet * In Hovey's Magazine of Horticulture, pp. 5053, vol. xvi., we have described the beautiful gardens at this place, from a visit which we gave them at that time. We have subsequently visited them, as well as many other places, and still consider them the finest gardens we have seen in America. They are made precisely in the style of modern 122 FORMATION OF GARDENS. In nearly all gardens, trellises and wood fences are employed instead of walls, as enclosures to the garden ground ; and these are well adapted for the purpose, as the fruits which require the protection of walls in England thrive and produce their fruit in greater perfection as open standards here. The utility of walls, however, around a garden, cannot be doubted, even in this country, especially as regards the protection they afford to trees trained on them, in early spring. Walls may be consid- ered as useful to plants trained on them, or near to them, in three ways: first, by the mechanical shelter they afford against cold winds ; secondly, by giving out the heat they had acquired during the day; and, thirdly, by preventing the loss of heat which the trees would sustain by radiation. [See Experi- ments by Dr. Wells, in the third part of this work, Section VI. Protection of Plant-houses during Night.] The same arguments which have been applied in favor of the best aspect for hot-houses, [see Section I.,] are equally appli- cable to walls. In the middle and southern states, we should think walls having a due southern aspect decidedly objectiona- ble, and, for tender and delicate kinds of fruit-trees, would decid- edly prefer either a south-eastern or a south-western aspect. The height of walls, or fences of any kind, round a garden, should always correspond to the space inclosed. Twelve feet may be taken as a maximum height. In England, low walls produce a greater effect in accelerating fruit than high ones ; i English gardens, surrounded with fine walls, with the principal range of hot-houses, about 300 feet in length, on the southern aspect of the wall on the north side of the garden, and a smaller range on the inside of the east and west walls, all lean-to houses. There are convenient back-sheds and other offices on the north side of the hot-houses. There is no wall on the south side of this garden, which we think is very appropriately dispensed with. "We regard this as a general rule, and more especially in gardens of small size, as it gives the enclosed spaces a less meagre and confined appearance. This garden, alone, of any which we have seen in this country, bears an impress of the style and genius cf Loudon. And though we have some faults to find with the surrounding grounds, nevertheless, we believe, taking it all in all, it is the most perfect specimen of modern European gardening in this coun- try. FORMATION OF GARDENS. 123 but in this country the great radiation of heat from the earth, during the heat of summer, would render low walls of little use. On the other hand, high walls have always a gloomy effect, and, where it is necessary to have high walls round a garden, it is better to relieve the monotony of the wall by making it of differ- ent heights. Hot, or flued, walls are very common in European gardens, and have been used upwards of a century; and, in our opinion, where walls can be of any importance in this country, in the practice of horticulture, it must be chiefly as flued walls. In summer, the protection of a wall is not required to ripen the common fruits, and in hot summers they are frequently injuri- ous, by the attraction and radiation of heat during the midday sun, by which the leaves are sometimes scorched. It must be as protectors of peach and apricot blossoms in spring, and accel- erating the ripening of grapes in autumn, in which they can be most serviceable to the horticulturist ; and for these purposes hot walls are of great benefit. [See Wall Heating, Part II., Sec. V.] Flued walls can be built as cheap, if not cheaper, than solid ones, and are invariably built of brick; indeed, a considerable saving of material is effected, as little more than half of the bricks required to build a solid wall will build a hollow or flued wall ; and, unless a flued wall be desired, it is better to dispense with a wall altogether, for although a wooden paling will not ab- sorb so much heat as a brick wall, as a structure for mechanical shelter it is in every way equal to it, providing it be boarded perfectly close, and sufficiently high. The comparative cheap- ness of wooden fences, for gardens, must give them the prefer- ence, and the comparative beauty of brick walls and wood palings is a matter of taste which must be decided by the pro- prietor. Walls, or close palings, must, in all cases, be faced with a light trellis, made of laths or wire, to which the trees can be trained. The injury resulting to trees nailed on walls, in our gardens, is owing to their touching the material of the wall. The branches should be trained at least six or eight inches from the surface, so as to admit a stratum of air between the wall 124 FORMATION OF GARDENS. and the branches. When this is attended to, no injury results to the foliage, even in the hottest of seasons. Boarded walls have long been used in northern countries, and are frequently made to incline considerably towards the north, so as to present a better angle to the sun's rays than if standing upright ; an expedient which here is unnecessary. We cannot help thinking that flued walls are worthy of more attention from horticulturists than they seem to have had, espe- cially when early fruit is desired, without the trouble and expense of a glazed structure, as an expedient for a hot-house. [See cut 50, in the next part of this work, page 245.] PART II. HEATING. SECTION I/ PRINCIPLES OF COMBUSTION. 1. To warm hot-houses, etc., most economically and efficiently, we must study not only the principles of heating, but, also, the principles of combustion. And as we are yet far from, having obtained a complete knowledge of the most profitable manner of submitting coal and other kinds of fuel to the process of combustion, or, of applying the caloric so obtained to increase the temperature of hot-houses, it will, therefore, be desirable to begin at the beginning of this part of our work, and before treat- ing on the different mechanical contrivances in common use for the generation and diffusion of heat by combustion, let us first consider the principles upon which these ends are to be obtained. The subject before us involves a consideration of the nature and properties of the various kinds of fuel. It examines the chemical action of their several constituents on each other. It applies those inquiries to the class of chemical results which may be useful, and avoids those which are injurious. It involves also, in an especial degree, the closest observation on the sepa- rate influences which each of the constituents of atmospheric air exercises on combustible bodies, in the generation of those extraordinary elements of nature, heat and light. And, finally, it investigates the cause and character of flame and smoke, and the influence these have on the former. Economy of fuel being one of the most important points to be sought for in a heating apparatus, we must inquire whether our common furnaces be so constructed as to give us the maximum quantity of caloric, for the fuel that is consumed. We, there- 126 HEATING. fore, must look into the furnace, and consider chemically as well as practically, the operations which are there going on, so that we may improve its arrangements, and adapt them so as to give full practical effect to the several processes which constitute combustion. To enable our practical readers to obtain a more accurate knowledge of the processes going on in the furnace, and of the results of the common mode of managing the fires of extensive forcing houses, we will enter more fully upon the constituents of coal, and the gases thereby generated, which form such an important part of the fuel itself, and which, by their escape into the atmosphere from the chimney, or into the atmosphere of the house from the flue, become the source of immense loss of heat. And, in the latter case, the loss is more than doubled, as they are destructive in the highest degree to every kind of vegetable life. In undertaking to show how these evils may be remedied, we must not be understood to concur in the exploded opinion, that these gases may be consumed by the methods hitherto used for that purpose, viz., by passing the smoke over a body of red-hot fuel at a distance from the burning and smoking mass. And however desirable it may be to know of some way of preventing smoke from being emitted in clouds from the chimney of hot- houses, yet, if we can discover no other method of obviating the evil, except " burning it," according to the common acceptation of that word, I fear we must continue to put up with the loss and annoyance as it is. It is not our purpose here to show how the smoke from fuel may be burned ; but rather, we will attempt to show how fuel may be burned without smoke. And, let it be observed, this distinction involves the main question of economy of fuel. When smoke is once produced in a furnace or flue, we believe it to be as difficult to burn it, (and convert it to heating pur- poses,) as to burn and convert the smoke issuing from the flame of a candle to the purposes of light. If, indeed, we could collect the smoke and unconsumed gases of a furnace, and separate them from the products of combustion which the flues carry off, they might, subsequently, be made instrumental to the purposes PRINCIPLES OF COMBUSTION. 127 of heat ; but, by the common method of constructing furnaces, their collection is impossible. When we see smoke issuing from the flame of an ill-adjusted common lamp, the heat and light are diminished in quantity. Do we attempt to burn that smoke ? No ; it would be impossible. Again, when we see a well-adjusted lamp burn without pro- ducing any smoke, the flame is clear and white. But here, the lamp has not burned its smoke ; it has burned without smoke; and it remains to be shown why the same methods may not be employed with regard to common furnaces, whereby they may burn without smoke, and thereby give out a greater quantity of heat, as in the case of the common and Argand lamp, since the elements of combustion in both cases are the same. 2. In pointing out the leading characteristics in the use of coals, it is unnecessary to enter into detail of the various pro- cesses of gasefaction. We will, however, give this part of our subject a little attention, as the greater portion of the practicable economy in the use of coal, and the management of furnaces, will be found more or less connected with the combustion of the gases which arise from the combustion of fuel, and as the numer- ous combinations of which they are susceptible embrace the whole range of temperature, from that of flame down to the refrigeratory point. The subject of gaseous combinations, then, is undoubtedly an important part of our inquiry. And those who would study the economy of fuel, and the obtaining from it the greatest quantity of heat, cannot altogether dispense with the part of our subject which at present lies before us. Though it may not appear equally interesting and important to every one, it is, neverthe- less, the alpha and omega of the whole process of combustion. The gardener may say, what has this to do with gardening ? But we tell him, plainly, that this is an essential part of his business, which will be generally admitted by intelligent men, that so long as a furnace is connected with a hot-house, and fuel consumed in that furnace, this must necessarily be a part of his business. On the application of heat to bituminous coal, the first result 128 HEATING. is its absorption by the coal, and the consequent disengagement of gas, from which all that subsequently bears the character of flame is exclusively derivable. This gas, whether it be in a close retort, or in a furnace, is associated with several other substances, more or less tending to deteriorate its inflammable properties and powers of giving out heat and light. In the preparation of gas, or smoke, for illuminating purposes, these impurities are separated, and the pure gas alone is used. As, however, this separation cannot be effected in a common furnace, arid, as the entire gaseous products of the coal, good and bad, are indiscriminately consumed together as they are gener- ated, it is the more incumbent on us to be cautious, lest, by any injudicious arrangement, we force these impurities into more active energy, and thus increase their deleterious power. We will not stop here to consider the nature of those impuri- ties arising out of the unions of sulphur, and the other injurious constituents of coal, although they exercise a mischievous in- fluence on the calorific effect of the gas burning in the furnace, but will consider those constituents alone, which unite in form- ing the useful gases, and from which we are to derive heat. These constituents are the hydrogen and the carbon. And the unions which alone concern us here, are, first, carburetted hydrogen; and, second, bi-carburetted hydrogen, commonly called olefiant gas. These two, and their unions with the air, in the process of combustion, we will shortly examine. Gases, as well as other bodies, endowed with the power of giving out heat and light, have been called combustible. This term has been a source of much error in practice, from a mis- conception of its meaning, under the received impression that combustibles possess, in some undefined manner, and within themselves, the faculty of burning. And, though every person knows that they will not burn without air, still the part which air acts in the process is but little inquired into. It is but lately that the nature of this union of the gas with the air has come to be fully understood ; and, although the abstract question as re- gards the immediate cause of that chemical action, which we ,?all combustion, may continue to be disputed, and new theories continue to be broached, still, for all practical purposes, it is sufficiently defined and understood. PRINCIPLES OF COMBUSTION. 129 And here we are called on to inquire, wiih reference to the gases under consideration, whether there are any peculiar conditions which can influence the amount of heat to be ob- tained from them ? and, if so, what they are ? This, again, involves other questions in reference to air, and the part which it has to act in the process ; and thus we find ourselves intro- duced into the chemistry of combustion. One advantage of receiving the subject in this light, is, that we shall see how idle would be any calculations or arrangements as to the dimensions or details of a furnace, before we had well examined and understood the rationale of that process on which these details must necessarily be contingent. For what chemist would begin by deciding on the dimensions of his retort, or other apparatus, before he had considered the particular purposes to which they were to be applied ? Yet such is the every-day practice of those who profess to instruct us in these matters. The absurdity of this practice, and the dangers into which it leads practical men, will be more apparent when we come to consider the nature of heating apparatuses, and the powers and properties which belong to each. Combustibility, then, is not a quality of the combustible taken by itself. It is merely a faculty which may be brought into action through the instrumentality of a corresponding faculty in some other body. It is, in the case now before us, the union of the combustible with oxygen, and which, for this reason, is called the supporter. Neither of which, however, when taken alone, can be consumed. To effect combustion, then, we must have a combustible, and a supporter of combustion. Strictly speaking, combustion means union; but it means chemical union, one of the ac- companying incidents of this union being the emission of heat and light. What the nature of heat is, or how it is liberated during chemical action, it is not our province to consider; nor does it relate much to our present inquiry. Sufficient for our present purpose, is the fact, that the chemical union of the com- bustible, (the coal,) and the supporter of combustion, (the oxygen of the air,) is the cause of heat being given off; and, further, 130 HEATING. that exactly in the ratio that such union is complete, is the quan- tity of heat increased. But we have not the means of obtaining this necessary sup- porter in sufficient quantity, in a separate state, except at an ex- pense which would render it incompatible with the purposes of a furnace. Our only alternative then is to apply to the atmos- phere, of which it forms a part, in order to satisfy our wants. Had we to purchase this oxygen, we would, necessarily, be more economical of its use, and inquire more respecting its application. But, finding an abundant supply at hand, in the atmosphere, and obtaining it without expense, we are careless of its use, and unconscious of its value, and take no note of the large quantity of the noxious ingredients with which it is accompanied, or loss sustained, by diminishing the supply; and hence, many of the evils, such as bad apparatus, bad fuel, and bad furnaces, might be easily remedied, were the properties of these gases fully understood. The unions we have now to consider are those which take place between the constituents of the coal and the atmospheric air, namely, the hydrogen and carbon of the former, and the oxygen of the latter. Dr. Ure calls the carbonaceous part of coal, " the main heat-giving constituent." In this he must be understood to include that portion of the carbon which forms one of the constituents of the gases alluded to, and, although, for the purposes of the furnace, so much value is set upon the solid part the coke we must not, on that account, undervalue the heat-giving properties of the gas. Indeed, the extent of those powers is strikingly brought before us, by the fact, that for every ton of bituminous coal no less than 10,000 cubic feet of gas are obtained. When we consider the immense heating powers of such a mass of flame as would be produced by 10,000 feet of gas, we cannot resist the conclusion, that there must be something es- sentially wrong in the mode of bringing it into action within a furnace, as compared to its well known efficacy in an argand burner. That this is the fact, will appear manifest as we pro- ceed. And one of our objects is to show how greater heat may be obtained by the combustion of the volatile products of the PRINCIPLES OF COMBUSTION. 131 coal, than by allowing the whole body of gas to escape into the atmosphere. Let us bear in mind, that smoke is always the same, whether it may be generated in a common fire-place, in a furnace, or in a retort; and that, strictly speaking, it is not inflammable, as by itself it can neither produce flame nor permit the continuance of flame in other bodies, as is proved from the fact that a lighted taper being introduced into a jar of coal gas, (or smoke,) is instantly extinguished. How, then, is it to be consumed or prevented, and rendered available for the production of heat ? The answer is, solely by effecting a chemical union, not with the air merely, as is the dangerous notion, but with the oxygen of the air, the "sup- porter" of flame, the heat-giving constituent of the air, in given quantities, and at a given temperature. This at once opens the main question, What are these quan- tities, and what is this temperature ? and, are there any other conditions requisite for effecting the chemical union of the oxygen of the air with the inflammable gas, to the best advantage ? Effective combustion, for practical purposes, is, in truth, a question more as regards the air and the gas ; and the former, as referable to our object, would appear better entitled to the term combustible than the latter, inasmuch as the heat is in- creased in proportion to the quantity of air we are enabled to use advantageously. Besides that, we have no control over the gas after having thrown the fuel on the furnace, but we can exercise a control over the air, as we shall show, in all the essentials of perfect combustion. It is this which has done so much for the perfection of the lamp, and may be rendered equally available for the furnace. Now, although this control, and the management arising out of it, influences the question of perfect or imperfect combustion, and, therefore, affects that of economy, yet, strange to- say, in an age when chemical science is so advanced, and in a matter so purely chemical, this is precisely what is attended to in practice. The how, the when, and the where, this controlling influence over the admission and the action of the air is to be exercised, 12 132 HEATING. are points demanding the most attentive consideration from all who are interested in these matters. Much confusion at present prevails in all that regards hot- house furnaces, as well in their practical working as regards the admission of air and the combustion of fuel. In commenting briefly upon the constituents of coal smoke, or coal gas, car- buretted hydrogen, and the quantity of air required for their combustion, we will be as explicit as possible, without going more into scientific detail than is consistent with the means and opportunities of that class of practical men for whom we write. 3. The first step towards effecting the perfect combustion of any combustible gas, is the ascertaining the quantity of oxygen with which it will chemically combine, and the quantity of air re- quired for supplying such quantity of oxygen. Here, then, we are called on for strict chemical proofs these several quantities depending, not on the dictum of any chemist, but on the faculty which each particular gas possesses of combining with certain definite proportions of the other the supporter ; these respec- tive proportions being termed " equivalents" or combining vol- umes. This doctrine of equivalents must, therefore, be under- stood before we can be prepared to admit the necessity of any precise quantities. This question, as to quantity, is also the more important when we consider that the quantity of effective heat obtained by the combustion of any body, will be in exact relation to the quantity of oxygen with which it will chemically combine. Let us begin, then, by inquiring into the constitution of the coal gas, and the relative proportions in which its constituent elements are combined, as these necessarily govern the propor- tions in which it will combine with the oxygen of the air. Now, the doctrine of " equivalents," that all-convincing proof of the truths of chemistry, being clearly defined and understood, reduces, to a mere matter of calculation, that which would otherwise be a complicated tissue of uncertainties. And let no mechanic feel alarmed at this introduction to " elementary atoms " and " chemical equivalents," or imagine it will demand a deeper knowledge of chemistry than is compatible with his sources of PRINCIPLES OF COMBUSTION. 133 information; neither let him suppose he can dispense with the knowledge of this branch of the subject* if he has anything to do with the combustion of coal. Without it, he is at the mercy of every speculative "smoke-burning" pretender; whereas, with it, his mind will be at once opened to the simplicity and efficiency I may^ add, to the truth and beauty, of nature's processes, as regard combustion. There is not, indeed, a more curious or instructive part of the inquiry than that respecting the conditions and proportions in which the compound gases enter into union with the constituents of the air; neither is there one more intimately connected with the practical details of our furnaces. These introductory remarks are, therefore, necessary for those who are not already familiar with it. Indeed, without some information on this head, the unions of the gases might appear capricious or uncertain; whereas, in fact, they are regulated by the most exact laws, and subject to the most unerring calculations/* * Mr. Parkes observes : " We are unfurnished with any definite, determinate experiments regarding the proportions in which air and fuel unite during combustion. We are, practically speaking, altogether ig- norant of the mutual relations which subsist between the combustible and the supporter of combustion^ (the fuel and the oxygen ;) and, though we know that, without oxygen, we cannot elicit heat from coal, we have yet to discover the most productive combinations of the two elements. " Here, then, remains a wide field for research and experiment, wor- thy, and, indeed, requiring the labors of a profound chemist." These matters are now better understood, and those "most productive combinations " rendered familiar and certain, by the labors of that " pro- found chemist," John Dalton, who first drew the attention of the chemical world to the subject of equivalent proportions, and taught us the impor- tance and necessity of ascertaining those proportions in fact, of " reasoning by the aid of the balance." Dalton's papers were first read before the Manchester Philosophical Society, and published in their memoirs, in the year 1803. These vol- umes are very scarce, and I have not been able, anywhere, to meet with a complete copy of them. The Royal Institution, where Davy brought his great discoveries to light, contains but the five volumes of the first series. These volumes, or, at least, the papers of Dalton, should be re- published, for the purpose of showing the correct chain of reasoning by which the mind of that acute philosopher proceeded. 134 HEATING. Much of the apparent complexity which exists on this head arises from the disproportion between the relative volumes, or folk, of the constituent atoms of the several gases, as compared with their respective weights. For instance, an atom of hydrogen (meaning the smallest ultimate division into which it is supposed to be resolvable) is double the bulk of an atom of carbon vapor ; yet the latter is $ix times the weight of the former. Again, an atom of hydrogen is double the bulk of an atom of oxygen ; yet the latter is eight times the weight of the former. So of the constituents of atmospheric air, nitrogen and oxygen. An atom of the former is double the bulk of an atom of the latter ; yet, in weight, it is as fourteen to eight. A further source of apparent complexity arises from the faculty of condensation, or diminution of bulk, which, in certain cases, attends the union of the gases. For example, one volume of oxygen and two volumes of hydrogen, when united, condense into a volume equal to that of the hydrogen alone, (the weight being, of course, the sum of both ;) that is to say, one cubic foot of oxygen chemically combined with two cubic feet of hydrogen condense into the bulk of two cubic feet : and so on, each union bearing its now ratio of volume and weight. This apparent complexity, however, we shall soon see give way to a systematic consideration of the subject. We have stated that there are two descriptions of hydro-carbon gases, in the combustion of which we are concerned ; both being generated in the furnace, and even at the same time, namely, the carburetted and bi-carluretted hydrogen gases. For the sake of simplifying the explanation, I will confine myself to the first, as forming the largest proportion of the gas to be consumed, namely, the carburetted hydrogen, or common coal gas, as I shall call it for the sake of brevity. Now as, during combustion, the atoms of this gas become decomposed, and its constituents separated ; and as these will be found to exercise separate influences during the process, it is essential that we examine them as to their respective properties, weights, and volumes. On analyzing this mixed gas we find it to consist of two vol- PRINCIPLES OF COMBUSTION. 135 umes of hydrogen and one of carbon vapor; the gross bulk of these three being condensed into the bulk of a single atom of hydrogen ; that is, into two fifths of their previous bulk, as shown in the annexed figures. Let figure A represent an atom of coal gas carburetted hydrogen with its constituents, carbon and hydrogen ; the space enclosed by the lines representing the rela- tive size or volume of each ; and the numbers representing their respective weights hydrogen being taken as unity both for vol- ume and weight/* Carburetted Hydrogen. Bi-carburetted Hydrogen. A. its constituents. 1 atom of Hydrogen, weight 1. 1 atom of Hydrogen, weight 1. 1 atom of Carbon, 6. its constituents, 1 atom of Hydrogen, weight I. 1 atom of Hydrogen, weight 1. 1 atom of Carbon, 6. 1 atom of Carbon, 6. * " Ce gaz (carburetted hydrogen) est compose de 75.17 parties (by weight) de carbone, et 24.33 d'hydrogene; ou, d'un volume de carbone gazeux et quatre volumes de gaz hydrogene, condenses a la moitie due volume de ce dernier, ou, aux 2/5 du volume total du gaz, de maniere que de cinq volumes simples, il n'en resulte pas rflus de deux de la com- binaison." Berzelius, vol. i., p. 330. 136 HEATING. Or they may be represented thus : Carburetted Hydrogen. The above Gas , I its constituents, Bi-carburetted Hydrogen. its constituents, Although not intending to take any further notice, in this place, of the bi-carburetted hydrogen, I have, however, annexed the above diagrams, representing this gas and its constituents, that both may be under view at the same time ; and by which it will be seen, that although, in volume, the two gases are precisely the same, there is yet double the quantity of carbon in the bi-car- buretted that there is in the carburetted hydrogen : this circum- stance is of great importance, and must be kept in our recollec- tion, as these proportions will be found to have a considerable influence during the subsequent process of its combustion. ^ * The mode of representing the volumes of gas, by rectangular figures, as adopted by Mr. Brande and other chemists, is favorable, so far as tingle atoms are concerned, inasmuch as the eye at once recognizes the PRINCIPLES OF COMBUSTION. 137 I would here observe on the importance of keeping in mind this double relation of weight and volume, and the atomic consti- tution of these gases, as it will prevent much of that confusion which too often embarrasses those who are not familiar with the subject of gaseous combinations. Let us now, in the same analytical manner, examine an atom of atmospheric air, the other ingredient in combustion. Atmospheric air is composed of two atoms of nitrogen and one atom of oxygen : and here again we find a great disproportion between the relative volumes of these constituents ; one atom of nitrogen being double the volume of an atom of oxygen, while their relative weights are as 14 to 8 : the gross volume of the nitrogen, in air, being thus four times that of the 'oxygen; and in weight^ as 28 to 8, as shown in the annexed figure. Atmospheric Air, (or thus,) Atmospheric Air. r 1 atom of Nitrogen, Jl weight 14. 8 1 atom of Nitrogen, equal to II 51 weight 14. o 1 atom of i Oxygen, 8. Here we are relieved from the complexity arising out of any difference in volume between these constituents, when united and when separate. In the coal gas we found the constituents con- densed into two fifths of their gross bulk when separate : this, we see, is not the case with air ; an atom of which is the same, both as to bulk and weight, as the sum of its constituents. relation between volumes and half volumes. As, however, I shall have to do with masses of these gases, I have adopted circular figures, the rela- tion between the sizes of the volumes of the different gases being the same. 138 HEATING. Thus, we find, the oxygen the heat-giving constituent of the air bears a proportion in volume to that of the nitrogen, as 1 to 5 ; there being, in fact, but 20 per cent, of oxygen in atmos- pheric air, and no less than 80 per cent, of nitrogen ; a circum- stance which should never be lost sight of in all that has to do with its admission and application. Having shown the composition of coal gas, and also of air, with the weights and volumes of their respective constituents, we now proceed to the ascertaining the separate quantity of oxy- gen required by each of those constituents, so as to effect its per- fect combustion, and produce the largest quantity of available heat ; in other words, to find the " chemical equivalent" or vol- ume of air, required for the saturation of this mixed gas. Now, this is to be decided, not by the quantity of air we may admit or force into the furnace, but solely by the faculty with which each of these constituents is endowed of uniting chemically with the oxygen. With respect to this power, or faculty of reciprocal saturation, the first great natural law is, that bodies combine in certain fixed proportions only, a remarkable feature in this law, as far as gaseous bodies are concerned, being, that it has reference both to volume and weight ; thus, by their concurrence, establishing the principle which now no longer admits of any doubt. * The important bearings of this great elementary principle of proportionate combination cannot be more strikingly illustrated, or its influence rendered more familiar, than in the several com- * " L'experience a demontre que, de meme que les elemens se com- .binent dans des proportions fixes et multiples, relativeraent a leur poids, fls se combinent aussi, d'une maniere analogue, relativement a leur volume, lorsqu'ils sont a 1'etat de gaz : en sorte qu'un volume d'un element se combine, ou, avec un volume egal au sien, ou avec 2, 3, 4 et plus de fois son volume d'un autre element a 1'etat de gaz. En com- parant ensemble les phenomenes connus des combinaisons de substances gazeuses, nous decouvrons les memes lois des proportions fixes, que celles que vous venons de deduire de leurs proportions en poids : ce qui donne lieu a une maniere de se representer les corps, qui doivent se combiner, sous des volumes relatifs a 1'etat de gaz. Les degres de combinaisons sont absolument les memes, et ce qui dans 1'une est nomme atome, est dans Vautre apelle volume} 1 Berzelius, vol. iv., p. 549. PRINCIPLES OF COMBUSTION. 139 binations of which the elements of atmospheric air are suscepti- ble, and the extraordinary changes of character and properties which accompany the changes, in the relative quantities alone, of the combining elements. FQT instance, oxygen unites chemically with nitrogen in five different proportions, forming five distinct bodies, each essentially different from the others, thus : Atoms. Weight. Atoms. Weight. Gross Weight. 1 of Nitrogen 14 unites with 1 of Oxygen 8 forming Nitrous Oxide . . 22 " 2 " 16 " Nitric Oxide . . 30 " 3 " 24 " Hyponitrous Acid 38 " 4 32 Nitrous Acid . .46 " 5 40 " Nitric Acid. .54 14 14 14 14 Or thus : ...Atmospheric Air. Nitrous Oxide. .Nitric Oxide. Hyponitrous Acid. Nitrous Acid. Nitric Acid. 140 HEATING. A description of the properties of these distinct bodies may be found in any chemical work of authority, and I only mention these unions to exemplify the importance of attending to the proportions in which bodies unite ; as we here find the very ele- ments of the air we breathe, by a mere change in the proportions in which they are united, forming so many distinct substances, from the laughing gas, nitrous oxide, up to that most powerful and destructive agent, nitric acid, commonly called aqua-fortis. This case of the combination of nitrogen and oxygen also shows the importance of the distinction between mechanical and chemical union ; these two elements being only mechanically united in forming atmospheric air, by which the essential prop- erties of its two constituents as preserved unaltered ; whereas, in the five bodies above enumerated, the union is chemical, and, consequently, the essential characters of their respective con- stituents are lost, and new ones obtained. Now, to apply these principles to the bodies under considera- tion, namely, the carbon and hydrogen, and ascertain the propor- tions of oxygen they respectively require to produce chemical union. These two constituents, though united in the one body the gas yet, not only separate themselves during combustion in a remarkable manner, but, by two distinct processes, form two essen- tially different unions. This is an important feature of the development of chemical action which the law of equivalents at once points out and enables us to satisfy, although this double process does not appear to be understood, much less to be pro- vided for, in practice, though familiar to every chemist. On the first application of heat, or what may properly be termed the firing or lighting the gas, when duly mixed with air, the carbon separates itself from its fellow-constituent, the hydro- gen, and forms a union with the former, the produce of which is carbonic acid gas. Now, the laws of chemical proportion teach us that carbonic acid is composed of one atom of carbon vapor, (by weight 6,) and two atoms of oxygen, (by weight 16,) the latter, in volume, being double that of the former, as in the annexed figure : PRINCIPLES OF COMBUSTION. 141 Carbonic acid. Thus, as far as the carbon is concerned, we obtain the infor- mation we sought, namely, its saturating equivalent of oxygen, and which we find to be just double its own volume ; or, by weight, as 16 is to 6. But, without the aid of chemistry, we should here have remained satisfied ; combustion would appear to have been complete ; there would be no smoke, and no visi- ble indication of an imperfect or unfinished process. Yet, chem- istry tells us, we have only disposed of the one. constituent of the gas, namely, the carbon, and that the hydrogen, the second constituent, remains yet to be accounted for, and converted to heating purposes. * It is true, the carbon was, in weight, equal to six parts out of eight (the original weight of the gas.) In bulk, however, it was but one fifth; and when it is recollected, that, although the illuminating properties of the carbon are superior to those of the hydrogen, yet that the heating properties of the hydrogen are far superior to those of the carbon, we can appreciate the loss sus- tained should these four fifths of the gas remain unconsumed. To this may be added, the probable injury done to the heat- ing powers of the flame by the conversion of any part of this otherwise valuable hydrogen into one of the most destructive compounds which can be met with in the furnace or flues, * I have here stated the case of the oxygen uniting with the carbon, before the hydrogen. Chemists are undecided on this point ; and, indeed, the evidence at present is quite contradictory. It is to be observed, however, that the argument, drawn from the combustion of the carbon before the hydrogen, or vice versa, is the same, as regards the point now under consideration. Whichever half passes off uncombined, is lost. 142 HEATING. namely, ammonia, composed of unconsumed hydrogen and a portion of the nitrogen liberated from the air. Thus we have a double motive for providing against the escape, unconsumed, of the hydrogen of the gas. What, then, is to be done ? Let us complete this second process as we did the first : let us supply this hydrogen, this remaining 80 per cent, in volume of the gas, with its own proper equivalent of oxygen, as we did in the case of the carbon. But what is this second equivalent ? By the same laws of definite proportions, we learn that the saturating equivalent of an atom, or any other given quantity of hydrogen, is, not double the volume, as in the case of the carbon, but one half its volume only the product being aqueous vapor, that is, steam ; the relative weights of the combining volumes being 1 of hydrogen to 8 of oxygen ; and the bulk, when combined, being two thirds of the bulk of both taken together, as shown in the annexed figure 8. * We thus find, that to saturate the one volume of carbon vapor, two volumes of oxygen are required ; whereas, to saturate the two volumes of hydrogen, one volume only of oxygen is required : thus, FIRST CONSTITUENT. Carbon. Oxygen. Vol. Atom. Weight. Vol. Atom. Weight. Vol. Atom. Weight. 1 . . 1 . . .6 unite with 1 . . 2 . . .16 forming ) . l 22 carbonic acid. ) SECOND CONSTITUENT. Hydrogen. Oxygen. Vol. Atom. Weight. Vol. Atom. Weight. Vol. Atom. Weight. 2 . . 2 . . .2 unite with 1 . . 2 . . .16 forming ) steam, j 2 ' ' 2 ' ' ' 18 Here we see, that, in the case of this first constituent, as above, the half volume of carbon and one volume of oxygen * Professor Brande puts this so clearly that I here give his own words : " The simple ratio which the weights of the combining ele- ments bear to each other involves an equally simple law in respect to combining volumes, where substances either exist, or may be supposed to exist, in the state of gas or vapor. " Thus, water may be considered as a compound of 1 atom of hydro- gen and 1 atom of oxygen, the relative weights of which are to each PRINCIPLES OF COMBUSTION. 143 become condensed into one volume of carbonic acid (as shown in the last figure) ; and that, in the second constituent, the two vol- umes (meaning double bulk) of hydrogen, and one volume of oxygen, become condensed into two volumes of steam, (as shown in the annexed figure.) other as 1 to 8. Hence, the equivalent of the atom of water will be, 1 hydrogen -f- 8 oxygen = 9. But oxygen and hydrogen exist in the gase- ous state, and the weight of equal volumes of those gases (or, in other words, their relative densities, or specific gravities) are to each other as 1 to 16 ; hence, 1 volume of hydrogen is combined with a volume of oxygen to form 1 volume of the vapor of water, or steam : for the specific gravity of steam, compared with hydrogen, is as 1 to 9. The annexed diagram, therefore, will represent the combining weights and volumes of the elements of water and of its vapor." Hydrogen, 1. = Steam, 9. Oxygen, 8. Steam. or thus, The following is also much to the point : "La composition de 1'eau cst un des elemens les plus necessaires aux calculs des chemistes, les derniers experiences de MM. Berzelius et Dulong out fourni pour sa composition des nombres qui sont adoptes t>ar t.ous les chemistes. Elle est formee d'apres eux de Oxygene 88.90 1 volume, oxygene. Hydrogene . . . .11.10 2 volumes, hydrogene. 100.00 1 volume eau. Parmi les nombreuses decouvertes que la science doit a M. Gay Lussac, on remarquera toujours la belle observation sur la composition de 1'eau, qui le conduisit a trouver les vrais rapports des gaz et des vapeurs dans leurs combinaison. Des experiences tres exactes, qu'il avoit faites con- jointement avec M. de Humboldt, lui prouverent que Ve.au formee (Pun volume d'oxygene et de deux volumes de hydrogene, resultat plainement confirme depuis par tous les phenomenes ou 1'eau joue un role actif, et qui s'accorde avec la composition trouve par MM. Berzelius et Dulong." Dumas, vol. i., p. 33. 13 144 HEATING. No facts in chemistry, therefore, can be more decidedly proved, than that one atom of hydrogen and one atom of oxygen (the former being double the bulk of the latter) unite in the formation of water; and, further, that one atom of carbon vapor and two atoms of oxygen (the latter being double the bulk of the former) unite in the formation of carbonic acid gas. Thus, the ultimate fact of which we were in search is, that the one condensed volume of the gas, as generated from the coal, requires two volumes, or double its bulk of oxygen, that being the quantity required for the saturation of its constituents when separated. Now, this is the entire alphabet of the combustion of the car- buretted hydrogen gas. Having thus ascertained the quantity of oxygen required for the saturation and combustion of the two constituents of coal gas, the only remaining point to be decided is, the quantity of air that will be required to supply this quantity of oxygen. This is easily ascertained, seeing that we know precisely the proportion which oxygen bears, in volume, to that of the air. For, as the oxygen is but one-fifth of the bulk of the air,^ne volumes of the latter will necessarily be required to produce one of the former ; and, as we want tivo volumes of oxygen for each volume of the coal gas, it follows, that to obtain those two vol- umes, we must provide ten volumes of air. Thus, then, by strict chemical proof, we have obtained these facts : First, that each volume of coal gas requires two vol- umes of oxygen ; secondly, that to obtain these two volumes of oxygen we must employ eight atoms of air ; thirdly, that these eight atoms of air are equal to ten volumes of the coal gas ; each volume of the latter, in fact, requiring ten volumes, or ten times its bulk of air : thus, Ten volumes of air are the same as eight atoms ; Eight atoms of air produce four atoms of oxygen ; Four atoms of oxygen are equal to two volumes of the same ; and Two volumes of oxygen saturate one volume of the coal gas : Therefore, ten volumes of air are required for each one volume of this gas. We now see why ten volumes of air are required for each PRINCIPLES OF COMBUSTION. 145 volume of gas, and why neither more nor less will satisfy the conditions of its combustion. For, if more, the excess, inde- pendently of the mischievous chemical unions that might enter into it in the furnace, would be the means of carrying away as much heat as it would take up by its expanding faculty. And if less, a corresponding quantity of either hydrogen or carbon would be deficient of its supporter, and necessarily pass off uncombined and unconsumed. The only observation here necessary to make on the difference between these two gases is, that as this latter gas contains two atoms of carbon instead of one, it follows that a proportionate additional quantity of oxygen will be required for this additional atom of carbon. Hence, if carburetted hydrogen requires two volumes of oxygen for combustion, the bi-carburetted hydrogen will require three volumes. And so of air : if ten volumes of air are required for the one gas, fifteen volumes are consequently required for the other gas. 4. We have seen that, in the formation of the carburetted hydrogen, a considerable portion of the carbonaceous constituent of fuel is separated, and carried away by the hydrogen in the gaseous form, forming the carburetted hydrogen ; the remainder of such carbonaceous matter is what we have now to deal with ; the difference as regards combustion between these two portions of carbon being so important as to demand especial notice. In observing this curious arrangement by which the saturation of the combustible atoms is effected, we perceive that three atoms of the combustible are apportioned to four of the supporter. This, we see, is the result of one atom of carbon requiring two of the supporters, while the two of hydrogen are satisfied with one each. Now, in this arrangement no excess or deficiency appears among the heat-producing ingredients. Could we have dis- pensed with or avoided the presence of such an excess of nitro- gen, (which is neither a combustible nor supporter of combus- tion,) the several unions would have been less embarrassed, their combustion more rapid and complete, and the intensity of their action much increased. That, however, was impossible. 146 HEATING. The presence of so large a quantity of nitrogen being the una- voidable condition of obtaining the oxygen through the instru- mentality of atmospheric air, It is to be observed that the process of combustion here described is the most perfect that could be produced, either in a furnace or lamp. Any deviation, therefore, by means of excess or deficiency, or from any interruption or interference, such as the interposition of another gas, must be more or less destructive to the desired effect, viz., the generation of the greatest quantity of available heat. 5. When we speak of mixing a given quantity of oxygen with a given volume of smoke, (or coal gas,) we do so because we know that such quantity of the former is required to saturate the latter, and by such saturation every atom of loth gases enters into union, without excess or deficiency of either, pro- ducing entire and complete combustion. So, when we speak of mixing a given volume of atmospheric air with a given volume of smoke, we do so for the same pur- pose, knowing that the precise quantity of air will provide the required quantity of oxygen. Thus, if we know that two cubic feet of oxygen are the exact saturating equivalent, or combining volume, for effecting the en- tire combustion of one cubic foot of coal gas, we know that ten cubic feet of atmospheric air will effect the same purpose, because ten cubic feet of air contain the required two cubic feet of oxygen. We require ten cubic feet of air to supply two cubic feet of oxygen, which, if the air be pure, effects the combustion of one cubic foot of coal gas, emanating from coals in the process of combustion in a furnace ; but if this quantity of air does not contain this 20 per cent., or one-fifth, of oxygen, it is clear we cannot obtain it. The air, in this case, may be said to be viti- ated, or impure. It is therefore desirable that the air admitted into a furnace should be direct from the atmosphere ; otherwise, the oxygen contained may be deficient, although the volume of air admitted be sufficiently large. PRINCIPLES OF COMBUSTION. 147 Let us now inquire how far the ordinary mode of constructing and managing our furnaces enables us to satisfy this condition. In ordinary furnaces, the supply of air is obtained by means of the ash-pit ; and the larger the ash-pit, the greater the quan- tity of air admitted. The ash-pit is made larger, under the mistaken notion that the more air we give, the better will be the draught, the more complete the combustion, and the greater the quantity of heat produced. There can scarcely be a more absurd practice than is involved in this one-sided view of the principles of combustion, even sup- posing that the introduction of air is tantamount to the introduc- tion of oxygen. It is manifest, however, that there are two different processes going on in the furnace, and two different combustibles, requiring their respective volumes of oxygen to consume them, namely, the gas or smoke generated in the body or cavity of the furnace, and passing off by the flues, and also, the solid carbon resting on the bars, both of which require sepa- rate volumes of oxygen to effect their combustion. All that seems to be concluded in practice is, that air is essential to combustion ; and that if air be admitted to the fuel, through between the bars, it will work out the process of com- bustion satisfactorily in its own way. And hence the many errors and absurdities of the present system of practice. There can be no greater mistake than letting a large quantity of air act directly on the burning fuel, which acts like a blast upon the red-hot mass, driving off the gases more rapidly, but also driving off the contained heat, and consuming the fuel with unnecessary rapidity. It seems to be taken for granted, that if air, by any means, be introduced to the fuel in the furnace, it will, as a matter of course, mix with the gas, or other combustible, in a proper man- ner, and assume the state suitable for combustion, whatever be the nature or state of such fuel, and without regard to time or other circumstances. Now, it might as well be supposed, that by bringing large masses of nitre, sulphur, and charcoal to- gether, we could form gunpowder. We know that it is by the proper mixture and incorporation of the different elementary atoms that simultaneous action is imparted to the whole ; and 13* 148 HEATING. so, also, by bringing different kinds of gases into a state of preparation for simultaneous action. The complete combustion of a body depends upon the chemical union of its atoms, or elementary divisions, with their respective equivalents of the supporter, oxygen; and which necessarily implies the bringing together, and the mixing of such atoms, previous to the mixture being fired for combustion. It is not our purpose to enter upon the theory of atomic mix- tures, or the time required to effect their combination, which will be found in the numerous chemical works of the present day. We will now proceed to consider the means by which air may be introduced to the furnace, to effect the combustion of the gases therein generated. In looking for a remedy for the evils arising out of the hurried state of things which the interior of a furnace naturally presents, and observing the means by which the gas is effectually con- sumed in the Argand lamp, it seemed manifest, if the gas in the furnace could be presented by means of jets to an adequate quantity of air, as it is in the lamp, the result would be the same, namely, a quicker and more intimate mixture and diffu- sion, and consequently a more extensive and perfect combustion. The difficulty of effecting a similar distribution of the gas in the furnace, by means of jets, however, seems insurmountable. One alternative alone remains : since the gas cannot be intro- duced by jets into the body of the air, the air might be intro- duced by jets into the body of the gas ; and this will be an effectual remedy. FIG. 33 is a section of Williams' furnace for the prevention of smoke. In this furnace, the fuel, as will be seen from the cut, is thrown immediately upon the grate bars, and through them the air finds admission to it for the purpose of consump- tion. The gases pass over the bridge C ; here they meet a cur- rent of air entering just beyond the bridge, which has been admitted by the air-tube , below the ash-pit /, into the air- chamber d, and from thence escaping through a great number of small apertures in the diffusion plate above. The force with which the air enters through this series of jets or blow-pipes enables it to penetrate into the gases, and HEATING. 149 150 HEATING. obtain the largest possible extent of contact-surfaces for the air and gases ; which is important, since the short time allowed for the diffusion would otherwise be insufficient, in consequence of the rapid passage of the smoke and gases over the diffusion plates ; e is the spy-hole for ascertaining the state of the smoke. FIG. 34 is an apparatus invented by Mr. Jeffreys, of Bristol, as long ago as 1824, for precipitating the lamp-black, metallic vapors, and other sublimated matters from smoke, by washing the latter by means of a stream of water. Where the necessary supply can be secured, this plan is both effectual and economi- cal, and well adapted for situations where the presence of smoke, as well as the impurities produced by it, is an annoyance. In the vertical section, B B is the smoke flue. The smoke passing in the direction of the arrows at A, the flue turns down- ward; and at the top of this vertical portion is a cistern E, the perforated bottom of which lets down a constant stream of water, after it is set to work. The shower, in its descent, carries all the smoke and the sublimated matter which has passed from the fire, which runs off at the bottom, F. The flue may then turn upwards, or enter a common chimney ; but little or nothing will pass up it, providing the water be kept constantly running. This apparatus is easily constructed, and is admirably suited for hot-houses situated in the midst of pleasure-grounds, where smoke is unsightly and disagreeable. Whether these methods of consuming the gases generated in the furnaces and flues of hot-houses may be considered worthy of general adoption, we cannot tell. It is, nevertheless, pre- sented to the consideration of the ingenious mechanic, not doubting that were the subject fully taken up by energetic fur- nace builders, something good would be the result. That immense quantities of fuel are wasted by imperfect combustion, cannot be doubted, when we see the dense volumes of smoke proceeding from chimneys where much heat is required. Professor Brande says, " when air is admitted in front of the furnace, or through or over the fuel, it obviously never can effect those useful purposes, which are at once obtained by admitting it in due proportion to the intensely heated inflamma- ble vapors and gases, or, in other words, to the products of the HEATING. 151 Fig. 34. 152 HEATING. distillation of coal, at such temperatures that they may take fire in its contact." If a number of jets of air be admitted into a heated inflammable atmosphere, as the body of a furnace, its combustion will be attained in such a way as to produce a great increase of heat, and, as a necessary consequence, destroy the smoke. In some of the large gardens of Europe, as well as in some manufactories, attempts have been made to consume the smoke or gases of the furnaces, by bringing them in contact with a body of glowing incandescent fuel, producing a result the reverse of what was expected, namely, the absorption of heat by their expansion and decomposition, instead of giving out heat by their combustion. It is strange that this erroneous notion should be persisted in, even at the present day, when any chemical work of good authority would satisfy any one wishing for such knowl- edge that decomposition, not combustion, is the effect of a high temperature being applied to hydro-carbon-gases ; that no possible degree of heat can consume carbon ; that it is a well- known property of both the varieties of carburetted hydrogen, that they deposit charcoal, (carbon) virtually become smoke, when heated ; that the amount of carbon deposited is propor- tioned to the increase of temperature, and that its combustion is merely produced by, and is, in fact, its union with, oxygen, which these smoke-burners take no care to provide.^ * Numerous methods have been devised for burning smoke, and patents have been issued for supposed inventions of this kind, showing the want of chemical knowledge on this subject. One consists in hav- ing a double set of fire-bars, so that when the fuel is red-hot, it is thrown back on the innermost bars, and the smoke of the fresh coal in front passing over this incandescent fuel, is supposed to be consumed in its passage. Another proposes a sliding carriage for this purpose, working on castors inside the furnace. Others of a similar kind have been put forward, and all on the same principle ; all manifesting the same neglect or ignorance of chemistry, for chemistry teaches us that heat has nothing to do with the combustion of smoke beyond this, that a certain temperature is essential to the development of chemical action between the combustible and the supporter, when they are brought together. But producing heat is not producing air ; and decom- position is not, in this respect, combustion. PRINCIPLES OF COMBUSTION. 153 The neglect of chemistry when treating of combustion, and the results of this neglect in these smoke-burning furnaces, can- not be too strongly exposed ; neither can its study be too strongly enforced, seeing that it is practically within the reach of all. For chemistry is no longer the mysterious alchemy that it was a century ago ; it is now a mere rigid inquiry into nature's pro- cesses and laws, by the aid of those proofs and illustrations which nature herself has supplied. It has taken its place among the exact sciences, and now recognizes no man's dictum or opin- ion, apart from experimental tests, and strict, substantial evidence. Looking, then, to chemistry, we would add, in reference to these smoke-burning expedients, that, in seeking to obtain heat from gas, (or smoke,) the bringing it into connection with ignited carbonaceous matter, or to anything approaching the temper- ature of incandescence, is absolutely useless, if not injurious, until we are assured of having the means of contact with air fully provided for. The mere enunciation of a plan " for consuming smoke" is prima facie evidence that the inventor has not studied and con- sidered the subject in its chemical relations. Chemists can understand a plan for the prevention of smoke ; but as to its combustion, it is so unscientific, not to say impossible, (if there be any truth in chemistry,) that such phraseology should be avoided. The popular phrase, " A furnace burning its own smoke," may be justifiable, as conveying an intelligible mean- ing ; but, in a work having any pretensions to science, or from any one pretending to teach those who are unable to distinguish for themselves, and who may easily be led into error, is wholly objectionable. 6. Construction of Furnaces. From what has been already said, in the preceding part of this section, it will be seen that the construction of furnaces is a matter of great importance in the economy of heat. To investigate the various varieties of furnaces which have been recommended, would occupy too much of our space at present, especially as we shall have to refer to them hereafter, when treating of the different methods of heat- ing; besides, in small apparatuses, the intense heat required 154 HEATING. for large boilers is unnecessary. A very moderate heat, ap- plied on the most economical principle, and the furnace so con- structed as to make the fuel burn for a long time, without much attention, and without much escape of smoke, is the grand desideratum, and which is easily accomplished, with a moderate degree of care and skill in the erection. Passing over, then, as unnecessary for our purpose at present, the many ingenious forms which have been given to furnaces, we will proceed to describe the most simple plan, which, in our experience, is the most effectual in the combustion of the fuel, as well as the least expensive in the construction. It should be an object of consideration, in building the fur- nace, to confine the generated heat within the cavity of the furnace as much as possible, so that the gases generated by the combustion of fuel may be prevented from passing too rapidly along the flue ; this is more especially requisite with boiler furnaces. The throat of the furnace should be contracted as much as possible. In furnaces where the only entrance for air is by the bars, provision should be made for the entrance of enough but no more than enough for the combustion of the fuel, and the entrance should, in all cases, be regulated by a damper, on the ash-pit door. It should be considered, that the rarity of the heated gases causes them to force their pas- sage through the throat of the furnace, just in the proportion of its size. We have already shown that any air entering through the door of the furnace reduces the intensity of the heat, although it is supposed by some that the passage of air over the burning fuel promotes the more perfect combustion of the gaseous pro- ducts of the coal. But even if this be correct, the heat will be reduced, and less heat will be generated in a given time, than if the whole gaseous products escaped by the chimney. The kind of fuel to be burnt must, in all cases, determine the width of the bars ; and as a certain open area is necessary for the admission of air to effect combustion, it is desirable that this area should be known. Supposing the ordinary kind of furnace bars to afford about thirty inches of opening for air for every square foot of surface, PRINCIPLES OF COMBUSTION. 155 then supposing you wish to erect a hot water apparatus the relative proportions between the area of the bars and the length of pipe would be as follows : Area of Bars. 75 square inches will supply 100 " " 150 " " 200 " " 250 " " 300 " " 400 " " 500 " " 4 inch pipe. 3 inch pipe. 2 inch pipe. 150 feet, or 200 feet, or 300 feet. 200 266 400 300 400 600 " 400 533 800 " 500 666 1000 " 600 800 1200 " snn mfifi < t 1600 " 1000 1333 " 2000 " Thus, suppose there are six hundred feet of pipe, four inches in diameter, in an apparatus, then the area of the bars should be three hundred square inches, so that thirteen inches in breadth, and twenty-three inches in length, will give the re- quired quantity of surface. When it is required to obtain the greatest heat in the shortest time, the area of the bars may be a little increased. In order to make the fire burn for a long time without atten- tion, the furnace should extend beyond the bars, both in length and breadth ; and the coals, which are placed on this blank part of the furnace, in consequence of receiving no air from below, will burn slowly, and will only enter into complete combustion when the rest of the coal, on the bars, has been consumed. It may be observed, that as the maximum effect of the furnace is seldom required, the register on the ash-pit door, and the damper in the flue, must be used to regulate the draught, and thus limit the consumption of fuel. 14 SECTION II. PRINCIPLES OF HEATING HOT-HOUSES. 1. Effects of artificial heat. The effects that are produced upon the functions of vegetables, by atmospheric air that has passed over intensely heated surfaces, are perceptible to the most casual observer. The changes, therefore, that are produced upon atmospheric air by subjecting it to a high temperature, are of the utmost importance to the horticulturist, and consequently demand our particular attention. When common air passes over highly heated surfaces, the small particles of animal and vegetable matter, (organic mat- ter,) which are always held in suspension by it, are decomposed by the heat, and resolved into various elementary gases. This is one of the causes of the unpleasant smell which results from this method of heating, as in common stoves, Polmaise furnaces, &c. But, in addition to this, the aqueous vapors of the atmos- phere are almost entirely decomposed, the oxygen entering into combination with the iron, and the hydrogen mixing with the air. The changes which have thus taken place, render the atmosphere extremely deleterious to both animal and vegetable life. The mixture of the hydrogen thus disengaged is even more injurious to the plants than the alteration which has taken place in its hygrometric state, as this will be partly supplied by the moisture contained in their tissue, until it be restored to the atmosphere by evaporation, which is easily effected. The particles of animal and vegetable matter as we have said are decomposed by the heat; and they then produce extraneous gases, consisting of sulphuretted, phosphuretted, and carburetted hydrogen, with various compounds of nitrogen and PRINCIPLES OF HEATING HOT-HOUSES. 157 carbon, which, in the state in which they exist, are highly inim- ical to vegetable life. * The quantity of hydrogen which is eliminated by the decom- position of water contained in the air is one thousand three hun- dred and twenty-five cubic inches for every cubic inch of water that is decomposed ; and if the dew point of the air be 45 at an average, this quantity will be given out from every seventy-two cubic feet of air wliich passes over the heated surface. It is, therefore, not difficult to account for the effects produced on vegetation by hot-air stoves, in consequence of the air, when thus artifically dried, abstracting too much moisture from their leaves. It is also clear that the injury must increase in propor- tion to the length of time the apparatus continues in use, by the plants being surrounded by, and compelled to inhale, these extra- neous gases, which are evolved from the decomposition of the constituents of the atmosphere. The extreme dryness of the air, after it has been deprived of * I am unable to ascertain the exact nature and extent of the change which atmospheric air undergoes by being passed over intensely heated metallic bodies j but whatever be the chemical alteration which occurs, a physical change undoubtedly takes place, by which its electrical con- dition is altered. From some experiments recorded in the Philosophical Transactions of the Royal Society, made with a view of ascertaining the effect produced on the animal economy by breathing air which has passed through heated media, it appears that the air which has been heated by metallic surfaces of a high temperature must needs be exceedingly unwholesome. A curious circumstance is related, in reference to these experiments, which is illustrative of this fact. " A quantity of air, which had been made to pass through red-hot iron and brass tubes, was collected in a glass receiver, and allowed to cool. A large cat was then plunged into this air, and immediately she fell into convulsions, which, in a minute, appeared to have left her without any signs of life ; she was, however, quickly taken out and placed in the fresh air, when, after some time, she began to move her eyes, and, after giving two or three hideous squalls, appeared slowly to recover. But on any person approaching her, she made the most violent efforts her exhausted strength would allow to fly at them ; insomuch, that, in a short time, no one could approach her. In about half an hour she recovered, and became as tame as before." 158 PRINCIPLES OF HEATING HOT-HOUSES. its hygrometric vapor by passing over a hot-air stove, such as polmaise, is productive of the worst consequences to growing plants. To remedy this evil, a trough of water is laid over the heating surface, which in some degree mitigates this evil. The evil, however, cannot be entirely got rid of by this means ; for even if the proper quantity of moisture can be again restored to the air, the effects which result from the use of extraneous gases are in no way removed. When the surface of radiation is an iron plate, these injurious effects are much greater. The heating by means of brick flues is, in some respects, similar to the effects produced by hot-air stoves, but only when the flues are heated to a high temperature, which is unneces- sary. In the latter case, an unwholesome smell is also produced, by the decomposition of the organic matter in the atmosphere, and in some cases, probably, by a small portion of sublimed sulphur from the bricks, as well as by the escape of various gases through the joints or accidental fissures of the flues. These contingent causes may, however, be in a great measure avoided. The hygrometric vapors of the atmosphere are not decomposed by this system of heating, as by a hot-air stove, because when the flues are warmed to a common temperature, the heat is perfectly pure, and the materials of which the flues are built having but little affinity for oxygen, they are conse- quently more healthy than hot-air stoves. Air passing over a highly heated surface of iron is, therefore, more injurious than when passed over any other body, as stone, or brick, as the power of iron to decompose water increases with the temperature to which it is heated. The limit to which the temperature of any metallic surface ought to be raised, for warm- ing horticultural buildings, (or indeed any other buildings,) is 212, if a healthy, uncontaminated atmosphere be desired. The importance of this rule cannot be too strongly insisted on, for upon it entirely depends the healthiness of every system of artificial heat. 2. Laws of Heat. Heated bodies give off their caloric by two distinct methods radiation and conduction. These are governed by different laws ; but the rate of cooling or parting PRINCIPLES OF HEATING HOT-HOUSES. 159 with heat by both modes, increases in proportion as the heated body is of greater temperature above the surrounding medium. The cooling of a heated body, under ordinary circumstances, is evidently the combined effects of radiation and conduction ; the conductive power of the air is, evidently, owing to the ex- treme mobility of its particles, for otherwise it is one of the worst conductors with which we are yet acquainted, so that when confined in such a manner as to prevent its freedom of motion, it becomes useful as a non-conductor. The proportion which radiation and conduction bear to each other has, in general, been very erroneously estimated. Count Rumford considered the united effect, compared with radiation alone, was as five to three, and Franklin supposed it to be as five to two. No such general law, however, can be deduced, for the relative proportions vary with the temperature, and with the peculiar substance, or surface, of the heated body ; for, while the cooling effects of the air, by conduction, is the same on all substances, and in all states of the surface of those substances, radiation varies very materially, according to the nature of the surface. The influence of the air, by its power of conduction, varies also with its elasticity. The greater its elastic force, the greater also is its power of cooling, according to the following law : When the elasticity of the air varies in a geometrical progres- sion whose ratio is 2, its cooling power also changes in a geo- metrical progression whose ratio is 1.366. The same law holds with all gases, as well as with atmos- pheric air ; but the ratio of the progression varies with each gas. To show the relative velocities of cooling at different temper- atures, the following table, constructed from the experiments of Petit and Dulong, is given. The first column shows the excess of temperature of the heated body above the surrounding air ; the second column shows the rate of cooling of a thermometer with a plain bulb, and the third column gives the rate of cooling when the bulb was covered with silver leaf. The fourth column shows the amount due to the cooling of the air alone ; and by deducting this from the second and third columns respectively, 160 PRINCIPLES OF HEATING HOT-HOUSES. we shall find what is the amount of radiation under the two different states of surface, noticed at the top of the second and third columns. * Excess of temperature of the thermometer above that of the air. Centigrade Scale. Total velocity of cooling of the na- ked bulb. Total velocity of cool- ing of the bulb cov- ered with silver leaf. Amount of cooling due to conduction of air alone. 260 24-42 10-96 8-10 240 21-12 9-82 7-41 220 17-92 8-59 6-61 200 15-30 7-57 5-92 180 13-04 6-57 5-19 160 10-70 5-59 4-50 140 8-75 4-61 3-73 120 6-82 3-80 3-11 100 5-57 3-06 2-53 80 4-15 2-32 1-93 60 2-86 1-60 1-33 40 1-74 96 80 20 77 42 34 10 37 19 14 Some very remarkable effects may be perceived by an inspec- tion of the above table. It appears that the ratio of heat lost by contact of the air alone, is constant at all temperatures ; that is, whatever is the ratio between 40 and 80, for instance, is also the ratio between 80 and 160, or between 100 and 200. This law is expressed by this formula : where t represents the excess of temperature, and n a number which varies with the size of the heated body. In the case represented in the foregoing table, n = 0.00857. Another remarkable law, is that the cooling effect of the air is the same, for the like excess of heat, on all bodies, without regard to the particular state or nature of their surface. This * The temperatures of this table are expressed in degrees of the Cen- tigrade thermometer, as the zero of this thermometer is the freezing point of water, and from that to the boiling point of the same fluid is 100. In order to find the number of degrees on Fahrenheit's scale, which answers to any given temperature of the Centigrade, multiply the number of degrees of Centigrade by 9, and divide the product by 5 j add 32 to the quotient thus obtained, and this sum will be the number of degrees of Fahrenheit required. PRINCIPLES OTF HEATING HOT-HOUSES. 161 was ascertained by Petit and Dulong, in a series of experiments, not necessary here to detail, but which proved the accuracy of the deduction. By comparing the second and third columns of the above table, it will be immediately perceived that the loss of heat by radiation varies greatly, with the nature of the radiating sur- face ; though, whatever be the nature of the surface, the loss of heat is the same in all cases, though in a different ratio. It should be observed, that, in this table, the second, third, and fourth columns show the number of degrees of heat which were lost per minute by the body which was subject to the experiment ; and, therefore, these numbers represent the velocity of cooling. The fact, already adverted to, that the ratio of cooling in those bodies that radiate least is more rapid at low tempera- tures, and less at high temperatures, than those bodies that radiate most, is, perhaps, one of the most remarkable of the laws of cooling. It was first deduced experimentally by Petit and Dulong, arid it may be mathematically proved from their for- mula ; but it is unnecessary here to enter into the investigation. It appears, however, that when the total cooling of two bodies is compared, the law is more rapid at low temperatures for the body which radiates least, and less rapid for the same body at high temperatures ; though separately, for conduction and radia- tion, the law of cooling is, for the former, irrespective of the nature of the body, and for the latter, that all bodies preserve at every difference of temperature a constant ratio in their radi- ating power. It is not our purpose to enter minutely into detail on the laws of heat, which will be found in modern works on chemistry, and which ought to form part of the studies of all young gar- deners who wish to become acquainted with the principles of hot-house management. We will now proceed to consider the specific properties of air and water as agents in the heating of horticultural structures. 3. Specific heat of air and water. Very erroneous notions are entertained by many persons as to the absolute quantity of 162 PRINCIPLES OF HEATING HOT-HOUSES. heat taken up by different substances. To ascertain, therefore, the effect a certain quantity of water will produce in warming the air of a hot-house, there appears to be no better method than that of computing from the specific heat of gases compared with water. Every substance has its peculiar specific heat. Now, one cubic foot of water, by losing one degree of heat, will raise the temperature of 2990 cubic feet of air the extent of one degree ; and, by the same rule, by losing 10 of its heat, it will raise the temperature of 2990 cubic feet of air 10 degrees; and so with similar quantities in similar proportions. In order to know the time it will take to heat a certain quan- tity of air any required number of degrees, by means of hot water contained in metal pipes, we must calculate the effect from direct experiment ; and, as the radiating and conducting powers of different substances differ considerably, it is necessary that the experiment be made with the same material as the pipes for which we wish to estimate the effect. From data obtained by experiments on the cooling of iron pipes, it appears that the water contained in a pipe 4 inches in diameter loses -851 of a degree of heat per minute, when the excess of its temperature is above 125 degrees above that of the surrounding air. There one foot in length of a pipe 4 inches diameter will heat 222 cubic feet of air one degree per minute, when the difference between the temperature of pipe and the air is 125 degrees. To calculate from this data, however, the length of a pipe, of any given size, that will be necessary to warm a house, and to maintain it at any given temperature under a certain external temperature, it will be necessary to estimate the heat lost by the conducting and radiating power of the glass, and of any metallic substance used in the structure. Heating horticultural structures is a very different matter from heating solid opaque buildings ; and here many erectors of heating apparatus fall into error. They suppose, because an apparatus of certain power heated a large building, a church or a hall, one of proportionate dimensions should warm a hot- house of proportionate size, without taking into full considera- PRINCIPLES OF HEATING HOT-HOUSES. 163 tion the great difference of the external radiation, and the con- duction of heat by the materials of the building. The loss of heat by buildings covered with glass is very great. It appears, by experiment, that one square foot of glass will cool down 1-279 cubic feet of air as many degrees per minute as the internal temperature of the house exceeds the temperature of the external air; thus, if the difference between the external temperature and the temperature of the house be 30 degrees, then 1-279 cubic feet of air will be cooled 30 degrees by each square foot of glass ; or, more correctly, as much heat as is equal to this will be given off by each square foot of glass, for, in real- ity, a very much larger quantity of air will be affected by the glass, but it will be cooled to a less extent. The real loss of heat, however, from the house will be what is here stated. There are various causes likely to affect these calculations, such as, High winds, which are found to reduce the internal tempera- ture more than actual cold, or even frost ; Condensation of moisture on the glass, which prevents the escape of heated air ; and, when a certain temperature is main- tained within, prevents radiation from the glass to a great degree ; The extent of wood in the roof of the house, which also pre- vents radiation and conduction, as in the case of metallic roofs. These circumstances will be found to affect, in a greater or less degree, the air of the house, though, under general circum- stances, these calculations will be nearly correct. In estimating the quantity of glass surface contained in a building, the extent of wood surface must be carefully excluded. This is particularly necessary in all horticultural buildings, where the maximum of heating power is dependent upon the estimate taken. The readiest way of calculating, and suffi- ciently accurate for ordinary purposes, is to take the square sur- faces of the sashes, and then deduct one eighth of the amount for wood work. In the generality of horticultural buildings, the wood work fully amounts to this quantity. When the frames and sashes are made of metal, the radiation of heat will be quite 164 PRINCIPLES OF HEATING HOT-HOUSES. as much from the frame as from the glass ; therefore no deduc- tion is required in such cases. From the preceding calculations the following corollary may be drawn : The quantity of air to .be warmed per minute in habitable rooms and public buildings, must be 3^ cubic feet for each person the room contains, and l cubic feet for each square foot of glass. For conservatories, forcing-houses, and all buildings of this description, the quantity of air warmed per minute must be l cubic feet for each square foot of glass the structure contains. When the quantity of air required to be heated has thus been ascertained, the length of pipe to heat it by hot water may be found by the following table : Table of the quantity of pipe 4 inches diameter which will heat 1000 cubic feet of air per minute, any required number of degrees. The temperature of the pipe being 200 Fahrenheit : Temperature of exter- nal air. Temperature at which the house is required to be kept. Fahrenheit's scale. | 45 | 50 J | 55 60 J | 65 | 70 | 75 80 J | 85 | 90 10 126 150 174 200 229 259 ; 292 328 367 409 12 119 142 166 192 220 251 283 318 357 399 14 112 135 159 184 212 242 274 309 347 388 16 105 127 151 176 204 233 265 300 337 378 18 98 120 143 168 195 225 256 290 328 368 20 91 112 135 160 187 216 247 281 318 358 22 83 105 128 152 179 207 238 271 308 347 24 76 97 120 144 170 199 ' 229 262 298 337 26 69 90 112 136 162 190 220 253 288 327 28 61 82 104 128 154 181 211 243 279 317 30 54 75 97 120 145 173 202 234 269 307 Freezing point 32 47 67 89 112 137 164 193 225 259 296 34 40 60 81 104 129 155 184 215 219 286 36 32 52 73 96 120 149 175 206 239 276 38 25 45 66 88 112 138 166 196 229 266 40 18 37 58 80 104 129 157 187 220 255 42 10 30 50 72 97 121 148 178 210 245! 44 3 22 42 64 85 112 139 168 200 235 46 15 34 56 79 103 130 159 190 225 48 7 27 48 70 95 121 150 181 215 50 19 40 62 86 112 140 171 204 52 11 32 54 77 103 131 161 193 To ascertain, by the above table, the quantity of pipe required to heat 1000 cubic feet of air per minute, find, in the first column, PRINCIPLES OF HEATING HOT-HOUSES. 165 the temperature which corresponds to that of the external air, which may be the medium (or average) of your locality. Then, in the other column, find the temperature required in the house ; then, in this latter column, and on the line which cor- responds with the external temperature, the required number of feet of pipe will be found. Supposing, now, that a forcing-house is to be kept at 75 de- grees, and the average of the external thermometer in the coldest weather, taken at 10 (Fah.) ; then, by the foregoing table, we find, under the column 75, and on the line 10, for external temperature, the quantity 292, which is the number of feet of pipe required to heat 1000 cubic feet of air per minute, the proposed number of degrees. Of course, the volume of air in the house must be previously ascertained. Any other differ- ence of temperature may be found in the same way. It will thus be perceived, that the amount of heat required for warming a glazed structure is much greater than that re- quired for warming an opaque building of the same size, in consequence of the radiation of heat from its surface ; and the difference is much greater than the allowance made by erectors of heating apparatuses, under general circumstances. To ascertain the effect of glass windows in cooling the atmos- phere of a house, the following experiments were made, with a vessel as nearly as possible the same thickness as the glass ordinarily used for glazing. The temperature of the house, in these experiments, was 65 ; the thickness of the glass was .0825 of an inch ; the surface of the vessel measured 34-296 square inches, and it contained 9-794 cubic inches of water. The time in which this vessel cooled, when filled with hot water, is shown as follows : Thermometer cooled. Observed time of cooling. Calculated time of cooling. Average rate of the observed time of cooling. from | to 150 150 150 150 140 130 120 110 6' 40" 14 50 23 30 34 & 51" 14 43 23 40 34 1-176 per minute, at an excess of 65 above the tempera- ture of the air. From the average rate of cooling here given, the effect of glass in cooling the atmosphere of a room may easily be calcu- 166 PRINCIPLES OF HEATING HOT-HOUSES. lated, as the specific heat of equal volumes of air and water is as 1 to 2990. The above average will show that each square foot of glass will cool 1-279 cubic feet of air one degree per minute, when the temperature of the glass is one degree above that of the external air. But by this we can only find the effect of glass in a still at- mosphere, and, therefore, to find the effect of glass in cooling the volume of a hot-house, especially when exposed to the action of winds, further experiments are necessary, of which we shall treat in a subsequent part of this work, in connection with " pro- tection of hot-house roofs during the night." , SECTION III. HEATING BY HOT WATER, HOT AIR, AND STEAM. 1. THE practice of employing hot water, circulating through metallic tubes, or wooden troughs, for diffusing artificial heat in horticultural structures, though of recent origin, has now become so general, that its merits are fully acknowledged as the best method that has yet been invented, to effect the purpose with efficiency and economy. Until the last few years, although its powers and properties were fully known, it had been chiefly confined to a few cases of experiment, rather than to any general or useful purpose. The present day, however, has fully revealed its merits, and shown the great, the unlimited, extent of its practical application and general utility. When we see such an immense structure as the great Palm house, lately erected at Kew Gardens, in London, heated with hot water in preference to all other modes ; when we see the lately applauded mode of heating by steam abandoned; when we see the powerful, but unsuccessful, at- tempt to establish a new system of heating by hot air, called Polmaise, by some of the first horticulturists of England ; when we see this system, notwithstanding its powerful supporters, driven into obscurity, and all but annihilated, by the well-tried superiority of hot water, which maintains its proud preeminence over all other methods of heating, and has its superiority ac- knowledged, even by its enemies. One of the greatest advantages which this mode of heating possesses over all others, is, that a greater permanency of tem- perature can be obtained by it, than by any other method. The difference between an apparatus heated by hot water, and one heated by steam, is not less remarkable, in this particular, than in its superior economy of fuel. 15 168 HEATING BY HOT WATER, HOT AIR, AND STEAM. 2. Comparison of heat in water and steam. The heating of horticultural buildings by steam had its day and its admirers, though both are now numbered among the things that were. Even if the original outlay were equal, the additional outlay for fuel, the risk of explosion from neglect, and the want of perma- nency in the apparatus to maintain the heat for any length of time, are insuperable objections to its adoption. Among many instances that could be given of this method of warming large houses, we might mention the large Palm house, in the Royal Botanic Garden of Edinburgh, which was erected when heating by steam was in the height of its fame. This house is about fifty feet high and seventy-five feet wide, in the form of an octagon ; the pipes are laid around the side of the wall. There is a contrivance, however, resorted to here, in connection with the system, to which its success in heating the house may be somewhat, if not entirely, attributed. The steam is thrown into large iron boxes, loosely filled with stones and pieces of brick, for the retention and absorption of the heat. These iron boxes are placed underneath the shelf that surrounds the house, and close by the side of the wall, and at regular distances from each other. By this contrivance, the temperature .of the house is kept up for a considerable time longer than would be by the circulation of the steam alone. Indeed, we believe it was found perfectly impracticable to maintain the proper temperature, dur- ing cold nights, until this expedient was adopted, viz., of filling the boxes with absorbing materials. We have known conservatories, in which steam apparatuses had been erected, taken down, and their place supplied with others of hot water, merely in consideration of the consumption of fuel and extra attention required by a steam apparatus, keep- ing the danger of explosion out of the question. It seldom happens that the pipes of a hot-water apparatus can be raised to so high a temperature as 212 ; in fact, it is not desirable to do so, because it is unnecessary to generate steam, which would only escape by the air vent, without affording any available heat. Steam pipes, on the contrary, must always be above the temperature of 212, otherwise steam will not be gen- erated ; and here the grand point to be attended to in artificial HEATING BY HOT WATER, HOT AIR, AND STEAM. 169 heating is nullified, namely, the diffusion of heat at a low tem- perature. A given length of steam pipe, however, will afford more heat than one heated by hot water, by the aggregate cal- culation of its specific heat. But, if w consider the relative permanency of temperature, we shall find a very remarkable dif- ference in favor of pipes heated by hot water ; and the calculations here given are fully confirmed by experience and observation. The weight of steam, at the temperature of 212, compared with the weight of water at 212, is about as 1 to 1694, so that a tube that is filled with water at 212 contains 1694 times as much matter as one of equal size filled with steam. If the source of heat be withdrawn from the steam pipes, the temper- ature will soon fall below 212, and the steam immediately in contact with the pipes will condense ; but, in condensing, the steam parts with its latent heat, and this heat, in passing from the latent to the sensible state, will again raise the tem- perature of the pipes ; but, by the withdrawal of the heat from the boiler, the action of the cold air on the pipes quickly con- denses the whole of the steam contained in them, which, when condensed, possesses just as much heating power as the same bulk of water at a similar temperature. This water now occu- pies only T ^g- P art f t ^ le s P ace which the steam originally did in the pipes. The specific heat of uncondensed steam, compared with water, is, for equal weights, as -8470 to 1 ; but the latent heat of steam being estimated at 1000 degrees, we shall find the relative heat obtainable from equal weights of condensed steam and of water, reducing both from the temperature of 212 to 60, to be as 7-425 to 1 ; but for equal bulks it would be as 1 to 228 ; that is, bulk for bulk, water will give out 228 times as much heat as steam, reducing both to the temperature of 60. A given bulk of steam, therefore, will lose as much of its heat in one minute, as the same bulk of water will lose in three hours and three quarters. It must be considered, however, that when the water and steam are both circulated in iron pipes, the rate of cooling will be somewhat different from this ratio, in consequence of the 170 HEATING BY HOT WATER, HOT AIR, AND STEAM. much larger quantity of heat contained in the metal, than in the steam with which the pipe is filled. The specific heat of cast iron being nearly the same as water, the water being 1000 and the iron 1100, if we take two similar pipes, four inches in diameter and one fourth of an inch thick, the one filled with water and the other with steam, each at the temperature of 212, the one which is filled with water contains 4-68 times as much heat as the one which is filled with steam. Therefore, if the pipe with the steam cools down to the temper- ature of 60 in one hour, the one filled with water would require four hours and a half, under the same circumstances, before it reached the like temperature. But this is merely reckoning the effect of the pipe and the fluid contained in it. In a steam apparatus, this is all that is effective in giving out heat ; but in a hot-water apparatus there is likewise the heat from the water contained in the boiler, and even of the brick-work around the boiler, all which tends to increase the heat of the pipes, long after the fire is extinguished. In the one, the heat will continue to circulate through the pipes as long as any heat remains about the fire-place, because the circulation will continue in the pipes until the whole apparatus is cooled down. But, in the case of steam pipes, as soon as the water in the boiler falls below the boiling point, (212,) circula- tion ceases, and the pipes then begin to cool, the remaining heat in the boiler and furnace goes for nought. From these causes the difference in permanency of hot water and steam will be clearly apparent, and the fact of a house heated with hot water keeping up its temperature at least six times as long as one heated with steam, will be fully understood by those interested in the matter. These considerations are of the utmost importance to those erecting horticultural build- ings, or, indeed, any other kind of buildings requiring artificial heat. This admirable property, which water possesses, of re- taining its heat, of carrying it to any distance, and, without difficulty, giving it out gradually, or retaining it for many hours, renders it of vast importance to gardeners, and prevents the necessity of that constant attention to the fire, which forms so serious an objection in all other methods of heating. HEATING BY HOT WATER, HOT AIR, AND STEAM. 171 We find, by experience, that no system of heating horticul- tural buildings in all respects answers the purpose so well as a hot-water apparatus, well constructed, and judiciously arranged, in regard to the amount of work it has to do, so that it may not be necessary to strain it, on exigencies, to its maximum point of strength. In whatever point of view it may be regarded, it is, undoubtedly, the best for all practical purposes ; and the best possible evidence of its utility is derived from the fact, that no case has ever come under our knowledge, wherein it has failed to give complete satisfaction, when it has been properly con- structed, rightly managed, and judiciously arranged, in regard to supplying a sufficient amount of radiating surface for the work it has to do. 3. Comparison of hot air with hot water, as a mode of heating horticultural structures. Various erroneous opinions and prin- ciples have been theoretically and practically promulgated, in regard to hot-air heating ; and, carrying with them, in general, some degree of plausibility, and in some cases emanating from men of learning, have led many, who have not studied the mat- ter attentively, into very great errors. However invidious, therefore, may be the task of pointing out such errors, we con- sider it our duty, when treating on the subject at large, not only to exhibit what we consider to be the true principles, but to show where erroneous principles have been adopted. This must serve as an apology for the freedom with which the advocates of Polmaise, and other methods of hot-air heating, and the sys- tems they approve, are descanted on in this section. We have already observed that the cooling of a heated body, under ordinary circumstances, is evidently the combined effect of radiation and conduction. The conductive power of the air is principally owing to the mobility of its particles, for, otherwise, it is one of the worst conductors we are acquainted with. Atmospheric air, in passing into a house over a highly heated surface, must necessarily lose a large quantity of its contained moisture ; [see 1. Effects of Artificial Heat, of the preceding section ;] and, as its capacity for taking up moisture is increased according to its temperature, it follows that a great demand 15* 172 HEATING BY HOT WATER, HOT AIR, AND STEAM. must be made upon the moisture of the house, upon the plants, and upon everything else within its influence capable of giving off moisture. This is also the case with hot-water pipes. But here the advantage of the latter is plainly illustrated ; for while a hot-air stove abstracts the moisture, in excess, from that part of the house nearest to the aperture of ingress, hot-water pipes radiate the heat at a low temperature equally over the whole surface, and, as the temperature at which the heat is radiated is comparatively low, little or no moisture is abstracted. Some suppose that they get a fine moist heat from hot-water pipes. This, however sound and sensible it may appear, is, nevertheless, a practical fallacy, the fact of the case being this, that, instead of the moisture of the house being taken up by the air, as in the case of Polmaise, and other stoves, the warm air of the pipes being so much lower in temperature than that of the stoves, it cannot take it up, and hence the moisture remains with the plants and the atmosphere in its original purity. In fact, there is no difference between the heat radiated from stone, brick, or iron, unless it be mixed with extraneous gases, by heat- ing these bodies to a high temperature. To supply the moisture required by the heated air, water may be placed in evaporating pans, in connexion with the current of ingress; but, as we have already shown, though moisture may be supplied, the hydrogen of the rarefied air still remains uncombined, and, until the air be replaced by a fresh volume from the external atmosphere, its impurities still remain. With regard to the motion and circulation of the atmosphere of a hot-house, the system of heating by hot air possesses, the- oretically, some advantages over all others. Strictly speaking, however, this has scarcely a practical foundation. If hot air be admitted in currents, the atmosphere will be agitated, certainly, but the house will be very unequally heated, as the heated air will pass upward in currents, at the aperture of its entrance, without diffusing itself over the lower surface of the house. Air expands, when heated, ^^ of its bulk for each degree of Fahrenheit, and the velocity of its motion is equal to the addi- tional height which a given weight of heated air must have, in order to balance the same weight of cold air ; and as all rare HEATING BY HOT WATER, HOT Affi, AND STEAM. 173 bodies tend to rise vertically, in a dense medium, it follows, that when heated air enters a house by an aperture at one part of it, a very large portion of the heated air thus entering, must rise immediately towards the roof; and in practice we find this to be exactly the case. For, let any person examine the roof of a hot-house in a frosty night, heated by a hot-air stove, and he* will perceive the part immediately above the entrance of the air quite warm by the ascending heat, while all the rest of the roof may be covered with ice or snow. But the atmosphere of a house heated artifically, by whatever means, is always in motion ; with hot-water pipes it may be less perceptible, for the reasons already stated, but it is not the less real. The motion given to the atmosphere of a house depends upon the difference of temperature between the two bodies of air, externally and internally ; therefore a motion must continue in the air of a house artificially warmed, so long as the house requires warming, that is, as long as any difference ex- ists between the internal and external atmospheres. Some advocates of hot-air heating found their arguments upon the fact that air can be raised to a higher temperature, in a given time, by a given amount of caloric, than water. This is probably true, if we calculate according to the bulk, without re- gard to the density, of the respective bodies ; but, supposing it to be true, then we know that, by the law already referred to, its rapidity in warming will just be in exact proportion to its rapid- ity in cooling, and vice versa. It is, therefore, manifest, that this property militates against it as an agent in heating horticultural buildings, as it is well known to be an all-important point, in warming these structures, to obtain an equilibrium of heat for the greatest length of time, and with the least possible amount of attention, and experience has fully concluded that this is most effectually and most easily obtained by the circulation of hot water through wooden and metallic radiators and conductors. Suppose, for instance, that a house, containing 4000 cubic feet of air, is required to be heated, from 32 to 60, and suppose the external thermometer to remain stationary at this point ; then, by calculation, we find that it requires double the amount of fuel to heat the atmosphere through the 28 degrees 174 HEATING BY HOT WATER, HOT AIR, AND STEAM. between these two points, by means of water, that it does through the medium of air, i. e., by direct communication, in each case the calorific action being in pretty exact ratio to the combustion, and both acting under the most favorable circum- stances. This would, at first view, decide us in favor of hot air as a means of heating, in preference to hot water ; and the fact that the heat becomes more rapidly sensible by hot air, has induced many to come to a premature conclusion on this point. Let us, however, take another view of the position here alluded to, and consider the two methods in regard to their per- manency of heating power. We find also, by calculation, that while the temperature of the house is maintained at 60 for 3*25 hours by hot air, with the same amount of combustion the temperature of 60 is maintained for 10 hours by hot water, or three times the period that the equilibrium is maintained by hot air. The same experiment shows that 2 bushels of coal will warm an equal volume of air in a hot-house the same length of time that 5-067 bushels will warm by direct connexion of its particles with the source of heat. Now, in a large house, or number of houses, this saving of fuel would, in a few years, amount to the difference of cost be- tween the two apparatuses, keeping out of the question the saving of labor, the cleanliness and neatness of the one compared with the other. In regard to these numbers, we may remark, that the calculations of some experienced and intelligent gardeners, drawn from accurate observation, have made the difference be- tween the two methods still greater, in regard to the consump- tion of fuel, placing this position in still stronger light than by the calculation here given. This remarkable difference in the retention of heat is owing to the following causes. First. The power possessed by the water [as already ex- plained, see " Comparison of Water and Steam "] of absorbing and retaining a large amount of heat, and giving it off gradually, as the atmosphere requires it. Secondly. Owing to the body of metal with which the water is surrounded, which also absorbs and retains a large amount HEATING BY HOT WATER, HOT AIR, AND STEAM. 175 of heat, and parts with it slowly to the air by which it is sur- rounded. Water is a better conductor of heat than air. Every gardener well knows how rapidly a wet mat, or any other wet substance, will carry off the heat in a frosty night, if laid over a hot-bed, or green-house. In fact, the temperature of a frame under such covering will fall quicker than if fully exposed. Yet the case is different if the mat be dry, because the apertures of the mat, and also the space between it and the glass, are filled with air at rest, because the latter is a bad conductor of heat, and the former a good conductor. In a tank of water in a hot-house, the thermometer will indicate a temperature probably 10 above the atmosphere, while, by plunging the hand in the water, it will feel about 10 lower. This arises from the power possessed by the water of conducting the heat from the hand immersed in it. The effect in all these cases may appear different, but the prin- ciple of action is the same. Water conducts heat rapidly from a body warmer than itself, and conveys it to a colder one. Let a stream of air be forced through a tube 100 feet in length, entering at the temperature of 150; by the time it has travelled, by its own specific gravity, to the end of the tube, it will be reduced to the temperature of the external atmosphere. A stream of water, under the same circumstances, will travel to the end of the tube with a very slight diminution of its tem- perature, probably only a few degrees, and will have heated the tube, if a good conductor, to nearly the same temperature as itself during its passage. SECTION IV. HOT-WATER BOILERS AND PIPES. 1. Size of Boilers, and surface necessary to be exposed to the fire. In adapting the boiler of a hot-water apparatus, it is not necessary, as in the case of steam boilers, to have its capacity exactly in proportion to the quantity of pipe that is attached to it. On the contrary, it is sometimes desirable to invert this order, and to attach a boiler of small capacity to a considerable length of pipe. We do not mean, however, in recommending a boiler of small capacity, to propose, also, that it should be of small superficies ; for the efficiency of a boiler very much depends upon the quantity of surface exposed to the fire. The larger the surface exposed to the action of the calorific influence, the greater will be the economy of fuel, and, therefore, the greater will be the effect of the apparatus. In proposing the adoption of boilers of small capacity, how- ever, it is necessary to accompany the recommendation with \ caution against running into extremes, for this error has been the cause of the inefficiency of apparatus in many instances. In some boilers, we have seen the space allowed for the water so very small that the boiler was thereby rendered completely useless. Too small a quantity of water, and too large a surface exposed to the fire, give rise to various evils, among which are the depo- sition of neutral salts and alkaline earths by the water which evaporates, contracting the water-way, and impeding circulation ; and also preventing the full action of the fire on the exposed surface of the boiler. But perhaps the greatest evil arising from this state of things, .is from the repulsion of heat by the metal of the boiler. The quantity of water it contains being so small, and the heat of the fire very intense upon it, a repulsion is caused between HOT-WATER BOILERS AND PIPES. 177 the iron and the water, and, consequently, the latter does not receive the full quantity of heat. The repulsion between heated metals and water has been ascertained to exist, even at low tem- peratures, being appreciably different at various temperatures below the boiling point of water. But as the temperature rises the repulsion increases with great rapidity ; so that iron, when red-hot, completely repels water, scarcely communicating to it any heat, except, perhaps, when under considerable pressure. It is obvious that the extent of surface exposed to the fire should be in proportion to the amount of water contained in the boiler and the pipes ; and it is easy to estimate these relative proportions with sufficient accuracy, notwithstanding the various circumstances which modify the effect. Calculating the surface which a steam boiler exposes to the fire at 4 square feet for each cubic foot of water evaporated per hour, and calculating the latent heat of steam at 1000 degrees, we shall find that the same extent of boiler surface that would evaporate a cubic foot of water, of the temperature of 52, into steam, of which the tension is equal to one atmosphere, would supply the requisite heat to 232 feet of pipe, 4 inches diameter, when its temperature is to be kept at 140 degrees above that of the surrounding air. The following proportions for the surface which a boiler for a hot- water apparatus ought to expose to the action of the fire, will be found useful. Surface of boiler exposed to the fire. 4 inch pipe. 3 inch pipe. 2 inch pipe. 3 square feet will heat 200 feet, or 266 feet, or 400 feet. 5k " " " " 300 " 400 " 600 " 7 " " " " 400 533 " 800 " 8i " " " " 500 " 666 " 1000 " 12 " " " 700 933 " 1400 " 17 " " " " 1000 " 1333 " 2000 " A small apparatus ought, perhaps, to have rather more sur- face of boiler, in proportion to the length of pipe, than a larger one, as the fire is less intense, and acts with less advantage, than in large furnaces. It depends, however, upon a variety of cir- cumstances, whether it will be expedient to increase the quan- tity of pipe, in proportion to the surface of the boiler, beyond 178 HOT-WATER BOILERS AND PIPES. what is here stated ; for, although many causes tend to modify the effect, the above calculation will be found a good average proportion, under ordinary circumstances. The effect very much depends upon the quality of fuel, the force of draught, the con- struction of the furnace, &c., which, from what has been already said on these matters, will show that they will, in a great measure, influence the intensity of the heat received by the boiler. It is always safest, however, to work with a larger sur- face of boiler, at a moderate heat, than to keep the boiler Avork- ing at the maximum of its power. There is another cause, however, that will tend to modify the proportions which may be adopted. The data from which the calculation of the boiler surface is made assumes the difference to be 140 between the temperature of the pipe and the air with which it may be surrounded ; the pipe, in this calculation, being 200, and the air 60. But if this difference of temperature be reduced, either by the air in the house being higher, or by the apparatus being worked below its maximum temperature, then, in either case, a given surface of boiler will suffice for a greater length of pipe. For, if the difference of temperature between the water and the air be only 120, instead of 140, the same surface of boiler will supply the requisite degree of heat to one sixth more pipe ; and if the difference be only 100, it will sup- ply one third more pipe than the quantity stated in the table. It will, therefore, frequently occur, in practice, that the quan- tity of pipe, in proportion to a given surface of boiler, may be considerably increased beyond the amount which is given in the preceding table ; because, in forcing-houses, the temperature of the air may sometimes be above the number of degrees here given, and frequently the temperature of the water may be below 100, the pipe not being required to be worked at its full heat; and, therefore, in both these cases, a larger proportion of pipe may be worked by a given sized boiler. In order to estimate the quantity of surface which is acted upon by the fire, an allowance must be made for the flues which circulate round the exterior of the boiler, (and all boilers should be so erected as to admit of the action of the heat round their sides.) Thus, suppose an arch boiler (Fig. 35) to be 30 inches HOT-WATER BOILERS AND PIPES. 179 Fig. 35. long, there will be about 8- square feet of surface exposed to the fire, that is, to its direct action underneath ; and suppose, also, that there are four external flues, one on each side, or sup- posing that the flue went all round the boiler, top and all, we may calculate that nearly one half of the effect is produced by these flues which would have been obtained had the direct action of the fire been employed on a like extent of surface ; therefore the flues will be equal to 5 square feet, making altogether 13| square feet as the available heating surface of a boiler of this shape and size, which we consider far superior to the old form of boiler, as shown in the following cut, (Fig. 36.) A boiler of the size Fig. 36. here described (Fig. 35) would be sufficient to heat about 800 feet of pipe, 4 inches diameter, when the excess of its tempera- ture above that of the surrounding air is 140, as before stated : a boiler of the same shape, 24 inches, has about 11 square feet of surface directly acted upon by the fire ; one 36 inches long has 16 square feet of surface ; and one 42 inches long has 19 square feet of surface ; the increase being directly proportioned in the simple ratio to the length. IB 180 HOT-WATER BOILERS AND PIPES. A circular boiler, 30 inches diameter, with a 9 inch circular flue running round the outside, will expose nearly the same extent of surface to the fire as the one just described, (Fig. 35,) both being the same length, and therefore the one will be as effective as the other ; a slight diminution on the perpendicular length of the curve makes but little difference to its capacity for radiating caloric. The surfaces of any size of this shaped boiler can easily be calculated by the same rule ; but, instead of varying in the sim- ple ratio of the length or diameter, it will be found to be propor- tional to the square of the diameter, so that the proportion of surface increases more rapidly than in the arched boiler. Thus, a circular boiler, 24 inches diameter, has 8J square feet of sur- face exposed to the fire ; a 30 inch has 13f square feet ; a 36 inch has 19f square feet ; and a 42 inch has 26f square feet exposed to the fire ; the small sizes having proportionally less surface, and the large sizes more than the high-arched boilers. The rules which are here given regarding boilers, are framed to suit common occurrence, arid intended to guide practical men who have the management and working of common hot-water apparatus. There are some cases, however, where apparatus of great magnitude is necessary, in which these rules will not apply without modification. But as such instances are com- paratively rare, and, moreover, as no person that is a novice in the practical application of this principle of warming, will be likely to undertake, for his first essay, the responsible erection of an apparatus of great dimensions, it is the less necessary to enter at length into such cases as may be supposed to render any alterations of these principles necessary. It may, however, be observed, that cases may occur where a peculiar construction of apparatus may be desirable ; for instance, where, from a large quantity of required surface a furnace of very great power would be necessary ; and, in that case, a boiler which exposes a large surface, while it possesses but a small capacity, would obviously be injudicious, because the intense heat acting on a small body of water would probably generate steam to a high degree of elasticity in the boiler, and not only I HOT-WATER BOILERS AND PIPES. produce much inconvenience, but neutralize the effects of what might otherwise be an efficient apparatus. The nearer the rules here laid down for regulating the size of boilers are acted upon, the more efficient will be the working of the apparatus. There is no advantage whatever gained by using a larger boiler than is necessary to heat the pipes to their maximum temperature, even though this temperature may never be required, for, as the return-pipe should (if the appara- tus be working right) bring in a fresh supply as rapidly as the flow-pipe takes it away, the boiler is always kept full. It may be observed, that the circulation will be more rapid from a minimum boiler than from a maximum one, that is, from a boiler whose capacity is rather below the proportion ; while a boiler whose capacity is above the proportion of the pipes, has a slower circulation ; and for all horticultural purposes, though the former has some little advantage in the time of heating the latter is decidedly to be preferred. In the following section, (Sect. V.,) further information will be found on boilers, etc., where different methods of heating, in practical operation, are figured and fully described. We may here state, in regard to the material for boilers for horticultural purposes, that cast-iron boilers, if properly made, will last much longer, and be also somewhat cheaper in the first instance, than malleable-iron ones, be the plates ever so good ; the principle of durability resting on the former not being injured by oxydation so much as the latter. In both cases, however, the durability depends very much on the kind of water used ; that least liable to form a deposition on the boiler being the best. 2. Size and arrangement of hot-water pipes. Some contro- versy has arisen, among engineers, gardeners, and others, respect- ing the size of tube most suitable for the purposes of heating hot-houses. 2, 3, 4, 5, and 6 inch pipes have been used, and experiments instituted respecting the merits of each ; from which it has been found that 4 inch pipes radiate more heat than any of tKe other sizes ; and, consequently, the 4 inch pipes are now most generally used. 182 HOT-WATER BOILERS AND PIPES. The unequal rate of cooling of the various sizes of pipes, however, renders it necessary to consider the purpose to which they are applied. If it be desired that the heat shall be retained for a great many hours after the fire is extinguished, then pipes of larger dimensions must be used. Where a conservatory is very much exposed, and liable to fall below the minimum tem- perature during a cold night, then 5 inch pipe may be used, which will retain the heat longer than one of smaller size ; but a double length of pipe should always be used in doubtful cases. But, as a general rule, no pipe should be used of more than 4 inches diameter, as the larger the pipe the greater the consump- tion of fuel, and more heat will be given out by 4 inch pipes, in proportion to the consumption of fuel, than by pipes of any other size. The ordinary method of arranging hot-water pipes is by placing the furnace and boiler at one end of the house, and lead- ing them along the front within a few inches of the wall. If the house be span-roofed, the pipes ought to travel completely round both sides ; if single, or lean-to house, the pipe should pass along the front and return the same way; i. e., the flow and return pipes should be placed beside each other, as will be seen in the figures in the next section. The pipe ought never to run by the back wall of a house, except there be some reason to fear the entrance of frost in that quarter, which, in houses with thin walls, or those constructed with clapboards, is quite likely. In general cases, the heat rises with sufficient rapidity from the front, to prevent the entrance of frost at the back wall, unless it be near the bottom of the wall. In general, hot-water apparatus is so constructed that when the smoke leaves the boiler, it passes immediately up the chim- ney, by which an incredible amount of heat is lost. I have seen the thermometer rise to 200 when placed at top of a chimney of this kind, and an amount of heat thereby lost nearly equal to the whole amount radiated in the atmosphere of the house. This is the case with many heating apparatuses, without the smallest notice being taken of the fact. On making this remark, lately, to a most intelligent gardener, he doubted the fact of losing any heat by his chimney ; while, on trying the thermome- HOT-WATER BOILERS AND PIPES. ter at the top of his chimney, we found it rise in a few minutes to 137, after having travelled through 20 feet of flue through the back wall of the house. Whatever apparatus be employed in heating a hot-house, the flue should always be taken advantage of. It must be remem- bered that smoke will not travel through a flue, neither up nor down, without first being rarefied by heat. The smoke, as already described, is, in fact, a body of gases emitted from the fuel by the action of heat, and a portion of this it takes along with it on leaving the furnace. In its passage, it com- municates this heat to other bodies, as the flue ; and more so, as the flue is in a position more or less horizontal. A flue, there- fore, should, if possible, be carried the whole length before giving egress to the smoke, by which a great amount of fuel may be economized. In laying down hot-water pipes, it is necessary to allow suffi- cient room for their elongation and expansion when they become hot. Want of attention to this has caused several accidents ; for the expansive power of iron, when heated, is so great, that scarcely anything can withstand it. The linear expansion of cast-iron,by raising its temperature from 32 to 212,is -0011111, or about one nine hundredth part of its length, which is nearly equal to If inches in 100 feet. Therefore, it is necessary to leave the pipes unconfined, so that they shall have freedom of motion lengthways ; and, instead of confining, as has frequently been done, facilities should be provided for their free expansion, by laying them on small rollers, or pieces of rod-iron, between them and the bearers on which they rest ; for the contraction on cooling is always equal to the expansion on heating, and unless they can readily return to their original position when they become cool, the joints are apt to become loose and leaky, as indeed all cast-iron pipes do, that are exposed to sudden extremes of temperature. Every hot-water apparatus should be provided with a supply- cistern attached to the boiler, or the pipes; the pipe leading from the supply-cistern should flow either into the return-pipe, or into the boiler, near the bottom. In no case should it enter the flow-pipe, as it is more likely to emit vapor, and the steam, 16* 184 HOT-WATER BOILERS AND PIPES. that may sometimes be generated on the surface of the water in the flow-pipe, would find egress, unless the supply-pipe were bent in the shape of an CQ to prevent it, which is a very good plan ; and, as a small lead pipe of about l inch bore is suffi- cient to supply a boiler of considerable size, the pipe can easily be bent in any shape to answer the purpose. 3. Impediments to circulation, fyc. The power which pro- duces the circulation of the water in the pipes is the specific gravities of the two bodies in the return and flow-pipes ; whether this force acts on a pipe 100 feet in length, or on one only 5 feet in length, the result is precisely similar. Now it is evident that if this unequal pressure is the vis viva, or motive power, which sets in motion the whole quantity of water in the apparatus, in order to ascertain the exact amount of this force, it is only necessary that we know the specific gravities of the two columns of water, and the difference will, of course, be the effective pressure, or motive power. This can be accurately determined when the respective temperatures of the water in the boiler and in the descending or returning pipe are known. As this difference of temperature rarely exceeds a very few degrees in ordinary cases, the difference of the weight of the two columns must be very small. But, probably, the very trifling difference that exists between them, or, in other words, the extreme smallness of the motive power, is very imperfectly com- prehended, and will, perhaps, be regarded with some surprise, when its amount is shown by exact computation. In order to ascertain, without a long and troublesome calcula- tion, what is the amount of motive power for any particular apparatus, the following table has been constructed. An appara- tus is assumed to be at work, having the temperature in the descending pipe 170, and the difference of pressure upon the return-pipe is calculated, supposing the water in the boiler to exceed this temperature, by from one to twenty degrees. This latter amount will exceed the difference that usually occurs in practice. By referring to the annexed table, it will be found that when HOT-WATER BOILERS AND PIPES. the difference between the temperature of the flowing returning columns is 8 degrees, the difference in weight is grains on each square inch of the section of the return- supposing the height of the boiler A (Fig. 36, / B) to be 12 inches. This height, however, is/ only taken as a convenient standard from I which to calculate; for, probably, the height \ may, in many instances, be more than this, \ though it will seldom be less. Now, suppose that, instead of 12, 18 inches was the distance between the two pipes, that is, between the top of the upper and the centre of the lower pipe, and the pipe 4 inches in diameter ; if the difference of temperature between the water in the boiler and the return-pipe be 8 degrees, the pressure on the return-pipe will be 153 grains, or about one third part of an ounce ; and this will constitute the whole amount of motive power of the apparatus, whatever be the length of pipe attached to it. If such an apparatus have 100 yards of pipe 4 inches in diameter, and the boiler contains, say, 30 gal- lons of water, there will be in all 190 gallons, or 1900 Ibs. weight of water, kept in continual motion by a force equal only to one third of an ounce. This calculation of the motive power will vary under different circumstances ; and, in all cases, the velocity of the circula- tion will vary simultaneously with it. and 8-16 pipe, 186 HOT-WATER BOILERS AND PIPES. Difference in weight of two columns of water each one foot high, at various temperatures. Difference in temp, of the two columns of water in Difference in weight of two columns of water contained in different pipes. Difference of a column one foot high. degrees of Fah.'s scale. 1 in. diam. 2 in. diam. 3 in. diam. 4 in. diam. 5 in. diam. per sq. inch. 2 grs. weight 1*5 grs. weight 6-3 grs. weight 14-3 grs. weight 25-4 grs. weight 33-6 grs. weight 2.028 4 3-1 12-7 28-8 51-1 110-1 4-068 6 4-7 19-1 43-3 76-7 211-7 6-108 8 6-4 25-6 57-9 120-5 250-0 8-160 10 8-0 32-0 72-3 ' 128-1 317-5 10-200 12 9-6 38-5 87-0 154-1 376-1 12-264 14 11-2 45-0 101-7 180-1 390-9 14-328 16 12-8 51-4 116-3 205-9 449-1 16-392 18 14-4 57-9 131-0 231-9 522-0 18-456 20 16-1 64-5 145-7 258-0 700-0 20-532 The above table has been calculated by the formula given with table IV., (see Appendix,) for ascertaining the specific grav- ity of water at different temperatures. The assumed tempera- ture is from 170 to 190. It will be observed, in the foregoing table, that the amount of motive power increases with the size of the pipe ; for instance, the power is four times as great in one of 4 inches diameter as in one of 2 inches, and nearly six times as great in one of 5 inches. The power, however, bears exactly the same relative proportion to the resistance, or weight of water to be put in motion, in all the sizes alike ; for, although the motive power is four times as great in pipes of 4 inches as in those of 2 inches, the former contains four times as much water as the latter. The power and the resistance are, therefore, relatively the same. These calculations are given with the view of showing how trifling a cause may impede the proper circulation of the hot water in pipes, and that, when once obstructed, how impossible it is for an apparatus to work. Trifling as this power may appear, yet upon its action depends entirely the efficiency of an apparatus. Seeing that the motive power is so small, it is not surprising that, by an injudicious arrangement of its parts, the motion may frequently be impeded and even destroyed ; for the slower the circulation of the water, the more likely is it to be interrupted in its course. HOT-WATER BOILERS AND PIPES. 187 There are two ways by which the motive power may be increased. One, to allow the water to cool a greater number of degrees between the time of its leaving the boiler and the period of its return through the descending pipe. The other, by increasing the vertical height of the ascending and descending columns. The effects produced by these two methods are pre- cisely similar; for, by doubling the difference of temperature between the flow and return pipes, the same increase of power is obtained as by increasing the vertical height. There are two methods of increasing the difference of temper- ature between the flowing and returning pipes. First, by increasing the quantity of the pipe, so as to allow the water to flow a greater distance before it returns to the boiler. Secondly, by diminishing the diameter of the pipe, so as to expose more surface in proportion to the quantity of water contained in it, and by this means to make it part with more heat in a given time. The first of these methods, although the most practical, is ncessarily limited, in some instances, to the length of the build- ing to be heated, to which the length of pipe must be adjusted, in order to obtain the required temperature ; and, as to the second, we have already enumerated many objections against the use of small pipes. Where the motive power, therefore, is not of sufficient strength, the increase of the height of the col- umn ascending from the boiler must be depended on for an additional motive power. In all cases, the rapidity of circulation is proportional to the motive power, and, in fact, it is the index and measure of its amount. For, if, while the resistance remains uniform, the motive power be increased in any manner, or in any degree, the rapidity of circulation will increase in a relative proportion. Now, the motive power may be augmented, as we have seen, either by increasing the vertical height of the pipe, by reducing its diameter, or by increasing its length. If, by any of these means, the circulation be doubled in velocity, then, as the water will pass through the same length of pipe it did before, in one half the time, it will only lose half as much heat as in the for- mer case, because the rate of cooling is not proportional to the 188 HOT-WATER BOILERS AND PIPES. distance through which the water circulates, but to the time of transit. If, then, by raising the pipes vertically, the difference between the temperature of the flow and return pipes be in- creased, it appears to be the most practical method of increasing the velocity of motion. The increased velocity, therefore, is indicative of increased power, and in a hot-water apparatus it is the velocity of circulation which enables it to overcome any extraordinary obstructions. Neither the principle nor the practice of an apparatus is in the least affected by having an additional number of pipes lead- ing out of, or into, the boiler ; the effect is the same, whether there be more flows than return pipes, or, conversely, more return than flow pipes. 4. Level of Pipes. Some persons have supposed that if the pipes be inclined so as to allow a gradual fall to the boiler in its return, additional power is gained. This appears very plausible, particularly with regard to some forms of apparatus, but the principle is entirely erroneous. This error appears to arise from treating the subject as a simple question of hydraulics, instead of a compound result of hydrodynamics. If the question were only as regards a fluid of uniform temperature, then the greatest effect would be obtained by using an inclined pipe; but the water in the pipes we are now treating of, is of varying density and temperature, which very materially alters the results. Contrary to the ideas of some persons, the circulation of the water first takes place in the lower pipe ; in consequence of the water in the boiler becoming lighter by the absorption of heat, the column of water in the return-pipe, being of greater density, forces its way into the boiler, when the water in the upper pipe falls into its place. Now, suppose the distance between the entrance of the return-pipe and that of the flow-pipe be 12 inches. This distance is neither increased nor diminished by any incli- nation of the return-pipe towards the boiler, the effective pressure being in both cases the same. Discarding the erroneous hypothesis that the motion of the water commences in the upper pipe instead of the lower one, and the motion commences at the entrance of the lower pipe into HOT-WATER BOILERS AND PIPES. 189 the boiler, which we have frequently proved, it is, therefore, evident that there can be no advantage by making the pipe to incline from the horizontal level ; for whether the water descends through a vertical or through an inclined tube, the force of gravity will only be equal to the perpendicular height ; there must, therefore, be an equality of pressure on the boiler under all circumstances, whether the pipe entering the boiler be on a level, or inclined from its junction with the flow-pipe. When it is necessary to sink the return-pipe below the level of the boiler, there must be a sufficient weight of water in the pipes, above its level, to overcome the perpendicular column that exists below the level of the boiler, otherwise the tendency of the lower column will be to a retrograde motion. The only way is to raise the pipe sufficiently to afford a perpendicular return- ing column of sufficient pressure to raise the water in the per- pendicular pipe attached to the boiler. If the flow-pipe be carried on a horizontal level with the boiler, and the return-pipe carried below the level of the boiler, it is scarcely possible to obtain any circulation ; and if this depth be much, no circulation at all can be obtained. We have seen some costly apparatuses completely useless on this account ; and those erectors of heating apparatus, unacquainted with the principles of hydrodynamics, are very apt to commit similar mistakes. The velocity of circulation in such apparatus will be just in proportion to the difference of weight between the columns above and below the boiler. It must not be supposed that water will not circulate in pipes below the level of the boiler; and much trouble and expense have frequently been incurred in consequence of being ignorant of this position. All that is necessary is to give the upper section of pipe a sufficient preponderance to raise the water in the lower one, allowing for the superior density of the water in the lower pipe. It, however, requires considerable judgment in adopting any such forms of apparatus as this, for many concurring cir- cumstances are essential to complete success. It should, there- fore, never be adopted when a common horizontal working apparatus can be introduced. 190 HOT-WATER BOILERS AND PIPES. 5. Accumulation of air in pipes. It is necessary to make provision for the escape of air in the pipes, which sometimes so accumulates as to prevent circulation. This is more especially the case when the apparatus is complicated, and has many turn- ings and vertical bends in the pipes. It generally collects at the upper bends of the pipe, but this will depend very much upon the mode of supplying the apparatus with water. It frequently requires the greatest care and the closest attention to discover where the air is likely to lodge, as the most trifling alteration in the position of the pipes will entirely alter the arrangements in regard to the air-vents. Want of attention to this has been the cause of many failures, and the discovery of the places where the air accumulates is sometimes a matter of difficulty. For although it be true, in a general sense, that air will rise to the highest part of the apparatus, it will frequently be prevented from getting to the highest part by alterations in the level of the pipes, and by other causes. As water, while boiling, always evolves air, it is not sufficient merely to discharge the air from the pipes on first rilling them, because it always accumulates ; and, in many instances, it is desirable to have the air-vent self-acting, either by using a valve, or small open pipe ; but we have generally found a cock most convenient. The size of the vent is not material, as a very small opening will be sufficient to allow the escape of air. The rapidity of motion in fluids is inversely proportional to their specific gravi- ties, as water is 827 times more dense than air ; an aperture which is sufficiently large to empty a pipe in 14 minutes, if it contained water, would empty it, if it contained air, in one second. Air being so much lighter than water, it is of course necessary that the vents provided for its escape should be placed at the highest parts of the apparatus, for there it will always lodge when no impediment occurs to prevent it; but it will sometimes be found necessary to have several in different parts of the apparatus. Though it is perfectly easy to provide for the discharge of the air from the pipes, as far as the mere mechanical operation is concerned, it requires much consideration arid careful study to HOT-WATER BOILERS AND PIPES. 191 direct the application of those mechanical means to the exact spot where they will be useful. We have frequently seen mechanics, who, though well acquainted with the practical details of the apparatus they were erecting, yet were perfectly ignorant of the principles on which it works ; hence the success of such an apparatus must be entirely a matter of chance. Wherever alterations of the level occur, vents should be pro- vided for the escape of air; and, as we have said, a small tap (or cock) will be the most convenient method of outlet. In a complicated arrangement of hot-water apparatus, it is sometimes so very difficult to detect the various causes of inter- ference, and the impediments which arise are often so apparently insignificant in their extent, that when ascertained they are frequently neglected. Those, however, who bear in mind how very small is the amount of motive power in any apparatus of this description, will not consider as unimportant any impedi- ment, however small, which they may detect ; moreover, they will immediately see the propriety of having the evil in ques- tion put right. But, in the more complicated forms of the apparatus, so many causes become operative in impeding the circulation, that the real cause of impediment may elude the detection of even an experienced practitioner. We will now proceed to give a description, in detail, of various methods of heating, which come within the range of our own experience, accompanying the descriptions with sketches, by which their details will be more easily understood. 17 SECTION V. VARIOUS METHODS OF HEATING DESCRIBED IN DETAIL. THE heating of hot-houses, by any of the ordinary methods of warming these structures, has hitherto been attended with extravagant expense. The difficulty of obtaining, at a reason- able price, the means of keeping up the desired temperature, during long and severe winters, the expense of the apparatus, the annual cost of repairs, the continual outlay for fuel, together with the incidental expenses and trouble of working them, has, in many instances, proved a barrier to their erection, and has induced many to abandon the attempt, who had well nigh carried it into execution. Many lovers of exotic gardening have thus been diverted from the enjoyment of this pleasant and healthful pursuit; and hence it is of the utmost importance, especially to amateurs and others having small establishments, and who do not keep a regular gardener, that the internal ar- rangements of a plant-house, and, above all, the heating arrange- ments, should be so constructed as to be dependent upon the very smallest possible amount of time and attention, and likely to produce the least injury by neglect. Among the numerous systems of heating lately applied to horticultural buildings in England, is one called Polmaise, from its having originated at a place in Scotland of that name, the seat of the late Mr. Murray, near Sterling. The principles upon which this method is founded are not new, and the system itself, in other modifications, dates from a period much more remote than any other with which we are acquainted. This system is applied, in a more practical and perfect form, to the warming of many public and private buildings in this country. The very general adoption, however, of this system, does not, in the smallest degree, give us a warrant against its defects. It VARIOUS METHODS OF HEATING DESCRIBED IN DETAIL. 193 has been ascertained that air heated to a temperature of 300 de- grees, becomes so deprived of its organic matter, and otherwise changed in its properties, as to be unfit for the sustenance of either animal or vegetable life, in a state of healthy and vigorous development, for any length of time ; and hence the admission of a current of highly heated air into a dwelling room, or into a well glazed hot-house, if no means are taken to restore its original properties, must, in a short time, become sensibly in- jurious to the animals and vegetables that are compelled to breathe it. And this we find to be practically the case. Every gardener, on entering a hot-house so heated, is immediately sensible of the presence of contaminating gases in the atmosphere, whether arising from the combustion of fuel, or otherwise, and he is too well acquainted with its effects on vegetative beings to allow his tender plants to absorb it ; hence he takes immediate meas- ures of modifying what he cannot possibly prevent. It can scarcely be doubted, that a vast amount of sickness and diseases of the respiratory organs is, in a great measure, attributable to the same circumstance, especially in people of sedentary habits, who confine themselves to close chambers, warmed by currents of hot air, or highly heated stoves. The latter, in this respect, is probably worse than the former ; for, in the one, the supply of air to be heated is drawn from the external atmosphere, and, consequently, is less likely to contaminate the air of the room, although, when conducted into the room at high temperatures, the atmosphere of the latter, without egress as well as ingress of air, must ultimately become so. In the case of stoves, how- ever, it is different, for by them the same atmosphere is heated over and over again, by convection. The particles of air in contact with the stove first become heated, these expand with the heat, and, consequently, becoming lighter, rise, and the colder particles supply their place, which also expand, rise, and are in their turn replaced by others. Here the supply of air to be warmed is drawn directly from the room itself; thus com- pelling the inmates to inhale the same contaminated atmos- phere for days together, without mixture or admission of fresh air, except the small portion that finds an unwelcome entrance 194 VARIOUS METHODS OF HEATING DESCRIBED IN DETAIL. by the occasional opening of the door ; and in the severe weather of our winters, with the thermometer below zero, this portion is frequently small indeed. The pleasure and ability of exercising our physical functions in cold weather, will be in exact propor- tion to the frequency of practice ; and it is truly surprising, that with so much positive proof of direct injury resulting from con- tinued confinement over highly heated stoves, many will, never- theless, persist in so pernicious a custom, a custom which is truly national, and which renders the influence of these stoves as baneful as that of the Upas tree, and sends thousands an- nually to an untimely and premature grave. I have observed, by some articles that have lately appeared in an excellent horticultural periodical, (Downing's Horticulturist,) that this much talked of system of warming horticultural struc- tures with hot air, called Polmaise, has been adopted by some individuals in this country. These individuals have been misled by the extravagant statements, or rather raw-statements, that have from time to time appeared in the Gardener's Chronicle, (of England,) by its talented editor and others under his influ- ence. Those who have been in the habit of reading that paper in this country, and noticed the laudatory articles that have so frequently appeared in it, in favor of this method, yet unac- quainted with the practical opposition it has received by num- bers of experienced men, in every way qualified to decide upon its merits, can scarcely be blamed for adopting a system said to possess so many advantages over all others ; and when it is con- sidered that the gardening journal, which represents the opinions of practical men in that country, is but little read in America, in fact, I may say, almost unknown, save by a few individuals, it is not surprising that they should have been betrayed into the system supported by such authority. It is difficult, indeed, to account for the strong-headed and one-sided policy of the advo- cates and promoters of Polmaise. The fact is well known, that the system, and the defects connected with it, were thoroughly established many years before it was applied at the place from which it takes its name. In many places it had been tried, and found inferior, and far more fickle than the common smokf VARIOUS METHODS OF HEATING DESCRIBED IN DETAIL. 195 flue.^ It originated at Polmaise Gardens, from the following circumstances : A church in the neighborhood of that place had been warmed by a hot-air furnace, similar to those used in dwelling-houses in this country. A gardener at that place examined it, and thought it a good plan to warm his hot-houses ; accordingly, he applied something of the same kind to heat his vinery. The thing was entirely new to the worthy gardener, as well as to his employer, who sent an account of it to Dr. Lindley, of the Gardener's Chronicle, who forthwith espoused the system, extolled it to the skies, and induced various individ- uals to adopt it ; and those who would not, he straightway de- nounced as interested and dishonest men. The gardening com- munity arose in arms, and waged war against their theoretical foes, until its so-called originators were confounded at the amount of opposition excited. No controversy connected with gardening was ever carried on with so much virulence as this one on Pol- maise heating ; and no system has been so severely tested, to * The premature encomiums so liberally lavished upon this system, by the zeal of its promoters, have neither shamed imposture nor reclaimed credulity. Deceptions seldom stand long against acpurate experiments, and the mere charm of novelty soon vanishes, when economy and util- ity are both against it. The desire of notoriety, if nothing else, has too often induced parties to impose on the credulity of those who have not science enough to investigate its principles, nor practice enough to dis- cover its defects. Nothing can more plainly show the necessity of doing something, and the difficulty of finding something to do, to obtain these paltry ends, than the getting up of this method of heating hot-houses ; and this, too, by those who know, or ought to know, better, and who ought to have rejected it with contempt. When a system has no intrin- sic value, it must necessarily owe its attractions to theoretical embellish- ment, and catch at all advantages which the art of writing can supply. Trifles always require exuberance of ornament ; the building which has no strength or utility, can be valued only for the novelty of its charac- ter, or the money which it cost. It is certain that the advocate of a new system is less satisfied by its failure, than its success, even when no part of its failure can be imputed to himself, and when the fruits of his labor are tested by those who can discover their real worth. No man has a right, in things admitting of gradation, to throw the whole odium upon his opponents, and totally to exclude investigation and in- quiry, by a haughty consciousness of his own excellence. 17* 196 VARIOUS METHODS OF HEATING DESCRIBED IN DETAIL. prove its worth. Gardeners, amateurs, and all, entered the arena of experiment and discussion. Still its promoters would not flinch from their original position, and, right or wrong, would cram it down gardeners' throats, whether it was digestible or not; and that, too, without one tittle of evidence in favor of it, except ripe grapes in September, a period when grapes would ripen themselves, without any artificial heat at all. Yet its cheapness and simplicity were its recommendation, and for some successive winters many went to work Polmaising their hot-houses, tearing down their furnaces, flues, &c., and con- verting them into Polmaise stoves, hot-air drains, and other appurtenances of Polmaise ; but, after a short trial, and a good deal of plant-killing, they one and all abandoned the sys- tem with disgust. Still, amidst all this dust and dirt, and smoke and gas, created by the cracking of plates and the breaking of tiles, the Doctor maintained his ground, until, like the conquered hero, he was left alone in his glory, in the midst of the wreck and ruin he had created. What seems very strange, he never erected one, or caused one to be erected, at the Horticultural Society's garden, where he had unlimited con- trol, and ample opportunity of so doing ; and those who erected them by his recommendation and advice, were obliged to ac- knowledge them unqualified failures, notwithstanding all their alterations and improvements upon the original plan, which was simply this : A hot-air furnace is placed behind the back wall, about the centre of the house; immediately opposite the stove there is an aperture in the wall, for the admission of the heated air into the house ; directly in front and above this aperture, a woollen cloth is suspended, which is kept constantly moist by a number of worsted skeins depending from a small gutter, fixed on a frame of wood, which supports both the gutter and the cloth, the lower end of the latter reaching the ground. The cloth is made thicker in the middle, in order to equalize the heat, an arrangement which is absolutely necessary ; for if the cloth was an equal thickness all over, the centre of the house would be heated to a scorching degree, (by the rush of hot air,) while the ends would be comparatively cold. By means of drains under the floor, the fire-place is supplied with air from VARIOUS METHODS OF HEATING DESCRIBED IN DETAIL. 197 Fig. 37. a 198 VARIOUS METHODS OF HEATING DESCRIBED IN DETAIL. inside the house, part of which is used for the combustion of fuel ; the rest passes over the heated stove and enters the house through the apertures above noticed. Fig. 38. Such is the original system of Polmaise heating, which has created so much sensation in England, but which is now aban- doned for some one or other of the many improved methods to which it gave rise, the most perfect and scientific of which, I have represented in the accompanying cuts, Figs. 37, 38, and 39. The arrows marked a, in the three figures, show the entrance of the cold air from the external atmosphere ; and its passage to the fire-place, beneath the floor of the house, is further shown by the arrows b, in Figs. 37, 38, and 39. Its passage over the hot plate, through the chamber, under the bed, and thence into the house, is marked by c, attached to each arrow in the three figures ; d, the fire-place ; e, a tank containing water, imme- diately over the cast-iron plate ; /, a small funnel, or tube, for supplying water to the tank ; g, (Fig. 38,) shows the bed on which the plants are placed, resting on cross-bars, and filled with pieces of brick, having a layer of sand or sawdust on top ; this can be converted into a stage, if desired. This is Mr. Meek's modification of Polmaise, from whom the drawing ap- peared in the Gardener's Chronicle, and was there represented as something very near perfection in heating, if not perfection itself. The above sketch is somewhat altered and simplified in VARIOUS METHODS OF HEATING DESCRIBED IN DETAIL. 199 D the formation of the drains; and yet, in all conscience, it is complex and compound enough for a heating appa- ratus, as any person can see by a glance at the above sketches. It is difficult to discover wherein lies its superiority over the old smoke-flue, and it is clearly evident, that it has neither cheapness, simplicity, nor economy in fuel, to recommend it; and, as to its working, it is infinitely more precarious than the common flue, and the loss of heat is certainly much greater. This loss has been stated, by those who have tested its merits, to be at least one fourth of its whole heating power. Mr. Ayres, one of the most enlightened gardeners in England, stated, in a paper on that subject, published in the Gar- dener's Journal of 1847, that Mr. Meek wasted more heat from his one house, than he (Mr. Ayres) did from one fire that had nine different arrangements to work; and in a Polmaise apparatus that Mr. Ayres had erected, the waste of heat was enormous ; that in ten min- utes after the fire was lighted, he could ignite a piece of paper at the top of the chimney with the greatest ease; and when the same gentleman asked one of its strongest advocates the following question, " If you had a range of houses to heat in the best possible manner, would you abandon hot water for Pol- maise ? " he was answered, " No, cer- tainly not." I have quoted the opinion of Mr. Ayres, because he is well known to be one of the best authorities on matters of practical D 200 VARIOUS METHODS OF HEATING DESCRIBED IN DETAIL. importance, connected with horticulture, at the present day, and his opinions are endorsed by almost every gardener of note in England. Mr. Fleming, of Trentham, and Mr. Paxton, of Chatsworth, as well as many others, regarded it as a thing ut- terly unworthy of notice. Mr. Ayres, in the same paper already quoted, puts to the advocates of Polmaise the following conclu- sive and unanswerable query. If Dr. Lindley, or any other of its advocates, can point to one place where the apparatus is at work, and as efficacious as a hot-water apparatus ; if they can refer us to any one place, where we can see better productions than what have resulted from the use of hot water, why, says he, I am ready to spend five sovereigns to go to see it, and be convinced of my error in opposing it ; bat until then, it is mere nonsense to suppose that any responsible person will adopt it. As an example of a combination of hot water and hot air, applied in a practical and scientific manner, the following sys- tem is superior to any other with which I am acquainted, espec- ially for small houses. It supplies heat, moisture and air, either singly or combined. It consists of a cast or plate iron boiler, 0, for containing the water ; in shape it is not unlike a pretty large inverted flower-pot, with a hollow between its sides, about four or five inches wide, having one pipe entering near the top for the flow, and another at the bottom for the return, with a tube entering quite through to the fire-chamber, as represented at b, c, and d ; then there is a hot-air chamber round the boiler and fire-place, as shown at e, e, e, Figs. A, B, and C ; the boiler rests on a circular course of bricks, forming the furnace/,/, Figs. B and C. The whole is enclosed by the hot-air chamber, from which the air is conducted into the house, at k, and is supplied with cold air, both for the combustion of fuel, and drawing off the heated air, at i, i, Fig. C. The fire is fed through the door in the chamber, j, opposite which is a smaller door in the furnace, at k. In Fig. C is shown the door of the ash-pit, Z, through which the ashes are drawn. We know of no apparatus, where a small green-house or conservatory is required to be heated, that will do it so effectually and economically as this. No particle of heat generated is lost, and in its simplicity is everything that a novice could desire. Here is nothing more VARIOUS METHODS OF HEATING DESCRIBED IN DETAIL. 201 Fig. 40. 202 VARIOUS METHODS OF HEATING DESCRIBED IN DETAIL. than a cone of cast or plate iron, with hollow sides, one hole for a flow, and one for a return pipe, (these pipes can branch into several directions, if necessary, on leaving the boiler,) and a channel through it, with a flange, or neck, on which to fix the smoke pipe ; build the boiler, thus formed, on a fire-place, with just distance sufficient below the edge of the cone for a door, to supply fuel ; this door should be quite narrow, in order to let the edge of the boiler as far down as possible. The hot-air chamber should be built of brick, and, if exposed to the atmos- phere, should be at least one foot thick. In fact, the thicker the wall of the hot-air chamber is made, the better will the heat be retained. A tank of water is placed over the hot-air entrance, inside the house, for evaporation. If this system be not bungled in the construction, it will be found as cheap as any other, and the expenditure for fuel is but trifling. The cir- culation of the water is complete, and the air in the chamber is neither roasted nor burned, as it is chiefly received through the boiler, and, consequently, is possessed of more natural purity, which is so essential to vegetable life ; and it requires so little attention that any amateur can manage it without much trouble. Even in pretty severe weather, when set fairly agoing in the evening, it wants no more attention till morning ; set it right in the morning, and you may safely leave it again till night. Nor is it liable to accident or derangement. Not the least of its recommendations is its economy of fuel, a circumstance of con- siderable importance, especially where the cost of fuel is high ; and, therefore, the economy thereof is of double moment to the proprietor. We have never seen this system applied to large structures, but we have no doubt, were the apparatus made in proportion to its work, it would answer as well in large as in small houses ; at all events, there is no reason why furnaces and boilers of every description should not be chambered round in a similar way ; a very great amount of heat, that is now lost, would be turned to advantage, and I think it is not too much to say, that hot-houses could be heated at one half the expenditure of fuel. The system of heating two, three, or more, houses with one boiler, is one of those valuable improvements which science, VARIOUS METHODS OF HEATING DESCRIBED IN DETAIL. 203 combined with mechanical ingenuity has devised, and which has been carried out in practice with the most gratifying success, so much so, that in some places, separate apparatuses have been torn down, and this system adopted instead, merely on account of the fuel economized thereby. Among the many systems brought before the public, under the fine-sounding name of improved, it is doubtful whether any of them have given so entire satisfaction as the above, where it has been properly con- structed. The facility so admirably afforded by this method of heating any of the connected houses in the space of a few min- utes after it is found necessary, is certainly a great recommen- dation in its favor. In short, you have only to turn a tap, and the thing is accomplished. Fig. 41 represents the ground plan of four houses heated in this way, and most efficiently. It will be seen from the plan, that the two end houses on the front are heated by the pipes flowing and returning into the pipes which supply the hot water for the two houses standing on the back. This is easily accomplished by having a tap on each pipe where it enters the house, so that either house may be heated, or both together, if required. In the extensive forcing-establishment of Mr. Wilmot, at Isle- worth, near London, no less than seven ranges of houses, each ninety feet in length, are heated by one boiler, and all are heated effectually, and that too for the purpose of forcing grape-vines. In many other places, in England, we know that this method has been adopted with the very best results. In the plan here given, the box, (Fig. 42,) which is given on a larger scale, is situated immediately over the boiler. It may, however, be on the same level, or nearly so, a*id situated in any corner out of the way. The boiler here used is a common saddle boiler, and with a large apparatus, is probably the best boiler for general purposes. The apartments, g g, in the cut (Fig. 41) are offices for the garden, tool-house, potting-room, fruit-room, &c., and may be used as a mushroom-house. As the hot-water pipes pass through them, they are kept slightly warmed, and may be made useful as store-rooms and other kinds of garden offices. In some places in England, no less than eight or ten different 18 204 VARIOUS METHODS OF HEATING DESCRIBED IN DETAIL. I 6 VARIOUS METHODS OF HEATING DESCRIBED IN DETAIL. 205 Fig. 42. departments are heated by one boiler ; some of them going at one time, and some at another, and sometimes all going together, and each having abundance of heat.^ The convenience of this system cannot be too highly appreciated, especially when there are a number of small plant-houses situated near each other. For instance, suppose the boiler to be at work for one of the houses, which may be a plant-stove or forcing-house ; well, you Fig. B. * Fig. B shows the common method of placing supply-cisterns. They may be placed in some convenient situation and attached by a small pipe to the apparatus. To prevent the escape of vapor, it is desirable to bend the pipe into the form shown at a b, as the water in the part of the inverted syphon at a, will remain quite cold. 206 VARIOUS METHODS OF HEATING DESCRIBED IN DETAIL. go out before bed-time, and find the sky clear and frosty, con- trary to your anticipations in the early part of the evening, and how often do we find this really to be the case, you enter into your green-house, and you find the thermometer travelling down rather quickly towards the freezing-point. Kindling fires is generally an unpleasant business at this time of night, and we are pretty often inclined to let the plants take their chance, rather than be at the trouble of doing it, even if it should cost us half a night's sleep through anxiety. Here, this unpleasant business is dispensed with, and the anxiety too, as well as the sitting up till the house is heated arid safe for the night. You go to the tank or box, which is generally situated so as to be easily got at, in a recess made in the wall, perhaps, or immedi- ately over the boiler, as represented in Fig. A ; but, in any case, it should be so arranged as to be always of easy access from the houses. The arrangement of the pipes makes no difference, providing the accumulating tank be sufficiently elevated. The moment the water is put on, the circulation commences; in flows a delightful stream of hot water, warming the pipes as it proceeds through the flow and return; a vivifying glow of warmth pervades the chilly atmosphere of your green-house, and you can retire to rest without being troubled with anxious thoughts about your plants, let the weather turn as it may. It may appear, that, by this arrangement, a larger quantity of fuel will be required for a single house, than if that house had an apparatus for itself. Not so, however ; for, by close observa- tion, it is found that the consumption of fuel is pretty nearly in proportion to the water heated, and that the heat given off by the pipes is in direct ratio to the heat absorbed by the boiler from the fire. Thus, if one house only be at work, there is only the water of one arrangement to be heated ; and, consequently, only one return of cold water into the boiler, the' rest being shut off. Now, if the water be shut off into the box, that is, the mouths of the flow-pipes stopped, there is no circulation ; hence, there is no return of the cold water into the boiler, and, conse- quently, no absorption of caloric or combustion of fuel. Of course, more fuel is required to heat the four houses, than would be required to heat one, for the reasons stated, that the larger VARIOUS METHODS OF HEATING DESCRIBED IN DETAIL. 207 the body of cold water flowing into the boiler, and the larger the body of warm flowing from, it, the more heat is carried away ; hence, the more specific caloric is required, and the more combustion of fuel to produce it. But the proportion of fuel consumed to the proportion of heat generated by the pipes is found to decrease as the radiating surface is increased. This decrease amounts to nearly one third ; for it is found that eight separate houses, or departments of a house, can be heated by the same quantity of fuel which it formerly required to heat five. This calculation was supplied to me by an intelligent gardener, of extensive experience, who made it from strict investigation into the working of the system under his own charge ; and the statement is corroborated by the fact, that no case has occurred, to my knowledge, among many with which I am acquainted, and have examined, that has failed to give satisfaction. This system has not the complex character which some have assigned to it, and which, at first sight, it would appear to pos- sess ; and, as to its cheapness, I believe little can be said about it, when placed in comparison with other hot-water apparatuses. I have had no means of calculating the difference, if any, between this apparatus and as many single ones as it may be substituted for. But it certainly appears, that four houses heated with one boiler and one furnace, would be cheaper than four houses heated with four distinct boilers and furnaces, the quantity of piping in both cases being equal ; for then, three boilers and furnaces, or the cost of them, would be saved. This difference, however, will depend very much upon the distance the pipes must travel before entering the different houses. When the houses are situated close to each other, the difference must be very considerable. Some apparatuses of this kind have no box attached to them, and work directly to and from the boiler. I consider the box, however, as a very important appendage ; not only because it affords greater facility for working the apparatus, but because any of the other arrangements may be repaired more easily, and parts may even be taken away with- out in the least affecting the working of the rest. As I have already stated, pipes, in reality, radiate a very dry heat ; though many think otherwise, because the air of a hot- 18* 208 VARIOUS METHODS OF HEATING DESCRIBED IN DETAIL. house, so heated, is generally less arid than one heated by a hot- air stove. This arises from the fact, that, by hot-water pipes, a much larger radiating surface is presented to the atmosphere of the house than by any other method, and the heat is radiated at a lower temperature, and more equally diffused; hence, less moisture is carried upwards by currents of heated air and deposited on the glass by condensation. Thus, it is clear, that the larger the heating surface that is acted upon by the air, and the lower the temperature of that surface, the less moisture will be drawn from the plants and the atmosphere of the house. It is always desirable, however, to provide against aridity in the atmosphere, as heated air will have its supply of moisture, come from where it will ; and if it cannot draw it from anywhere else, it will draw it from the plants, or whatever can supply the larg- est quantity under its influence. For this purpose, a number of troughs are made to fit on the pipes, made of zinc or gal- vanized tin. These troughs may likewise be made of earthen ware, and perhaps more cheaply than of zinc, though more lia- ble to be broken. They may be filled with a syphon from the pipes, or by a common water-pot. When moisture is required in the house, an agreeable evaporation will be given off, and which can be rendered still more healthful, by putting in a few bits of carbonate of ammonia among the water, or common pigeon's dung, or guano. As the water warms, ammonia will be evolved into the atmosphere and greedily absorbed by the plants. In recommending this system to the notice of those who may be entering upon the erection of hot-houses, we would state that we recommend it not only upon our own experience, but also upon that of others, whom we consider much better qualified to decide upon its merits. Nor do we mean to assert that it is the ne plus ultra of a heating apparatus, although, under certain circumstances, it is the nearest approach to it that has yet come under our observation. In making this statement, we do not wish to dispute the judgment of those who think differently, and who have opposed it more from a feeling of groundless distrust, than from any fact they can bring to bear against it. We have conversed with many who would prefer heating each house with VARIOUS METHODS OF HEATING DESCRIBED IN DETAIL. 209 its own fire and boiler, and be at the additional trouble of attend- ing them too. This, however, springs from a fear entirely with- out foundation, and we are convinced that a little experience in the working of this double system of heating would so prove it. It is a very singular attachment which some people have for old methods and customs, that they will unflinchingly adhere to them, however little merit they may have to recommend them. Some individuals, with a self-sufficiency altogether incompatible with knowledge, will smile or sneer at what they are pleased to call the folly of enthusiasm, and, without seeming to be in any way sensible of the importance of whatever tends to the im- provement of horticulture, regard these innovations merely as idle speculations of men who have nothing else to do but invent them ; and while we cannot guard too much against the adoption of methods that will prove inconvenient in practice, although supported by theory, it is an injury to gardening, as an art, to give an unqualified opposition to systems that have proved their superiority, and are still capable of great improvement. This plan is not introduced under the deceptive cognomen of cheap- ness. Its cost will very much depend upon the circumstance of position, and may, after all, be much less than some of the costly and cumbrous apparatuses that are now in use. The easiness with which it is worked adds an additional item to its worth, for, when once set agoing, and understood, the veriest novice could manage it. * * It is the common fate of new systems connected with the art of horticulture, that they are eulogized beyond their real merits by their advocates, and decried as strongly by their opponents j for every new system has always both friends and foes, each of whom are unwilling to adhere to the naked truth, and equally incapable of appreciating its merits with exactness. When a person invents, or fancies he has invented, something new, he is too much inclined to set a high value upon it ; for, if it has cost him much labor, he is unwilling to think he has been diligent in vain. He, therefore, magnifies what is merely an alteration into an improvement, and probably prevails upon the imagi- nation of others to fall into a false approbation of the system, and to regard that as a valuable desideratum which, at the best, was only a novelty. If durability and econo'my in working be allowed to constitute any part of excellence in a system, then this one has especial claims to our notice j a fact which cannot be said of many others. 210 VARIOUS METHODS OF HEATING DESCRIBED IN DETAIL. Fig. 43. VARIOUS METHODS OF HEATING DESCRIBED IN DETAIL. 211 Fig. 43 shows a house wherein provision is made for increas- ing the heating surface, when more than a very moderate degree of heat is required from the pipes. A box or tank, made of wood or zinc, is placed under the stage, and passes all round it on a level with the pipes. This tank is supplied by a branch pipe ,, for carrying off extraneous gases and vapors that may be generated by artificial heat, it should be introduced, by all means, with great caution ; and some expedient should be adopted for supplying it with moisture, as well as to warm it slightly on its passage inwards, more especially in cold, frosty, or windy weather. If it is only introduced for the purpose of lowering the tem- perature, as in mild and genial spring and summer weather, it may be admitted without any such precaution ; and the freedom of admission should be in proportion as the external and inter- nal temperatures approach each other in equality. In hot, sultry weather, air should be sparingly admitted, as EFFECTS OF VENTILATION. 267 the same effects are produced by the excessive evaporation as by the currents of cold air, as will be afterwards shown. 5. Ventilation is also required, in winter, in pits and frames where soft and succulent plants are grown, especially in pits and frames warmed with fermenting materials. In this case, much care and caution are necessary ; the object here being to carry off the superfluous moisture, in order that the succulent tissue of the plants may not absorb more aqueous matter than they can decompose and assimilate. Although these kinds of plants will bear a high degree of atmospherical moisture in summer, when the days are long and the sun bright, and when, consequently, all their digestive energies are in full activity, yet they are by no means able to endure the same amount in the dark, short days of winter, when their powers of decomposition, or diges- tion, are comparatively feeble. 6. The thermometric changes are by no means satisfactory guides for regulating the admission of air in hot-houses, as the effect required by the indications of the thermometer may be produced without resorting to the admission of air. In hot- houses, we have full control over the state of the atmosphere, both as regards its moisture and temperature ; and the means of exercising this power ought to be known and familiar to every gardener. But there are many circumstances which ought to be duly considered in the exercise of this power, and some unsuspected results arise from the unlimited use and exer- cise of it ; and, as has been already said, by far the greater number of gardeners attach too much importance to the mere opening and shutting of sashes, windows, etc., without duly studying the rationale of the practice. We will show that the practical effects of ventilation are not only different from what many suppose, but are actually injurious. During winter we are in the habit of raising the temperature of our hot-houses, by artificial heat, to 45 or 50 ; then, for six or seven hours during the day, we open the lights and admit a large quantity of cold air. This is also a stumbliag-block, on which a great many gardeners fall ; for it is not solely to the 23* 268 EFFECTS OF VENTILATION. temperature, but rather to the hygrometrical state of the atmos- phere, we ought to look. We ought to regulate the admission of air, not solely by the thermometer, but also by the hygrome- ter ; for, upon the latter condition, the health of the plants, and the perfection of their flowers and fruit, very much depend ; and, consequently, it is a matter which ought to be studiously considered. Nothing is more injurious than the admission of currents of air when the external temperature is lower than the internal one ; and more especially so to plants that have been for a considerable time subjected to a high temperature by arti- ficial heat. The causes which operate in rendering the atmosphere of hot-houses unnaturally arid may be said to be two-fold. The first is the condensation of moisture upon the glass, arising from the action of the external cold upon its upper surface. The second is the escape of heated air through the laps and crev- ices of the glass, and otherwise. This heated air escaping, car- ries along with it a large quantity of contained moisture, the loss of air being supplied with cold, dry air, which finds access by the same means. The loss of heat and moisture sustained by these means is far more than would be supposed by those who have not calculated the amount. 7. We have seen that the quantity of moisture a cubic foot of air will hold in invisible suspension depends on its tem- perature ; and as the temperature is increased, so is its capacity for moisture. Suppose, then, that this capacity is doubled between the temperature of 40 and 60 ; that is to say, every cubic foot of air that enters the house at 40, and escapes at 60, carries with it just double the quantity of moisture it brought in. Now, every one must be sensible that these circumstances, con- tinued for any length of time, must render the atmosphere of the house too arid for healthy vegetation ; and, consequently, if the deficiency of moisture so occasioned be not supplied by artificial evaporation, then the plants must part with their secre- tions to supply the atmospheric demand, and the soil and other materials in the house will also be drained of their moisture, to make up the deficiency. The .greater the difference between EFFECTS OF VENTILATION. 269 the internal and external temperature, the greater will be the demand for moisture. Thus, if the external air be at the freez- ing point, (32,) and the air in the house heated to 50 degrees, then there is three times more moisture carried away by escap- ing air than is brought in by the returning quantity; and, escaping at 90, it carries away four times as much, and so on, in proportion to the difference of the two atmospheres ; the ex- ternal air, however, increasing in ratio as it decreases in tem- perature. According to these calculations, atmospheric air, entering a house at 32, and escaping at 100, carries away nearly six times as much moisture as it brings in. This, in a short time, would render the atmosphere of a house deleterious to either animal or vegetable life ; and in large and lofty houses this is practically the case. We have managed a lofty plant-house, where the plants on the side shelves were nearly frozen, while the thermometer, hung in the angle of the roof, about 45 feet high, stood at 100 degrees. Now this heated air, escaping at the top of the roof, as is generally the case as well as here, carried away more moisture than the small evaporating surface could supply ; the effects were, consequently, ruinous to the plants. However imperfect the above calculations may be, they are within the bounds of truth, and are sufficiently accurate to show the im- portance of this subject to exotic horticulture ; and it will more effectually impress upon our minds the amount of care and con- sideration which the ventilating of hot-houses demands. If air must be admitted, for the purpose of regulating the internal temperature, every precaution should be taken to prevent it from entering in strong currents, and it should be taken in from the warmest side of the house, and, if possible, over a warm surface, as hot-water pipes, or whatever heating apparatus may be employed, so that the internal atmosphere may be gradually reduced ; and, at the same time, the utmost precaution should be used to prevent the escape of heated air, at least as little as possible, by direct ascension ; this is easily accomplished by the improved methods of ventilation now adopted, some of which I shall hereafter endeavor to describe. Thus the cultivator is enabled to modify the two atmospheres, previous to their com- 270 EFFECTS OF VENTILATION. bination, and by raising the humidity in the atmosphere of the house, to compensate for that carried away by the egress of heated air, the plants will breathe an atmosphere more con- ducive to their healthy development, and will be benefited by the change. 8. Every gardener has observed the water on the under sur- face of the glass, in the morning, before the sun has risen, warmed the glass, and driven it off again, in the form of aqueous vapor. This affords us a good illustration of the immense quan- tity of moisture carried upwards by the heated air, and depos- ited upon the glass, by condensation. This moisture is, of course, taken away from the plants, and other bodies capable of giving it off, and is demanded by the air as it becomes warm, and capable of carrying a larger quantity than when no fire was applied, or rather, when the temperature of the house and the temperature of the external air were alike, for in such case no condensation on the glass would take place ; and, as I have remarked, the proportion of water deposited will be in exact ratio to the intensity of the external cold ; thus, the greater the difference, the greater the deposition ; for then the action of the external cold upon the upper surface of the glass being greater, and the two atmospheres being brought into more rapid proximity, the particles of heated air are cooled as quickly as they ascend to the under surface of the glass ; they then fall to supply the place of others, leaving the contained moisture upon the cooling surface, in the form of dew, the same process being repeated through the whole night, or until an equality of temperature is established ; the quantity thus deposited amounts to immense volumes of water. 9. Experiments have proved that each square foot of glass contained in the roof of a hot-house will cool down 1J cubic feet of heated air per minute as many degrees as the temper- ature of the internal exceeds that of the external atmosphere. Suppose, for instance, that the external air stands at 40, and that of the house 60 ; then, for every square foot of glass contained in the house, one and one fourth cubic feet of EFFECTS OF VENTILATION. 271 air will be cooled down the 20 degrees ; thus, 60 minus 40 gives the difference, which is 20. If the house contains 800 square feet of glass, presented to the action of the external atmosphere, 1000 cubic feet of air will lose 20 degrees of heat ; consequent- ly, the moisture this air held in invisible solution, in virtue of its 20 degrees of temperature, will be condensed by the external cold, and deposited on the glass ; and it will also be found, that the greater the difference between the external and internal temperatures, the greater will be the amount of condensation. The quantity of moisture abstracted from plants, at high tem- peratures, is enormous. This fact is sufficiently demonstrated in a hot summer day, when the leaves of the trees are wilted, and the garden vegetables flag and droop their leaves. The earth gives out its moisture, and the atmosphere carries it away. The same thing takes place in hot-houses ; the moisture is ab- stracted by the heated air, and is carried off in the form of invisible vapor, till its upward progress is arrested by the glass, and the cold again reduces it to water. If we take, for example, the roof of a hot-house, comprising 750 superficial feet of glass, and calculate that every square foot of that glass will cool down 1| cubic feet of heated air 36 per minute, and calculating the internal temperature at 65, we shall find that 937 cubic feet of air will be cooled down 36 de- grees per minute. Now air, saturated at the temperature of 65 degrees, contains about 6-59 grains of water per cubic foot, and at the temperature of 30 degrees, it is saturated with 2-25 grains ; this gives 4-34 grains of water lost, in condensation on the glass, per minute ; or further, each square foot of glass con- denses 1 J cubic feet, or about 5-42 grains of water, per minute ; and supposing the atmosphere of a house, such as we have de- scribed, to be constantly supplied with moisture, by evaporation, or otherwise, there would be abstracted from it about of "a pint of water per minute, which is about 12 quarts per hour, or at the rate of nearly 72 gallons in 24 hours. This enormous amount of water, evaporated into the atmosphere of a hot-house, when reduced to calculation, and displayed in plain figures, seems to startle the imagination, and looks very like exaggera- tion; although it is much below the mark which, by a more 272 EFFECTS OF VENTILATION. accurate calculation, it would certainly reach, yet the accuracy of these calculations will appear sufficiently obvious to any one who has paid studious attention to the subject. I say studious attention, because a person may be tolerably observant of atmos- pheric phenomena, and yet not form anything like an accurate idea of this extraordinary process going on in his presence, and the effect thereby produced on the vegetable system. When we enter a hot-house, on a cold, frosty morning, after a strong fire has been kept up during the night, we are very apt to regard the moisture condensed upon the lower surface of the glass as an evidence of a healthy atmosphere and luxuriant veg- etation ; and often have I heard it stoutly asserted, that it was merely the effect of an excess of moisture in the atmos- phere of the house. This may be partly true, but the conclusions which are drawn from the fact are founded on misconception, that the moisture thus deposited on the glass has already per- formed its purpose of benefit to the plants. SECTION III. METHODS OF VENTILATION, &C. 1. If we admit the truth of the foregoing calculations, (and we cannot justly reject them, until they are disproved by calcu- lations more accurate, and observations more extended,) then we must acknowledge, also, that the old methods of ventilating hot-houses, which are still in common practice, are contrary to what we know to be right. Hence the question arises, How are these methods to be improved ? Now, I would remark, that the mere system on which a house may be ventilated is of com- paratively little importance, for no method of ventilation will be good, if the atmosphere be unskilfully managed. Various plans have been employed to modify the influence of draughts, or currents of air, many of which can hardly be termed improve- ments, since the general effect is the same as by the old method of opening the top and bottom sashes, which admits a current to rise up beneath the under surface of the glass, and, as it pro- ceeds towards the aperture made by letting down the upper sashes, it carries the ascending moisture along with it, without in the slightest degree mixing with, or purifying, the volume of atmosphere contained in the lower portions of the house. 2. It has long been an object among gardeners to obtain a motion in the atmosphere of a hot-house ; and to secure this, even machinery has, in some instances, been employed, and, under certain conditions of the atmosphere, these machines may go on very well. But subject to those vicissitudes of climate, so prevalent in many parts of this continent, the consequent result of their adoption is, a complete derangement of all that equalizing regularity which they were intended to secure. It appears to us a matter of considerable difficulty to lay down a definite rule, or propose a particular system of ventilating a house, since almost every locality has some characteristics pecu- 274 METHODS OF VENTILATION. liar to itself. It is true, the elements of the atmosphere may be nearly the same in one place as in another ; but they are influ- enced by various circumstances, in different localities, and hold soluble matters in suspension in very different proportions ; and in places much screened by trees, buildings, and similar objects of shelter and obstruction, air may be admitted with greater impunity than in situations exposed to wind from every quarter of the compass, the latter condition, as a matter of course, re- quiring more care, not only in the adjustment of the apertures of admission, but also in the admission itself. The course of the current of air, by the common methods of ventilation, that is, by opening the front, and letting down the top sashes, is ex- ceedingly variable ; sometimes the actual motion created in the atmosphere is little more than a foot, or fourteen inches, below the surface of the glass. This motion can be easily determined by holding the flame of a candle in the current, when the flame will incline towards the aperture of egress ; lower it gradually down, till it assumes and maintains a perpendicular position, being no longer affected by the current, the volume of air being, in fact, stationary, except there be some aperture of ingress else- where. We have found this simple operation exceedingly useful in determining the currents of air in large houses, and, in most cases, it seldom fails in giving an accurate indication of their course. However desirable a motion may be in the atmosphere of a hot-house, and I do not doubt but it is beneficial, yet it is not necessary that we should run headlong either upon Scylla or Charybdis. There is a great difference between a motion in the atmosphere created by the warm particles ascending, and being replaced by the denser and colder air, and that created by a tornado sweeping through the house. The former motion is only perceptible to the eye of the attentive and experienced cul- tivator, and he can tell at a glance, by the quivering of the leaves, that they are fanned by a gentle zephyr. I am aware that some gardeners have a peculiar fancy for seeing their plants and vine-leaves bristling about by a good wind, and may be very successful, too, in their productions; but it cannot be as- serted that it is compatible with a high state of gardening skill, METHODS OF VENTILATION. 275 or with that perfection in horticulture at which it is our duty to aim ; inasmuch as the revelations of science are against it, as has already been shown, and practice has hitherto given no evi- dence to prove it beneficial to tender plants. 3. In large and lofty structures, and especially in dome-shaped houses, the management of the atmosphere becomes a matter of much more importance than in small houses. During mild and temperate weather, things may go on very well, as at such times the external air may be allowed to circulate through the house with greater impunity ; but during the heat of summer, and the cold of winter, the atmosphere is much more difficult to equalize. With a frosty air externally, and the temperature at the surface of the earth down to zero, it is impossible to maintain a proper degree of temperature, in all parts of the house, without positive injury to those plants that may be growing, or have their branches extended into the upper regions of the house. In fact, without the precaution of covering, or some such expe- dient, mischief is absolutely unavoidable. What has already been said, upon the nature and properties of air, will sufficiently explain the cause ; and, although it has been repeatedly asserted by theorists, that one part of a house being heated by radiation, from a body radiating heat, the equalizing law of nature will heat all parts of the house to the same temperature, and as speedily, too, yet we must enter our decided protest against the practical correctness of such a statement ; at least, in our own practice, we have never found it so, under any circumstance, or by any system of heating. And hence, whatever the natural law of equality may be, the practical effects cannot be mistaken, or disputed, as far as regards hot-houses. We know that heated bodies tend to an equality of temperature ; but, as has been already observed, air, of all other bodies, possesses peculiar properties in this respect in regard to heat, and in nothing is this peculiarity more strikingly illustrated than in the case under consideration. 4. With regard to the motion of the atmosphere in a hot- house, we know that the greater the difference between the tem- 24 276 METHODS OF VENTILATION. perature of the air entering the house and the atmosphere of the house itself, the greater will be the movement produced among the particles. The motion is in exact proportion to the difference of temperature ; and hence the necessity of admitting the external air, in small quantities, when the external ther- mometer is low. The slightest cause that disturbs the equilib- rium of the air produces a motion. It is more sensible than the most delicate balance. It is put in motion by the slightest inequalities of pressure, and by the smallest change of tempera- ture. It is speedily rarefied by heat, and thereby rendered specifically lighter than the neighboring portions, so that it descends, while colder, and consequently denser, flows in, to re- store the equilibrium. It will be easily seen, from the very nature of this law, that an equilibrium cannot be maintained in the artificial atmosphere of a hot-house, since the source of radiation must necessarily be confined to too small a surface to equalize the ascending heat; and, on the other hand, the con- densation by cold is too irregular throughout the heated vol- ume. This irregularity, produced by its unequal action on different parts of the house, must ever render it impossible to obtain an equality of temperature throughout an atmosphere heated by artificial means ; and the larger the house, the greater will be the difficulty of maintaining an equilibrium in its various parts. So much so is this the case, that, as has been already stated, the difference has been found to amount to 100. "Gaseous bodies expand equally for an equal increase of temperature, as measured by the thermometer. Gay Lussac showed that 100 measures of atmospheric air, heated from the freezing to the boiling point, became 137.5 measures ; conse- quently, the increase for 180 Fahrenheit is \^ of its bulk. Dividing this quantity by 180, we find that a given quantity of dry air expands T ^ of the volume it occupied at 32, for every degree of Fahrenheit. New experiments have been made by Kudberg, within a few years, giving ^ T as the ratio of expan- sion for one degree of Fahrenheit ; and these results are con- firmed by Regnault. This last number may be adopted as the true increment. "If we wish to ascertain the volume which 100 cubic inches METHODS OF VENTILATION. 277 of a gas at 40 would occupy at 80, we must remember that it does not expand f T of its bulk at 40 for each degree, but T of its bulk at 32. Now, 491 parts of air at 32 become 492 at 33, become 493 at 34, and so on. Hence we can institute a proportion between the volume at 40 and that at 80. * Vol. at 400. Volume at 803. Cubic inch. Cubic inch. 491 + 8 : 491-J-48 : : 100 : 108 5. The annexed cuts represent an improved method of ventilating lean-to houses, and by which the Fig. 52. Fig. 53. Fisr. 51. * Wyman on Vent. 278 METHODS OF VENTILATION. whole house may be aired in the space of one minute ; or as many houses as may be in the range. This is effected by a rod passing along the whole length of the house. A pulley is fixed immediately above each ventilator, and another placed opposite it upon the rod, as shown in Fig. 51. A piece of chain or cord is attached to the ventilator at one end ; and passing over the pulley, as shown at a, Fig. 52, is then fixed to the pulley placed opposite it upon the rod. A larger wheel, or pulley, is fixed at one end of the rod, (,) to which is attached a chain, connected with a crank, situated within the reach of a person standing on the floor. This crank is fixed on the back wall, as seen at c, Fig. 52. From the foregoing cuts and description it will be perceived that, by giving the crank (d) a few turns, the whole of the ven- tilators will be opened. The crank is provided with a racket, so that they may be opened to any distance, from half an inch to the full height. The ventilators in the front wall may be opened and shut by the same method, and may be, for convenience, brought from the outside. Any length of house, or any number of houses, may be ventilated at once by this method, providing the apertures are in a straight line; their perpendicular distance from the horizontal shaft makes no difference in their facility of working. The pulley cords of the higher ones only require to be length- ened according to the distance, the diameter of the wheel on which the cord turns being equal all along the shaft. 6. Figures 54 and 55 represent a method of ventilating span-roofed houses. It is employed in the houses at Frogmore, Fig. 54. METHODS OF VENTILATION. 279 in England. Fig. 54 represents the end section of the house, with the ventilator in proportion to the other parts. Fig. 55 Fig. 55. shows the sectional view of the ventilator, enlarged : a a sue openings of admission, and are covered with lattice-work, to break the force of the current of ingress ; b, the movable shut- ter, which regulates the admission to and egress from the house. It is scarcely necessary to observe that these houses have been ventilated on the most approved principles; and it appears that several advantages are gained by this method. For in- stance, the current of heated air is arrested, in its progress outwards, by the depending glass at c c, and is, in some meas- ure, thrown downwards, preventing also the escape of its con- tained moisture. There is no doubt this method is very com- mendable for span-roofed houses ; and one of its advantages is, that the house can be aired, at any time, without the plants being saturated with rain. It is very possible that these compound systems of ventilation may excite a smile from some who have, all their lifetime, been accustomed to pull heavy sashes up and down for the purpose of giving air. But if we include, in one computation, the labor, the time, and the advantages of giving a range of houses three or four hundred feet long, air at the proper time, and all at the same moment, we will find a value in the system worthy of something more than the mere smile of passive silence, which is too frequently all that is at first accorded to such improve- ments. In some establishments, instead of pulleys, toothed wheels are fixed to the shaft, which are made to work in a curved handle 24* 280 METHODS OF VENTILATION. attached to the front sash by means of a hinge. This curved rod is toothed on the lower side to answer the wheel, and is kept in its place by an iron staple, having an eye through which the sash-handle passes, as seen at a, Fig. 56. A crank and rachet- Fig. 56. wheel is provided, at one end of the shaft, by turning which the sashes are simultaneously opened and shut, to any distance. This method is simple and efficient. It has been extensively carried out in the unique assemblage of horticultural buildings at Frogmore ; and, as an improvement in the modes of ventilat- ing hot-houses, is considered, by competent judges, the most valuable contrivance that has been introduced during the last half century. By the turning of a small windlass, (which any child may do,) any quantity of air may be admitted, and in- creased or diminished at pleasure, throughout the whole range of buildings. The ventilation of forcing-houses, by this compound method of opening the whole sashes at once, is very liable to produce serious results, before the person in charge becomes fully acquainted with the management of it. This, like many other really valuable improvements in gardening, has been adopted, bungled in the construction, mismanaged afterwards, then, lo ! it is condemned, with all the pomp and dignity of practical experience ! The present moment affords an ocular demonstra- tion of this too common fact, Some people suppose, if they can only get mechanical contrivances to accomplish certain ends, METHODS OF VENTILATION. 281 that all is right. It is certainly desirable to employ mechan- ical contrivances, whenever they can, as in the present case, be applied advantageously. But mechanism can never make a gardener, inasmuch as the chief part of what constitutes a real gardener springs from mental, not physical, activity. It is a very easy matter to open and shut the ventilators of a hot- house ; but it requires something more than mere mechanical power to do so with certain benefit to the inhabitants within. This will be rendered clear by a common illustration. Let a dwelling-room be warmed to a temperature of 60 ; and suppose it to be tolerably well filled with individuals, by the animal heat and respiration of whom the room by and by becomes somewhat raised in temperature, and contaminated in its atmosphere. Then, all at once, let the windows be thrown open, and the con- sequence is not only disagreeable, but highly dangerous, as is manifest by the murmur which very soon pervades the assem- bled party. Now, the case is precisely similar in a hot-house, only with this difference, the unfortunate plants cannot speak in audible sounds to tell the injuries that are perpetrated upon them ; yet they bear a language, imprinted on their leaves, no less truthful, nor less understood by the attentive observer. The above common occurrence is a plain illustration of what I have often seen, and have been forced to perform, in the ventilation of forcing-houses, and which is more likely to be exemplified by the compound methods which I have described. Science may enable us to be more watchful of atmospheric phenomena, and may draw our attention to facts which mere practice might pass unnoticed. But this is a practical operation which science has not yet approached, and which, in all her discoveries, she never can approach, i. e., to tell us the precise quantum of air to admit at different times and under different temperatures. The method of mixtures does not come near it, and the combination of gases gives the gardener little scientific assistance. We must know the nature and properties of air at all times and tempera- tures ; but the quantities and proportions in which we are to admit it must be learned by experience and strict observation. We must watch its effects upon the plants, and admit it in 7/0-1 ..li '.- 282 METHODS OF VENTILATION. proportions which appear, by oft-repeated trials, to be most beneficial. 7. We could describe several other systems of ventilation, by what we have called the compound method, which have a greater number of wheels and rachets, and other kinds of ma- chinery about them, but which possess no advantage over either of the methods we have described. One system, in particular, has received some countenance, which consists in opening by the aid of a spring instead of the toothed rod, as shown in Fig. 56. We have managed various houses ventilated by this method, but we must say that it worked badly, although much care had been taken to have the machinery properly fitted up ; for instance, where the springs are of unequal strength, and by constant use they very soon become so, you will find a very great irreg- ularity in the airing of the house, some of them requiring to be opened nearly full length, before the others will open a few inches. Again, if some of the sashes be stiff to open, those that are not so will open freely, while the ones that are hard to move will not open at all. This has frequently caused us much annoyance. It can never occur with the toothed wheel, as an equal force is exerted on each ventilator or sash, and every sash is opened to a regular distance. But if any of the sashes be stiff to open, then the whole power applied is directed upon them alone, until the whole move together. The only supposed advantage of the springs is, that they do the work silently, whereas a little noise is made by the rachet-wheels, a matter, in most cases, of so trifling importance, as to be unworthy of consideration ; but, as drowning men catch at straws, so the most insignificant circumstance is eagerly seized, and magnified into momentous import, by would-be inventors, for the purpose of palming off their so-called invention upon the community, and sustaining its sinking reputation. The less machinery there is about a hot-house, the better ; and that system which does its work in the most efficient manner, with the smallest amount of labor, and is least likely to get out of order, is decidedly to be preferred. This is a commendation which cannot be justly given to some late inventions ; and, without wishing to throw METHODS OF VENTILATION. 283 anything in the way of improving our present systems, or discouraging the application of new mechanical inventions to aid the practical operations of horticulture, we would say that some of these methods lately brought into notice may be justly compared to the putting of extra wheels to a carriage, increasing the rattling and complexity of the machine, but add- ing neither to the strength of the structure nor the rapidity of its course. ,-, SECTION IV. MANAGEMENT OF THE ATMOSPHERE 1. NOTWITHSTANDING all the discussion which has taken place upon the abstract question of atmospheric motion, and which, under certain temperatures, as we have already seen, cannot be disputed, the true principles of ventilation still remain unsettled ; and the mechanical operation of admitting the air in larger or smaller quantities with facility does not, in the slightest degree, remove the general objections that have been urged against its effects on the internal atmosphere. In considering, therefore, the question, how far the admission of external air into forcing-houses is practicable and proper, it is necessary to ask, in the first place, For what purpose is the admission of external air resorted to under certain circum- stances ? and, secondly, How does it act upon the atmosphere when admitted ? The first of these questions is of comparatively easy solution : the latter requires more deep consideration, and more close investigation, before we can find a satisfactory reply. First. The necessity for ventilation arises from two prime causes, which are briefly these : to regulate and reduce the internal temperature ; and to allow the escape of impure air, or that portion from which some of the essential constituents have been abstracted by the plants, or in which the natural equiva- lents have been changed in their proportions, and consequently the health-imbuing balance destroyed, an effect which may arise from various causes. The first of these points is a distinct consideration, forming an important branch in vegetable physi- ology: the others constitute a different branch of scientific research ; but in relation to our present subject, they both merge into one. MANAGEMENT OF THE ATMOSPHERE. 285 The admission of cold air as the sole or principal agent in regulating the internal temperature of a hot-house during win- ter, seems to be perfectly unjustifiable. There are, indeed, times when it can hardly be avoided, during the application of artificial heat ; but these are exceptions, rather than the rule. Heat, when applied in early forcing, or to maintain the temper- ature of plant-houses, is artificial, and, therefore, so far unnatu- ral. And it appears still more unnatural to apply more than is necessary, for the purpose of admitting the external to cool down the internal atmosphere, without having secured any equivalent advantage, but rather lost, by the change. It is much more reasonable, as well as economical, to apply as much heat, and no more than is necessary, to raise the temperature to the minimum point, or, at least, as near this point as is possible. It may be supposed that it would be unsafe to keep the tempera- ture so close to the minimum point, lest the sudden external changes, to which we are subjected in this country in winter, might have an unfavorable effect upon the internal atmosphere ; and, under certain circumstances, this would be the case, such as an imperfect heating apparatus, a badly glazed house, or a want of skill in the management of it. The necessity of main- taining the minimum rather than the maximum temperature has been already adverted to in the preceding chapter; and, instead of being the exception to a general rule, it is rapidly becoming the rule itself. We must consider that the object to be kept in view is to improve upon the means at present in use to obtain these results, and to obviate the risk and inconvenience which might otherwise ensue by their adoption. It will be observed, that it is not when the mild and genial weather of spring is experienced that these remarks have any forcible effect, but when the outward elements are unfavorable to the development of vegetable life. 2. The atmosphere of a hot-house is very much influenced in winter by the glazing of the sashes, and the adjustment of its various parts. When the laps of the glass are open, there is a continual egress and ingress movement in the atmosphere adja- cent to the apertures, extending generally over the whole of its 286 MANAGEMENT OF THE ATMOSPHERE. interior surface, but not always affecting seriously the internal volume, except in carrying off the rising particles of heated air, the greater portion of which is condensed by the cold air imme- diately as it escapes from the house. The consideration which refers to the escape of air in a deteriorated state, and the conse- quent necessity of admitting a fresh volume in its place, does not appear to offer any insurmountable difficulties to the belief that the admission of fresh air in the months of winter is very frequently carried to an injurious excess. Although plants, in the process of their growth, and in the discharge of their vital functions, abstract matters from the atmosphere around them, there is nothing, even in this, to render the admission of cold air in large volumes at all necessary. In considering the nature of the atmosphere in its relation to heat and cold, its elastic and all-pervading properties must not be lost sight of. Under any circumstances, a considerable effect will be produced by the external upon the internal atmosphere, by radiation alone ; and with the evidence before us of the successful growth of plants in situations so much closed up as in Wardian cases, we cannot do otherwise than believe that the interchange which takes place between the volumes by these causes is sufficient to secure the health and vigor of the plants, so far as the admission of air alone is concerned. If it be argued that deterioration will take place by means of evaporation from flues, or pipes, or any substances confined within the structure, or from the decomposi- tion of organic matter, the same fact is presented of an inter- change continually going on, and is sufficient to meet the case, so far as to show, that, on this ground, at least, the admission of external air in large volumes is not essential. Besides, with proper management, the gases that are generated by artificial heat, or by the decomposition of substances which should find a place in .hot-houses, may be combined with others having an affinity for them, and thereby not only purifying the atmosphere by preventing an excess of particular agents, but also turning those agents to their legitimate purpose, and rendering them beneficial, rather than detrimental, to vegetable life. And, therefore, it can only be in cases where misapplication or gross MANAGEMENT OF THE ATMOSPHERE. 287 mismanagement of some kind or other exists, that they can possibly be productive of injury, or even of inconvenience. These considerations, then, would seem to point out the fact r that the admission of air to any extent in forcing-houses in win- ter, or at a very early period of the season, cannot be said to be a matter of urgency, or necessity ; neither can it be grounded on the plea that many of our practical operations have for their foundation, viz., an expedient for a better, and probably more tedious, method of effecting the same results. Whatever impro- priety may appear in the above statement, it will be fully justi- fied by its truth, if a dozen years' extensive practice in the management of hot-houses, both large and small, and in the working of forcing-houses throughout the winter, be worth any- thing, as well as the evidence of many of the best practical gardeners of the present day. Then we would say that the influx of large volumes of cold air is decidedly hurtful, even on other grounds than those advanced in a former part of this chap- ter. But, on the other hand, the opposite extreme must also be avoided. The process may not be altogether dispensed with, although every means ought to be taken to modify its immedi- ate effect upon the internal atmosphere. It does appear, never- theless, that the regulation of the internal temperature, i. e., the prevention of too powerful a degree of heat, when the source of that heat is the sun, is the only legitimate end to be effected by the practice. If there are any other real advantages, they are certain to follow. If air is admitted with this only in view, and these advantages are not likely to be lost if air is not admit- ted when not required to effect this primary purpose, periods of bright sunshine, then, may be regarded as the only instances in which a recourse to the practice is absolutely necessary. 3. From a full investigation and consideration of this sub- ject, the conclusion at which we hare arrived is, that, with a proper system and routine of management, as regards the application of atmospheric humidity and heat, the admission of large volumes of the external air into the interior of hot-houses is not by any means so essential as it is generally represented to be. Whatever other differences of opinion may exist with 25 2SS MANAGEMENT OF THE ATMOSPHERE. respect to this practice, it cannot be denied that a risk is in- curred, and frequently an injury sustained, when cold air comes in contact with the active organs of tender plants. And, there- fore, if no other advantage be gained from the practice than the regulation of the temperature, then, except in cases where the heat is" increased by the influence of the sun, and therefore uncontrollable, it would be a much wiser practice to apply a less amount of heat by artificial means, thus rendering it less neces- sary to allow the superabundant portion to escape, and conse- quently exposing the plants in a less degree to the risk to which we have alluded. 4. Even in those cases in which it is really necessary to have recourse to the practice of admitting air, much injury will be sustained, though it may not be apparent at the time, by admitting it in a rash and improper manner. It should be con- trived so that the change to be effected may be brought about gradually, and the cold and heated volumes should be made to intermingle regularly together, and in a way that the internal volume will be equally affected by it. Thus, if it be desirable to admit a quantity of air equivalent to the reduction of 20 of temperature, then the first consideration ought to be the external temperature ; and the apertures of admission ought to be regulated according to the calculations given at pp. 164 and 165, and in such a manner that the volume of air within the house will not be deteriorated thereby, nor deprived of those gases which are essential to vegetable existence. Secondly. How does the external air act upon the internal at- mosphere, when so admitted ? This portion of our subject is of more difficult solution, and requires a closer investigation, inas- much as it is influenced by various causes, such as the form of the structure, the method of admission, and the material of which the interior part of the house is composed ; for example, a house presenting a large surface of glass to the morning sun requires to be sooner ventilated than one whose largest glass surface has a western aspect, and a small quantity of air admit- ted early in the morning will keep the temperature down for a MANAGEMENT OF THE ATMOSPHERE. 289 longer period, than a larger portion, when the temperature of the house has increased ten or twelve degrees higher. Again, if the top sashes be opened first, which is generally done, then a much larger quantity of oxygen and aqueous vapor is carried off than at any other period of the day. We believe it is the practice of nineteen out of every twenty gardeners, to open the top sashes first; then, when the internal temperature rises, and more external air is necessary, the top sashes are opened still more ; and, last of all, the front sashes are opened to make a cir- culation; a circulation, indeed! By the time the front sashes are opened, the two atmospheres are generally equalized. Now, I would ask, how is this circulation produced, and what are its effects ? Not by the superior density of either atmosphere, for both are the same, but by currents of wind, and draughts created by other causes ; arid their effect is to carry off the moisture already too much reduced. The annexed figure represents a Fi?. 57. n 1 V"; It V rn method of admitting fresh air into a house which obviates the evil here complained of. The air enters through the side-walls at a a, then passes along beneath the floor, and enters the house in the centre of the floor, at b. In this instance, no air is admitted at the top; hence, the air, passing through these drains, enters the house at a higher temperature than if admitted at the sides or top, and, becoming gradually warmed as it ascends through the aperture in the floor, rises until it is again cooled by action of the external air upon the glass, then falls towards both sides of the house, producing a motion somewhat similar to that 290 MANAGEMENT OF THE ATMOSPHERE. shown by the arrows in the foregoing figure. By this method, air may be introduced into a house at any period of the day, or even at night; and while every advantage arising from the admission of external air is gained, the disadvantages are done away with, save and except by the crevices in the structure. In winter, if cold air must be introduced to regulate the internal temperature, some such method as that given above should be adopted; but at a more advanced season of the year, when a larger supply of air is necessary, provision must be made at the sides for that purpose. As to opening the top sashes first, and keeping them open till the last, it is a practice for which we are unable to obtain any satisfactory reason, and which we think will not bear a strict investigation. But, it may be asked, how is the temperature to be reduced, where, at an advanced period of spring, the sun shines more powerfully, and when the tem- perature of a hot-house will suddenly rise ten or fifteen degrees above the maximum point? To answer this question, it is necessary to consider whether there be any other method of reducing the temperature than by expelling the heated air, by the opening of the top sashes. From what has already been said on this point, we think we are fully justified in disposing of this question in the affirmative. Of course, we do not allude to the ventilation of houses in summer, but in the months of autumn, winter, and spring. By introducing the external air in the manner described in the last figure, the atmosphere of a hot- house will be reduced to any given point as effectually, though not so rapidly, as if the heated air was expelled through the sashes at the top of the house. This is accounted for by the circumstance already explained, viz., that when two columns of air of unequal temperatures are mixed together, the tempera- ture of the whole is reduced, while its density is increased ; and hence, so long as the atmosphere continues to be heated by reflection or radiation, this cold air will continue to cool it down, so that nothing is lost, while all the essentials of vegetation contained in the atmosphere are retained. 5. The materials of which the internal part of the house is composed have also a powerful influence on the ventilation of a MANAGEMENT OF THE ATMOSPHERE. 291 hot-house. Those houses whose internal bases are composed of open soil require less ventilation than those that are paved with stone or tiles ; and those that are paved with tiles, or other soft materials, require less than those formed of hard and highly reflecting bodies ; dark-colored walls, also, are longer in raising the temperature of houses than walls painted white, and for this reason white is preferred to any other color, as well as for its clean and light appearance when contrasted with the dark-green foliage of the plants. But in houses that are perfectly transpa- rent on every side, and admit abundance of light, there is no reason to suppose a dark color would not be preferable to a light one, although we are well aware that some scruples may be raised against it. Its propriety, however, can only be ques- tioned as a matter of taste, not of utility ; for, with the advan- tages above alluded to, in a well-constructed green-house, so far as the management of its atmosphere is concerned, we would decidedly prefer a house having the interior painted with a dark color, although we are very sensible that the effect produced would be meagre and dull, and but little calculated to harmo- nize with the floral inhabitants of the house, or the feelings of those who admire them. Fig. 58. 6. The above cut represents a house ventilated by the com- mon method, i. e., the upright sashes at the sides and the top sashes along the roof, which, in span-framed houses, are gener- ally about four feet long, or nearly square. In summer this method answers perfectly ; but in winter and early spring it is 292 MANAGEMENT O'F THE ATMOSPHERE. next to impossible to admit air without injury to the plants, and incurring the evils which have been already detailed. Such a house as this should, by all means, have these sashes made to open, when requisite, but should also be provided with an under-ground method of admitting air, when the weather is unfavorable for opening the top arid side sashes ; and, in this country, this may be said to be the case for at least three months out of the twelve, during which time air can seldom be admitted in anything like a sufficient quantity, without a posi- tive, though perhaps at the time an imperceptible, injury to exotic plants. Various other methods have been adopted for imparting to the atmosphere of a hot-house all the freshness of the natural atmosphere, without a reduction of temperature corresponding to the amount of cold air admitted, and also to effect this with- out an increased consumption of fuel. The following simple method has been carried out with pretty favorable results : 7. Suppose a house already heated by the common flue. We would propose that a square chamber be built over the top of the furnace, and embracing the neck of the flue for two or three feet, if practicable. This chamber should have a drain, not straight, but of a serpentine or zig-zag form, laid through it, one of its ends communicating with the external air, and the other communicating with the interior of the house. Into this latter opening, a pipe, made of tin or zinc, should be fitted, of sufficient size for the admission of a good volume of air. Let this pipe be laid along the lateral surface of the flue nearest the front wall of the house, not in immediate contact with, but sup- ported by bricks, or some other means, at the distance of a few inches from the flue. Let that portion of the tube which passes along the front be perforated with holes, to facilitate the escape of the warm air, with which it will be filled, into the interior of the house. This done, let a number of small tubes, say one for each light, or one for each alternate light, be fixed through the front wall, or otherwise as may be convenient, one end communicating with the external atmosphere, and the other entering the perforated tube. These smaller tubes should be MANAGEMENT OF THE ATMOSPHERE. 293 provided with valves to open and shut at pleasure, to any extent within the limits of their diameter, so that the apertures of ingress for the cold air may be regulated by the operator accord- ing to the state of the weather and the quantity of air required. The size of these tubes will Depend upon the size and situation of the house. For instance, if the house contains a large inter- nal volume of atmosphere, the perforated tube would require to be at least eight inches in diameter, and the smaller about one half the size of the large ones. And now for its mode of action. It will be evident, that when fire is applied to the furnace, its cover (which forms the floor of the chamber) will become heated to a considerable degree. As soon as this takes place, the external valve of the drain, which communicates with the main tube, should be opened, when the external air will immediately rush in ; and, by having to traverse the heated floor of the chamber aforesaid, will expand along the large tube connected with it, which, from being in contact with the heated air, will itself become warm. The radiation of heat, too, from the surface of the flue directly beneath it, will assist in maintaining the tem- perature of the tube ; so that, although a portion of the heated air will escape through the perforations in its upper surface, enough will be retained to effect the purpose intended, which is, to neutralize the effects of the cold air that will be admitted through the medium of the small lateral tubes, and which may be admitted in any quantity, to the full volume of their admis- sion. As the warm air rushes along the tube, it will mingle with that admitted by the small tubes; and the cold air, enter- ing by the latter, will thus be modified, while a supply of fresh air will at the same time be circulated through the atmosphere of the house. 8. The advocates of what has been called a " free system of ventilation" have, like many others, in practising and advocat- ing a favorite theory, in their excess of zeal, completely defeated the objects they sought to secure. The sole object of some of the advocates of the free system appears to be the prevention of a stagnant atmosphere. They admit an unlimited quantity of atmospheric air, at all seasons, to prevent this most terrible 294 MANAGEMENT OF THE ATMOSPHERE. evil they call stagnation, and denounce the system of sealing up plants (as some of them have termed it) from all atmospheric influence but that exerted over them by their own tainted arti- ficial atmosphere. Now, a stagnant atmosphere, or any con- dition in the atmosphere of a hot-house approaching to stagna- tion, certainly cannot be otherwise than injurious to vegetation. This is a statement the truth of which will scarcely be called in question. But, although the prevention or removal of it has always been the chief object of every scientific gardener, it can- not be said that every gardener, having this aim in view, has taken the right way to effect his purpose ; for, certainly, what is called "free" ventilation is very far from being the proper mode of obviating the evil ; and, in questioning the propriety of the system upon these grounds, it may be deemed necessary to enter into an explanation of the results attributed to this sys- tem of ventilation, which is said to be requisite in order to adapt an artificial air to the circumstances of the plants growing in it, and which is supposed by some to be in exact harmony with the laws of vegetable physiology, and with all that science has unfolded to us respecting the effects of the atmosphere upon vegetable life. The direct effects of ventilation, of any description, are two- fold, mechanical and chemical. The former embraces the influ- ence which motion possesses over the growth of plants ; and this influence has never yet been accurately defined or explained whether it be injurious or beneficial, and in what particular degree it ceases to be so. The latter comprehends the effects of the various gases, and their influence upon the vital functions of vegetable beings. To illustrate the effects of the first of these agents, viz., motion, we may refer to the circumstance that is well known, that trees trained upon a wall, in ordi- nary circumstances, do not grow to such size as those standing in isolated places ; but their fibre is sooner matured, and also their fruit earlier, as well as larger and more saccharine. It has been asserted that wall trees do not arrive at so great an age as others standing in exposed situations, an assertion as founda- tionless as it is absurd ; for it is a well-known fact, that wall trees have outlived others of the same kind, planted in similar soil, MANAGEMENT OF THE ATMOSPHERE. 295 and at the same period with themselves. And yet this assertion has been made the basis of an argument in favor of free ven- tilation. [Experiments of Knight, in Philosophical Tramac- tions.] Surely a system must be in a tottering condition when such far- fetched arguments are resorted to for its support. Nor is this a solitary instance of irrelevant arguments being brought to sup- port untenable systems, when in a sinking condition. When a plant is in a healthy and vigorous state, its sap is propelled through its various tissues by its own vital principle, aided by the combined influence of light, and heat, and moisture. And while its vital principle remains unimpaired, and these essentials of its existence unexhausted, its functions will continue in a state of activity, until some cause, known or unknown, occur to destroy them. Let us rehearse an argument which has been advanced to overthrow the above theory. " When a plant is young and suc- culent, through all its parts, then all goes on very well ; but when the plant becomes more matured, and its vessels less per- vious to the flow of sap, from its increased bulk, its approach to maturity, and probably its deadened susceptibility to the action of light and heat, it is evident that to prolong the existence of such a plant, a new impulse must be communicated to its sap, by a different species of agency from that which was necessary in the case of the young plant. This impulse is imparted by motion, and that motion is created by the winds and currents of the atmosphere." Such is the sum and substance of an argument which involves the solution of a most important problem in vegetable physiol- ogy ; and, to the merely superficial reader, it has something very plausible in its appearance, but, unfortunately, it will not stand to be strictly investigated, for then the very breezes that are brought to support it, would sweep it away. This is more especially true when the illustration is applied to the atmos- phere of hot-houses, upon which point enough has been already said in this chapter, regarding the mechanical effects of currents, to render further enlargement on this subject unnecessary. SECTION V. CHEMICAL COMBINATIONS OF THE ATMOSPHERE. 1. WITH respect to the chemical effects of ventilation, upon an artificial atmosphere, there are two important things to be kept in view, in providing an artificial atmosphere for plants in a glazed structure ; namely, the nourishment they ought to receive from it, and how to maintain it in this nutrient state. It is needless, in this place, to enter upon the minute detail of the various substances which enter into the composition of plants, or of the various elements which combine to form the different bodies of which they are composed, bodies, in them- selves so different in their qualities, yet so identical in their for- mula, and consisting of the same elements, united together in the same proportions. This is one of those facts in chemical science which appear so very remarkable to those who have not directed their attention to chemistry, but are scarcely capable of being clearly comprehended and explained, even by those who have profoundly studied this branch of natural science. Starch and sugar how different their properties ! how unlike their uses ! how unequal their importance to the human race ! Yet they consist of the same weights, of the same substances differently conjoined. The skilful architect can put together the same proportions of the same stone and cement; and the painter can combine the same colors, to produce a thou- sand varied impressions on the sense of sight. But in the hand of the Deity matter is infinitely more plastic. In his hands, and at his bidding, the same particles can unite in the same quantities, so as to produce the most dissimilar impressions, and on all our senses at once. A knowledge of the above close relations, in composition among a class of substances occurring so abundantly in plants, imparts a degree of simplicity to our ideas of this otherwise so very complicated subject. It does not appear so mysterious that CHEMICAL COMBINATIONS. 297 we should have woody fibre, and starch, and gum, and sugar, occurring together in variable quantities, when we know that they all are made up of the same materials, in the same pro- portions ; or that one of these should occasionally disappear from a plant, to be replaced in whole or in part by another. A further question arises in our minds, in connection : Are these elements formed in an artificial atmosphere, such as that of a hot-house, from the same combinations of matter as in the natural atmosphere ? A reply, though probably not a satis- factory one, may be drawn from the following considerations : During the day plants assimilate carbonic acid, and evolve oxygen ; and during the night this system is reversed, although we have no accurate data from which to conclude that the rela- tive proportions of these gases are, at all times and under all circumstances, the same. From the latest experiments, we are induced to suppose that, in an artificial atmosphere, oxygen is the most important element to be attended to, in the regulation of its elements; and from the ^ fact .that its presence, to the amount of 21 per cent, in common atmospheric air, is essential to the existence of animals and plants, there can be little doubt that it is more frequently in deficiency, than in excess, in an artificial atmosphere, and that hot-house plants are more frequently injured by the want of a proper supply, than by an excess of it in the atmosphere, when we consider the quantity of this substance which nature has stored up for the use of plants and animals. Nearly one half of the solid rocks which compose the crust of our globe, of every solid substance we see around us, of the houses in which we live, and of the stones on which we tread, of the soils which we daily culti- vate, and much more than one half by weight of the bodies of all living animals and plants, consist of this elementary body, oxygen, known to us only in the state of a gas. It may appear surprising that any one elementary substance should have been formed, by the Creator, in such abundance as to constitute nearly one half by weight of the entire crust of our globe. But this is not so surprising, when we consider that it is on the presence of this element that all animal and vegetable life depends ! Nor is it less wonderful that a substance, which we know only in a 298 CHEMICAL COMBINATIONS state of thin air, should, by some extraordinary mechanism, by bound up and imprisoned, in such vast stores, in the solid moun- tains of the globe, be destined to pervade and refresh all na- ture, in the form of water, and to beautify and adorn the earth in the solid parts of animals and plants. But all nature is full of similar wonders, and every step we advance in the study of the principles of our art, we cannot fail to perceive the united skill and bounty of the same great Contriver. 2. It has been stated by some philosophers, that when the leaves of plants are in a state of rest, their respiration is reduced to its minimum point, and that it increases within certain limits, as motion is communicated to them by the action of a current of air. Now this may be perfectly correct, and very likely is so ; although, under natural conditions, the suspension of respi- ration has never been accurately ascertained. Various physiol- ogists have attempted to discover the minimum of respiratory suspension, under certain atmospheric conditions, but without any satisfactory results. But it does not require the discovery of this delicate point, to decide on the propriety or utility o atmospheric motion. That a certain motion in the atmosphere is beneficial, we know; but then, it becomes a question of degree. We know that the gentle zephyr is favorable to vegetation, and, even in a hot-house, we have some reason to suppose it is so, under certain circumstances, and to a certain extent. Now it is under the uncertain circumstances, and the uncertain extent to which this practice is carried, that we have any objections ; for such circumstances, and such indiscriminate abuse of the prac- tice, we know to exist ; and hence the chemical effects of venti- lation, in the majority of cases, instead of promoting respiration, rather tend to prevent it, by depriving the atmosphere of the principal element that nature has designed to carry on the work. The mechanical and chemical influences are intimately con- nected with each other, so that to secure the chemical ad- vantage of ventilation, I presume consists in maintaining the proper equivalents of the atmosphere, which nature has deter- mined as essential to the development of vegetation. If this view be correct, the grand and important practical question OF THE ATMOSPHERE OF HOT-HOUSES. 299 suggests itself, whether, in the atmosphere of hot-houses gener- ally, these essentials to the growth of plants be suitably provided. By chemical research, we find that nitrogen forms only a small portion of plants, but it is never entirely absent from any part of them ; even when it is not found in any particular organ,, it is found to be present in the fluids that pervade it. Many experiments have been instituted, with the view of ascertaining expressly, by what particular organs nitrogen entered into the plant, and in what form it enters. Indeed, this is a question which at present occupies much attention. It is well known that the leaves of plants absorb gaseous elements largely from the atmosphere, both free and in a combined state, and we might, therefore, expect that some of the nitrogen of the air would, by this channel, be admitted into their circulation. This view, however, is not confirmed by any of the experiments heretofore made, with the view of investigating the action and functions of the leaves. We are not at liberty to assume, therefore, that any of the nitrogen which plants contain, has in this way been derived directly from the atmosphere. It may be the case, but it is not yet proved. There is little doubt, however, that nitro- gen enters the roots of plants, in a state of solution ; but the quantity they thus absorb is uncertain ; it is supposed to be small, and must be variable. Therefore, by whatever organs it finds an entrance into plants, and in whatever quantity it may be present, the question still remains, that it is the ammonia of the atmosphere that chiefly furnishes nitrogen to plants. 3. In a former part of this treatise, while treating on the subject of heating, by means of fermenting manure, we have alluded to the extraordinary effects of ammonia upon plants. It is unnecessary, at present, to recapitulate what has already been said on that interesting point. It has, we think, been clearly established, that the difference between a hot-bed of manure, and that heated by any other means, does not lie in the quality of the heat generated ; as we know full well that a hot-bed of manure, warmed beyond a certain point, will burn the roots of plants as quickly as one heated by any other method to the same temperature ; nor does it consist in any life-giving proper- 300 CHEMICAL COMBINATIONS ties, possessed exclusively by stable manure, for we know, also, that by placing living plants in a hot-bed, newly made, even if the heat of the bed be kept from injuring the roots, they will soon cease to exist as living beings, purely from an excess of those very gases which, in proper proportions, add so much to their natural luxuriance. Plants are more sensitive, and more easily affected, with regard to life and health, than many living animals. Many persons, who have paid little attention to veg- etable physiology, may be dubious of this fact, but it is, neverthe- less, true. The atmosphere of a hot-house may be impregnated with ammoniacal and other gases, beneficial to vegetable life, without being offensive to the ordinary visitor, or even detected by him in the atmosphere of the house. Besides, it is so quick- ly absorbed by the plants, that it has to be saturated almost to excess before much smell is sensibly felt. We have carried on the practice daily, of impregnating the atmosphere of a green- house with carbonate of ammonia, by dissolving it in water and sprinkling through the house, without the ammonia being de- tected, except by the acute olfactory organs of the experienced chemist, except, perhaps, when the atmosphere was impregnated to an excess, which, by way of experiment, was sometimeF the case. 4. This subject now resolves itself into the following consid- erations : (1.) Which gases is it necessary to generate artificially, for the purpose of increasing the capacity of the atmosphere of a hot-house to sustain vegetable life in a state of vigor and health- fulness ? (2.) How are we to determine the precise proportions of each, so that we may keep as near as possible to that point of health- fulness, which lies midway between deficiency and excess ? In replying to the first question, it is not necessary to enter into an elaborate detail of the various volatile gases which arise from the combination of the prime elements of the organic world, in different proportions, and which are absorbed by plants. It may be sufficient for my present purpose, to notice that grand stimulus of vegetation already alluded to, viz., ammonia, which, OF THE ATMOSPHERE OF HOT-HOUSES. 301 as we have already seen, plays such an important part in the progress of vegetable life. This gas, though composed of hydrogen and nitrogen, is very unlike these, or, indeed, any other gases with which the chemist is yet acquainted. It is possessed of a most powerful penetrating smell, which is familiar to almost every one as hartshorn and smelling-salts. In excess, it suffocates living animals, though it requires a very considera- ble preponderance in the atmospheric volume to destroy either animal or vegetable life. Illustrations of this fact we have fre- quently observed in fumigating a pit, or house, for the destruc- tion of aphides, and other insects ; but it destroys both, much more rapidly, when evolved at a high temperature, as we fre- quently find it in hot-beds of dung, when plants have been placed in them before the gas and heat had somewhat subsided, as well as in vineries, which we have seen filled with ammoni- acal gas, when the atmosphere was near 100 degrees, when the edges of the tender leaves appeared as if they had been nipped with frost, but the insects were not entirely destroyed. In fumigating frames and pits with this and other gases, we have seen some kinds of tender-leaved plants completely destroyed, while many of the insects, tenacious of life, were uninjured, which has fully satisfied me of the truth of the statement already made, i. e., that the. generality of tender plants are more sensi- tive of noxious gases than living animals, although few may be inclined to believe it, and their disbelief is too often manifested in the treatment their plants receive. There can be little doubt that it is this gas, in a certain proportion of atmospheric air, that produces the luxuriance of plants, when combined with the mild heat of a dung-bed. Were we to ask a chemist, What are the manures which, in a fluid or gaseous state, can in these forms be presented to the atmosphere, and diffused among living plants, in a hot-house ? he would answer, " Ammonia, obtain it from whatever source you may, either in a simple or combined state ;" and as hitherto our chief supply of this substance, which we have had to deal with in the common operations of garden- ing, has been found in our hot-beds of stable manure, resulting from the decomposition of vegetable matter, principally the nitro- geneous substances contained in corn and other matter on which 302 CHEMICAL COMBINATIONS the horses have been fed, with the compounds of salts and ani- mal matter, all of which contain within themselves a tendency to rapid putrefaction, and necessarily evolve a large amount of ammonia. This is the principal source which gardeners have had to draw upon for a supply of this agent ; and, although exercising the most striking effects, it is rather remarkable that the cause of these effects should, until lately, remain a mystery to gardeners in general, and that the same elements, in a more concentrated state, should not, in other circumstances, be applied to produce the same results. The second question is, perhaps, of more difficult solution. Plants are living, organized, beings, and acted upon, atmospher- ically, chiefly by the glands that cover the surface of the leaves ; and abundant evidence exists, that they are as susceptible of either injury or benefit, through the medium of the atmosphere to which they are exposed, as animal life, and our ignorance of the effect of houses artificially heated, upon the delicate organ- ism of plants, is only accounted for from the fact, that com- paratively little attention has yet been paid to this branch of horticultural science by practical gardeners, and still less has it been applied to the culture of exotic plants. If, for instance, we take a plant from the open ground, where it is fully exposed to the pure air, plant it in a pot, and place it in a close living room, or in a hot-house, the effect will be rendered obvious by the altered appearance of the plant. Again, if we take a plant newly potted, and otherwise disturbed in the roots, and set it in an arid situation, and fully exposed to the air, the leaves will be withered and dried up in a few hours, and probably the death of the plant will be the issue. But if the plants are placed in a close, moist atmosphere, the results will be very different. Now these illustrations are common, and, in themselves, exceed- ingly simple, so much so, that we frequently observe them, and, if asked the cause, we give a kind of generalizing reply, by attributing it to the sun, or some such cause, which is well known to be the principal origin of heat, yet they serve to show how susceptible plants are of influences which, strictly speak- ing, are neither dependent upon heat nor cold, although these two latter elements are almost the only ones which we are in OF THE ATMOSPHERE OF HOT-HOUSES. 303 the habit of supplying to our plants by measure, and that, too, in the most unnatural proportions, while the ammoniacal and hygrometrical condition of the atmosphere is generally left to uncontrolled transmutations of chance. 5. It may be asked, " What guide have we to ascertain the condition of the atmospheric gases ? " In the present state of our knowledge of gaseous bodies, their presence or preponderance in the atmosphere of hot-houses must be little else than a matter of conjecture. An experienced gar- dener, on entering his hot-house in the dark, can tell pretty accurately what degree of temperature the atmosphere of the house is standing at, by the sensation produced upon his face, or by the wave of his hand in the air. Now, in regard to the excess of volatile gases floating in the atmosphere, the organs of smell are much more delicate indicators than the sense of feeling. This is more especially the case when the house is close, and the temperature pretty high; for then the ammonia, being little more than half the weight of the common atmosphere, [more nearly three fifths, its specific gravity being 0.59, that of air being 1 J hence, when liberated on the floor, or on the flue, pipes, tank, or other heating apparatus, it readily rises and mingles with the atmosphere ; and although it requires a considerable proportion of it in the atmosphere to be injurious, or even offensive to the senses, it is, nevertheless, easily detected by those acquainted with this gas, even when present in small quantities, and the experienced organs of the practical man have no difficulty in deciding whether or not it is present in excess. On entering a hot-house, when oxygen arid aqueous vapor are deficient in the atmosphere, this fact is at once detected by the oppressive burnt smell which pervades the house. Saturate the atmosphere with water, oxygen is generated, and the smell ceases. The carbonic acid, which previously existed in excess, combines with the oxy- gen, and is transformed into carbonic acid gas, in which state it is assimilated by the plants. In the state of vapor, water exer- cises a wonderful influence over the atmosphere of a hot-house, and ministers most materially to the life and growth of plants. It is in the form of water, indeed, that nature introduces the 26* 304 CHEMICAL COMBINATIONS greater portion of the oxygen which performs so important a part in the numerous and diversified changes which are contin- ually taking place in the interior of plants. Few changes are really more wonderful, in chemical physiology, than the vast variety of transmutations which are constantly going on through the agency of the elements of water. It rarely, perhaps never, happens that we find the same unhealthy and disagreeable smell in the external atmosphere, which we frequently perceive in forcing-houses after a strong fire has been kept up during the night. Sometimes this condi- tion may occur in the confined streets of closely-built cities, and in the vicinity of chemical works, where the heavier gases rise into the air in a rarefied state, and, on cooling, fall again to the surface of the earth, producing sometimes injurious conse- quences. The combustion of fuel for the production of artificial heat produces also carbonic acid gas in great abundance. And to form this gas the oxygen is drawn from the plants to form the combination ; and in this way the deficiency of oxygen, so much felt in forcing-houses, may partly be accounted for. Oxy- gen must exist ia the atmosphere to the amount of 21 per cent, of its bulk to be capable of supporting animal and vegetable life in a state of vigorous development ; and when this proportion is reduced, the plants under its influence must suffer accordingly. The most convenient method of supplying the atmosphere with oxygen is by saturation with water, which latter element con- tains a very large amount of this gas, every nine pounds of this liquid containing no less than eight pounds of oxygen. In the interior of plants, water undergoes continual decomposition and recomposition. In its fluid state it finds its way and exists in every vessel and in every tissue; and so slight, it would appear, in such situations, is the hold which its component elements have upon each other, or so strong their tendency to combine with other substances, that they are ready to separate from each other at every impulse, yielding now oxygen to one, now hydro- gen to another, as the production of the several compounds which each organ is destined to elaborate respectively demands. Yet with the same readiness do they re-attach themselves, and cling together, when new metamorphoses require it. 6. In the constitution of the natural atmosphere we are at OF THE ATMOSPHERE OF HOT-HOUSES. 305 no loss to discover its beautiful adaptation to the wants and structural development of animal and vegetable life. The excit- ing effect of pure oxygen on the animal economy is diluted by the large admixture with nitrogen; the quantity of carbonic acid present is sufficient to supply food to the plant, while it is not so great as to prove injurious to the animal; and the watery vapor suffices to maintain the flexibility of the parts of both orders of beings, without being in such a proportion as to prove hurtful to either.^ The air, thus charged with these gases, by its subtilty diffuses itself everywhere. Into every pore of the soil it make its way. When there, it yields its oxygen, or its carbonic acid, to the dead vegetable matter existing therein, or to the living roots. When the soil is heated by the sun, the gases that are impris- oned therein expand arid partially escape, and are as before replaced by other particles of air when the heat of the sun is withdrawn. By the action of these and other causes, a constant circulation is kept up, to a certain extent, between the atmosphere on the surface, which plays among the leaves and stems of plants, and the air which mingles with the soil and ministers to the roots; * The mutual influence of animal and vegetable life is well illustrated by the following experiment. Into a glass vessel, filled with water, put a sprig of a plant and a fish. Let the vessel be tightly corked, and placed in the sun. The plant, under the influence of solar light, will soon commence the process of liberating oxygen. This being absorbed by the water is respired by the fish, which, in its turn, gives out car- bonic acid to be decomposed by the plant. Remove the vessel from the sun-light ; the plant will cease to give out oxygen, and the fish will soon languish, and revive when placed in the light. The moving power of this beautiful system is the solar light. The balance is thus pre- served ; and the atmosphere, even if of limited extent, cannot be sensibly changed through all time. It is not intended to intimate that it is in the removal of carbonic acid from the atmosphere that plants are most essential to animals, the supply of organic matter ready for assimilation is of more immediate importance than this, but to show that their influence is mutually conservative, preventing that change in the constituents of the atmos- phere which would eventually be fatal to organic life. [Wyman on Ventilation.} 306 CHEMICAL COMBINATIONS and will also suffice to show the absolute necessity of maintain- ing an adequate supply of aqueous vapor in the atmosphere of our hot-houses, as well as the imperative necessity of studying and making ourselves acquainted with the nature and qualities of the atmospheric elements. Science has already done much, and is still doing more, for the art of horticulture. We have the thermometer, by which we can deal out heat and cold by the measure. We have the barometer, by which we can ascer- tain to a decimal the weight or density of the air. We have, also, the hygrometer, by which we can tell the precise amount of its contained moisture, although this latter instrument is but little used in practical horticulture, and we hope the time is not distant when it will find a place side by side with the thermometer in our hot-houses, to which it does not yield one iota of importance, of interest, or of utility. When shall we have an instrument, equally simple and efficient as these, with which we may ascertain the proportions of its gaseous elements, so that we can regulate the constituents of an atmospheric volume as easily as we can do its heat and moisture ? Such an instru- ment is much wanted by exotic horticulturists, and we trust something of the kind will be yet brought into use. Such an instrument could be applied to excellent purpose, and would be an incalculable boon conferred on gardening, one almost un- equalled in importance at the present day, and would be of immense utility in all the higher and more difficult branches of exotic horticulture. 7. There is, probably, no individual branch of natural science so useful in itself to the practical gardener as a knowledge of the various atmospheric phenomena which occur in hot-houses, as well as out of doors ; and without we study the one, we can have but little knowledge of the agencies which regulate the other. That a practical foreknowledge or intuitive perception of the ordinary changes of the atmosphere is an acquirement which may certainly be obtained, to a very considerable extent, without the aid of science, is beyond a doubt. We find that the untutored savage, taught only by his own observation, or instinct- ively, is regulated in his movements by an unerring perception OF THE ATMOSPHERE OF HOT-HOUSES. 307 of the coming changes of his own peculiar climate, and many of the lower animals are also highly sensitive of changes ap- proaching, especially the feathered tribes. Every person is more or less familiar with these facts. We reason, therefore, from the lesser to the greater; and if, in the absence of compara- tive calculation, or the comparison of the results of one season with another, if, in fact, we consider what are the attainments of instinctive knowledge alone, we are justified in believing that, from established principles, the result of learned inquiry and deep investigation, and the application of science extending over many successive years, many useful facts are already known and clearly explained for our practical guidance. Aided by these researches, man's ingenuity has already turned these elements to a useful account, and made them subserve his pur- pose, powerful though they be. But, in rendering these pow- erful and all-pervading elements subservient to our will, the object of that will must be undeviatingly directed to the imita- tion of nature. To exceed, or even reach, in every case, the perfection of the pattern, is impossible ; but the more closely it is kept in view, and the more nearly it is attained in our artifi- cial performances, the more perfect will that performance be, and the more exactly will our own ends be answered. Any departures from the principles suggested by the examples set before us in nature, through an over-hasty desire to arrive at the object by a nearer road, not only defeats the intended pur- pose, but also makes the ultimate attainment of that object much more troublesome and expensive. The subject of this treatise affords too many examples of this fact; and, though these examples may remain unnoticed by some, and uncared for by others, their baneful influence on the progressing art of horticulture is neither distant nor obscure. The various structures for cultivation are, indeed, much improved of late years ; so, also, are the methods of applying heat, air, vapor, and water. All are so easy, and so much improved, that we sometimes hear practical men observe, that this or that principle or system cannot be beaten or improved ; yet the very best con- structed apparatus, and the most perfect methods of applying heat, vapor, air, and light, are capable of astonishing improve- 308 COMBINATIONS OF THE ATMOSPHERE. ments. And no doubt the next twenty years will bring many a hidden treasure to light, and, in that time, even our most approved systems of applying heat, etc., will be altogether economized and reformed. SECTION VI. PROTECTION OF PLANT-HOUSES DURING COLD NIGHTS. 1. BEFORE concluding this brief treatise on horticultural buildings, we will just cursorily advert to one more topic con- nected therewith, which we are inclined to think is of far more importance than is generally credited, at least, it certainly is so, if we are to judge from the degree of its practical application, viz., the protection of plant-houses, and, more especially, forcing- houses, during cold nights, both with a view to the economizing of fuel, and the equalization of heat. If duly considered, the advantages of such covering are obvious. The low degree of night temperature, which the best cultivators of the present day agree in regarding as being most favorable to the healthfulness and general welfare of their plants, would depend upon the com- bustion of fuel, so much less, in proportion, as the escape of the internal heat, by radiation and otherwise, was prevented by means of a covering exterior to the conducting surface of the glass. The manifest advantage of such a protecting body does not wholly consist, in the economizing of fuel. In such a variable climate as we have in the New England States, with the exter- nal atmosphere acting on the glass at a temperature of 25 or 30 degrees below the freezing point, it is, then, almost under any system of heating, unavoidably necessary to apply an excess of artificial heat, to ensure the safety of the plants against injuri- ous depressions of temperature. Now, if a covering of non- conducting materials be employed to intercept the action of the changing atmosphere upon the surface of the glass, the plants will be as safe at a much lower internal temperature, as if no such protection were afforded them, with a high temperature. The plants, therefore, will, under these circumstances, be in a 310 PROTECTION OF PLANT-HOUSES DURING COLD NIGHTS. condition more conducive to their health, than if their safety from excess of cold had involved their submission to a higher degree of artificial heat during the night. Night coverings, moreover, seem to afford facilities for night ventilation, a time when ventilation, of all others, appears to be most necessary; for then, deleterious gases are generated in the greatest abundance, and the agitation and circulation of the atmosphere is most required. We have seen that motion and interchange of atmospheric particles are, to a certain extent, beneficial to the health of plants ; and as their functions are in a state of activity during the night, motion and circulation are as necessary during that time as at midday. If a close confined atmosphere be injurious to plants in the daytime, it must be more so during the night, especially when artificial heat is in process of generation. This fact is now beginning to be recog- nized by the sounds, which are echoing in our ears though as yet but faintly, the injunction, to keep a little air on all night; and which is responded to by the practice of the best cultivators of the present day. Under ordinary circumstances, where artificial heat is neces- sary, there is some risk in following these recommendations. A chilly blast, which cannot be refused admission when the bar- rier to ingress is removed, would deal death and desolation around ; and if this would be liable to happen in the daytime, when attendants are at hand, the risk would be still greater at night, when none were present to guard*igainst it ; and, under the most favorable circumstances, night ventilation, if carried to any extent, would involve a great loss of heat. It becomes, therefore, a question, if the motion and circulation of the inter- nal atmosphere during the night could not be so far facilitated by other means, as to secure the chief advantage of an actual interchange of air, without the internal heat being carried off by the cause that produced it ? Whatever prevents the radiation of heat from the interior to the exterior atmosphere through the conducting agency of glass, decreases in the same ratio the amount of required heat, and hence, saves the plants from being subjected to unnecessary excitement. The principle upon which a covering acts effi- PROTECTION OF PLANT-HOUSES DURING COLD NIGHTS. 311 ciently, is that of enclosing a complete stratum of air between it arid the glass, this body of air being entirely shut off from the surrounding outer atmosphere, as far as may be practicable to do so ; and as air is a bad conductor of heat, the warmth of the interior is prevented from passing to the exterior atmosphere, by means of direct radiation from the glass ; or, in other words, the exterior atmosphere, being prevented from coming in contact with the glass, cannot absorb from the interior any sensible por- tion of its heat. To secure this advantage, it will be evident that the covering must be kept some distance from the glass, and should be on every side where the structure is formed of glass ; the coverings, in fact, should form a complete case to all the glazed portion of the structure.* So far, so good. As a matter of protection, and nothing else, this is all very well. The advantages of such a covering will be obvious to every one ; and, as a matter of protection alone, it deserves every word that can be said in its favor. Whether it * In the different experiments, it appears that the cooling effect of wind at different velocities on a thin glass surface, is very nearly as the square root of the velocity. In these experiments, the velocity of the air was measured by the revolutions of the vanes of a fan. The tem- perature of the air was 68 3 , the time required to cool the thermometer 20 was noted for every different velocity, and the maximum tempera- ture of the thermometer in each experiment was 120. In still air, it required 5' 4.5" to cool the thermometer this extent, and Table VIII. in the Appendix shows the time of cooling by air in motion. In consequence of the large quantity of glass used in the construction of horticultural buildings, the cooling effect of wind is of considerable importance. We find, however, that, with an increased velocity, the cooling effect is considerably less in proportion, on glass, than on metal, and it will be very much less on window-glass than even what is stated in the table. As glass is an extremely bad conductor of heat, the increased thickness which window-glass possesses over that which forms the bulb of a thermometer, will make a material difference in the quantity of heat lost by the abduction of the air, there will be, as in this case, a greater difference between the temperature of the external and the internal surface. The cooling effect of wind, therefore, is not near so considerable as is generally supposed ; and the effect of wind in hot-houses is very much increased by open laps and accidental fissures in the glazing of the sashes. 27 312 PROTECTION OF PLANT-HOUSES DURING COLD 'NIGHTS. can practicably be made the means of admitting the external air into the house at an increased temperature, and thereby creat- ing a motion in the internal atmosphere, is a question which, as yet, we are unable to prove from experience, although we mean to take an early opportunity of testing the plan which we are about to describe. 2. The best material which we have seen used for this pur- pose is canvas, or any other kind of strong coarse cloth, painted with two or three coats of pitch, wax, and oil boiled together, and applied in a warm state to the cloth ; this makes an efficient and durable covering. Asphalte felt is also used extensively in England and Germany for this purpose. This latter mate- rial is fixed on light wooden frames, about the size of a sash, or larger, as may be found convenient ; and for covering frames and pits it answers admirably, as it is quite impervious to wet, and if taken care of, will last for some years. But for covering the roofs of large houses, we would decidedly prefer the cloth, which can be more easily drawn off and put on, and, if well painted, will be as impervious to air and wet, as wooden shut- ters, or asphalte frames, and will be cheaper than either. Suppose, then, that a glazed cloth, of the requisite dimensions, is prepared. We would provide means for securing it against wind, by loops, etc., and fix on parallel strips of wood over each rafter, about nine inches from the glass. The cloth should be made to fit quite close at the top, and to reach the ground on all sides of the house, which, formed of conducting materials, or side-pieces, must be made to fit closely over the over-lapping edge of the upper one, and the lower edge secured against the admission of air. The house is now in a case, impervious both to air and water, and enclosing a stratum of air, which gradu- ally becomes warmer than the external atmosphere, and effectu- ally prevents the latter from abstracting the heat from the inte- rior of the house. Then let there be square holes made along the cloth, near the bottom, say one for each alternate light, each aperture made about ten inches square, and provided with a shutter of the same material, to close it when necessary. All these apertures, or any number of them, may be opened, accord- PROTECTION OF PLANT-HOUSES DURING COLD NIGHTS. 313 ing to the wind, or other circumstances likely to affect the inter- nal atmosphere. Then small apertures may be left open in different parts of the house, during the night, whereby an inter- change of the atmospheric volume would take place, without exposing the plants to immediate contact with the cold air. By this plan, we conceive that direct benefit would accrue to the plants, because the air between the covering and the glass, although not cold, would nevertheless be of greater density than that of the house, and would consequently find its way into the interior, by the ventilators left open for that purpose. This would also enable us to maintain a much lower night tempera- ture than could possibly be otherwise done, with regard to the safety of the plants, which the fear of sudden changes during the night, and consequent injury from frost, prevent from being realized in this changeable climate. It is truly remarkable how very slight a covering is required to exclude a pretty severe frost. " I have often," observes Dr. Wells, " in the pride of half-knowledge, smiled at the means fre- quently employed by gardeners to protect tender plants from cold, as it appeared to me impossible that a thin mat, or any such thin substance, could prevent them from attaining the tem- perature of the surrounding atmosphere, by which alone, I thought them liable to be injured. But when I had learned that bodies on the surface of the earth, become, during a still and serene night, colder than the atmosphere, by radiating their heat to the heavens, I perceived immediately a just reason for the practice which I had before deemed useless. Being desirous, however, of acquiring some precise information on this subject, I fixed perpendicularly in the earth of a grass plot four small sticks, and over their upper extremities, which were six inches above the grass, and formed the corners of a square, the sides of which were two feet long, fixed a thin cambric handkerchief, so as to cover the included space. In this disposition of things, there- fore, nothing existed to prevent the free passage of air from the surrounding grass to that which was sheltered under the hand- kerchief, except the four small upright sticks supporting it, and there was no substance to radiate heat downwards to the grass beneath but the cambric handkerchief. The temperature of the 314 PROTECTION OF PLANT-HOUSES DURING COLD NIGHTS. grass, which was thus shielded from the sky, was, upon many nights afterwards, examined by me, and was always found higher than the neighboring grass which was uncovered, if this was colder than the air. When the difference in temperature between the air several feet above the ground and the unshel- tered grass did not exceed 5, the sheltered grass was about as warm as the air. If that difference, however, exceeded 5, the air was found to be somewhat warmer than the sheltered grass. Thus, upon one night, when fully exposed grass was 11 colder than the air, the latter was 3 warmer than the sheltered grass, And the same difference existed on another night, when the air was 14 warmer than the exposed grass. One reason for this difference, no doubt, was, that the air which passed from the exposed grass, by which it had been very much cooled, had passed through that under the handkerchief, and deprived the latter of part of its heat. Another reason might be given, that the handkerchief, from being made colder than the atmos- phere, by the radiation of its upper surface to the heavens, would remit somewhat less to the grass beneath, than what it received from that substance. But still, as the sheltered grass, notwith- standing these drawbacks, was, upon one night, as may be seen from the preceding account, 8, and upon another, 11, warmer than grass freely exposed to the sky, a sufficient reason was now obtained for the utility of a very slight covering, to protect plants from the influence of frost or external cold." # * As the elevation of temperature, induced by the heat of summer, is essential to the full exertion of the energies of the vital principle, so the depression of temperature, consequent upon intense cold nights, has been thought to suspend the exertion of the vital energies altogether. But this opinion is evidently founded on a mistake, as is proved by the example of such plants as protrude their leaves and flowers in the winter season only, as well as by the dissection of the yet unfolded bud, at different periods of the winter, which proves regular and progressive develop- ment ; even in the case of such plants as protrude their leaves and flowers in the spring and summer, and in which, as we have said, there is a gradual, regular, and incipient development of parts, from the time of the bud's first appearance, till its ultimate opening in the spring. The sap, it is true, flows much less freely, but it is not wholly stopped. Du Hamel planted some young trees in the autumn, cutting off all the PROTECTION OF PLANT-HOUSES DURING COLD NIGHTS. 315 We have instituted numerous experiments with the view of ascertaining the capacity of various substances for the protection of plants and horticultural structures, by which we find that bodies of soft and open texture, as woollen netting, thin cloth, &c., will, on dry, clear nights, afford an amount of protection equal to 7 of frost. But if the covering should become wet before the frost sets in, it will afford very little protection to the plants beneath it. Coarse cloth, which had been coated with paint, kept out 10 of frost, and several kinds of plants, which, at the freezing point, would suffer injury, were kept alive during the whole winter, with the thermometer occasionally indicating 22 of frost. These plants were frequently frozen, but the covering was never removed during several months, although the air circulated freely under- neath the glass. In protecting plants, or glazed structures of any description, it is essential to observe that the covering should always be placed so that a stratum of air may always be confined between the covering and the objects to be protected ; this is an important part of he matter, as, if the covering be laid immediately on the glass of a frame, or green-house, which it is wished to protect, the cold will be conducted by the covering to the glass, which in turn will cool the air beneath it. The covering should never touch the object to be sheltered, though, from what we see around us, this point appears to be very little attended to. A covering of thin cloth, or woollen netting, when suspended in a vertical position over trees, &c., will afford better protection than the same substance laid horizontally over the surface. In this manner, wall trees are protected in the British Isles from spring frosts, and we have frequently seen the blossoms of peach, apricot, and pear trees completely uninjured under woollen or hair netting, when the hardiest trees of the woods were nipt with frost, and the tender vegetables of the garden were entirely smaller fibers of the roots, with a view to watch the progress of the for- mation of new ones. At the end of a fortnight he had the plants all taken up and examined, with all possible care, to prevent injuring them, and found that, when they did not actually freeze, new roots were always uniformly developed. 316 PROTECTION OF PLANT-HOUSES DURING COLD NIGHTS. destroyed. Peaches and other fruit trees might frequently be protected in this way, and the crop, at least, partly saved, instead of being, in one single night, blasted for the season. Common bass mats afford the best and cheapest protection for frames and small pits; but they have the fault of absorbing moisture very readily in wet weather, and then become very bad protection. They should never be laid on the glass in a wet state, as they are sure to do more injury than good. We have found it an excellent method, in covering frames and small houses with mats, to have a thin water-proof covering to lay over the mats, which not only prevents the escape of the con- fined air, but also keeps the mats always dry, and thus, one of the very best protectors is obtained. Large structures are more difficult to cover than pits, and the difficulty which thus presents itself has, in general, prevented every attempt to overcome it. We have seen various plans put in operation, besides that which we have already described ; all more or less effectual. The difficulty of getting common rollers to work in frosty weather has made them all but useless, in the protection of hot-houses by rolling blinds, or screens of oil-cloth. Nevertheless, this plan is not only an effectual one, but one which is cheap and easily adopted. And the cloth can be drawn off, in the mornings, and spread out to dry on the snow, or hung on a fence, during the day. When the time comes for covering at night, it might be so arranged as to be drawn up by cords passing through a pulley at each end of the house. We have succeeded in arrangements of this kind ; and the saving of fuel in a severe winter, with the certainty of the plants, being safe from injury, either from frost or from fire, is ample compensation for the trouble which it costs. Whatever kind of object it is wished to protect, whether a house or a plant, the protector should always be at least one foot from it. A considerable difference of temperature is always observed, on still and serene nights, between bodies sheltered from the sky by substances touching them, and similar bodies which were sheltered by a substance a little above them. "I found, for example," says Dr. Wells, " upon one night, that the warmth of grass sheltered by a cambric handkerchief, raised a PROTECTION OF PLANT-HOUSES DURING COLD NIGHTS. 317 few inches in the air, was 3 greater than a neighboring piece of grass, which was sheltered by a similar handkerchief, which was actually in contact with it. On another night the difference between the temperatures of the two portions of grass, sheltered in the same manner as the two above mentioned, from the influ- ence of the sky, was 4. Possibly," says he, " experience has long ago taught gardeners the superior advantages of defending tender plants from the cold of clear and calm nights, by means of substances not directly touching them, though I do not recol- lect ever having seen any contrivance for keeping mats, and such like bodies, at a distance from the plants which they were meant to protect." We know this to be a fact ; for gardeners seldom take any thought whether the plant is protected or not, provid- ing it be covered, with mats or something else, from the external atmosphere. Straw, and corn stalks, afford good protection to trees and half hardy shrubs, when properly arranged, so that the covering may be water-tight. The air that lodges among the straw, and in the interstices of the stalks, keeps the plant within, always at a regular temperature, and prevents sudden freezing and thawing, which prove the destruction of tender plants. Bodies, however, capable of absorbing heat during the day, and parting with it at night, when the temperature of the atmos- phere falls, are also useful as a means of protecting plants, &c. Among such bodies may be classed the walls of houses, which may be regarded useful in two ways ; namely, by the mechani- cal shelter they afford against cold winds, and by giving out the warmth, during the night, which they had absorbed during the day. It appears, however, that on clear and calm nights, those, on which plants frequently receive much injury from cold, walls must be beneficial in another way; namely, by preventing, in part, the loss of heat, which the plants would sustain from radiation, if they were fully exposed to the sky. The following experiment was made by Dr. Wells, for the purpose of deter- mining the justness of this opinion. A cambric handkerchief having been placed, by means of two upright sticks, perpendicu- larly to a grass plot, and at right angles to the course of the air, a thermometer was laid upon the grass close to the lower edge 318 PROTECTION OF PLANT-HOUSES DURING COLD NIGHTS. of the handkerchief on its windward side. The thermometer thus situated was, for several nights, compared with another lying on the same grass plat, but on a part of it fully exposed to the sky. On two of these nights, the air being clear and calm, the grass close to the handkerchief was found to be four degrees warmer than the fully exposed grass ; on a third night the difference was six degrees. An analogous fact is men- tioned by Gersten, who says that a horizontal surface is more abundantly dewed than one which is perpendicular to the ground. Snow forms an excellent covering, and seems to be a provis- ion of nature for the protection of many tender roots and plants which would otherwise perish. Its usefulness as a plant-pro- tector may be disputed, from the fact of their tops being exposed to the influence of the atmosphere, while their roots and lower parts only are protected. In reply to this, however, we may observe, that it prevents the occurrence of the cold, which bodies on the earth acquire in addition to that of the atmosphere, by the radiation of their heat to the heavens, in still and clear nights. The cause, indeed, of this additional cold, does not constantly operate, but its presence during only a few hours, might effectually destroy plants which now pass unhurt through the winter. Again, as things are, while low-growing vegetable productions are prevented, by the covering'of snow, from becom- ing colder than the atmosphere, in consequence of their own radiation, the parts of trees and tall shrubs which rise above the snow are little affected by cold from this cause ; for their outer- most twigs, now that they are destitute of leaves, are much smaller than the thermometer suspended by us in the air, which, in this situation, seldom became more than two degrees colder than the atmosphere. The large branches, too, which, if fully exposed to the sky, would become colder than the extreme parts, are in a great degree sheltered by them, and, in the last place, the trunks are sheltered both by the larger and smaller parts, not to speak of the heat they derive by conduction through the roots, from the earth kept warm by the snow. In a similar man- ner is partly to be explained the way in which a layer of straw PROTECTION OF PLANT-HOUSES DURING COLD NIGHTS. 319 or earth preserves vegetable matters in the fields from the inju- rious influence of cold during severe winters. * When frames and such places are covered with snow, it should be allowed to remain on till it melts away by the influ- ence of the atmosphere. In like manner, trees and shrubs should never have the snow drawn from their branches, during snow storms, except where the branches are likely to be broken down by the weight of snow lying upon them. Snow is not only the best, but also the most natural, covering during the winter months. * That the warmth of the soil acts as a protection to plants may be easily understood. A plant is penetrated in all directions by innumera- ble microscopic air-passages and chambers, so that there is a free com- munication between its extremities. It may, therefore, be conceived that, if, as necessarily happens, the air inside the plant is in motion, the effect of warming the air in the roots will be to raise the temperature of the whole individual, and the same is true of its fluids. Now, when the temperature of the soil is raised to 50 at noonday, by the force of the solar rays, it will retain a considerable part of that warmth during the night ; but the temperature of the air may fall to such a degree, that the excitability of a plant would be too much and too suddenly impaired, if it acquired the coldness of the medium surrounding it. This is pre- vented by the warmth communicated to the general system, from the soil through the roots, so that the lowering of the temperature of the air by radiation during the night, is unable to affect plants injuriously in consequence of the antagonist force exercised by the heated soil. SECTION VII. GENERAL REMARKS ON THE MANAGEMENT OF THE ATMOSPHERE OF HOT-HOUSES. 1. ONE of the most prevalent errors, and one of very consid- erable importance, consists in reversing the natural condition of the atmosphere in regard to the artificial regulation of the temperature during the night. The artificial climate is not rendered natural by adjusting it to the heat and light of the sun. In cloudy weather, and during night, the artificial atmosphere is kept hot by fires, and by excluding the external air; while, in clear days and during sunshine, fires are left off, or allowed to decline, the external atmosphere is admitted, and the internal atmosphere is reduced to the temperature of the air without. As heat in nature is the result of the shining of the sun, it fol- lows that when there is most light there is most heat ; but the practice in managing hot-houses is generally the reverse. "A gardener," observes Knight, "generally treats his plants as he would wish to be treated himself, and consequently, though the aggregate temperature of his house be nearly what it ought to be, its temperature during the night, relatively to that of the day, is almost always too high. " It is very doubtful if any point in exotic horticulture is less attended to than that which is involved in this question. We are too apt to forget that plants not only have their periodical rest of winter and summer, but they have also their diurnal periods of repose. Night and its accompanying refreshments are just as necessary to them as to animals. In all nature, the temperature of night falls below that of day, and thus, the great cause of vital excitement is diminished, perspiration is stopped, and the plant parts with none of its aqueous particles, although it continues to imbibe by all its green surface as well as by its roots. The processes of assimilation are suspended. No diges- THE ATMOSPHERE OF HOT-HOUSES. 321 tion of food and conversion of it into organized matter takes place, and instead of decomposing carbonic acid by the extrica- tion of oxygen, they part with carbonic acid, and rob the atmos- phere of its oxygen, thus deteriorating the air at night. It is, therefore, most important that the temperature of glass-houses of every kind should, under all circumstances whatever, be lower during the night than the minimum temperature of the day ; and this ought to take place to a greater extent than is generally imagined among practical gardeners. " Plants, it is true, thrive well, and many species of fruit attain their greatest state of perfection in some situations within the tropics, where the temperature in the shade does not vary in the day and night more than seven or eight degrees; but in these climates the plant is exposed during the day to the full blaze of the tropical sun, and early in the night it is regularly drenched with heavy wetting dews, and, consequently, it is very differently circumstanced in the day and night, though the tem- perature of the air in the shade, at both periods, be very nearly the same. I suspect," continues Knight, " that a large por- tion of the blossoms of the cherry and other fruit trees in the forcing-house often prove abortive, because they grow in too high and too uniform a temperature. I have been led," he says, " during the last three years, to try the effects of keeping up a much higher temperature during the day than during the night. As early in the spring as I wished the blossoms of my peach trees to unfold, my house was made warm during the middle of the day, but, towards night, it was suffered to cool, and the trees were then sprinkled, by means of a large syringe, with clear water, as nearly at the temperature as that which rises from the ground as I could obtain it, and no artificial heat was given during the night, unless there appeared a prospect of frost. Under this mode of treatment, the blossoms advanced with very great vigor, and, when expanded, were of a larger size than I had ever before seen on the same varieties. " Another ill effect of high night temperature is, that it exhausts the excitability of the tree much more rapidly than it promotes the growth, or accelerates the maturation, of the fruit, which is, in consequence, ill supplied with nutriment at the period of 322 GENERAL REMARKS ON THE MANAGEMENT its ripening 1 , when most nutriment is probably wanted. The Muscat of Alexandria grapes, and some other late grapes, are often seen to wither upon the branch in a very imperfect state of maturity, and the want of richness and flavor in other forced fruit is, we are very confident, often attributable to the same cause. There are few peach houses or graperies in this coun- try in which the night temperature does not exceed, during the months of April and May, that of the warmest valleys of Jamaica, in the hottest period of the year. And there are prob- ably as few hot-houses in which the trees are not more strongly stimulated by the close and damp air of the night, than by the temperature of the dry air of the noon of the following day. The practice which occasions this cannot be right; it is in direct opposition to nature."^ We have fully satisfied ourselves that a high night temperature is injurious to plants of any description, kept under glass, and that green-house plants not only expand their flowers more per- fectly, but continue much longer in bloom, when the temperature of the house is reduced at night by the admission of air or other- wise. In like manner, fruits are not only better flavored, a fact generally admitted, but also better colored, and more per- fect in form, by a low temperature at night. On the other * hand, too much air is generally admitted during the day. There is no doubt that gardeners frequently err in admitting the external air into their hot-houses, etc., during the day, par- ticularly in bright weather; and this error is so common as to form a portion of regular practice. We have seen graperies and green-houses fully exposed to the parching winds of a sum- mer day, without screen or shelter; while the plants subjected to this treatment plainly indicated, by their appearance, its inju- rious effects. The climate of this country is so different in respect to its atmosphere during the day, from that of Britain, we are too apt to follow the practice of that country, where this practice is also carried to too great extent.! * Loudon's Encyclopedia of Gardening. f The climate of the British Isles, relatively to others in the same lati- tude, is temperate, humid, and variable. The moderation of its temper- ature and its humidity are owing to its being surrounded by water, OF THE ATMOSPHERE OF HOT-HOUSES. 323 The striking difference which is exhibited between our con- servatories and green-houses in this country, and those of Eng- land, is not so much owing to the existing peculiarities of cli- mate, as to the methods of practice adopted by the gardeners themselves in the management of the atmosphere of their houses. However costly and faultlessly a conservatory, a hot- house, or a grapery, may be constructed, the whole success of the structure depends upon the subsequent management of its atmosphere. The imitation of warm climates in winter, for the purpose of preserving tender plants, must not be confounded with the arti- ficial climate created in a hot-house for the purpose of forcing or accelerating foreign or native productions. As two different objects are sought for, different courses of procedure must be adopted. All that is necessary for the preservation of green- house plants, is to keep the atmosphere at night a few degrees above the freezing point ; and, indeed, if a proper attention be paid to the plants, so as to avoid an excess of moisture, there is scarcely any kind of what are generally termed hot-house plants, that will not thrive well enough under similar treatment. We have often allowed our plant-houses to fall below the freezing point in very severe nights; and when long and continued frosts set in, the plant-houses should be gradually inured to bear even a few degrees of frost below 32 ; and this the plants will do without injury, if they be kept in a proper condition. When the external atmosphere is dry and mild, air should be admitted freely to the green-house during winter, but closed early in the which, being less affected by the sun than the earth, imbibes less heat in summer, and, from its fluidity, is less early cooled in winter. As the sea on the coasts of Britain never freezes, its temperature must always be above 33 or 34 ; and hence, when air from the polar regions, at a much lower temperature, passes over it, that air must be in some degree heated by the radiation of the water. On the other hand, in summer, the warm currents of air from the south necessarily give out part of their heat in passing over a surface so much lower in temperature. The variable nature of its climate is chiefly owing to the unequal breadth of watery surface which surrounds it, on one side a channel of a few leagues in breadth, on the other, the broad Atlantic Ocean. [London's Ency. of Gard.] 324 GENERAL REMARKS ON THE MANAGEMENT afternoon, so as to preserve a portion of the warmth generated by the sun's rays within the house, to maintain a slight degree of heat in the house before the heating apparatus is set to work. The accelerating, or forcing, of the vegetables and fruits of temperate climates into a state of premature production is some- what different, and more difficult, than the preservation of plants during winter. The constitutions of the various fruit-bearing plants, as vines, &c., require atmospheres of different tempera- ture and moisture, and their success is dependent upon many contingent circumstances, which never occur in the mere preser- vation of green-house plants. The two principal methods of accelerating fruits in hot- houses are, by planting them permanently in borders prepared for them, and by planting in tubs and large pots ; and keeping a succession of plants thus prepared, every year, to supply the places of those which had become unfruitful by the effects of forcing and producing a heavy crop of fruit. The first of these methods has long been practised, and is, undoubtedly, the best for permanent crops, as more fruit can be produced in a house by this method than by the potting system. When once planted out, however, and growing under the glass, they cannot be removed from the house, and, consequently, are dependent upon the cultivator for the elements of consumption, air and water. The grand effect is produced by heat, and the great aim is to supply just as much as will harmonize with the light afforded by the sun, and the peculiar condition under which the plants exist. All the operations must be natural and grad- ual, and a good cultivator will always follow the dictates and example of the natural world. He will never be anxious to force things on too rapidly, a very common error, and a frequent cause of failure ; he will likewise be careful to guard against sudden checks, either by a sudden decrease of temperature, or the reverse ; but he will endeavor to continue the natural course of vegetation uninterruptedly through foliation, inflorescence, and fructification. The skilful balancing of the temperature and moisture of the air, in cultivating the different kinds of fruits in forcing-houses, and the just adaptation of the various seasons of growth and OF THE ATMOSPHERE OF HOT-HOUSES. 325 maturity, constitute the most complicated and difficult part of the gardener's art. There is some danger in laying down any general rules on this subject, so much depends upon the pecu- liarities of the kind under cultivation, and the endless train of considerations connected with the process of forcing. The following rules, however, may be safely stated, as deserv- ing especial attention from the gardener in charge of hot-houses : 1. Moisture is most required in the atmosphere by plants when they first begin to grow, and least when their periodical growth is completed. 2. The quantity of atmospheric moisture required by plants is, cceteris paribus, in inverse proportion to the distance from the equator of the countries which they naturally inhabit. 3. Plants with annual stems require more than those with ligneous stems. 4. The amount of moisture in the air most suitable to plants at rest, is in inverse proportion to the quantity of aqueous matter they, at that time, contain. Hence the dryness required in the atmosphere, by succulent plants, when at rest. Moisture in the atmosphere, then, is absolutely necessary to all plants, when they are in a state of rapid growth, partly be- cause it prevents the action of perspiration becoming too violent, as it always does in a high and dry atmosphere, and partly because, under such circumstances, a considerable quantity of aqueous food is absorbed from the atmosphere, in addition to that drawn from the soil by the roots. Excessive moisture is injurious to vegetables in winter, when their digestive and decomposing powers are feeble, and evapora- tion from the soil should rather be intercepted than otherwise, except when the atmosphere is dried to an unhealthy degree, by the use of fire heat. One of the causes of the Dutch method of winter-forcing is, undoubtedly, their avoiding the necessity of winter ventilation, by intercepting the excessive vapor that rises from the soil, and would otherwise mix with the air. For this purpose they inter- pose screens of oiled paper between the earth and the air of their houses ; and in their pits for vegetables, they cover the surface of the ground with the same oiled paper, by which means vapor 326 GENERAL REMARKS ON THE MANAGEMENT is effectually intercepted, and the atmosphere preserved from excessive moisture. The difficulty of keeping succulent plants in damp cellars, during winter, is also owing to the same cause. Moisture, without a sufficiency of light to enable plants to decompose it, quickly destroys them. On the other hand, the difficulty of keeping up that necessary degree of humidity in the atmosphere of a dwelling room, dur- ing the summer months, is the cause of the unhealthiness of plants kept in them ; and the fact of their position being gener- ally in the window, where there is always a current of air from without, during the day, contributes, in a great measure, to exhaust the plants of their contained moisture, and then they gradually decline. Could the atmosphere around them be kept sufficiently moist, with plenty of light, there is no reason why they should not thrive as well as in the green-house. We have already alluded to the injurious effects of maintain- ing a high temperature in green-houses and conservatories dur- ing winter. If we look over the different climates of the world, we shall find, that in each there is a season of growth, and a season in which vegetation is more or less suspended, and that these periodically alternate with the same regularity as our summer and winter. I do not know that in nature there is any exception to this rule ; for even in the Tierra Templada of Mex- ico, where, it is said, that, at the height of 4000 to 5000 feet, there constantly reigns the genial climate of spring, which does not vary more than 8 or 9 of temperature, intense heat and excessive cold being alike unknown, the mean temperature varying from 68 to 70; we cannot suppose that, even in that favored region, a season of repose is wanting; for it is difficult to conceive how plants can exist, any more than animals, in a season of incessant excitement. Indeed, it is pretty evident that these countries have periods when vegetation ceases, for Xalapa belongs to the Tierra Templada, and we know that the Ipomea purga, an inhabitant of its woods, dies down annually, like our native Convolvuli. From what has already been said on this subject, it is evident that the natural resting of plants from growth is a most impor- OF THE ATMOSPHERE OF HOT-HOUSES. 327 tant phenomenon, of universal occurrence, and that it takes place equally in the hottest and in the coldest regions. It is, there- fore, a condition necessary to the well-being of a plant, not to be overworked, under any circumstances whatever ; and there cannot be any good gardening where this is not attended to, in the management of plants under glass. Rest is effected in two ways ; either by a very considerable lowering of temperature, or by a degree of dryness under which vegetation cannot be sustained. In treating on the various conditions of the atmosphere, and its effects on vegetation, we have already sufficiently explained these influences ; which renders it unnecessary to recapitulate them in this place. In practice we find that the effects of a very dry atmosphere are, necessarily, an inspissated state of the sap of the plant, and this, in all cases, if not carried to an injurious extent, leads to the formation of blossom-buds, and of fruit. This influence, however, must be controlled by the cultivator, otherwise it will lead to inevitable failure, as the sap of the plant may be so much dried up as to prevent its accumu- lation in sufficient quantity, in the smaller branches, to form fruit buds. It is, nevertheless, true, that a low temperature, under the influence of much light, by retarding and diminishing the expenditure of the sap in growing plants, produces nearly similar effects, and causes an early appearance of fruit. All the operations may be very essentially influenced by these facts, when they are fully understood to the cultivator, and, by a skilful alteration of the periods of rest, we are enabled to break in upon the natural habits of plants, and to invert them so completely, that the flowers and fruits of summer may be brought to perfection at the opposite season of the year. By carrying out these principles, we have, for several years, succeeded in fruiting grape-vines in the months of March and April, without any extraordinary degree of excitability being exercised at any period of their growth. The whole secret of success consists in preparing the plants the preceding season, by ripening their wood at an early period of the season, and ex- posing them to such an amount of heat and dryness as can be ob- tained by presenting them, unwatered, to the influence of the sun, 328 GENERAL REMARKS, ETC. at an early period of summer ; then, after the leaves have ripened, keep them as cool as possible for some time ; thus causing a sufficient accumulation of excitability by the end of October, instead of the following month of May, at which period the fruit will be ripe. . SECTION VIII. VENTILATION WITH FANS. IN a preceding part of this work, [see Part II., Sec. V.] we have described a method of warming hot-houses practised in Ger- many, in which a fan is used as a means of propelling the heated air into the apartments required to be warmed, and by which the volume of air to be heated is drawn from the external atmosphere. As an auxiliary to a heating apparatus, however, the complicated arrangements of this machine, the cost of its construction, and the expense and trouble of working it, must ever continue to prevent its adoption as a method of warming horticultural buildings, however extensive they may be. But as an auxiliary of ventilation, and as a means of creating that con- tinual motion in the air, which some cultivators so much admire, it is undoubtedly superior to all other methods. Fans are so common as to require very little description. The kind of machine generally used for this purpose is merely a light circular kind of wheel, composed of as many vanes or blades as the size will admit. By the constant revolution of this wheel, a movement is created in the atmosphere, which causes a change in the position of the atomic particles of the atmosphere of the room in which it is at work ; but does not, as some suppose, tend to its equalization. Fans are of two kinds, and have different methods of action. The one is termed blowing fans ; the other, exhausting, or suction fans. In the first case, the air in the house is driven outwards from the fan, or blown away ; in the other, it is drawn towards it. It will appear evident, however, that, in applying this machine to the creation of a movement in the atmosphere of a hot-house, various requisites must be had, namely, a moving power, con- stantly and steadily acting, and completely under control ; and when it is to be applied to night ventilation and motion, which appears to us the most adaptable use to which it can be applied 330 VENTILATION WITH FANS. in relation to any kind of horticultural structures, then a supply of warmed air must be kept up by means of the heating appa- ratus, and a channel of conduction for the vitiated air to escape by. In places where the mechanical power for moving a fan can be easily obtained, this machine may be turned to excellent advantage. The question, therefore, is not as to the adaptability of the machine, but as to the means of working it so as to bring it within the reach of hot-house adaptation, at a cost which would justify us in recommending it. There are various points to be considered in relation to draw- ing in fresh, and expelling foul, air from a hot-house, namely, that we must not only expel the vitiated air from the house, but we must introduce pure air into its place ; and that pure air must be warmed before it is introduced. We have heard and read a good deal about this and the other method of introducing warm air into a hot-house ; and, in theory, many of these notions are very plausible, but when we come to apply them to practice, they are entire failures. The principal objects to be obtained by an efficient system of night ventilation may be classed as follows : 1. The expulsion of a certain quantity of vitiated air, in a certain time, from the whole volume contained in the house ; and, as the impure air rises by rarefaction to the upper regions of the house, means must be provided to carry it away, with- out creating counter-currents, or admitting any cold air, by the channels of conduction thus made. 2. A quantity of air must be introduced to the internal vol- ume equal to the quantity expelled ; otherwise the remaining internal volume will expand, by its increased temperature, and fill the space occupied by the decreasing volume, and thus the air becomes more vitiated than if none had escaped. The air thus brought in must be introduced without acting in a direct current upon the vegetable productions within the house. 3. The air thus introduced must be warmed to a certain tem- perature, before it enters the house. This temperature should be regulated by the temperature at which it is desired to main- tain the internal atmosphere. If the desired temperature be VENTILATION WITH FANS. 331 50, the air entering should not be under that temperature, but rather a few degrees above it. 4. If the house be heated by pipes laid round the side of the house, the air thus admitted should be introduced so as to pass upward, by the side of the pipes, on entering the house. This air should pass regularly and consentaneously upwards ; not in sudden blasts and currents, which have always an injurious influence on the internal atmosphere. To effect this, a hot-air chamber should be placed in connec- tion with the heating apparatus, from which must be laid air channels, or conduction tubes, all around the house, having apertures for the egress of the air, at distances of six or eight feet apart. Within this chamber a fan might be used for drawing in the external air and driving in the warmed air through the tube. This fan might be driven by a small windmill con- structed for the purpose. When air is under the control of a moving power, it will take any direction that is desired. It will move horizontally, or ver- tically, either upwards or downwards, and even in both direc- tions, at the same time. It is essential, however, that the supply to be warmed should be drawn from the external atmosphere ; and here the fan may be used to great advantage. In no case should the supply of air be drawn from the interior of the house. The vitiated air, as it passes upward, should be allowed to pass off freely into the atmosphere. In this country, however, the fan cannot be so advantageously applied in the ventilation of horticultural buildings, as in north- ern Europe, and only at night, the period when ventilation is most needful. The large amount of artificial heat necessary in our New England climate, in severe nights, is more injurious to green-house plants than the excessive heat of summer. There is no impossibility, however, in producing a constant and equa- ble motion in the atmosphere of green-houses, at night ; and this may be effected by the means which we have just ex- plained. Fans may also be beneficially employed in producing a cool- ing effect in the air at the top of the house. The injurious 332 VENTILATION WITH FANS. effect of the highly-heated air in the upper regions must be obvious. We have measured the temperature of a house 45 feet in height, and have found the temperature at the floor of the house to be 38, while the temperature of the upper stratum was 103, showing a difference of 65. In many other cases, we have found the temperature of the upper stratum of air in a house, above 120, while the water cistern, at the floor of the house was covered with ice. The application of a fan may be beneficial in reducing this temperature, and expelling the foul air collected in the upper portions, at apertures lower down the house. Various other mechanical contrivances, besides the fan, have been used for producing motion in the atmosphere of houses. Among these may be mentioned common windmills, of which we have already spoken. The windmill ventilator is a very adaptable machine, and may be constructed very simply, in con- nection with a hot-house, and applied in moving the atmosphere of the house, or in propelling the warmed air through the con- duction tubes with greater velocity than it would acquire by its own specific gravity. The windmill, of course, is turned by the force of the wind outside the house, and is entirely depend- ent upon the motion of the external air, for the power it exer- cises over the internal atmosphere. In hot-houses, with dome- shaped roofs, it is well adapted for drawing off the highly- heated air at the top of the house, and may be made something like the screw propeller of the steamboats, and situated directly in the apex of the roof. Pumps have also been used for drawing off the foul air from buildings, although we are not aware that they have ever been employed for ventilating hot-houses, for which they are not at all adapted. Chimney shafts are well adapted for producing motion in the air, by the draughts. None of these methods, however, are so useful as the fan, when mechanical means are to be applied ; though, for the practical purposes of ventilation, in horticultural structures, the common process of spontaneous ventilation must, in general cases, suffice ; and, therefore, the question is, as to the means of admitting the air, and the temperature at which it VENTILATION WITH FANS. 333 is to be admitted. The movements of the atmosphere, caused by the difference of temperature between the external and inter- nal volumes, have been already considered ; and we now leave the subject to the consideration of those who are engaged in the practical operations of exotic horticulture. APPENDIX TABLE I. TABLE of the Expansive Force of Steam, in Atmospheres, and in Ibs. per square inch ; for temperatures above 212 of Fahrenheit. N. B. The steam is supposed to be in contact with the water from which it is formed, and the water and steam to be alike in temperature. DO Pressure. g . Pressure. S Pressure. 1! m !'! w gjj tri 2 o S E flj " a f Ibs. C A S Ibs. c J3 " a i Ibs. jfi | fl 1 |5 I 212 251 1 2 15 30 431 436 23 24 345 360 646 655 150 160 2250 2400 275 3 45 439 25 375 663 170 2550 294 4 60 457 30 450 671 180 2700 308 5 75 473 35 525 679 190 2850 320 6 90 487 40 600 686 200 3000 332 7 105 499 45 675 694 210 3150 342 8 120 511 50 750 700 220 3300 351 9 135 521 55 825 707 230 3450 359 10 150 531 60 900 713 240 3600 367 11 165 540 65 975 719 250 3750 374 12 180 549 70 1050 726 260 3900 381 13 1^5 557 75 1125 731 270 4050 387 14 ?10 565 80 1200 737 280 4200 393 15 Vk>5 572 85 1275 742 290 4350 399 16 ^40 579 90 1350 748 300 4500 404 17 255 586 95 1425 753 310 4650 409 18 270 592 100 1500 758 320 4800 414 19 285 605 110 1650 763 330 4950 418 20 300 616 120 1800 768 340 5100 423 21 315 627 130 1950 772 350 5250 427 22 330 636 140 2100 *** The above Table is deduced from the experiments of MM. Dulong and Arago. Their calculations extend only as far as 50 atmos- 29 336 APPENDIX. pheres ; from thence the pressures are now calculated to 350 atmos- pheres by their formula, viz. : 7153 where e represents the pressure in atmospheres, and t the temperature above 100 of Centigrade. In this equation each 100 of Centigrade is represented by unity. In reducing these temperatures from Centigrade to Fahrenheit's scale, vhere the fractions amount to -5, they have been taken as the next legree above, and all fractions below -5 have been rejected. TABLE II. TABLE of the quantity of Vapor contained in Atmospheric Air, different Temperatures, when saturated. at _o o t* c 1- *: 1- o y r in inches of mercury. Tempe- rature. Force of va- x>r in inches of mercury. 32 0-2000 49 0-3483 65 0-6146 33 0-2066 50 0-3600 66 0-6355 34 0-2134 51 0-3735 67 0-6571 35 0-2204 52 0-3875 68 0-6794 36 0-2277 53 0-4020 69 0-7025 37 0-2352 54 0-4171 70 0-7260 38 0-2429 55 0-4327 71 0-7507 39 0-2509 56 0-4489 72 0-7762 40 0-2600 57 0-4657 73 0-8026 41 0-2686 58 0-4832 74 0-8299 42 0-2775 59 0-5012 75 0-8581 43 0-2866 60 0-5200 76 0-8873 44 0-2961 61 0-5377 77 0-9175 45 0-3059 62 0-5560 78 0-9487 46 0-3160 63 0-5749 79 0-9809 47 0-3264 64 0-5944 80 1-0120 48 0-3372 NOTE TO TABLE XII. (See next page.) To determine the dew-point, take two thermometers, the scales of which agree, cover the bulb of one with thin muslin, and wet it with water ; swing both thermometers in the air, that they may be exposed under similar circumstances, and note the height of the mercurial column in each, after it has become stationary. Ascertain the difference between the heights of the two columns. In the following table, find a number at the top corresponding to the difference of heights, and in the left hand column the degree answering to the temperature indicated by the dry bulb thermometer ; the figure at the intersection of the two lines is the dew-point. Suppose, for instance, the dry bulb indicated 70, and the wet bulb 61 ; 70 61=9, which is found at the top of the table ; in the column beneath, and against 70, is 55, the dew-point. 346 APPENDIX. TABLE XII. Table for ascertaining Dew-point by Temp, of air 1 2 3 4 5 6 7 8 9 10 11 12 (13 14 ~90 88-7 87-5 86-1 85-1 83-8 82-581-2 79-9|78-e 77-S 751 J7T 473- 71-5 89 87-7 86-5 85-3 84-0 82-7 81-480- 78-8i77-4 76-f 74-e > 73-2 71-6 70-3 88 86-7 85-5 84-3 83-0 81-7 80-479- 77-7|76-2 74-S 73- 72- 170-b 69-1 87 85- 84-5 83-2 81-9 80-6 79-378-1 76-675-2 73-g 72-4 70-969-4 67-9 86 84- 83-5 82-2 80-9 79-6 78-276-9 75-5,74-1 72-7 71-2 69-768-2 66-6 85 83- 82-4 81-1 79-8 78-5 77-275-8 74-4,73 71-5 70-068-567-0 65-4 84 82- 81-4 80-1 78-8 77-5 76-li74-7 73-3,71-8 70-4 68-S 67-365-" 64-1 83 81- 80-4 79-1 77-8 76-4 75-073-6 72-0 70-7 69-267*7 66-164-5 62-8 82 80- 79-4 78-1 76-7 75-3 73-9I72-5 71-0 69-6 68-1 66-564-963-2 61-5 81 79-7 78-3 77-0 75-6 74-2 72-871-4 70-0 68-4 66-965-3 63T62-0 60-3 ~80~ 78-6 77-3 76-0 74-6 73-2 71-7170-3 08-8672 65-7|64-l 62-4 60-7 58-9 79 77-6 76-3 75-0 73-5 72-1 70-769-2 67-666-1 64-5 62-8 61-l i 59-4 57-6 78 76-6 75-3 73-9 72-5 71-0 69-5 68-0 66-565-0 63-3 ! 61-6 59-858-1 56-2 77 75-6 74-2 72-8 71-4 69-9 68-4 66-9 65-363-7 62-1 60-358-556-7 54-8 76 74-6 73-2 71-8 70-3 68-9 67-3 65-8 64-262-5 60-8 59- 157-2 55-3 53-4 75 73-6 72-2 70- 69-2 67-7 66-2 64-b 63-061-3 59-5 57-7 55-9 54-0 52-0 74 72-6 71-1 69- 68-2 66-6 65-163-4 ul-860-1 58-3 56-4 54-5 52-5 50-4 73 71-5 70-1 68- 67-1 65-5 64-0 62-3 60-658-8 57-0 55-153-1151-1 49-0 72 70-5 69-1 67- 66-0 64-4 62-8 61-1 59-357-5 55-753-7 51-7149-6 47-3 71 69-5 68-0 66- 64-9 63-3 61-6 59-9 58-156-2 54-452-4 50-348-1 45-7 70 68-5 67-0 65- 63-8 62-2 60-5 58-7 StHJBlFo 530 51-0 48 46-5 44~1 69 67-4 66-0 64- 62-7 61-0 59-3 57-5 55-6 53-7 51-6 49-5 47-3 44-9 42-4 68 66-4 64-9 63- 61-6 59-9 58-1 56-3 54-352-3 50-2 48-0 45-7!43-2 40-5 67 65-4 63-8 62-2 i)0-5 58-7 56-9 55-0 53-051-0 48-8 46-5 44-1141-5 38-8 66 64-4 62-7 61-1 59-3 57-5 55-7 53-7 51-7 49-6 47-3'45-042-4i39-7 36-8 65 63-3 61-7 60-0 58-2 56-4 54-5 52-5 50-4 48-2 45-843-4 40-7i37-9 34-8 64 62-3 60-6 58- " 37- 1 55-2 53-2 51-2 49-046-7 44-341-7 39-0:36-0 32-7 63 61-3 59-6 57-8 55-8 54-0 520 49-8 47-645-1 42-740-1 37-1134-0 30-5 62 60-3 58-5 30-7 54-8 52-8 50-7 48-5 46-2 43-7 41-138-3 35-2j31-9 28-2 61 59-2 57-4 55-5 53-6 51-5 49-4 47-1 41-7 42-2 39-4 36-4 33-2 29-7 25-7 60 58-2 56-3 54.4 5JF4 50-3 48-1 45-7 IJFS 40-6 3T734-63M 27-3 23-0 59 57-2 55-3 53-3 51-2 49-1 46-8 44-3 41-739-0 35-9 32-6 28-9 24-8 0-1 58 56-1 54-2 52-2 50-0 47-8 45-4 42-9 40-1 37-2 34-0 30-5'26-6|22-l 7-0 57 55-1 53-1 51-0 18-8 46-5 44-041-4 38-5 35 5 32-1 28-3 24-1U9-2 3-5 56 54-0 52-0 49-8 47-6 45-2 42-6 39-8 36-833-6 30-0 26-0 '21-4I16-1 9-5 55 53-0 50-8 48-6 16-3 43-8 41-138-2 35-1 31-8 27-823-4 18-4 12-4 4-9 54 51-9 49-7 47-5 45-0 42-4 39-636-6 33-329-7 25-620-8 15-3 8-5 -0-2 53 50-9 48-6 46-2 43-8 41-1 38-134-8 31-327-423-0 17-8 11-5 3-7 52 49-8 47-5 15-1 42-4 39-6 36-633-2 29-725-320-5, 14-8 7-8 -1-4 51 48-8 46-4 43-S 41-1 38-2 35-031-4 27-422-9 17-7 11-3 3-3 50 4T7 45-2 126 39~7 3lH3 33-3:29-5 25-320-4 14-7 7-4 -2-0 49 46-6 44-1 41-3 38-4 35-1 31-627-5 23-0 17-7 11-2 2-9 48 45-5 42-9 40-0 37-0 33-5 29-7)25-5 20-614-7 7-3 -2-4 47 44-4 41-7 38-7 35-5 31-9 27-9J23-3 7-9 11-4 3-1 46 13-4 40-5 37-4 34-0 30-1 25-7 20-8 4-8 7-4 -2-6 45 12-2 39-3 36-1 32-5 28-4 23-9 18-5 2-0 3-6 44 il'l 38-1 34-7 30-9 26-2 21-7 15-8 8-5 -1-2 43 40-1 36-8 33-2 29-3 24-7 19-4 129 4-6 -7-0 42 38-9 35-6 31-8 27-6 22-7 16-9 9-7 0-2 41 37-8 34-3 30 3 25-8 20-b 14-3 6-2 -5-0 40 36-7 33-0 28-8 23-9 18-1 11-4 2-2 APPENDIX. 347 TABLE XII. Continued. Observations on the Wet and Dry Bulb Thermometer. Temp, of air. ~9(F 15 TtM) 16 681 17 66^8 18 6T2 19 6T6 1 20 21 (JHJifiO 7 ! 22 sFs 23 56 : 4 24 1 25 1 26 27 5TI 52 4 50 3i48 7 28 4F6 89 68-8 67-2 65-6 63-9 62-2 60-558-7 56-9 54-9 52-9 50-8 48-5146-2 88 67-5 65-9 64-3 6-6|60-9 59-157-2 55-3 53-3,51-2 49-0 46-7J44-3 87 86 66-3 65-0 64-7 63-3 63-0 61-6 61-3 59-9 59-5 58-1 57-7:55-8 56-2 : 54-2 l53-g 52-2 51-7 49-6 47-3 44-9 50-1 47-8 45-5 43-0 42-4 85 63-7 62-0 60-3 58-5 56-6 54-7:52-7 50-6 48-446-1 43-641-0 84 62-4 60-7 59-0 57-1 552 53-2;5l-l 48-9 46-644-2 41-638-9 83 61-1 59-4 57-5 55-6 53-7 51-649-5 47-2 44-8,42.339-6 82 59-8 58-0 56-1 54-2 52-1 50-047-8 45-4 43-040-337-4 81 58-5 56-6 54-7 52-7 50-6 48-446-1 43-6 41-038-2 35-2 80 5T 7 ! 55^2 53^2 5FI 49-0 46 7 7 4F3 IP? 39-0;36-0 79 55-7 53-7 51-7 49-6 47-3 45-042-4 39-7 36-833-7 78 54-2 52-2 50-1 180 45-6 43-1 40-4 37-6 34-6 31-2 77 52-8 50-7 485 46-2 43-8 41-238-4 35-4 32-2 76 51-3 49-2 46-9 44-5 42-0 39-2 36-3 33-1 29-7 75 49-8 47-6 45-2 42-7 40-1 37-2 34-1 30-7 27-0 74 48-2 46-0 43-5 40-8 38-1135-0 31-7 28-1 73 146-6 44-2 41-6 39-0 36-OJ32-829-2 25-3 72 45-0 42-5 39-8 36-9 33-8 30-4 26-5 22-3 71 43-3 40-6 37-8 34-8 31-4 27-8 237 ~TQ~ 41-5 SS 7 ? 35^8 321) 25 lil <-> -^3 -> 11 c l - i -5;tic2t:"-' S- H <^ .-. -c OK < -U o o p T O lO T*< **! ^H TABLE A Table of the Anal !* ^* "^ o 4 O ^-^COO us V I u APPENDIX, 349 TABLE XIV. Constitution of the Atmosphere. Dumas and Boussingault analyzed atmospheric air by fixing its oxygen on copper, which was weighed j the azote was also collected and weighed. 1000 parts of air at Paris contained by weight : Oxygen. Azote. April 27, fair weather, 229.2 770-8 " " " " 229.2 770.8 " 28, " " 230.3 769.7 " " " 230.9 769.1 " 29, " " 230.3 769.7 " " " " 230.4 769.6 May 29, rainy, 230.1 769.9 July 20, mid-day, rainy, 230.5 769.5 " 21, midnight, clear, 230.0 770.0 " 26, mid-day, clear, * * . * . 230.7 769.3 769.8 792=1000 Consumption of Oxygen and Formation of Carbonic Acid. From experiments of Dumas on himself, it appears that about twenty cubic inches were received into the lungs at each inspiration, and from Hfteen to seventeen inspirations per minute. The expired air contained from three to four per cent, of carbonic acid, and had lost from four to six per cent, of oxygen. These data, for each day of twenty-four hours, give, 16 insp. X 20 cubic inches = 320 cubic inches expired per minute. 19,200 " " " hour. 460,800 " " " day 350 APPENDIX. TABLE XV. A Table of Mean Temperatures of the hottest and coldest months. Mean' Pemp. of Latitude. Longitude. est Month. Coldest Month. Authorities. St. Petersburgh, 59 56 N. 30 19 E. 65-660 8-600 Humboldt. Moscow, 55 45 N. 37 32 E. 70-52 608 " Melville Island, \ 74 47 N. 110 48 W 39-08 42-41 -35-52 -32-19 Hugh Murray. Ed. Phil. Journal. Copenhagen, Edinburgh, 55 41 N. 55 57 N. 12 35 E. 3 10W. 65-66 59-36 27-14 38-30 Humboldt. Geneva, 46 12 N. 6 8E. 66-56 34-16 it Vienna, 48 12 N. 16 22 E. 70-52 2660 M Paris, 48 50 N. 2 20 E. 65-30 36-14 H London, Philadelphia, 51 SON. 39 56 N. 5 W. 75 16 W. 64-40 77-00 37-76 32-72 ! New York, 40 40 N. 73 58 W. 80-70 25-34 Pekin, 39 54 N. 116 27 E. 84-38 24-62 1 Milan, 45 28 N. 9 11 E. 7466 36-14 ( Bordeaux, 44 50 N. 34 W. 73-04 41-00 1 Marseilles, 43 17 N. 5 22 E. 74 '66 44-42 ( Rome, 41 53 N. 12 27 E. 77-00 42-26 1 Funchal, 32 37 N. 16 56 W. 75-56 64-04 1 Algiers, 36 48 N. 3 1 E. 82-76 6XHH ' Cairo, 30 2 N. 30 18 E. 85-82 56-12 Vera Cruz, 19 11 N. 96 1 W. 81-86 71-06 1 Havanna, 23 10 N. 82 13 W. 83-84 69-98 ' Cumana, 10 27 N. 65 15 W. 84-38 79-16 (( Canton, 23 10 N. 113 13 E. 84-50 57-00 Anglo-Chinese Calendar. Macao, 22 10 N. 113 32 E. 86-00 63-50? it Canaries, 23 30 N. 16 00 W. 78-90 63-70 Brande's Journal. Lohooghat (5800 ) feet above the > sea,) ) 29 23 N. 79 56 E. 6934 43-57 $ Trans. Med. Phya. Soc. ? Calc. Fattehpur, Gurrah Warrah, 25 56 N. 23 ION. 80 45 E. 79 54 E. 74-94 87-45 58-74 60-23 Gleanings in Science. Calcutta, \ 22 40 N. 88 25 E. 85-70 86-86 66.20 70-10 Journal As. Soc. Ava, 21 51 N. 95 98 E. 88-15 64-12 Gleanings in Science. Bareilly, 28 23 N. 79 23 E. 91-91 5650 ti n Chunar, 25 9 N. 82 54 E. 90-00 58-00 Ed. Ph. Journ. Cape of Good ) Hope (Feld- [ 34 23 S. 18 25 E. 74-27 57-43 Herschel (MSS.) hausen.) ) Bahamas, 26 SON. 78 30 W. 83-52 6907 Hon. J. C. Lees (MSS.) Swan River, Bermuda, 32 00 S. 32 15 N. 115 50 E. 64 SOW. 78-00 76-75 54-84 57-90 Milligan. Col. Emmett. APPENDIX. 361 TABLE XVI. The following proportions between the Mean Temperature of the earth, as indicated by springs, and that of the atmosphere, have been collected from various sources. Names of Places. Authority. Temp, of Earth. Mean Temp, of Atmos- phere. Berlin, Wahlenberg, . . . 49-28 46-40 u 47-30 42-03 Upsal, ii 43-70 42-08 Paris, (Catacombs,) ... 53-00 51-00 Charleston, ...... 63-00 68-00 u 53-00 53-42 Virginia (i 57-00 57-00 Dewev. 47-21 44-73 Volney, 44-00 56-00 Raith (Scotland,) Ferguson, .... 47-70 47-00 Watson, 52-46 51-42 Kendal (do ) n 47-20 47-04 Keswick, (do.) tt 46-60 48-00 Leith, (Scotland,) 47-30 48-36 South of England, Rees' Cyclo, . . . 48-00 50-62 Torrid Zone, 63-00 81-50 30* 352 APPENDIX. TABLE XVII. Showing the Specific Gravity of different kinds of timber. I. H. Box, _ 942 Plum-tree, 872 Hawthorn, 871 Beech, 852 Ash, 845 670 Yew, 807 744 Elm, 800 568 Birch, 738 Apple, 733 734 Pear, 732 Yoke-elm, 728 Orange-tree, 705 Walnut-tree, 660 Pine, 657 763 Maple, 645 Linden-tree, 604 559 Cypress, 598 Cedar, 561 Horse chestnut, 551 Alder, 538 White poplar, 529 Common poplar, 383 387 Cork, 240 * # * The column I., in the above table, exhibits the specific gravity of different woods, adopted by the Annuaire du Bureau des Longitudes. The second column contains the results obtained by M. Karmarsch. APPENDIX. 363 TABLE XVIII. Solutions for the impregnation of wood which is exposed to the atmos- phere, for the purpose of preserving it from decay. Tar. Sulphate of Copper. Sulphate of Zinc. Sulphate of Iron. Sulphate of Lime. Sulphate of Magnesia. Sulphate of Barytes. Sulphate of Soda. Alum. Carbonate of Soda. Carbonate of Potash. Carbonate of Barytes. Sulphuric Acid. Acid of Tar, (pyroligneous acid.) Common Salt. Vegetable Oils. Animal Oils. Coal Oil, (Naphtha,) Resins. Quick-lime. Glue. Corrosive sublimate.* Nitrate of Potash. Arsenical Pyrites water, (water containing arsenical acid.) Peat Moss, (containing tannin.) Creosote and Eupion. Crude Acetate, or pyrolignite of iron. Peroxide of Tin. Oxide of Copper. Nitrate of Copper. Acetate of Copper. Solution of Bitumen, in oil of tur- pentine. Yellow Cromate of Potash. Refuse Lime-water of Gas-works. Caoutchouc, dissolved in naptha. Drying Oil. Beeswax, dissolved in turpentine. I Chloride of Zinc. * Corrosive sublimate is one of the most efficient of all these antiseptic applications. It was proposed by Mr. Kyan as a preventive of dry rot, under the idea of its acting as a poison to the fungi and insects, which were the supposed cause of the disease. But thia explanation of the action of corrosive sublimate is no longer tenable, as it is generally admitted that the fungi and insects are not to be considered the origin, but the result, of the dry rot. It has been suggested that its action depends on the forma- tion of a compound of lignum, or pure woody fibre, with corrosive sublimate, which re- sists decomposition in circumstances where pure lignum is liable to decay. But pure lignum possesses no tendency to combine with corrosive sublimate. The action of this substance is in reality confined to the albumen, with which it unites to form an insoluble compound, not susceptible of spontaneous decomposition, and, therefore, in- capable of exciting fermentation. Vegetable and animal matters, the most prone to decomposition, are completely deprived of their property of putrefaction and fermenta- tion by the contact of corrosive sublimate. It is on this account advantageously em- ployed as a means of preserving animal and vegetable substances. Its expensivenesg in this country is a great obstacle to its extensive employment on timber used for build- ing purposes, for fences, bridges, &c. There is scarcely any antisceptic application so effectual. By Mr. Kyan's process, the timber to be impregnated, is sawed up into planks, and soaked for seven or eight days in a solution containing one pound of cor- rosive sublimate to five gallons of water. The impregnation may be easily effected in an open tank ; though the best way is to impregnate the timber by placing it in an air-tight box, from which the air has been exhausted as much as possible by a pump. The solution then enters the pores of wood freely, being pressed into them by a force equal to about one hundred pounds to the square inch. ParneW s Applied Chemistry. 354 APPENDIX. JBT3t "3J8AT TABLE XIX. Table showing the Heating Power of different kinds of Wood, drawn by MM. Peterson and Schodler, from the quantity of Oxygen required to burn them. Names of Trees. Oxygen required to burn them. Tila Europea, lime, 140-523 Ulmus suberosa, elm, 139-408 Pinus abies, fir, 138-377 Pinus larix, larch, 138-082 JEsculus hippocastanum, horse-chestnut, 138-002 Buxus sempervirens, box, 137-315 Acer campestres, maple, 136-960 Pinus sylvestris, Scotch fir, 136-931 Pinus pinea, pitch pine, 136-886 Populus nigra, black poplar, 136-628 Pyrus communis, pear tree, 135-881 Juglans regia, walnut, 135-690 Betula alnus, alder, 133-956 Salix fragilis, willow, 133-951 Quercus robur, oak, , . 133-472 Pyrus malus, apple-tree, 133-340 Fraxinus excelsior, ash, 133-251 Betula alba, birch, 133.229 Prunus cerasus, cherry-tree, 133-139 Robinea pseudacacia, acacia, 132-543 Fagus sylvatica, white beach, 132-312 Prunus domestica, plum, 132-088 Fagus sylvatica, red beach, 130-834 Diospyros ebenum, ebony, 128-178 APPENDIX. TABLE XX. Difference in Weight of two columns of Water, each one foot high, at various Temperatures, Difference in temperature of the two columns of water in degrees of Difference in weight of two columns of water, contained in different sized pipes. Difference of a column one foot high. Fahrenheit's Scale. 1 inch dia. 2 inches dia. 3 inches dia. 4 inches dia. per square inch. grs. weight. grs. weight. grs. weight. grs. weight. grs. weight. 2 1-5 6-3 14-3 25-4 2-028 4 3-1 12-7 28-8 51-1 4-068 6 4-7 19-1 43-3 767 6-108 8 6-4 25-6 57-9 102-5 8-160 10 8-0 32-0 72-3 128-1 10-200 12 9-6 38-5 87-0 154-1 12-264 14 11-2 45-0 101-7 180-0 14-328 16 12-8 51-4 116-3 205-9 16-392 18 14-4 57-9 131-0 231-9 18-456 20 16-1 64-5 145-7 258-0 20-532 * # * It will be observed in the above table that the amount of motive power increases with the size of the pipe j for instance, the power is 4 times as great in a pipe of 4 inches diameter as in one of 2 inches. The power, however, bears exactly the same relative proportion to the resistance, or weight of water to be put in motion in all the sizes alike j for, although the motive power is 4 times as great in pipes of 4 inches diameter, as in pipes of 2 inches, the former contains 4 times as much water as the other. The power and the resistance, therefore, are rela- tively the same. INDEX TO THE ILLUSTRATIONS. PART I. SECTION I. Fio. PAGE. 1 Elevations of hot-house roofs, 35 2 Difference of elevation of the sun's rays at Philadelphia and London, . . 36 3 End section of a forcing pit, . . . 39 4 Ground plan and elevation of forcing pit, 41 5 End section of a stove, 42 6 Polyprosopic forcing house, 43 7 Cambridge pit, 43 8 Saunders' pit, 48 9 Curvilinear cold pit, . . . 46 10 Dung bed with frame for forcing, 46 1 1 Portable glass frame, 48 12 Portable plant protector, 43 13 Ground plan ofan extensive framing ground, 51 14 Range of graperies at Clifton Park, 53 15 Single-roofed grapery, 55 16 Span-roofed house on the same scale, 55 17 Single-roofed curvilinear grapery, 57 18 Double-roofed house of the same plan, 57 19 Polyprosopic grapery, 61 20 Ground plan of do., 61 21 Ridge and furrow roof, 65 22 Range of small houses, 69 23 Ornamental grapery, 71 24 End section of green-house, 76 25 Perspective view of span-roofed green-house, 77 26 Range of plant-houses, 78 27 Ornamental plant-house, 80 SECTION II. 28 Roof trellises, . 85 29 Interior trellises, 86 30 Upright trellises, 87 31 Rooftrellises and open border planting, 87 32 Interior ground plan of a conservatory, 95 ILLUSTRATIONS. 357 PART II. FIG. PAGE. 33 Williams' furnace for prevention of smoke , 149 34 Jeffreys' smoke-precipitating furnace, 151 35 Improved arch boiler, 179 36 Common boiler, 179 36 A Circular boiler and pipes, 185 37 Ground plan of polmaise heated green-house, 197 38 End section of do., 198 39 Longitudinal section of do., 199 40 Combination of hot water and hot air, 201 41 Four houses heated with one boiler, 204 42 Boiler and supply box, 205 B Supply cistern, 205 43 Tank method of heating, 210 44 Tank of galvanized zinc, 214 45 Wooden tank for retention of heat, 216 46 End section of do., 216 47 Plant pits heated by wooden tanks, 227 48 Arched borders heated with hot-water pipes, 230 49 Chambered border heated with tanks, 233 50 Covered hot wall, 241 PART III. 52 Method of ventilating lean-to houses by pulleys, 277 53 End section, showing the apertures for ventilation through the walls, . 277 54 End section of span roof, showing ventilation at top, 278 55 Showing the ventilator enlarged, 279 56 Front ventilation by rachet wheel, 280 57 Movement of the atmosphere from the floor of the house, 289 SB Common methods of ventilation .291 TABLES. I. Table of the expansive force of steam in pounds per square inch, for tem- peratures above 212 Fahrenheit, 335 II. Table of the quantity of vapor contained in atmospheric air at different temperatures, when saturated, 336 III. Table of the expansion of air and other gases by heat, when perfectly free from vapor, 337 IV. Table of specific gravity and expansion of water at different temper- atures 338 V. Table of specific heat, specific gravity, and expansion by heat of different bodies, 339 VI. Table of the effects of heat, 340 VII. Table of the quantity of water contained in 100 feet of pipe of different diameters, 341 VIII. Table showing the effects of wind in cooling glass, 341 IX. Experiments on the cooling effect of windows, 342 X. Weights of watery vapor in one cubic foot of air, at dew points from to 100 Fahrenheit, 344 XI. Dalton's table of the force of vapor, from 32 to 80, 345 XII. Table for ascertaining dew point by observations on the wet and dry bulb thermometer, 346 XIII. Table of the analysis of confined air, 348 XIV. Constitution of the atmosphere ; consumption of oxygen, and formation of carbonic acid, -. 349 XV. Table of mean temperatures of the hottest and coldest months, . . 350 XVI. Mean temperature of the earth and of the atmosphere, 351 XVII. Specific gravity of different kinds of timber, 352 XVIII. Solutions for the impregnation of wood which is exposed to the at- mosphere, for the purpose of preserving it from decay, 353 XIX. Heating power of different lunds of wood, drawn from the quantity of oxygen required to burn them, 354 XX. Difference of weight of two columns of water, each one foot high, at various temperatures, 455 INDEX. PART I. -CONSTRUCTION. SECTION I. SITUATION. Site and position. What is to be understood by site and position. Cir- cumstances to affect the position of a hot-house. Avoid bare, elevated spots. Reasons for so doing. For shelter. For beauty and effect, 13 Terraces. Their origin, and use round horticultural buildings. The un- sightliness of turf terraces. Architectural terraces. Description of a terrace at a gentleman's residence. Effect of trees. Effect without trees. Choice of position decided by other circumstances, 15 Aspect. Best aspect for lean-to houses. Reasons for choosing a south- eastern aspect. Aspect for span-roofed houses. The aspect of conserva- tories. Unsuitable conservatories, 20 SECTION II. DESIGN. General principles. Object of hot-houses. Agents of vegetative growth. Reasons wnybad structures are so generally erected in this country. Mansion architects. Their incapacity for erecting horticultural buildings. Fitness for the end in view. Solid, opaque conservatories. Conservatory at Brookline. Absurdity of spending large sums on conservatories. Observations of an architect. Massive conservatories, 25 Light a primary object. Wonderful effects of light on vegetables. Theory of the transmission of light. Rays of light reflected from transparent sur- faces. Action of light upon plants. Effects of different rays. Light which has permeated yellow media. Light which has permeated red media. Light which has permeated blue media. Difficulty of obtaining pure colors. Amount of assimilation and perspiration in plants. Necessity ot making plant- houses transparent on all sides, 29 Slope of hot-house roofs. Much depends on the angle of elevation. Prin- ciples to guide the inclination of hot-house roofs. Elevations of roofs in England. Figure representing different elevations. Figure showing the dif- ference of latitude between London and Philadelphia. Application of these 360 INDEX. principles. Error committed in laying hot-house roofs too flat. Table show- ing the number of rays reflected at different angles. Circumstances on which the slope of roofs depends, 34 SECTION III. STRUCTURES ADAPTED TO PARTICULAR PURPOSES. Forcing-houses, culinary-houses, &c. Purposes of their erection. Section of a forcing-pit figured and described. Large forcing-pit figured and described. Dimensions of winter forcing-houses. Skill required in the forcing of fruit in winter. Polyprosopic forcing-houses figured and described. Advantages of pplyprosopic roofs, 39 Pits. The Cambridge pit. Saunders' forcing-pit figured and described. Curvilinear roofed cold pits. Dung beds. Temporary frames. Plant pro- tectors. Figures and descriptions of them, 43 Framing ground. Its purposes. General condition of this department. Appropriate site for it. Ground plan and disposition of framing ground, . 49 Orangeries, graperies, &c. Latitude given in their construction. Repre- sentation of a range of cold-houses at Clifton Park. Size of cold-houses. Figures of lean-to and span-roofed houses. Figures of double and single- roofed curvilinear houses, 54 Objections raised against curvilinear houses in England. Properties pos- sessed by curvilinear houses. Reflection and refraction of light by them. Their adaptability for grape-growing. Gable ends. Objections to them, . 58 Polyprosopic houses. Figures and descriptions of do. Double-roofed houses of this kind. Cold vineries. Disadvantages attending them. Front wall of hot-houses. The height of do. Objections to upright fronts. Parapet walls, 60 _ Ridge and furrow-roofed houses. Figure and description of a house of this kind. Directions for building ridges and furrows. Glazing of do. Advan- tages of do. Principle of their construction, 64 Cold vineries. Range of small booses figured and described. Advantages of small houses over large ones, 67 Green-houses, conservatories, &c. Distinction between green-houses and conservatories. Amalgamation of the two together. Appropriation of green- houses in summer. Span-roofed green-houses preferable to single-roofed ones. Beauty of well-grown plants. Impossibility of growing plants well in opaque houses. Proportions of a green-house, " 73 Plan of green-house, and description. Prospective view of green-house. Range of green-houses. Height of plant-houses. Errors in making them too high. Conservatory at Regent's Park Botanic Garden. Principles of design and taste displayed. Advantages of low-roofed plant-houses, 76 SECTION IV. INTERIOR ARRANGEMENTS. Arrangements for forcing-houses, culinary-houses, &c. Trellises and meth- ods of fixing trellises. Roof trellises. Centre trellises. Cross trellises. Trellises for double houses, 84 Interior of green-houses. Slope of sfreen-house stages. Green-houses for promiscuous plants. Width and height of green-house shelves. Stages for small plants, &c., 87 Conservatories, Orangeries, &e. Houses for growing large plants. Con- servatory beds. Level of do. Objections to the general form of conserva- tory beds. Irregular method of laying out the interior of conservatories. This method illustrated in the conservatory at the Royal Botanic Garden, Re- gent's Park. Ground plan of a conservatory laid out in the irregular style. Advantages resulting from this method, 89 INDEX. 361 SECTION V. MATERIALS OF CONSTRUCTION. Workmanship. Bad foundations, &c. Temporary nature of horticultural erections. Consequence of bad constructed houses. Superior workmanship. Economy of building substantial houses, 99 Materials of const rueaon. Most suitable materials for building hot-houses. Metallic houses Superior to wood. Opposition to iron hot-houses. Objections raised. Objections answered. Expansibility of copper Of iron. Power of metals to conduct heat. Electricity an objection. Cost of iron hot-houses. Mr. Ressor's iron vinery. Horticultural structures in Europe of iron. Transportability of materials, &c., 101 SECTION VI. GLASS. The physical properties of transparent bodies. Glass of the palm-house at Kew. Report of Mr. Hunt, from Silliman's Journal of Science. Calorific influence of the glass chosen. Action of the non-luminous rays of light. Green glass of Melloni, 106 Evils consequent on -employing bad glass in hot-houses. Knotted and wavy glass. Its effects. Resources against bad glass. Painting and shad- ing the glass. Inconveniencies attending both these methods. Utility of using good glass. Propriety of manufacturers of glass making good mate- rial, 109 Glazing. Size of laps. Glazing roof-sashes,. Objectionable nature of broad laps. The most approved method of making laps. Curvilinear glaz- ing. Reversed curvilinear glazing. Puttying the laps. Glaring ridge aod furrow roofs. Anomalous surfaces, 110 Color of walls. Considerations in favor of a dark color. Influence of reflected light on dark walls. Retention of heat by dark-colored walls. Color of the rafters. Painting of the wood-work of the house with an anti- corrosive solution, 113 SECTION VII. FORMATION OF GARDENS. Form of the garden and disposition of the ground. Considerations neces- sary for fixing on the site. Walks. Entrance-walk. Formation of walks. Different kinds of walks. The durability and comfort of walks. Materials for the surface of walks. Form of the surface. Edges of walks, . . . .116 Borders and compartments. Width and size of do. General rule for lay- ing down borders. Size and number of compartments. Bad effects of small walks, . 119 Walls their 'ise. Forms of walls. Their height. Gardens of Mr. Cushing, at Watertown. Hot and flued walls. Wooden fences. Com- parative economy dt walls and fences, 121 362 INDEX. PART II. -HEATING. SECTION I. PRINCIPLES OF COMBUSTION. The nature and properties of fuel. Considerations on the subject. Char- acteristics in the use of coals pointed out. Result of the application of heat to coal. Disengagement of gas. Gases endowed with the power of giving out heat. Combustibility. What is combustion. The heating power of gas, 125 Inquiry into the combustion of coal gas. Doctrine of equivalents. Ob- servations of Mr. Parks. Disproportion between the volumes of the constituent parts. Different kinds of gases generated. Bulk of gases represented by figures 132 Atmospheric air. Its constituents represented by diagrams. The com- ponent parts of different gases represented by diagrams. Union of the con- stituents. Chemical law in relation to these gases. Carbon vapor, . . . 137 Formation of carburetted hydrogen. Excess and deficiency of heat-producing ingredients. The union of oxygen with smoke. Quantity of air required to supply the requisite quantity of oxygen. How ordinary furnaces are incapable of consuming coal perfectly. The complete combustion of bodies, .... 145 Argand lamp. Williams' smoke-preventing furnace figured and described. Jeffries' smoke-precipitating furnace figured and described. Their value considered. Application of these inventions in Europe. Methods of burning smoke, 148 Construction of furnaces. For heating large boilers. For making the fuel last a long time. Considerations necessary to be noticed in building the furnace. The kind of fuel to be consumed. Size and width of bars. Table for ascertaining the area of furnaces, 153 SECTION II. PRINCIPLES OF HEATING HOT-HOUSES. Effects of artificial heat. Changes produced by it. Animal and vegetable matter decomposed by it. Hydrogen eliminated by the decomposition of water. Experiments on the effects of heated air. Heat from brick flues. Iron radiators more injurious than others, 156 Laws of heat. Radiation and conduction. Combined effects of radiation. Proportion they bear to each other. Table showing the velocities of cooling at different temperatures. Experiments on cooling of iron pipes. Specific heat of air and water. Horticultural structures different from opaque build- ings. Causes of loss of heat, 158 Table showing the quantity required to heat given volumes of air. The effects of glass windows ascertained. Experiments on glass surfaces. Table showing the results. Specific heat of air and water. Application to hot- house buildings, 164 SECTION III. HEATING BY HOT WATER, HOT-AIR, AND STEAM. Practice of heating by hot water. Its merits considered. Temperature of hot- water pipes. Weight of steam. Calculations showing the superiority of hot-water pipes. Permanancy of heat by hot water, 167 INDEX. 363 Comparison of hot air with hot water, as a method of heating horticultural buildings. Air a bad conductor. Evaporating pans for supplying moisture. Considered in respect to motion in the atmosphere in respect to perma- nency of heating power. Water a better conductor. Experiments on air and water as modes of conducting heat, 171 SECTION IV. HOT-WATER BOILERS AND PIPES. Size of boilers, and surfaces necessary to be exposed to the fire. Adapta- tion of the boiler to the apparatus. Of the boiler and the quantity of water contained. The repulsion of heat by the metal of the boiler. Table showing the proportion the surface exposed to the fire must bear to the quantity of pipe, 176 Causes tending to modify the proportions to be adopted. Figures of boilers. Estimated action of the fire upon the boilers. Material for boilers, . . 179 Size and arrangement of hot-water pipes most suitable for the purposes of heating. Unequal rate of cooling in the various sized pipes. The ordinary methods of arranging hot-water apparatus. Advantage of taking the flue through the house. Laying down hot-water pipes. Expansion of pipes when heated. Supply cisterns, 181 Impediments to circulation. Causes of circulation. Amount of motive- power. Table showing the weight of water at different temperatures. Trifling cause renders an apparatus inefficient. Methods of increasing the motive-power. The rapidity of circulation in proportion to the motive- power 184 Level of pipes. Errors committed in the level of pipes. Circulation takes place first at the boiler. Methods of making water circulate in pipes below the level of the boiler, 188 Accumulation of air in pipes. Provision necessary for the escape of air. Want of attention to this the cause of failures. The size of air vents. Diffi- culty of finding the proper place to place the air vents, 190 SECTION V. VARIOUS METHODS OF HEATING DESCRIBED. Expense attending the ordinary methods of heating. Polmaise method of heating. Its adoption in houses in this country. Its origin. Means em- ployed to promote it in England. Description and figures of this method, . 192 A method of combining hot air and hot water together. Figured and de- scribed. Advantages of this method in the generation of heat and saving of fuel, 200 Compound method of heating. Seven ranges of houses heated by this method. Figure representing four houses heated by this plan. Figure of boiler and box. Of supply cisterns. Advantages of this mode of heating. Saving of fuel by it. Simplicity of working, 203 Tank methods of heating. Methods figured and described. Wooden and metallic tanks. The merits and properties of each. Utility and simplicity of do., . . .211 Fertilization of the atmosphere by tanks. Dissolving volatile gases in tanks. Their use in English nurseries for growing young stock. Their adaptation to amateurs, in small pits, 223 Representation of plant pits and description. Uses of these pits. Protec- tion of plants during winter in them, 226 Chambered vine borders. Argument in favor of them. Their utility under certain circumstances. Figure and description of a chambered border. Evidence in favor of them, 228 Cheap method of forming a chambered vine border. Comparison of cost of it with manure. Economy of their adoption. Method of managing them. Coverings of borders, 234 31* 364 INDEX. Construction of hot walls. Figure and description of hot wall. Various methods of building hot walls. Trial of hot walls, covered and uncovered. Foreign grapes may be grown on hot walls. Grapes produced on hot walls in England, 2iO New method of propelling heated air by means of machinery described by Mr. Marhock in Gardeners' Journal. The air propelled by means of a fan, 246 PART III. -VENTILATION. SECTION I. PRINCIPLES OF VENTILATION. Attention required from gardeners, &c. Its practical importance. Power of plants to withstand the^changes of climate. Power of vitality possessed by seeds. Power of plants to bear high temperatures. Of bearing delete- rious gases. Effect of winter-forcing on the odor of flowers and on the flavor of fruits, 248 Whether vegetation purifies the air. Opinions of Priestley of Dr. Dau- beny, of Oxford of Dr. Lindley, of London. Natural adjustment of the atmospherical elements. Atmosphere of cities. Benefits of large trees in the streets. New Haven, the effect of trees in it, 252 Power of plants to absorb carbonic acid. Gottingen springs. Property of charcoal for absorbing gases. Table of gases and the quantities absorbed by charcoal, 254 Power of plants to withstand the vicissitudes of temperature. Theories of physiologists. Dalton's chemical philosophy. His theory of the relations of the atmosphere to heat. The properties possessed by caloric, 256 SECTION II. EFFECTS OF VENTILATION. Effects of admitting cold air into a hot-house. Moisture carried away. Necessity of keeping the floors damp. Plants unlike animals in respect to ventilation. Ventilation not necessary as regards respiration. Air-tight glass cases for plants, 262 Knight's experiments on grape vines. The philosophy of this system. Evaporation of moisture on the glass. Contaminating gases in the atmos- phere. Experiments of Drs. Turner and Christison, 264 The abstraction of moisture in proportion to the rapidity of the motion of the air. Methods of counteracting this loss. Thermometric changes not sat- isfactory rules for the admission of air, 266 Quantity of moisture contained in the air. Its capacity for moisture. Estimated quantity of air escaped. Estimated quantity of moisture escaping in the air. Lofty plant-houses. Difficulty of managing the atmosphere in them, 269 SE CTION III. METHODS OF VENTILATION. Improvements of the present methods of ventilation. Plans adopted to modify the influence of draughts. Motion in the atmosphere. Machinery employed for this purpose. Detection of currents by a common candle. Propriety of a rapid motion disputed, 273 INDEX. 365 Difficulty of managing the atmosphere in large, dome-shaped houses. Covering necessary. To equalize the temperature. The natural law of equality ineffectual. The slightest cause disturbs the equilibrium of the air. The extreme sensibility of the air. Irregularity of its temperature in hot- houses. The causes of this irregularity. Experiments of Gay Lussac of Rudberg, 275 A new method of ventilation. Adapted to lean-to houses. Figured and de- scribed. Facility with which this method may be wrought, 277 Method of ventilating span-roofed houses. Adopted in the new hot-houses at Frogmore. Figures and description of this method, 279 Methods of airing by the rachet wheels. By springs. Superiority of the former. Necessity of having the machinery for ventilation properly erected. Its liability to get out of repair. Method applauded without merit. Neces- sity of guarding against the applauded inventions of any one, 280 SECTION IV. MANAGEMENT OF THE ATMOSPHERE. Atmospheric motion. Admitting large quantities of cold air. The results of this method. Questions arising out of these considerations. The quantity of air to be admitted. Motion affected by various circumstances. The atmosphere of a hot-house influenced by the glazing of the sashes. Effect produced by radiation. Growth of plants in Wardian cases. Deterioration of air by flues, &c., 284 Method of airing without opening the sashes. Figured and described. This method recommended for houses during cold weather in winter, . . . 288 Common method of ventilating figured and described. Evils resulting from this method. Action in cold weather, 290 Contrivance for admitting warmed air into the house over the heating appa- ratus. By a serpentine conductor. Size of the tubes necessary. Radiation of heat from the surface of the flue. The effects of the external air neutral- ized by this method, 292 The system of ventilation. Its object being to prevent a stagnation in the atmosphere. Evils of this method shown and explained. Mechanical and chemical effects of ventilation, 293 SECTION V. CHEMICAL COMBINATIONS IN THE ATMOSPHERE OF H OT-HOUSE S. Nourishment plants ought to receive from the atmosphere. How to receive it. Starch and sugar. Their different properties. Questions arising from considerations of their properties. Experiments on the atmosphere. The importance of oxygen to vegetable life, 296 Atmosphere from fermenting manure. Quality of heat generated by it. Impregnation of the atmosphere with ammonia. Experiments on the atmos- phere of a green-house with ammoniacal gas, . .299 Composition of ammonia. Excess of ammonia. Its suffocating influence. Illustrations of its effects. Fumigation of plant-houses and pits with ammonia. The cause of luxuriance in plants. Produced largely from fer- menting manure, &c., 300 What guides we have to ascertain the various changes in the atmospheric elements. Disagreeable smell on entering a hot-house. The cause, and how to remedy it. The important part played by oxygen in this process. Pro- portion of oxygen necessary to vegetables. Amount contained in atmospheric air and water. Affinity of its elements, 303 366 INDEX. Beautiful adaptation of the atmosphere to plants and animals. Effect of pure oxygen. Property of watery vapor in vegetable economy. Subtlety of the air. Necessity of maintaining an adequate supply of aqueous vapor in the atmosphere. Instruments for guiding us m regulating the atmosphere. In- struments much wanted for measuring the respective quantities of the gaseous elements, 305 SECTION VI. PROTECTION OF PLANT-HOUSES DURING COLD NIGHTS. Advantages of protecting bodies. Conditions of the plants at low temper- atures. Light coverings otherwise useful. Experiments on the cooling effects of wind. Materials for protecting glazed structures. Methods of protection, 309 Slight covering that is required to protect plants from frost. Experiments of Dr. Wells on coverings. Method of covering. Distance to keep the cov- ering from the object protected, ... 313 Effects of vertical coverings. Horizontal coverings. Coverings of straw, etc. Protection afforded by walls. Protection of snow. Warmth afforded by the soil to trees in winter, 31T SECTION VII. GENERAL REMARKS ON THE MANAGEMENT OF THE ATMOSPHERE OF HOT-HOUSES. Adjustment of the artificial to the natural atmosphere. Observations of Knight. Rest necessary to plants during night. Cause of the imperfect maturation of fruit-tree blossoms. High night temperatures exhaust the ex- citability of the trees. Plants continue longer in bloom in low night temper- atures. Admission of external air during day. Difference of climate be- tween this country and England, 320 Rules to be observed by the gardener in charge of hot-houses. Dutch meth- od of forcing. Excessive moisture its effects. Necessity for periods of rest to plants. Changing the period of fructification, 325 SECTION VIII. VENTILATION WITH FANS. Construction of ventilating fans. Methods of using them. Their adapta- tion to horticultural purposes. Different kinds of fans. Objects to be ef- fected by them. Requisites to the use of fans. Windmill ventilators. Their employment in horticultural buildings. Pump ventilators. Ventila- tion by means of chimney shafts, &c 329 UNIVERSITY OP CALIFORNIA LIBRARY, BERKELEY THIS BOOK IS DUB ON THE LAST DATE STAMPED BELOW R^nVa nnt returned on time are subject to a fine of 50c P er k vo?ute aS the third day oyerdug tocre^ng o $1 00 per volume after the sixth day. Books not in demand mly be renewed if application is made before expiration of loan period. AF 101931 MAY 31 If940 DEC 1 7 1379 NOV1219736 REC.C1R.JW 20 DEC 3 1978 REC. ClR. DEC 1 2 W ' 291979 -R&M iOV 2 3 1980 i OCT o 1 1980 50m-7,'29 667791 UNIVERSITY OF CALIFORNIA LIBRARY