THE LIBRARY 
 
 OF 
 
 THE UNIVERSITY 
 OF CALIFORNIA 
 
 PRESENTED BY 
 
 PROF. CHARLES A. KOFOID AND 
 MRS. PRUDENCE W. KOFOID 
 
AN 
 
 INQUIRY 
 
 INTO THE 
 
 CHANGES INDUCED 
 
 ON 
 
 ATMOSPHERIC AIR, 
 fa 
 

 * 
 
 Abernethy & Walker, Printers, 
 Old Bank Close. 
 
 
INQUIRY 
 
 INTO 
 
 THE CHANGES INDUCED 
 
 ON 
 
 ATMOSPHERIC AIR, 
 
 THE GERMINATION OF SEEDS, 
 
 THE VEGETATION OF PLANTS, AND 
 
 THE RESPIRATION OF ANIMALS. 
 
 BY DAJ&fifc ELLIS, 
 
 ? 
 
 SS. .- * 
 
 EDINBURGH : 
 
 PRINTED FOR WILLIAM CREECH ', 
 AND J. MURRAY, FLEET STREET, LONDON, 
 
 1807. 
 
 

 t flIA ^)TI 
 
 
Td 
 
 NICHOLAS NUGENT, M.D. 
 
 THIS TREATISE IS INSCRIBED, 
 
 IN TESTIMONY 
 OF RESPECT AND AFFECTION, 
 
 BY 
 
 THE AUTHOR. 
 
 

 
PREFACE. 
 
 . , L 
 
 
 ' ' 
 r 
 
 L HE investigation of the subject of the fol- 
 lowing Treatise Was suggested to the author by acci- 
 dentally observing the spontaneous recovery of an 
 animal in whom all the appearances of life had been 
 suspended by drowning. Reflecting on this part of 
 the pathology of respiration, and on the theories 
 which have been proposed to explain it, he was led 
 to consider with particular attention the physiology 
 of that function. The result of his inquiry termi- 
 nated in a conviction, that although many great 
 and important steps had been made, yet much hy- 
 pothetical conjecture was blended with established 
 fact, and many suppositions were admitted into our 
 theories which but ill accorded with the structure 
 and economy of the animal system. 
 
Vlll PREFACE. 
 
 By comparing attentively the chemical facts which 
 relate to this function with certain pathological ap- 
 pearances, and considering both in connection with 
 the actual structure of the respiratory organs, he was 
 induced, not only to reject the sufficiency of the ex- 
 planations which have been hitherto proposed, but 
 to form some opinions on the subject more conso- 
 nant, as it appeared to him, with the real designs of 
 nature. In the course of this investigation, analogy 
 readily suggested to him a comparison of the facts 
 ascertained in human respiration with those which 
 have been observed in the respiration of the inferior 
 animals ; and, from the lowest order of animal be- 
 ings, the transition to the analogous phenomena 
 which occur in the vegetable kingdom, was natural 
 and obvious. Thus, in a descending series, all the 
 great classes of animated nature were successively 
 brought under his review j and, arriving ultimately 
 at the most simple form of existence, he was led to 
 make it the first subject of investigation, and then 
 to retrace his steps through the more complex and 
 perfect forms of vegetable and animal life. 
 
 It has been the constant aim of the author to make 
 observation and experiment the basis of all his rea- 
 sonings, and to deduce his conclusions from a full 
 and distinct consideration of all the circumstances 
 which seemed necessarily to affect them. The facts 
 
PREFACE. *X 
 
 which he has introduced in support of these conclu- 
 sions have been collected from the best sources : and 
 where they appeared to him to be either erroneous 
 or defective, he has, in many cases, attempted to 
 correct or supply them by experiments of his own. 
 On every occasion of importance, where his results 
 differed materially from those of preceding authors, 
 he has endeavoured to compensate for the want of 
 experience by a frequent repetition of his experi- 
 ments : and, without regard to any particular opi- 
 nions, he has detailed their results with all the accu- 
 racy in his power. These results, it is hoped, will, 
 in general, authorise the conclusions which have been 
 drawn from them ; but, he is well aware, that many 
 points still remain which require farther elucidation^ 
 and are susceptible of more accurate proof. In the 
 present state of science, however, the physiologist 
 must often be content with probability, nor expect 
 to attain to demonstration but by very gradual ap- 
 proaches to truth. 
 
 oj.-\r < *W"il i i > ' ' * ( * * 
 
 Much attention has likewise been bestowed t<t 
 present the several facts, connected with the sub- 
 ject, in the order in which they have been suc- 
 cessively made known. By thus blending with 
 the recital of facts, the history of their gradual de- 
 velopment, an additional degree of interest is im- 
 parted to them, and an opportunity is, at the same 
 
X PREFACE* 
 
 jtime, afforded of doing justice to those eminent per* 
 sons who have preceded him in this branch of in- 
 quiry. To those also who feel a pleasure in tracing 
 the progress of the mind through the details of an 
 extensive and complicated investigation, it may be 
 interesting to observe, in what a slow and successive 
 manner the knowledge which we at present possess 
 has been acquired : how many individuals, often at 
 very distant periods, have been employed in adding 
 to it : and how many observations, apparently, at 
 first, trivial and insulated, have, in the progress of 
 science, assumed an unexpected importance, serving 
 to connect together a long series of facts, which, but 
 for such casual aid, might still have remained dis- 
 jointed and broken. Reflections such as these, 
 while they encourage every one to contribute his 
 share, however small, to the general stock of know- 
 ledge, may, at the same time, serve to abate some- 
 what of that unpardonable vanity which has led some 
 philosophers to assume the sole merit of having 
 raised the edifice, when, in fact, the foundation was 
 entirely laid by others, and they themselves have on- 
 ly assisted in arranging and giving form to the ma- 
 terials of which the superstructure may hereafter be 
 composed. 
 
 Without entering into formal and minute descrip- 
 tion, it has been a principal object through the 
 
PREFACE Xi 
 
 whole of this inquiry, to combine anatomical fact 
 with those reasonings which relate immediately to 
 the living system. Unfortunately for physiology, 
 this circumstance has not always been sufficiently 
 kept in view, and the laws which govern the move- 
 ments of inanimate matter have too frequently been 
 applied, without reserve or discrimination,' to explain, 
 the functions of organized and living beings. To a 
 certain extent, indeed, animated bodies are subject 
 to the same laws as all other material substances; 
 but the peculiar properties which they derive from, 
 the principle of life can never be explained on me- 
 chanical or chemical principles. Although, there- 
 fore, it will be found, that chemical phenomena have 
 received a due share of attention, yet they have at 
 all times been held in subservience to the establish- 
 ed truths of anatomy, and to those laws which pe- 
 culiarly belong to bodies possessed of the principle 
 of life. 
 
 In the present Treatise, no reference whatever 
 has been made to the theories which have been 
 proposed to explain the phenomena of vegetation 
 and respiration. Such theories, it is evident, must 
 rest entirely on a" knowledge of the changes pro- 
 duced by living bodies on the air ; and it is the 
 professed object of this inquiry to examine, with 
 more attention than has yet been done, into the na- 
 
Xli PREFACE. 
 
 ture and extent of these changes. It will be proper, 
 therefore, in the reader to abstract as much as possi- 
 ble from any preconceived opinions on these sub- 
 jects ; and to consider, that it is only by a scrupu- 
 lous examination of the facts adduced, and of the le- 
 gitimacy of the conclusions drawn from them, that the 
 views of the author can be finally established pr over- 
 thrown. To try the merits of these views, not by 
 the evidence on which they rest, but by anticipating 
 difficulties which they may be supposed hereafter to 
 create, would be an open departure from the only 
 mode in which we can ever hope faithfully to inter- 
 pret the laws of nature. 
 
 Whatever may be the ultimate judgment passed 
 upon the general merits of his performance, the au- 
 thor indulges the hope, that some advantage at least 
 may indirectly result from it. The extensive series 
 of facts which he has brought together, and the 
 analogies which, from the evidence of experi- 
 ment, he has endeavoured to trace anjong them, 
 may direct the attention of future inquirers to a 
 more comprehensive view of the subject than has 
 yet been taken, and impart a new degree of inte- 
 rest and utility to the research. The attempt also 
 to combine, in every instance, the demonstrations of 
 anatomy with the chemical phenomena which we ob- 
 serve, and to Consider both in connection and sub- 
 
PREFACE, Xill 
 
 servience to the laws which characterise living be- 
 ings, will, he trusts, meet with the approbation of 
 physiologists ; and tend to reduce within proper li- 
 mits the application of chemistry to this science. 
 At any rate, the present inquiry may serve to recall 
 to a new and more accurate examination, the facts 
 on which our knowledge of these subjects is founded ; 
 and in the present state of science, such an exami- 
 nation cannot fail to dispel much of the error and 
 obscurity in which they are still invplved. 
 
 EDINBURGH, ") 
 MARCH 20, 1807,) 
 
CONTENTS, 
 
 CHAP. Page 
 
 I. Of the Changes induced on the Air by the Germination of 
 
 Seeds, 1 
 
 II. Of the Changes induced on the Air ly the Vegetation of 
 
 Plants, 27 
 
 III. Of the Changes induced on ths Air by the Respiration of 
 
 Insects, Worms, Fishes, and Amphibious Animals, 61 
 
 iy. Of the Changes induced on the Air by the Respiration of 
 
 Birds, of Quadrupeds, and of Man, .... 95 
 
 V. Of the Source of the Carbon in Vegetables and Animals 
 
 ly 'which the Changes in the Air are effected, . 166 
 
 VI. Of the Phenomena which arise from the Changes induced 
 on the Air by the living Functions of Vegetables and 
 Animals . . 205 
 
 ERRATA. 
 
 Page 17 line 29 for injure read injures 
 
 20 1 1 renders render 
 
 58 7 air water 
 
 82 8 dele of this class. 
 
CHAP. I. 
 
 OP THE CHANGES INDUCED ON THE A.IR 
 BY THE GERMINATION OF SEEDS. 
 
 i. IT is a well-known fact, that dried seeds, 
 although exposed to heat and air, may be kept for a 
 great length of time without undergoing any sensi- 
 ble change ; nor do they in that state, as we found 
 by experiment, in the least degree affect the air in 
 contact with them. If, however, moisture have ac- 
 cess to them, they presently begin to swell, which is 
 the first step towards their evolution. This hap- 
 pens whether they be placed in water containing air, 
 or in that which has been previously deprived of its 
 air by long boiling ; for equal numbers of the same 
 peas, at the temperature of 60 Fahrenheit, being put 
 into separate phials of fresh and of boiled water, and 
 then corked and sealed, exhibited the same appear* 
 ances, and made a like progress. In a few hours 
 the seeds in both phials were much enlarged, and by 
 the next day greatly swollen : the radicles of many 
 
 A 
 
began to unfold, and they afterwards grew well, if 
 duly exposed to the air ; but when retained in this 
 situation for a few days, their outer coats began to 
 separate ; the water in each phial emitted a putrid 
 smell, and, on being added to lime-water, threw 
 down a copious precipitate, after which germination 
 did not take place. Mr Gough ascertained, that af- 
 ter five days steeping in water at 46, and even in for- 
 ty-eight hours, at temperatures from 60 to 66, pu- 
 trefaction in seeds came on, under which carbonic acid 
 and carburetted hydrogen gases were produced, and 
 the faculty of germinating was then destroyed *. 
 M. Huber found also, that both in boiled and distil- 
 led water, a small degree of germination might be 
 made to take place in peas ; but during their submer- 
 sion, the radicles never increased beyond three or 
 four lines, after which they began to decompose, 
 carbonic acid being first produced, and afterwards 
 carburetted hydrogen gas. The whole substance of 
 the pea, with the exception of its membranes, is, ac- 
 cording to Saussure, reduced into these two gases f- 
 It is evident from these facts, that the presence of 
 water alone, is essential to the commencement of 
 germination ; and that if its application be too long 
 and exclusively continued, it disposes to putrefac- 
 tion. 
 
 2. Besides water, a certain degree of heat is ne- 
 cessary also to the germinating process ; for no seed 
 
 * Manchester Memoirs, vol. iv. 
 
 f Mem. stir la Germination, par M. M. Huber et Sennebier. 
 p. 128, 157. 
 
3 
 
 can be made to grow at or below the freezing point, 
 and above that point, the degree of growth will be 
 always more or less influenced by that of tempera- 
 ture. Many seeds, however, will germinate as well 
 as ever after having been frozen, or having been 
 kept in frozen water. Although cold, therefore, 
 does not destroy the germinating faculty of seeds, 
 yet a certain degree of heat is essential to the dis- 
 play of it, and almost every species of seed seems 
 to require a degree peculiar to itself ; for each has 
 its peculiar season of germination, which season va- 
 ries with the temperature of the air. 
 
 3. Light is another agent which has been suppo- 
 sed to possess a considerable influence in the germi- 
 nation of seeds. The experiments of Hooke, Scheele, 
 and Herschell, have established the distinction be- 
 tween the heating and illuminating powers of the 
 rays of light : and a third portion of these rays has 
 been supposed to enter more peculiarly into combi- 
 nation with various substances, and thence have ac- 
 quired the denomination of the Chemical Rays. Dr 
 Jngenhousz and M. Sennebier considered the presence 
 of light to impede the growth of seeds ; but the 
 Abbe Bertholin affirmed, that the difference in the 
 germination of seeds in the shade and in the light 
 was owing not to the light itself, but to the greater 
 evaporation of moisture which seeds exposed to the 
 light suffered : and he added, that if the seeds in 
 both cases were kept equally moist, those in the sun 
 germinated sooner than those in the shade. M. Sen- 
 nebier, however, repeated his experiments, and em- 
 ployed every precaution to ensure an equality of 
 moisture in both situations, and he constantly found 
 A2 
 
the seeds in the shade to germinate soo'ner than those 
 in the light *. The more equable warmth which 
 seeds preserve, when excluded from the direct con- 
 tact of light, may be a principal reason why they 
 grow best in the dark ; and the evaporation which 
 takes place on exposure, to which the Abbe al- 
 ludes, would not only deprive the seed of the wa- 
 ter essential to its growth, but be at the same time 
 a considerable source of cold. In many trials we 
 found, that steeped seeds, when exposed in the open 
 air, rapidly lost their moisture, and made no effort 
 to germinate ; but the same seeds, when confined in 
 a glass-vessel, so as to check the progress of evapora- 
 tion, germinated freely, though exposed to the same 
 temperature, and the same degree of light. Hence 
 we infer, that light, simply, has little influence either 
 in promoting or retarding the germination of seeds. 
 
 4. But although water and heat appear to be the 
 only agents essential to the beginning of germina- 
 tion, yet after a certain period air becomes equally 
 necessary. Mr Boyle, Ray, and many other philo- 
 sophers, ascertained that seeds would not grow in 
 the vacuum of an air-pump.. Some lettuce-seeds 
 were sown upon earth in the open air, and some of 
 the same seeds, at the same time, on another portion 
 of earth placed in a glass-receiver, which was after- 
 wards exhausted of air. The seeds exposed to the 
 air grew nearly an inch and a half high in eight 
 days ; but those in the exhausted receiver not at all. 
 Air, however, being now admitted into the receiver, 
 
 * Thomson's System of Chemistry, vol. iv. p. 236. 1st Edit. 
 
it was found, that in the space of >ne week the seeds 
 had grown to the height of two or three inches *. 
 Mr Gough observed, that steeped seeds put into a 
 close vessel so as to fill it, did not grow, but that 
 when a smaller number v/as put into the vessel, and 
 its mouth covered loosely with a piece of glass, so 
 that the air had free access to the seeds, they ger- 
 minated very freely ; experiments which we have 
 often repeated, and always with the same results. 
 Since, then, air is essential to the progress of germi- 
 nation, and it is known that this elastic fluid consists 
 of two elementary parts, we may next inquire what 
 changes this air undergoes, and whether one only, 
 or both its parts, be required in this process. 
 
 5. It appears from the experiments of Mr Achard 
 and others, that no seed will germinate in nitrogen 
 gas, which gas forms nearly four-fifths of the air of 
 the atmosphere. Mr Gough placed some barley- 
 seeds, previously steeped in water, in a jar of this 
 gas. At the end of three days they had suffered no 
 change ; the bulk of gas was not perceptibly altered, 
 nor did it diminish by agitation in water, which 
 showed that no carbonic acid was formed : but when 
 the place of the nitrogen gas was supplied by atmos- 
 pheric air, the same seeds germinated freely, and 
 formed carbonic acid, just as if they had ne- 
 ver been confined in nitrogen gas at all. If, how- 
 ever, they remained three or four days in this gas 
 when the weather was warm, they began to putrefy 
 in the same manner as when retained too long in 
 
 * Lowthorpe's Abrid. Phil. Trans, vol. ii. p. <2Q6 f 
 A3 
 
Water, carbonic acid and carburetted hydrogen gases 
 being formed, after which they could not be brought 
 to germinate *. Mr Cruickshank introduced some 
 soaked barley-seeds into a jar containing nitrogen 
 gas, inverted over mercury. At the end of twelve 
 or fourteen days there was not the least appearance 
 of germination, but the gas had increased in bulk a* 
 bout one-fifth, containing from one-third to one- 
 fourth of its bulk of carbonic acid ; but neither the 
 original nitrogen gas nor the barley had undergone 
 any sensible change f- M. Huber found also, that 
 a quantity of carbonic acid was produced when seeds 
 were placed in a jar of nitrogen gas, and duly sup* 
 plied with water ; and that in a few days the radi- 
 cle in a small degree was protruded, which he con- 
 siders as a proof of germination];. But it appears that 
 the same protrusion of the radicle takes place when 
 seeds are submerged in water ; and M. de Saussure, 
 who repeated these experiments in pure nitrogen gas 
 inverted over mercury, observes, that the radicle e- 
 longatcd only in a small degree, which is the con- 
 sequence of the swelling of all the parts of the seed 
 from the imbibition of water, and must not be con- 
 sidered as a true germination . From these facts, 
 therefore, it may be concluded, that although seeds, 
 after being steeped in water, yield carbonic acid 
 when confined in nitrogen gas, yet that they are in 
 no respect affected by it ; neither does that gas itself 
 undergo any sensible alteration. 
 
 * Manchester Memoirs, vol. iv. p. 322. 
 
 f Experiments on Sugar, in Rollo on Diabetes, vol. ii. p. 218. 
 
 :f Sur la Germination, p. 175. Ibid. p. 184. 
 
& As thus the nitrogenous portion of the air seems 
 neither to produce nor to suffer any change in the 
 process of germination, let us next inquire into the 
 changes which the oxygenous part, the other ele- 
 mentary portion of our atmosphere, undergoes. Mr 
 Scheele and Mr Achard proved that oxygen gas 
 was absolutely necessary for the germination of aH 
 seeds. Mr Gough likewise observed, that steeped 
 barley-seeds, supported by means of a small hoop 
 in an inverted jar of atmospheric air, germinated 
 freely : the residual air, on the fifth day, being 
 passed through lime-water, precipitated the lime, 
 and, when thus freed from carbonic acid, was found 
 to have lost nearly one-sixth of its original bulk, and 
 the remaining air repeatedly extinguished a taper *. 
 Some grains of barley, which had been soaked in 
 water twenty-four hours, were introduced by Mr 
 Cruickshank, on the 1st of December, into a jar of 
 common air inverted over water, and preserved in a 
 temperature between 60 and 70. At the end of 
 five days they began to grow, and on the 28th day, 
 the greatest part of them had thrown out shoots half 
 an inch in length. Germination continued to pro- 
 ceed on the 7th of February, when the barley being 
 withdrawn, was found to be very sweet, and nearly 
 converted into the state of malt. The air in the jar 
 was found to consist only of nitrogen and carbonic 
 acid gases, the whole of the oxygen having entirely 
 disappeared. When the experiment was made in a 
 jar containing forty -six measures of very pure oxy- 
 gen gas inverted over mercury, the process of ger- 
 
 Manchester Memoirs, vol. iv. p. 319, 
 
8 
 
 initiation was considerably hastened : the column of 
 gas on the 10th day had suffered no apparent 
 change ; and the residual air then consisted of car- 
 bonic acid, mixed with only l-50th of its bulk of 
 oxygen. The barley was partly converted into malt * . 
 These experiments teach us, that oxygen gas is es- 
 sential to the process of germination ; that it gra- 
 dually and completely disappears during the conti- 
 nuance of that process ; and that a large quantity of 
 carbonic acid supplies its place. 
 
 7. M. de Saussure junior, instituted some experi- 
 ments to determine the proportion which the car- 
 bonic acid, produced in germination, bore to that of 
 the oxygen gas, which disappeared in that process. 
 He conveyed eighteen peas into a glass-vessel, the 
 mouth of which was plunged in mercury, and which 
 contained 11.5 cubic inches of atmospheric air, that 
 had been previously well washed in lime-water. 
 About one-fourth of a cubic inch of water was then 
 passed into the jar, and floated on the mercury, so 
 that the peas were about half immersed in it. In 
 ten days the radicles had sprouted about one-third 
 of an inch ; and the air in the jar, after making the 
 necessary corrections for pressure and temperature, 
 had undergone no sensible diminution of volume. 
 The residual air being submitted to analysis, lost 
 i|o by agitation with lime-water, and the remain- 
 der afterwards suffered a further loss of - by the 
 eudiometrical test of phosphorus. If the air he em- 
 ployed be held to contain ~ of oxygen gas, (which 
 
 Experiments on Sugar, p. 213. 215. 
 
9 
 
 that of his laboratory was found to do), the 11.5 
 cubic inches of it which he used in this experiment, 
 will, he says, consist, before the introduction of the 
 peas, of 3.105 of oxygen, and 8.395 of nitrogen : 
 and at the close of the experiment he states the com- 
 position of the air at 1.88 oxygen, 8.395 nitrogen, 
 and 1.035 carbonic acid ; consequently 1.255 of 
 oxygen were employed to form 1.035 cubic inches 
 of carbonic acid, a result as near to truth as can be 
 expected in observations on small volumes of air. 
 In other experiments made over water, nearly the 
 same results are said to have been obtained *. But 
 some error seems to have crept into the calculations 
 of M. de Saussure ; for, if with him we consider 
 the atmosphere as containing ^ of oxygen gas a 
 11.5 cubic inches of air will contain 2.415 of oxy- 
 gen, and 9.085 of nitrogen ; proportions very dif- 
 ferent from those which he has assigned, and which 
 necessarily affect all the subsequent calculations, and 
 the conclusions he has drawn from them. 
 
 8. With the view, therefore, of ascertaining this 
 point with more precision, we placed one dozen of 
 peas, which had been steeped in water thirty-six 
 hours, and whose surfaces were dried afterwards by 
 blotting paper, on a small whalebone hoop covered 
 with gauze, which was pushed half way up into a 
 glass-jar well dried, and of the capacity of 18.2 cu- 
 bic inches. This jar, filled with atmospheric air, 
 was then inverted into a deep saucer, where it stood 
 over a small glass-cup, containing half a cubic inch 
 
 Journal de Physique, torn, xlix. p. 92. 
 
10 
 
 of the xvater of potassa, which is well known ta 
 have a strong affinity for carbonic acid. A quantity 
 of mercury was now poured into the saucer, and 
 the jar was kept steady by a weight laid upon it. 
 The whole was then set aside in a room, where the 
 barometer stood at 29.3 inches, and the thermome- 
 ter at 62.5. By the following day, the peas had 
 sprouted about l-4th of an inch, the mercury in the 
 saucer had risen 3-10ths of an inch into the jar ; 
 and as the small cup with its- solution floated on the 
 mercury, it was necessarily raised into the jar with 
 it. By the end of the second day, the radicles were 
 S-4ths of an inch long, and the mercury had risen 
 nearly 7-10ths of an inch into the jar, as appeared 
 by a small paper scale, graduated to inches and 
 tenths, and pasted on the outside of the jar. At the 
 close of the third day, the radicles were to appear- 
 ance about an inch long, and the mercury in the 
 jar was nearly an inch in height. And by the fourth 
 day, the radicles had made little additional progress, 
 but the mercury stood nearly 1.1 inch in the jar. 
 During the following day, no apparent changes oc- 
 curred, and the mercury in the jar was now exactly 
 1.1 inch in height, when that in the saucer was 
 brought to a level with it. The barometer at this 
 period was 29.95, and the thermometer 65. 
 
 9. Having by other experiments ascertained, that 
 when the mercury had reached this elevation, and 
 became stationary, the oxygen gas of the air em- 
 ployed had completely disappeared, we proceeded 
 to determine the proportion which this diminution 
 In the bulk of air, evinced by the rising of the mer- 
 cury, bore to that of the oxygen gas which the air 
 
11 
 
 originally contained. The jar, therefore, was cau- 
 tiously raised out of the mercury, and the attraction 
 of carbonic acid by the alkaline solution was proved 
 by the brisk effervescence which the acetous and di- 
 luted sulphuric acids excited in it. The glass-cup, 
 containing a bulk of water equal to that of the solu- 
 tion employed, was now again put into the jar, pre- 
 viously replaced on its bottom, together with the 
 hoop and peas. Water was next poured in till it 
 reached that part of the jar to which the mercury 
 had risen^ and to raise it thereto required J2.5 cubic 
 inches, which, therefore, was the bulk of residual 
 air at the close of the experiment ; and afterwards, 
 to fill the jar completely, it required four cubic 
 inches more, which consequently was the measure 
 of the volume of air that had disappeared. But 
 these two quantities amount only to 16.5, and the 
 capacity of the jar has been stated to be 18.2 cubic 
 inches: the difference, therefore, equal to 1.7, will be 
 the space which the several substances occupied with- 
 in the jar, and which reduces the actual volume of 
 air employed in the experiment to 16.5 cubic inches, 
 'of which a portion equal to four, or ^ of the 
 whole, had disappeared. 
 
 10. But the volumes of elastic fluids are inversely 
 as the weights by which they are compressed ; and 
 for every degree of Fahrenheit's thermometer, Mr 
 Dalton has found that atmospheric air is dilated ~ 
 part of its bulk under common atmospheric pres- 
 sure*. Making therefore, on these accounts, the 
 
 * Manchester Memoirs, vol. v. 
 
necessary reductions, we have '' 12.777; 
 but ^| - ? = -0661 and 12.777 .0661 = 
 12.7109, which is the actual volume of air at the 
 end of the experiment when reduced to the same 
 pressure and temperature as it possessed at the be- 
 ginning. To find, farther, the proportion which 
 this loss of bulk bears to that of the whole air em- 
 ployed, we have 16.5 12.71O9 = 3.7891 and 
 ^~zz 4 ~ 5 - But atmospheric air contains ~ zz --- 
 
 of oxygen gas, so that the loss in the bulk of air by 
 this experiment is rather greater than the propor- 
 tional quantity of oxygen gas which the atmosphere 
 contains. In another trial made in the same jar, 
 and under similar circumstances, the loss of bulk 
 which the air was found to have suffered, after the 
 necessary reductions wer : made, was ^ l - of the 
 whole. Hence, therefore, if it be granted that the 
 loss of bulk which the air suffered arose from the 
 r.ttraction of the carbonic, acid by the water of pot- 
 assa ; and if it be also allowed, that all the oxygen 
 gas of the air employed had completely disappeared, 
 it must be concluded, that the bulk of carbonic acid ' 
 produced nearly corresponded to that of the oxy- 
 gen gas lost. This, too, will appear more directly, 
 by comparing the diminution which a given bulk of 
 air suffers in germination, with the proportion of 
 oxygen gas which the atmosphere contains : Thus, 
 10O : 22 :: 16.5 : 3.63, which comes near to 3.78, 
 the actual loss of bulk which the air in the foregoing 
 experiment underwent. 
 
13 . 
 
 11. That the whole loss of bulk which the air 
 suffered in the foregoing experiments, did actually 
 arise from the formation of carbonic acid, and its 
 subsequent attraction by the alkaline solution, ap- 
 pears almost certain, from the opposite results ob- 
 tained when no such solution was employed. In 
 the experiment of Mr Cruickshank, (6.), made in 
 pure oxygen gas, the original" volume of gas suffer- 
 ed neither increase nor diminution, although it was 
 almost entirely converted into carbonic acid ; and 
 the same observation was made in all the experi- 
 ments of Saussure. In a great number of experi- 
 ments also, we observed, that where no water of 
 potassa was placed within the jar, the seeds equally 
 grew ; the oxygen gas was completely destroyed, 
 and the residual air lost about l-5th of its bulk by 
 agitation in lim^-water, although the volume of air 
 was only in a small degree diminished. This trifling 
 diminution was probably owing to the necessary 
 condensation which oxygen gas experiences when 
 it combines with carbon, and which, according to 
 Crawford and Guyton, amounts to about l-7th of 
 the bulk of gas employed % although it is probable 
 that this estimate is considerably overrated. 
 
 12. We endeavoured to ascertain the amount of 
 this diminution in germination, by causing some 
 soaked peas to grow, supported on a whalebone 
 hoop, in a small flask with a narrow neck, which 
 was inverted into an ale-glass containing mercury. 
 When the peas had ceased to grow, the rise of the 
 
 Murray's "System of Chemistry, vol. ii. p. 352. 
 
14 
 
 mercury into the neck of the flask was accurately 
 marked, and the air was then analyzed. The de- 
 struction of its oxygen gas was found to be com- 
 plete, and the usual quantity of carbonic acid to be 
 formed. After deducting the space occupied by 
 the seeds and the hoop which supported them, the 
 volume of air, at the close of the experiment, was 
 compared with that which it possessed at the begin- 
 ning, and the difference noted. An estimate was 
 then made of the quantity of oxygen gas originally 
 contained in the air employed, and the loss of bulk 
 was considered to arise from the condensation at- 
 tending its conversion into carbonic acid, due atten- 
 tion being paid, at the same time, to corrections for 
 pressure and temperature. The average amount of 
 the diminution varied from 1-Sth to 1-1 2th ; or 
 the volume of carbonic acid produced was about 
 1-1 Oth less than that of oxygen gas which had dis- 
 appeared. In some trials the amount of the diminu- 
 tion was much greater, but this was found to depend 
 on the quantity of moisture given out by the seeds, 
 and the more or less vaporific state in which it ex- 
 isted, and which source of error can be obviated on- 
 ly by reducing the air to the same state with regard 
 to moisture at the end, as at the commencement of 
 the experiment. From the whole, however, we 
 may conclude, that a certain condensation attends 
 the conversion of oxygen gas into carbonic acid in. 
 the process of germination, as well as in combus- 
 tion ; and consequently that a part of the diminu- 
 tion which the air suffers, is to be attributed to this 
 cause. If this were not the case, the bulk of carbo- 
 nic acid produced would- appear greater than that of 
 
15 
 
 oxygen gas lost, which the experiments both of 
 Cruickshank and Saussure forbid us to believe, and 
 which, in all our own experiments, appeared to be 
 somewhat less. 
 
 13. If, then, we may be permitted to suppose, 
 that in the process of germination the oxygen gas 
 of the air loses about 1-1 Oth of its bulk by passing 
 into carbonic acid, it follows, that the 3.78 cubic 
 inches of this acid, which appeared to be attracted 
 in the foregoing experiment, (10.), will actually 
 consist only of 3.4O2 cubic inches. Now, one cu- 
 bic inch of carbonic acid weighs 0.467 of a grain, 
 and therefore 3.402 cubic inches will weigh 1.589 
 grains, which may be taken as the weight of the 
 carbonic acid actually produced. But farther, a cu- 
 bic inch of oxygen gas weighs O.3474 of a grain; 
 and therefore 3.62 cubic inches (the bulk of that 
 gas present in the experiment) will weigh 1.261 
 grains, which is less by 0.328 of a grain than the 
 weight of the carbonic acid produced. This greater 
 weight of the acid, therefore, must be attributed to 
 the carbon which united with the oxygen gas to 
 form it, and which makes the carbon to constitute 
 of the compound. According to Lavoisier, car- 
 bonic acid contains -^ zz ~~ of carbon ; and ac- 
 cording to Dr Priestley, the weight of the carbon is 
 about l-4th of the compound* : but Guyton, who 
 formed this acid directly by the combustion of dia- 
 mond, found that one part of diamond combined 
 
 Experiments on &ir Abridged, vol. iii. p. 37' 
 
16 
 
 with a little more than four of oxygen, and conse- 
 quently .that the proportion of carbon was only 
 l-5th of th'e compound*. Hence, therefore, if 
 these experiments and calculations be correct, the 
 quantity of carbon, which enters into the carbonic 
 acid formed by germination, is rather greater than 
 the experiments of Guy ton assign as the proportion 
 of that substance which forms this acid in combus- 
 tion. 
 
 34. M. Huber made experiments with the view 
 of ascertaining the composition of the atmosphere 
 best suited to carry on the germinating process. 
 He found, that an artificial atmosphere, composed 
 of the same constituent parts as common air, favour- 
 ed it precisely in the same manner ; that germina- 
 tion was retarded in proportion as the nitrogen gas 
 exceeded the usual quantity of that gas contained in 
 atmospheric air, but was little affected when the va- 
 riation was small. A superabundance of oxygen 
 gas also, although not destructive to germination, 
 nevertheless retarded the process ; for seeds were 
 found to germinate better in a mixture formed of 
 three parts nitrogen and one of oxygen, than in one 
 composed of three of oxygen mixed with one of ni- 
 trogen f. Hydrogen gas, in the course of these ex- 
 periments, was often substituted for nitrogen, and 
 answered precisely the same uses, germination be- 
 ing more vigorous in proportion as the hydrogen 
 came near to that of the nitrogen existing in the at-* 
 
 * Annales de Chimie, torn. xxxi. p. 108. 
 
 f Sur la Germination, p. 31. ct secj. 
 
17 
 
 mosphere ; but neither in pure hydrogen nor nitro- 
 gen did the process at all go on *. 
 
 15. M. Huber proceeded next to ascertain the 
 smallest quantity of oxygen that would suffice for 
 germination. He found, that seeds did not grow in 
 very small volumes of air ; and that the greater the 
 number of seeds confined in a given volume of air, 
 the more was their growth retarded ; that where the 
 oxygen gas constituted only one-eighth of the atmos- 
 phere, no effect was produced on lettuce-seeds, but 
 that one-sixth of it began to promote their growth. 
 Lefevre, however, is said to have found, that an at- 
 mosphere containing only ~ part of oxygen, favour- 
 ed the evolution of raddish-seeds, which renders it 
 probable that different seeds vary much in the pro- 
 portion of oxygen necessary to carry on their ger- 
 mination | When the atmosphere was impregna- 
 ted with the vapour of sulphuric ether, of camphor, 
 spirits of turpentine, assa-fcetida, vinegar and am- 
 monia, the process of germination was more or less 
 retarded. From the facts stated in this and the pre- 
 ceding paragraph, we learn, not only that air is ne- 
 cessary to the growth of seeds, but that the ordi- 
 nary state of its composition in our atmosphere is, 
 as M. Sennebier observes, the best adapted to pro- 
 duce a vigorous germination. This state, however, 
 is not determined with such rigour that a small va- 
 riation in the proportions or qualities essentially in- 
 jure the process : it admits, on the contrary, of great 
 latitude without much disturbance, which provision 
 
 * Sur la Germination, p. 58. f Ibid. p. 64% et seq. 
 
 B , 
 
18 
 
 affords a correction for those occasional variations 
 which our atmosphere experiences, and insures the 
 preservation of vegetables in all circumstances in 
 which they may naturally be placed *. 
 
 16. Such being the principal factfc in germination 
 which bear a relation to the changes which the air 
 suffers, let us next inquire in what manner they are 
 to be accounted for or explained ; and first of all, 
 what becomes of the oxygen gas that disappears ? 
 Mr Gough believes that seeds, during their germi- 
 nation, absorb oxygen gas from the air, retain a part 
 of it, and reject the remainder, charged with car- 
 bon f. Now although seeds have a power of im- 
 bibing and exhaling water, there is no evidence of 
 their possessing a structure fitted to absorb and expel 
 aeriform fluids ; neither is there any proof of such 
 fluids at any time existing in them. If the air be absor- 
 bed in its entire form, some receptacles ought to be 
 shown in the seed for retaining and decomposing it,. 
 and from which the nitrogen gas could be after- 
 wards expelled : or, if the oxygen gas alone be con- 
 ceived to enter, then it may be asked, by what 
 power of absorption it could be separated from the 
 nitrogen gas with which it was previously combined ? 
 Mr Cruickshank inclines also to the belief, that the 
 oxygen gas is chiefly absorbed by the seed, although, 
 part may, he says, be consumed in the formation gr 
 the carbonic acid \ ; and we learn from his experi- 
 
 * Sur la Germination, p. 119. et seq. 
 
 f Manchester Memoirs, vol. iv. p. 319. 
 
 J Experiments on Sugar, in Rollo on Diabetes. 
 
19 
 
 ments, (6.), that when seeds are made to grow in 
 oxygen gas, the bulk of carbonic acid produced is 
 equal to that of the oxygen gas which disappears. 
 Even granting, therefore, that a portion of this gas 
 is absorbed by the seed, this equality of proportion in 
 the carbonic acid produced forbids us to consider any 
 part of it to be retained ; and if it be admitted, that 
 the formation of this acid is in part effected without 
 a previous absorption of oxygen, why should we not 
 allow it to proceed to the fullest extent in which 
 it takes place, rather than have recourse to two such 
 opposite suppositions to account for the same pheno- 
 menon ? 
 
 17. To suppose this oxygen gas to be taken up 
 by the seed by the operation of chemical affinity, 
 necessarily implies its previous separation from the 
 nitrogen gas with v hich it was united ; but how 
 could this be done, unless the seed presented some- 
 thing to the air which had a stronger affinity for its 
 oxygen than the nitrogenous portion has ? And what 
 could it offer but moisture and carbon ? Moisture, 
 however, does not decompose air ; and if carbon be 
 the agent, must not carbonic acid be at once formed ? 
 And if this acid be thus formed, exterior to the 
 fceed, and out of the oxygen gas in contact with it, 
 bow can we hold that gas to be first singly taken in 
 by the seed, and expelled afterwards in the form of 
 carbonic acid ? To say that the air is attracted in 
 its undecomposed state, necessarily requires proof of 
 (he existence of certain cavities in the seed where it 
 can be retained ; for as the nitrogen gas neither 
 suffers (5.) nor produces change, it must be com- 
 pletely expelled after the oxygen is abstracted from 
 B2 
 
20 
 
 it. Lastly, M. de Saussure has endeavoured to 
 show, that the carbonic acid formed in germination 
 contains in it precisely the quantity of oxygen gas 
 that has disappeared : And although, from the dif- 
 ference of opinion which prevails concerning the ac- 
 tual proportions of the elements which constitute 
 that substance, this cannot be positively assumed ; 
 yet the near proportions which, in our own experi- 
 ments, as well as in those of Saussure and Cruick- 
 shank, the two gases bear to each other at the be- 
 ginning and end of the process, renders it extreme- 
 ly probable. If this opinion be well founded, no 
 part of the oxygen can be retained by the seed ; and 
 we may conclude, therefore, with M. de Saussure, 
 that none of it is either attracted or absorbed *. 
 
 1 8. If indeed oxygen gas were in part retained by 
 seeds, we might expect, since they deteriorate so 
 large a portion of it, that they would acquire weight 
 as well as other substances with which the base of 
 that gas combines. But Mr Gough found, that 
 steeped peas germinating in air, and sprouting to the 
 length even of two inches, did not increase in 
 weight ! We found, that if peas be steeped a suf- 
 ficient time in water, they almost exactly double 
 their weight, which increase of weight rapidly dimi-* 
 nishes if they be afterwards exposed freely to \he* 
 air. Six peas, weighing nineteen grains, were, after 
 forty-eight hours steeping in water, increased in 
 weight to thirty-eight grains, and in this state were 
 
 * Journal de Physique, loc. cit. 
 
 f Manchester Memoirs, vol. iv. p. 316, 
 
21 
 
 exposed on a table in a room at temperature 66* : 
 they gradually decreased in weight, and by the fifth 
 day had fallen back to their original weight and ap- 
 pearance. If, however, the direct effects of evapo- 
 ration be prevented, by inclosing them in a jar, they 
 still exhale moisture and diminish in weight, but 
 much more slowly ; for the peas in the foregoing 
 experiment (8.) were by their germination much di- 
 minished in bulk, and lost rather more than seven 
 grains in weight ; from all which it follows, that 
 no supposed absorption of oxygen adds, during their 
 growth, to the weight of seeds. 
 
 1 9. If then there be no proof that the oxygen gas 
 which disappears in germination enters, either by 
 absorption or by chemical affinity, into the seed, so as 
 to combine with it, it follows, that this gas must be 
 at once converted into the carbonic acid produced in 
 that process, without such previous combination ; and 
 the formation of the acid proceeding always in pro- 
 portion (7. 8.) to the disappearance of the oxygen gas, 
 sufficiently shows that it is derived from that source 
 alone. If the acid were formed independent of the 
 oxygen gas employed, the whole bulk of air ought 
 to be increased by germination : and accordingly, 
 Mr Cruickshank found, that when carbonic acid was 
 produced by steeped seeds, confined either in nitro- 
 gen (5.) or hydrogen gas, the bulk of air was in- 
 creased one-fifth, but nothing like germination then 
 took place. He even found that seeds, after being 
 soaked in water, and passed up into a tube of mer- 
 cury, formed carbonic acid in large quantity but 
 without undergoing any sensible change in their ap- 
 
 B3 
 
22 
 
 pearance *. This experiment we repeated by pass- 
 ing up half a dozen soaked peas into a graduated 
 tube filled with mercury, and immersed in a jar of 
 the same fluid. By the following day two-tenths of 
 a cubic inch of air were collected in the upper part 
 of the tube, and the experiment was continued till 
 the bulk of air was increased to 1.6 cubic inch, be- 
 yond which it did not proceed. Lime water was 
 then passed into the tube, and with the aid of a lit- 
 tle agitation, all the air except a small bubble was 
 rapidly attracted by it. The peas did not exhibit 
 the slightest sign of germination. These facts show, 
 that the production of this acid during the growth 
 of seeds is altogether different, both in its formation 
 and the consequences which attend it, from that pro- 
 duction of it which takes place where no living ac- 
 tion exists. Where this acid is formed independent 
 of germination, no change is effected in the seed, 
 and the bulk of air is increased : where it is pro- 
 duced by that process, a part of the air disappears ; 
 the evolution of the seed goes on ; and the quantity 
 of the air is somewhat diminished. Hence, therefore, 
 we conclude with De Saussure, that in germination 
 the seed does not form carbonic acid from its own 
 substance, but furnishes only one of the constituent 
 parts of it, namely, the carbon t ; and farther, that 
 when it does form this acid, independent of oxygen 
 gas, it is only under a state of decomposition, (5.), 
 
 * Experiments on Sugar, loc. cit. 
 
 f Journal de Physique, torn. xlv. p. 98. 
 
23 
 
 or in circumstances where no living action is going 
 on. 
 
 20. As therefore the carbonic acid met with in 
 germination is not emitted ready formed by the 
 seed, it must be produced exterior to it ; and for 
 this purpose carbon must be in some way furnished. 
 Dr Woodhouse supposes this carbon to proceed ei- 
 ther from the earth in which the seeds are planted, 
 from some decayed portion of the living leaves, or 
 from the cotyledons of the seed itself* ; but this 
 acid has been shown to be formed where no earth 
 is present, and before any leaves appear, (7. 8.), and 
 where the seed itself exhibits no sign of decay. It 
 is plain, therefore, that the living seed, during its 
 germination, must possess a power of supplying car- 
 bon ; and by the union of this carbon with the oxy- 
 gen gas of the air, the carbonic acid that is met with 
 must be formed. We find accordingly, that the 
 carbonic acid produced does actually exceed in 
 weight (13.) the oxygen gas that disappears; and 
 the carbon that occasions this excess can be derived 
 only from the living seed, which, if the experiments 
 of Hassenfratz are to be trusted, contains less car- 
 bon after germination than it did before it was sub- 
 mitted to that process ! 
 
 21. But it has been said, that when the air used 
 in this process contains no oxygen gas, or when all 
 which it contained has been converted into carbo- 
 nic acid, germination no longer goes on. This can?- 
 
 * Nicholson's Journal, July 1802, p. 151. 
 f Annales de Chirme, torn. xiii. p. 188. 
 B4 
 
24 
 
 not arise from the presence of nitrogen gas, for that 
 gas exerts (5.) no injurious operation. Neither, al- 
 though Mr Achard found, that germination did not 
 take place in carbonic acid, can the presence of that 
 substance be considered as positively destructive to 
 the process ; for the same effect occurs when nitro- 
 gen gas only is present, and even in vacua without 
 the direct agency of any gas at all. M. Huber too 
 found, that when oxygen gas or common air, in 
 certain proportions, was mixed with carbonic acid, 
 seeds grew in it very well * : and in all our experi- 
 ments, where the carbonic acid produced by germi- 
 nation was suffered to remain, the seeds ceased to 
 grow, not from the presence or abundance of that 
 acid, but from the diminished quantity or total ab- 
 sence of the oxygen gas out of which it was formed. 
 Since, indeed, the absence of that gas is sufficient to 
 account for the cessation of the process, it is unphi- 
 losophical, because unnecessary, to resort to the a- 
 gency of any secondary cause ; and as both carbo- 
 nic acid and nitrogen gas must always of necessity 
 be present in germination, it is not to be expected 
 that either the one or the other should exert any in- 
 jurious operation. 
 
 22. Neither, because carbonic acid is always pre- 
 sent, is it, on that account, to be held as at all essen- 
 tial to germination ; for the same might be urged in 
 favour of nitrogen gas, which neither produces (5.) 
 nor suffers change. The constant existence of this 
 acid in the atmosphere is no proof of its aiding ger- 
 
 Sur la Germination, p. 56. 
 
25 
 
 Tarnation. 1 By many its proportion has been rated 
 at ~ j but Berthollet considers this estimate as 
 much too high *, and Dal ton makes it amount only 
 to lo!^ a quantity much too inconsiderable to ex- 
 ert any sensible effect. When also M. de Saussure 
 abstracted the carbonic acid from the air, previous 
 to placing the seeds (7.) in it, germination went on 
 as well as where this had not been done ; and, on 
 the other hand, this process continued to proceed 
 when the acid was removed, as soon as formed (8.) 
 by an alkaline solution, as when it was suffered to 
 remain. It has been said, however, that when it is 
 abstracted, by means of lime, from the air in which 
 seeds are growing, the progress of their growth is 
 retarded. The lime, indeed, does abstract the car- 
 bonic acid in the same manner as we have seen the 
 alkaline solution to do : and, a firiori, there seems 
 no reason why its abstraction should in one case be 
 more prejudicial than in the other. But this ab- 
 straction of the acid by the lime is carried on only 
 after it has imbibed water; and we found, that when 
 a quantity of powdered lime was placed underneath 
 some peas growing in a jar inverted over mercury, 
 it presently began to swell, the jar continued quite 
 dry, and the peas germinated very imperfectly, be- 
 ing reduced nearly to half their bulk. Thus, they 
 \vere deprived too rapidly of their moisture to en- 
 able them to grow, in the same manner as when 
 they are too freely exposed (18.) to the air. . In 
 
 * Chemical Statics, vol. i, p. 375. 
 
 f Manchester Memoirs, vol. i. new series. 
 
26 
 
 fact, since carbonic acid is necessarily a product and 
 consequence of germination, it seems absurd to con- 
 sider it at the same time as an exciting principle and 
 a cause. 
 
CHAP. II. 
 
 OF THE CHANGES INDUCED ON THE AIR 
 BY THE VEGETATION OF PLANTS. 
 
 23. IN the former chapter it was shewn, 
 that water was a first and essential requisite to the 
 germination of the seed, and numerous facts prove 
 it to be equally necessary to the future progress of 
 the plant ; for if completely deprived of water, the 
 plant withers and dies, although every other requi- 
 site be present ; nor does it afterwards revive, even 
 although water be supplied. The quantity of this 
 necessary fluid that is daily absorbed by plants, va- 
 ries greatly according to their structure and habits, 
 to the temperature of the season, the supply of wa- 
 ter, and the state in which vegetation is going on. 
 After its absorption, it is again given out in great 
 part by transpiration ; and the degree in which this 
 latter process proceeds, must depend on the quan- 
 tity previously absorbed : and conversely, whatever 
 restrains the freedom of transpiration, must abridge 
 
28 
 
 the power of absorption ; for when the vessels arc 
 once filled with fluid, if none be carried off, no 
 more can enter. The absorption is carried on chief- 
 ly by the extremities of the roots, and the trans- 
 piration by the surfaces of the leaves. Dr Hales 
 found, that a sunflower, three and a half feet high, 
 lost by perspiration, on an average, 1 Ib. 14 oz. of 
 fluid ficr day. During a warm night, and without 
 dew, it lost only 3 oz. : when the dew was sensible, 
 it suffered no perceptible loss ; and if the dew was 
 great, or rain fell, it gained a few ounces in weight. 
 The surface of all the leaves of this plant was equal 
 to about 39 square feet, and that of its roots to 15.8 
 square feet. The depth of water absorbed by the 
 whole surface of the roots was ^ 7 part of an inch, 
 and the depth perspired by the surface above ground 
 y~ part. He found likewise, that different plants 
 varied greatly in the quantity of fluid which they 
 perspired ; that evergreens perspired much less than 
 other plants ; and that the perspiration was propor- 
 tional to the extent of surface of the leaves, and 
 ceased nearly if the leaves were removed *. The 
 experiments of M. Guichard shew, that it is chiefly 
 by the upper surfaces of the leaves that this transpi- 
 ration of fluid is carried on ; and those of the same 
 author, of Du Hamcl and Bonnet, demonstrate like- 
 wise that absorption, as far as the leaves are con- 
 cerned, is performed by their under surface. To 
 prove this, the superior surface of one leaf and the 
 inferior surface of another were covered with var- 
 
 * Statical Essays, vol. i. pp. 4. 6. 21. & 28.. 
 
29 
 
 nish, and the former, in a given time, suffered little 
 diminution of weight, but the latter became much 
 lighter. Similar leaves also, were laid on the sur- 
 face of water, and those which had their superior 
 surface inverted gained little weight, and for the 
 most part died in a few days ; while such as had 
 their inferior surface applied to the water, became 
 much heavier, and flourished many months. These 
 facts make it evident, that perspiration and absorp- 
 tion are not performed by the same vessels, but that 
 each has its peculiar organs *. The office of ab- 
 sorption is performed by fine hairs or points, placed 
 on the under surface of the leaves, which in many 
 plants are hollow tubes constructed for that purpose. 
 When leaves have no such points, small apertures 
 are found in their place t- The leaves of plants be- 
 come gradually less and less fit for carrying on these 
 functions ; not so much, however, from any decay 
 or inaptitude in their structure, as from the decli- 
 ning influence of another agent, whose operation we 
 are next to consider. 
 
 24. The fluids of plants which are taken up by 
 the roots, and afterwards perspired by the leaves, 
 require, for their due transmission through the stem, 
 a certain degree of heat : hence they flow most free- 
 ly in the day and in sunshine, and are checked du- 
 ring the night and in frost. Dr Hales observes, that 
 during moisture and warmth, the flow of the sap is 
 most vigorous, but that a cold easterly wind imme- 
 
 * Dr Bell, Manchester Memoirs, vol. ii. p. 411. 
 f Wildenovv's Principles of Botany, p. 298. 
 
30 
 
 diately abates it. When the sun was clouded du- 
 ring the rising of the sap, it would visibly subside 
 with great rapidity ; but when the sun-beams again 
 broke out, it would immediately return to its rising 
 state, just like the fluid in a thermometer *. A plant 
 of spearmint, placed in a syphon one-fourth of an 
 inch in diameter, absorbed, during the day, so much 
 water, that it fell into the opposite end to the depth 
 of an inch and a half; in the night only to one- 
 fourth of an inch ; during a rainy day still less ; 
 and, when the thermometer stood at the freezing 
 point, the plant absorbed no moisture at all f. That 
 it is to the influence of heat alone, and not tc? any 
 other peculiarity in the season, or in the plant itself, 
 that, the other requisites being duly supplied, its ve- 
 getation is to be ascribed, we learn from the fact, 
 that trees in a hot-house continue to grow during 
 the winter, while others of the same species in the 
 open air cease to do so. Even the branch of a tree, 
 planted in the open air, will, during winter, when 
 led into the hot-house, undergo the usual vegetative 
 changes, provided its stem be guarded from severe 
 frost ; but, on the contrary, a branch brought into 
 the open air from a tree that is flourishing in the hot- 
 house, will at the same season cease to grow J ; 
 affording sufficient evidence of the necessity and the 
 influence of heat. 
 
 * Statical Essays, vol. i. p. 122. f Ibid. p. 27. 
 J Philosophical Transactions, vol. Ixiii. part 1. 
 
31 
 
 25. With regard to light, we may observe, that 
 whatever doubts may exist concerning its effects on 
 the germination of seeds, there can be no question 
 but that it exerts a peculiar agency on the appear- 
 ance and qualities of plants. M. du.Fay is said to 
 have been the first philosopher who remarked how 
 much the green colour of plants depended on the 
 agency of light ; and from an observation made 
 about the year 1760, the late Professor Robison of 
 Edinburgh was led to infer, that it was very actively 
 employed in giving rise, not only to the colour, but 
 to the odour and combustibility of vegetables. Ha- 
 ving gone down into a coal-pit in the neighbour- 
 hood of Glasgow, he brought up some whitish-look- 
 ing plants, but no one knew what they were. After 
 being exposed to the light, however, the white leaves 
 died away, and were succeeded by green buds ; and 
 the plant acquired the smell of tansy. On further 
 inquiry, he found, that the sods on which the plant 
 grew had been taken down into the pit from the 
 part of an adjoining garden ; and although the plant 
 had continued to grow in its new situation, yet nei- 
 ther in colour, in odour, nor combustibility, did it 
 at all resemble plants of the same species which had 
 vegetated under an exposure to light. The etiola- 
 tion or blanching of the roots of celery, and of the 
 inner parts of cabbages and lettuce, are familiar ex- 
 amples of the same kind ; and Mr Davy ascertained, 
 that after the green colour is formed, it will again 
 disappear, if the plant be excluded from light. The 
 leaves of the common lettuce were in six days ren- 
 dered very pale, by being deprived of light, and at 
 the end of another week they were quite white : the 
 
32 
 
 growth of the plant was checked ; and its leaves, on 
 being analyzed, yielded more carbonic acid and wa- 
 ter, and less hydrogen and residual carbon, than an 
 equal weight of green leaves, results similar to 
 those obtained by Chaptal, Hassenfratz, and Senne- 
 bicr. The various colours of flowers are ascribed 
 also to the combination of light in different propor- 
 tions. Red rose trees, if carefully excluded from 
 light before their flowers begin to appear, will yield 
 flowers almost white: and marine plants are govern- 
 ed by the same laws with regard to colour as land 
 vegetables *. Scheele observed, that it is the violet 
 ray which acts most powerfully in reducing the o- 
 xide of silver t ' and Senncbier ascertained, that the 
 same ray has the greatest effect in producing the 
 green colour of plants J ; but according to Mr 
 Gough, the presence of oxygen gas is required to 
 aid its operation . The late Dr Hope found, that 
 the light produced by art effected changes in degree 
 similar to that which issues from the sun, which 
 has since been confirmed by others : and the light 
 of the moon has a similar effect. It follows, there- 
 fore, from these facts, that although light is not im- 
 mediately necessary to the growth of plants, yet that 
 many of their more peculiar and characteristic qua- 
 lities depend essentially upon its action. 
 
 * Davy in Beddoes' Contributions to Science, p. 186. ct 
 seq. t On Fire, p. 98. 
 
 J Mem. Phys. Chim. vol. iv. p. 72. 
 Manchester Memoirs, vol. iv. p. 501. 
 
33 
 
 26. But although water and heat alone be the 
 first requisites to the commencement of vegetation, 
 yet they are not of themselves sufficient to carry on 
 that process. Air likewise is required. Mr Papin 
 put an entire plant into the exhausted receiver of an 
 air-pump, and it soon perished ; but on keeping the 
 whole plant, except the leaves, which were exposed 
 to the air, in this vacuum, it continued to live a long 
 time *. ' Dr Hales, Du Hamel, and Mr Knight, 
 found that plants do not vegetate when deprived of 
 their leaves : and it is well known, that when the 
 leaves of a plant or tree are destroyed by insects, its 
 vegetation for that season is suspended, and the fruit 
 gradually decays. We have already seen, that when 
 the upper surface of the leaf was covered with var- 
 nish or laid on water (23.) it died in a few days ; and 
 Dr Darwin smeared the upper surfaces of the leaves 
 over with oil, and the plant died in a day or two af- 
 ter f. As thus without air plants do not vegetate, 
 and as they cease also to grow when deprived of 
 their leaves, even although air be supplied, we must 
 conclude, not only that air is essential to the growth 
 of plants, but that their leaves are the organs which 
 act upon this air. And farther, as their death takes 
 place if the upper surfaces cf their leaves be smear- 
 ed over with varnish or oil, we must also conclude, 
 that it is to some action going on between those sur- 
 faces and the air that the continuance of their life is 
 to be ascribed. Let us next then inquire into the 
 
 * Darwin's Phytologia, p. 51. f Ibid. p. 45< 
 
 c 
 
nature of this action, or the changes which the air 
 undergoes in the process of vegetation. 
 
 27. Dr Priestley, whose experiments and disco- 
 veries first opened the way to an investigation of the 
 reciprocal changes which take place between living 
 vegetables and the air, was led to conclude, that 
 plants not only lived in nitrogen gas, or what he 
 called phlogisticated air, but that, during their 
 growth, they restored to the air the pure part of which 
 it had been previously deprived by the processes of 
 combustion and respiration, and by the decomposi- 
 tion of organized bodies. He found, that if a sprig 
 of mint was put into a jar of air inverted over wa- 
 ter, and in which a candle had previously burned 
 out, the air was in a few days so far restored that 
 another candle would burn in it perfectly well. He 
 also put a sprig of mint to grow in air wherein mice 
 had died, and in eight or nine days a mouse would 
 again live very well in this air, though he died in 
 another portion of the same air in which no mint 
 had grown. In air strongly tainted by putrefaction, 
 sprigs of mint have sometimes presently died ; but 
 ii: this do not soon happen, they thrive in a surpri- 
 sing manner. These facts led him to conclude, 
 that plants, by their vegetation, reverse the effects 
 produced on the air by the several processes above 
 described, and thus become the chief means by 
 which the purity of the atmosphere is maintained *. 
 
 28. The experiments from which this conclusion 
 was drawn, were made so early as the year 1771, 
 
 * Observations on Air abridged, vol. iii. p. 251. et scq. 
 
35 
 
 before Dr Priestley had discovered any correct means 
 of analyzing the air ; nor does he seem to have been 
 aware, that the effects produced on the air by the 
 living functions of animals are very different from 
 those which arise from the decomposition of organi- 
 zed bodies. However much, therefore, we may be 
 disposed to admire the beautiful provision which 
 this supposed reciprocity of action would establish 
 between the functions of animal and vegetable life, 
 we must suspend our belief in its truth until better 
 proofs are brought forward than these experiments 
 afford. Even the fluctuation of opinion on this point 
 which Dr Priestley himself experienced, may well 
 suggest doubts of the correctness of his views ; for 
 towards the end of the same year, some experiments 
 of the same kind, he observes, did not answer so 
 well, and the restored air relapsed to its former 
 noxious state, which led him to suspend his judg- 
 ment concerning this power in plants *. In 1772, 
 on resuming his experiments, he had the most indis- 
 putable proof of the restoration of putrid air by ve- 
 getation ; but hearing that Mr Scheele and others 
 had obtained different results, he again repeated them 
 in 1778. The experiments of this year were unfa- 
 vourable to his former hypothesis ; for whether the 
 air had been injured by respiration, the burning of 
 candles, or other means, it was not rendered better 
 by vegetation, but worse ; and the longer the plants 
 continued in the air, the worse it became. He tried 
 a great variety of plants, as sprigs of mint, spinach, 
 
 Observations on Air abridged, vol. iii. p. 251. to 263. 
 C 2 
 
36 
 
 lettuce, and some others ; but with no better suc- 
 cess. His method was to put the roots of ihe plant 
 into a phial filled with earth and water, and then to 
 introduce it through water into the jar containing 
 the air on which he was making the experiment. In 
 some cases, the air was certainly meliorated, but Mr 
 Scheele, he adds, always found it injured : and he 
 concludes, that, upon the whole, it is jirobable that 
 the vegetation of healthy plants, growing in natural 
 situations, has a salutary effect on the air in which 
 they grow *. Thus we see, from Dr Priestley's own 
 representation of facts, related with his usual can- 
 dour, that nothing certain can be inferred from his 
 experiments in favour of this supposed purifying 
 power in plants : and that his conclusion was, even 
 at that period, in direct opposition to the experience 
 of the celebrated Scheele. 
 
 29. Notwithstanding, however, the uncertain, and, 
 in many respects, contradictory evidence on which 
 this conclusion has been shewn to rest, few opinions 
 in modern science have obtained a more general be- 
 lief: and both physiologists and chemists seem, in 
 this instance, to have satisfied themselves with con- 
 templating at a distance the beauty of thejinal cause , 
 instead of approaching to a nearer examination of 
 the facts on which the opinion has been maintained. 
 Accordingly, this opinion still keeps its ground, and, 
 in no publication that we have seen, has its accura- 
 cy been even questioned. In the experiments, 
 however, which we are about to detail, it will be 
 
 * Observations on Air abridged, vol. iii. p. 273. et seq. 
 
37 
 
 seen that their results are altogether opposed to the 
 notion that plants, by their vegetation, purify the 
 air ; and these experiments have been so often re- 
 peated, and with such uniformity of result, as to im- 
 press us with an entire confidence in their truth. 
 We proceed, therefore, to investigate the actual 
 changes which atmospheric air undergoes in vegeta- 
 tion ; and, first of all, the effects produced on its 
 nitrogenous portion. It has been already shewn, 
 that seeds (5.) do not in the smallest degree germi- 
 nate in nitrogen gas : and Dr Ingenhousz proved, 
 also, by experiment, that, in pure nitrogen gas, 
 plants do not grow. This conclusion has been more 
 lately confirmed by the experiments of Mr Gough. 
 He confined several succulent plants in jars of ni- 
 trogen gas when their flowers were just ready to ex- 
 pand ; but they died away without putting forth any 
 of their blossoms. Others were placed in similar 
 situations before the flower-buds formed upon them, 
 but made not the least effort towards vegetation ; 
 and many more, which grew very well in inverted 
 jars of atmospheric air, ceased to vegetate when 
 transferred into jars of nitrogen gas. But this gas, 
 although it does not aid vegetation, appears in no 
 degree to injure the faculty of growth in plants any 
 more than in seeds (5.) ; for a slip of spearmint, 
 which had remained twelve days in nitrogen gas, 
 recovered upon being restored to the air ; and an off- 
 set of semfiervivum vegetated freely after being remo- 
 ved from a jar of the same gas, in which it had been 
 kept from the 2d of April to the 2d of May *. As 
 
 Nicholson's Journal, November 1801, p. 218. 
 C 3 
 
38 
 
 thus it appears, that nitrogen gas exerts no direct in- 
 fluence on plants, so it is probable that they produce 
 no change upon it ; and we may conclude, there- 
 fore, that this gas, in its simple and uncornbined 
 form, is altogether inactive in the process of vege- 
 tation. 
 
 30. We have next to inquire into the necessity 
 and use of oxygen gas, and the changes which it 
 undergoes in vegetation. It has been shewn by Dr 
 Ingenhousz, that plants do not, any more than seeds, 
 grow in any species of air unless it contain a portion 
 of oxygen gas, and this gas have access to their 
 leaves : that they vegetate very well in atmospheric 
 air and in oxygen gas ; and, therefore, that this lat- 
 ter gas is necessary to vegetation *. What then are 
 the changes which it suffers in that process ? By the 
 experiments of Dr Woodhouse, we are taught, that 
 plants growing in earth and confined in a glass ves- 
 sel of atmospheric air render it impure, and that car- 
 bonic acid is formed : that when confined in oxygen 
 gas, previously well washed in lime-water, carbonic 
 acid is, in like manner, produced : and that the 
 quicker this acid is generated, the sooner does the 
 plant die f- These facts derive confirmation from 
 the following experiment. Some peas, growing in 
 a small pot of mould to the height of several inches, 
 were placed under a jar of atmospheric air inverted 
 over water ; and a common wine-glass, the foot of 
 which was broken off, was filled with lime-water, 
 
 * Thomson's System of Chemistry, vol. iv. p. 290. 
 f Nicholson's Journal, July 1802, p. 152. 
 
39 
 
 and Inserted into the mould in the midst of the 
 growing peas. The lime-water soon acquired a pel- 
 licle on its surface, which, by the fourth day, was 
 very thick, and, by the sixth day, had sunk down 
 to the bottom of the glass ; and the water into 
 which the jar was inverted, had risen in it to a con- 
 siderable height, proving that carbonic acid was 
 formed, and that a diminution in the bulk of air 
 had taken place. 
 
 31. As the mould, in which these plants grew ? 
 might by some be supposed to influence the produc- 
 tion of the carbonic acid, the experiment was next 
 varied in the manner following. Some mustard 
 seeds, which were raised on moistened flannel, and 
 had grown to more than an inch in height, were in- 
 troduced into a jar of atmospheric ai , and supported 
 about half way up it by a small whalebone hoop, 
 covered with netting. In a deep dish, filled with 
 water, and appointed to receive the inverted jar, was 
 placed a glass of lime-water, which stood directly 
 under the hoop. In four hours, a thickish pellicle 
 overspread the lime-water, and the inside of the jar 
 was bedewed with moisture. At the end of twenty- 
 four hours, the lime-water was very turbid, and the 
 water, into which the jar was inverted, had risen near- 
 ly half an inch. These appearances increased du- 
 ring the two succeeding days, at which time the 
 water stood nearly an inch high in the jar. The 
 plants now began to look sickly, gradually drooped, 
 and, by the ninth day, fell down in different direc- 
 tions against the sides of the jar, and, when with- 
 drawn from it, emitted a putrid smell. The resi- 
 dual air extinguished a taper, rendered lime-water 
 
 C 4 
 
40 
 
 turbid, and suffered thereby a farther loss of bulk. 
 This experiment was repeated, by confining mustard 
 plants in the same manner in jars of air inverted over 
 mercury. The lime-water was, in like manner, ren- 
 dered turbid, and the mercury for two or three days 
 continued to rise into the jars : after a certain time, 
 however, it became stationary, and then began to 
 fall, and the plants when withdrawn yielded a putrid 
 smell. 
 
 32. As by the preceding experiments it is proved, 
 that the pure part of the air disappears in vegetation, 
 and 'that carbonic acid is produced, it is desirable to 
 know the extent to which rhis destruction of the oxy- 
 gen gas proceeds. Some mustard seeds, therefore, 
 which had grown on moistened flannel to the height 
 of an inch, were conveyed into a jar, and supported 
 about half way up it by a small hoop in the manner 
 already described. The jar held 36.5 cubic inches 
 of air, and was inverted into a saucer, in which was 
 placed a small glass-cup, containing one cubic inch 
 of the water of potassa : mercury was then poured 
 into the saucer, so as to surround the mouth of the 
 jar. Three slips of paper, graduated to inches and 
 tenths, were pasted at equal distances on the outside 
 of the jar, to mark the ascent of the mercury : and 
 the whole was set aside in a room where the baro- 
 meter was at 29.2 inches, and the thermometer at 
 61.5. The inside of the jar was presently bedewed 
 with moisture, and, by the close of the twenty- 
 fourth hour, the mercury had risen in it of an 
 inch, bearing up the alkaline solution with it. By 
 the forty-eighth hour, the mercury stood at one inch, 
 when that in the saucer was brought to a level with 
 
41 
 
 it ; and at the end of the third day, it had under- 
 gone no variation. At this period, the barometer 
 was 29.75, and the thermometer 68. 
 
 33. In order to examine the air, the saucer and 
 jar were now immersed in the pneumatic trough, 
 and water being admitted into the jar, the mercury 
 fell, and the cup with its solution was withdrawn 
 through the water : the solution effervesced briskly 
 on the addition of diluted sulphuric acid. A portion 
 of the residual air being then transferred into a small 
 jar, was found to extinguish a lighted taper that 
 was plunged in it : and another portion was shaken 
 with lime-water without producing or suffering any 
 change ; neither did it afterwards experience any 
 perceptible loss by agitation with the liquid sulphur- 
 et of potassa, nor by being placed in contact with a 
 stick of phosphorus. The result, therefore, of this 
 analysis teaches us, that, during the growth of die 
 plants, the oxygenous portion of the air gradually 
 and completely disappeared ; and that the carbonic 
 acid that was formed was entirely attracted by the 
 water of potassa employed. 
 
 34. In the process of germination it has been 
 shewn, that the bulk of carbonic acid produced, bears 
 a constant proportion (7, 8.) to that of the oxygen 
 gas which disappears : and to ascertain whether the 
 same did not hold in vegetation, a similar method 
 was adopted. After the foregoing analysis of the 
 air was completed, the jar was allowed to run dry, 
 and being then re-inverted, it was set on its bottom : 
 and the plants, together with the glass-cup, and a 
 bulk of water equal to the solution, were re-placed 
 in it. Water, to the quantity of 25.4 cubic inches., 
 
was then poured in till it reached the height to 
 which the mercury had risen, which, therefore, was 
 the bulk of residual air : and afterwards to fill the 
 jar completely, it required 7.6 cubic inches more, 
 which was the bulk of air that had disappeared. 
 These quantities together amount only to 33.0, and 
 the 3.5 required to make up the 36.5 cubic inches 
 of air which the jar was capable of containing, must, 
 therefore, be allowed as the space occupied by the 
 several substances used in the experiment, which 
 reduces the volume of air actually employed to 33.O 
 cubic inches, of which a portion equal to 7.6 had 
 disappeared. The barometer, at the end of the ex- 
 periment, was, as already stated, 29.75, and the 
 thermometer 68. Hence, therefore, to make the 
 necessary reductions, we have '' 4 2 *;f' 75 25.8784, 
 
 2 
 6 ' 5 
 
 but ' 4 = .34826 and 25.8784 .34826 = 
 
 25.53014, the corrected volume of air at the close 
 of the experiment. But atmospheric air contains 
 
 O'> I 
 
 ^zz^-^of oxygen gas, and 33 25.53014=7.46986 
 
 and '' 4 ^ R - ~ ^jr t Or if we compare the diminution 
 which the air suffered, with the quantity of oxygen 
 gas which it originally contained, we have 100 : 22 
 : : 33 : 7.26, which comes near to 7.46, the esti- 
 mated amount of the diminution that actually took 
 place. In a second experiment, conducted in a si- 
 milar manner, the loss of bulk which the air suffer- 
 ed, after the necessary reductions were made, was 
 -~. From these facts it is inferred, that the loss of 
 bulk, which a given volume of air suffers in vegeta- 
 tion, corresponds very nearly with the proportion of 
 oxygen gas which that air contained : and if this 
 
loss be attributed entirely to the abstraction of car- 
 bonic acid, the quantity of acid produced will ra- 
 ther exceed that of oxygen gas lost. But on pass- 
 ing into carbonic acid, this gas suffers (11.) a small 
 degree of condensation ; and whatever future expe- 
 rience may determine this to be, it must of course 
 be deducted from the whole diminution which the 
 air suffers, and consequently will diminish some- 
 what the quantity of carbonic acid formed. 
 
 35. Having thus endeavoured to trace the na- 
 ture and extent of the changes which the air 
 suffers in vegetation, let us next direct our atten- 
 tion to the manner in which they take place. 
 Through the whole of the preceding experiments 
 (30. et seq.) we have seen that oxygen gas is essen- 
 tial to vegetation, and that it gradually and com- 
 pletely disappears, when, in a given bulk of air, 
 that process is made to take place. What then be- 
 comes of it ? By many it is supposed to be absorbed 
 by a necessary function of the plant. It may be 
 worth while, therefore, to inquire what is that struc- 
 ture of the plant by which such an absorption can 
 be effected ? Grew and Malpighi speak of sfiiral ves- 
 sels which they discovered in the wood, and which 
 later inquirers have found to extend to the minutest 
 branches, and to spread through every leaf. From 
 these vessels being always found empty, they sup- 
 posed them to be air-vessels, and called them tra- 
 c/iete ; but Dr Darwin observes, that if the end of 
 a vine-stalk, two or three years old, be cut horizon- 
 tally, these vessels may, by the help of a common 
 lens, be seen to be full of juice, which, in a minute 
 
44 
 
 or less, passes on or exhales, and then the vessels 
 appear empty, that is, filled with air. The same 
 author, however, speaks of horizontal vessels, first 
 noticed by Malpighi, which pass through the bark 
 of trees to the alburnum, and, as he supposes, ad- 
 mit oxygen gas * ; but, besides that these vessels are 
 not proved to extend to the leaves, which alone ope- 
 rate changes on the air, we have seen that the leaves 
 of themselves are able to preserve the life of the 
 plant when the stem is confined in vacm (26.) ; 
 and, on the contrary, that the stem is not sufficient, 
 when deprived of its leaves, or these are smeared 
 over with oil. Lastly, Dr Hales readily caused air 
 and water to pass through a piece of recent stick, by 
 setting one end of it in a cup of water placed under 
 a receiver, and then exhausting the receiver of its 
 air, and these vessels he imagined to perform a re- 
 spiratory function ; but the same vessels were found 
 by Malpighi to be present in the roots, where they 
 are not exposed to the air, and cannot, therefore, 
 serve the purpose of respiration. Neither, as be- 
 fore remarked, have we any proof that any other 
 part than the leaves of the living plant act upon the 
 air in contact with it to an extent necessary to main- 
 tain its life. 
 
 36. Have then these leaves any organization by 
 which an absorption of air can be effected ? It is as- 
 certained that the sap of plants ascends to the leaves, 
 and after undergoing certain changes in them, is 
 again returned to the branches and stern. Dr Dar- 
 
 Phytologia, p. 12, 13. 
 
45 
 
 win placed a plant of spurge (Euphorbia Hclmcofiia] 
 in a decoction of- madder, and in a few days the co- 
 loured fluid passed along the middle rib of the leaf, 
 and ran by several branches to the extremity of it 
 upon the upper surface ; while, on the under side, 
 a system of vessels was seen coming from the end 
 ef the leaf, and conveying a milky fluid, which de- 
 scended finally into the foot stalk* : and Mr Knight 
 has traced the vessels conveying this peculiar juice 
 from the leaves into the cortical layers of the inner 
 bark f Now the difference in the appearance and 
 qualities of this returning fluid was probably effect- 
 ed in the leaves by the agency of the air, and in this 
 way the succus firo-firius of plants seems to be pre- 
 pared. But can we suppose, either the air or its 
 oxygenous portion to be received into these minute 
 vessels, already filled with fluids, to be there decom- 
 posed, then transformed into, and at length emitted 
 as carbonic acid ? If so, by what passages can it en- 
 ter, and how afterwards can it be expelled ? It can- 
 not enter by the transpiring or exhaling pores, be- 
 cause the fluids constantly escaping from them must 
 obstruct its admission ; nor have we any instance of 
 any organ performing two contrary actions at one 
 and the same time, a condition which this supposi- 
 tion would involve. If, again, it enter by peculiar 
 vessels fitted for the absorption of air, (whose exist- 
 ence however is not proved), by what vessels can it 
 afterwards be emitted as carbonic acid ?. Not surely 
 
 * Phytologia, p. 43. 
 
 f Philosophical Transactions, 1801, p. 337. 
 
46 
 
 by the same vessels which absorbed it, for the reason 
 above given : and, therefore, the existence of other 1 
 peculiar vessels for the emission of air, ought like- 
 wise to be proved. By the common absorbents of 
 the leaf, it cannot be supposed to enter ; for they 
 are employed in taking up fluids, and are besides si- 
 tuated on the inferior surface (23.), while the changes 
 on the air are effected by the superior only. To 
 suppose that it gains admission through inorganic 
 pores, is contrary to all sound physiology ; for we 
 have no example of any healthy function being car- 
 ried on by animated beings, otherwise than by a li- 
 ving action, and by a peculiar organization destined 
 to perform that action. 
 
 37- Rejecting, therefore, the opinion that any 
 aeriform fluid obtains admission into the vascular 
 system of plants by any process resembling the living 
 function of absorption, let us next inquire how far 
 the supposition of chemical affinity will help to re- 
 move the difficulty. The juice of the leaves, w r hich 
 is supposed so powerfully to attract the oxygen gas 
 of the air, is contained within its peculiar vessels, 
 and these vessels are likewise covered by the green 
 colouring matter, over all which is spread the efiider- 
 mis. The minute structure of these parts has been 
 lately described by M. Jurine, as it appeared on mi- 
 croscopic examination * ; but the appearances ob- 
 ser v ed by him are so different from those represent- 
 ed by other authors, as to justify, in his opinion, the 
 adoption of new names, in almost every instance, to 
 
 * Phil. Mag. vol. xvi. 
 
47 
 
 express them, a circumstance which, while it be- 
 gets a doubt of their accuracy, tends greatly to 
 puzzle the student, without proportionally advancing 
 the science. It is sufficient lor our purpose to ob- 
 serve, that no direct communication is demonstra- 
 ted to exist between any pores on the surface of the 
 epidermis, and the vessels in which the vegetable 
 fluids move. If, therefore, chemical affinity operate 
 here, it must be between substances, not, as usual, 
 in actual contact, but at a certain assignable distance ; 
 and that distance intercepted by a very compounded 
 organized structure. Let us, however, suppose for 
 a moment the oxygen gas of the air to obtain ad- 
 mission into the juices of the plant, under all these 
 circumstances, by the operation of chemical affinity ; 
 by what power will the carbonic acid, which it 
 is there supposed to form, be afterwards emitted ? 
 Not by chemical affinity, surely, for where is the 
 external agent to effect it ? Not through a living ac- 
 tion, for where is the structure to perform it ? 
 Or how can the reception of air be thus carried on 
 by one law, and its expulsion by another so widely 
 different ? How, also, is this oxygen gas, previous 
 to its supposed attraction into the plant, to be sepa- 
 rated from the nitrogen gas with which it is natural- 
 ly united ? If, as is probable, the separation be 
 brought about only by the superior affinity of carbon 
 furnished by the leaf, then the carbonic acid, in- 
 stead of being emitted from the leaf, is formed exte- 
 rior to it, and out of the very oxygen gas which is sup- 
 posed first to enter into the leaf to form it. Those, 
 also, who favour this opinion of the attraction of 
 oxygen gas, cannot well suppose that plants likewise 
 
48 
 
 attract carbonic acid, retaining the carbon, and gi- 
 ving out the oxygen gas ; for this would not only 
 be making an attraction and expulsion of oxygen 
 gas to go on at the same time, but would be doing 
 the same by the carbonic acid also : and how could 
 that acid be ever formed and emitted at all, if the 
 oxygen gas which enters into it is first singly ex- 
 pelled, and the carbon, which is its other constitu- 
 ent, is permanently retained ? And if we suppose, 
 that, during the day, carbonic acid is received and 
 oxygen gas expelled by the leaves, and that during 
 the night the exact contrary takes place * ; it seems 
 very difficult to account for the death of plants (3O.) 
 confined in a given bulk of air at all, since they 
 would, in fact, be weaving Penelope's web, undoing 
 by night what they had done by day, and vice versa. 
 All these difficulties are greatly increased by the sup- 
 position, that the air enters into the vessels of plants 
 in its undccomfioscd state ; for since the nitrogen gas 
 remains unaffected (29.), what useful purpose could 
 its reception answer ? Or by what means, when se- 
 parated from its oxygen gas, could it again be ex- 
 pelled from the plant in the exact bulk which it be- 
 fore occupied ? On these grounds, we deny the en- 
 trance of aeriform fluids into the vascular system of 
 plants, by any power analogous to chemical affinity. 
 38. If from the experiments already related 
 (32.), it be granted, that, during vegetation, the 
 oxygen gas of the air gradually and completely dis- 
 appears, and that a bulk of carbonic acid, nearly 
 
 * Thomson's System of Chemistry, vol. iv. p. 29L 
 
49 
 
 equal thereto (33.) is formed ; and if it be admit- 
 ted, that none of this gas is received into the vessels 
 of the plant (35, 6, 7.)> we must embrace the belief, 
 not only that the acid is formed out of the oxygen 
 gas originally present in the air, but that it is not 
 emitted ready formed by the plant. If the acid 
 were formed independent of this oxygen gas, the 
 whole bulk of air ought to be increased, instead of 
 being diminished, by vegetation : if it be not so 
 formed, then as no oxygen gas is received into the 
 plant, no carbonic acid can be emitted from it. 
 Plants also, in nitrogen gas, do not vegetate, and 
 cannot, therefore, by a living process, form carbonic 
 acid ; and when, in atmospheric air or in oxygen 
 gas, this acid is formed by vegetation, that gas disap- 
 pears, and the plant for a time lives : whence it fol- 
 lows, that without the presence of oxygen gas, the 
 living plant is unable to form carbonic acid, and 
 that this acid, therefore, is formed by the reciprocal 
 changes going on between this oxygen gas and the 
 plant. If we do not, indeed, admit the conversion 
 of oxygen gas into carbonic acid, there is no way 
 in which its loss can be accounted for, since there 
 is no other new product formed, and none of the 
 gas, as we maintain, enters into the vascular system, 
 and combines with the fluids of the plant. In all 
 the foregoing experiments also, the deterioration of 
 the air increased in proportion to the continuance of 
 the vegetative process, until the whole of its oxygen 
 gas completely disappeared ; and the gradual rising 
 of the mercury into the jar (32.), indicated a cor- 
 responding production of carbonic acid, until the 
 bulk of the latter nearly equalled that of the former ; 
 
 D 
 
oO 
 
 which reciprocal action can in no way be accounted 
 for, but by that conversion of the one gas into the o- 
 ther, which is here supposed actually to have place. 
 
 39. To effect this conversion, however, of the 
 oxygen gas, carbon must be from some source sup- 
 plied. This Dr Woodhouse supposes, as in the 
 case of seeds, to proceed either from the organized 
 remains of the soil in which the plants grow, or from 
 the carbon of the decayed leaves, when confined a 
 considerable time in a given bulk of air * : but it 
 has been shewn, that carbonic acid is equally pro- 
 duced (31.), where there is no soil present to af- 
 ford carbon ; and that it appears also in a few 
 hours (31.), and before the leaves, therefore, exhi- 
 bit any si^n of decay. Indeed, there does not ap- 
 pear any good reason why oxygen gas should be so 
 essential to the life of vegetables, if it could be act- 
 ed on or changed by those parts of them only which 
 are already dead. To the living plant, therefore, as 
 well as to the seed (20.), \ve must look as affording 
 carbon, and by the union of this carbon with the 
 oxygen gas of the air, must we consider the car- 
 bonic acid met with in vegetation to be formed. 
 
 40. But as this carbonic acid is produced in ve- 
 lion, the oxygen gas proportionally disappears, 
 
 (3O. ct seq.), and when this substitution is com- 
 plete, the plant gradually declines and dies. To what 
 cause is this effect to be immediately ascribed ? It 
 must arise from the superabundance either of ni- 
 trogen gas, or of carbonic acid, or from the defi- 
 
 Nicbolson's Journal, July 1802, p. 
 
51 
 
 ficiency of oxygen gas. Now, it has been shewn (29. % 
 that although plants do not grow in nitrogen gas, 
 yet that gas suspends only, but does not destroy 
 the vegetating faculty : and precisely the same oc- 
 curs when they are placed in vacuo, where no direct 
 effect can be attributed to any air at all. In all the 
 preceding experiments also, vegetation did not cease 
 till all the oxygen gas had disappeared ; and ? there- 
 fore, the superabundance of nitrogen gas did not 
 prove fatal so long as any oxygen gas remained. 
 Mr Gough, indeed, has shewn (29.), that certain 
 plants retain the faculty of growth after being con- 
 fined for weeks in nitrogen gas. To the want or 
 absence, therefore, of oxygen gas, and not to the 
 presence of nitrogen gas in any proportion, is the 
 death of plants in a given bulk of air, as well as that 
 of seeds (21.), to be entirely attributed. How far 
 the proportions of nitrogen, and oxygen gas, as they 
 exist in atmospheric air, may be the best adapted to 
 the actual structure and constitutional habits of plants, 
 we have not experimentally attempted to determine : 
 but the general analogy to be derived from the case 
 of seeds (1'4, 15.), and the fitness of means to the 
 perfection of an end observed throughout all the 
 operations of nature, lead irresistibly to the belief, 
 that these proportions are in reality the best. 
 
 41 . The only other gas present in the preceding ex- 
 periments, by which the growth of plants can be af- 
 fected, is carbonic acid : and the experiments of 
 Priestley *, Ingenhousz, and others, clearly evince, 
 
 * On Air, vol. i. p. 36. and iii. p. 310. 
 D 2 
 
that when confined in this acid, plants will die. 
 This, however, is no more than what we have seen 
 to take place in nitrogen gas (29.), and in vacua, 
 and cannot therefore be received as proof of the 
 positive operation of carbonic acid. From the ex- 
 periments of the late Dr Percival, and Mr Henry, 
 we learn also, that although plants, when wholly 
 confined in carbonic acid, certainly died, yet that 
 where a small portion of oxygen gas was admitted, 
 they as certainly lived and nourished *. Neither do 
 plants, confined in a given bulk of atmospheric air, 
 die till all the oxygen gas disappears (30.) ; nor, con- 
 sequently, does the quantity of carbonic acid form- 
 ed prove fatal so long as any oxygen gas remains. 
 So far, indeed, is carbonic acid from being fatal 
 to vegetation, that many have deemed it highly salu- 
 tary f, and some even contend that it is essential to 
 that process, alleging that it is absorbed by the 
 leaves of plants during the day, decomposed within 
 their vessels, its oxygen being afterwards emitted, 
 while its carbon is retained as food J. 
 
 42. Against this opinion of the absorption and 
 emission of gases by the leaves of plants, when grow- 
 ing naturally in air, we have already, both on phy- 
 siological and on chemical grounds, been induced to 
 enter our protest : but the importance of the que- 
 stion, together with the high authorities by which it 
 has been supported, will, we trust, plead in excuse 
 
 * Manchester Memoirs, vol. ii. p. 341. et seq. 
 
 f Ibid. vol. ii. p. 331. 
 
 J Thomson's Chemistry, vol. iv. p. 283. et seq. 
 
53 
 
 for the additional remarks which the present occa- 
 sion enables us to offer. Passing over, then, for the 
 present, the anatomical and chemical difficulties 
 which beset this opinion, let us, for a moment, ad- 
 mit the capability of the leaves to absorb or attract 
 carbonic acid ; and then examine how far such a 
 supposition is consistent with reason and with fact. 
 That the same substance, carbonic acid, should, du- 
 ring the day, be absorbed by the leaf, and decompo- 
 sed within it as salutary; and, during the night, should 
 be formed within the same leaf, and emitted from it 
 as noxious *, seems to be not only inconsistent, but 
 absurd. Where would be the advantage in the 
 carbon of the acid being retained for twelve hours 
 as food, if, for the next twelve, it must again be gi- 
 ven out as excrementitious ? Or where is there an 
 instance, in the whole circle of existence, of a living 
 agent not only first forming its own food, but feeding 
 on its own excretions? If this carbon were, during the 
 day, retained as food, whence comes that composing 
 the acid which plants, when confined in a given bulk 
 of air (31.), are constantly forming ? If oxygen gas, 
 as these chemists suppose, be during the day con- 
 stantly emitted, why does that gas gradually disap- 
 pear as the process of vegetation proceeds (32, 3.) ? 
 And why at last is none to be met with, although 
 there is present an abundance of carbonic acid, 
 out of which it is supposed to be formed ? It has 
 been proved, that during the day carbonic acid, by 
 the act of vegetation (32, 3.), is constantly forming ; 
 
 * Thomson's Chemistry, vol. iv. p. 283. 
 D 3 
 
54 
 
 but if, at the same time, it be as constantly absorb- 
 ed by the leaves, how can its presence be mani- 
 fested in such quantity, and in such progression, as 
 experiment evinces that it is ? If plants do die in a 
 given bulk of air, and that, too, more or less rapid- 
 ly as carbonic acid is produced (30. ), with what 
 propriety can we hold that substance to be essential 
 to their life ? If they do live and flourish in air de- 
 prived of carbonic acid (30.), and this acid be a 
 consequence of their growth, on what principle can 
 we at the same time assign it as a cause ; or how can 
 we consider that substance to be essential to the ex- 
 istence of this process, when it did not itself exist 
 until produced by the very process to which its ex- 
 istence was essential. 
 
 43. But it is said, that M. Saussure, by putting a 
 quantity of lime into the glass-vessel in which plants 
 were vegetating, found that they no longer conti- 
 nued to grow, and that the leaves in a few days fell 
 off *. Admitting the correctness of this experiment, 
 is it any proof that the process ceased from the want 
 of carbonic acid? Would not the lime equally abstract 
 the moisture of the plant, and would not this be 
 more likely to occasion the fall of the leaf? The 
 leaf, indeed, might die from the absence of any elas- 
 tic fluid essential to its life ; but, if it did, it would 
 still adhere to the stem. The following experiment 
 is offered in support of this opinion. Two equally 
 sized pots of earth, containing four growing peas 
 each, were placed under two equal jars of atmosphe- 
 
 * Thomson's Chemistry, vol. iv. p. 283. 
 
55 
 
 nc air, one of which was inverted over mercury, and 
 the other over water. Each of the pots was sur- 
 rounded by a quantity of powdered lime, spread on 
 the board by which it was supported. The leaves of 
 the plants in the mercurial jar continued green for 
 several days, and became at length very much 
 curled, but the plants themselves had not increased 
 in height ; and when withdrawn from the jar on 
 the sixth day, their leaves were dry to the feel, 
 crumbled between the fingers, and were still very 
 green. The jar, during the greater part of the time, 
 remained dry and transparent ; the lime was much 
 increased in bulk ; and the mercury had risen consi- 
 derably into the jar. In the other jar, the water, by 
 the second day, had risen so as to moisten the lime, 
 and its whole inside was quite bedewed with mois- 
 ture, which appearance it continued to exhibit 
 through the whole period of the experiment. The 
 leaves of the plants did not continue dry or green as 
 in the former case, but assumed a yellowish appear- 
 ance, and, by the fifth day, the plant had increased 
 an inch and a half in length, the leaves becoming 
 still paler and more yellow ; and when withdrawn 
 from the jar, on the sixth day, they were quite 
 moist to the feel. From these experiments, it ap- 
 pears, that lime, placed in jars with growing plants, 
 abstracts their moisture, and thereby checks their 
 growth ; but that if the lime be previously moisten- 
 ed, the growth of the plants is not checked, and their 
 moisture is as great as usual. But water is as ne- 
 cessary to vegetation (23.) as air ; and, therefore, 
 granting unto lime the power of attracting carbonic 
 ucid as well as water, the death of the plants in M ? 
 
 D 4 
 
56 
 
 Saussure's experiments, may with as much justice be 
 attributed to this abstraction of water, as to that of 
 the carbonic acid. 
 
 44. That this event did not arise from the ab- 
 straction of the carbonic acid, is farther proved by 
 the experiments which follow. Two glass-jars, each 
 containing about 4O cubic inches of atmospheric air, 
 were inverted over water, and into each of them 
 was introduced some mustard plants, growing on 
 flannel, which were supported, as before, by a small 
 hoop fixed half way up the jar. Under the hoop 
 in one jar was placed a small cup, containing about 
 an ounce of water of potassa : the other jar contained 
 the growing plants only. In 24 hours, the water 
 in the jar with the alkaline solution had risen 7-1 Oths 
 of an inch, and in 48 hours H inch ; but in the other 
 jar, in the same time, it had risen only 6-1 Oths of an 
 inch. The plants and alkaline solution were now 
 withdrawn under water from the first jar, and five 
 cubic inches of the residual air being then passed in- 
 to the eudiometer filled with lime-water, produced 
 not the smallest discoloration, nor suffered the least 
 diminution in bulk ; but the same quantity of the 
 air of the other jar, being afterwards treated in like 
 manner, rapidly made the lime-water milky, and 
 suffered a loss of nearly l-5th of its bulk. The 
 air of neither jar, after washing with lime-water, 
 experienced any loss of bulk by agitation with the 
 sulphuretted solution of potassa, and in all respects 
 the plants in both jars presented the same appear- 
 ance, a few only reviving on exposure to fresh air. 
 In both these jars, therefore, carbonic acid was 
 formed by the act of vegetation, but in one case it 
 
57 
 
 was abstracted as soon as formed by the alkaline 
 solution, while in the other it remained in contact 
 with the plants ; but neither the presence nor the 
 absence of this acid caused any difference in the 
 growth or appearance of the plants ; and their de- 
 cline is referable only to that complete abstraction 
 of oxygen gas, which has been before shewn to be 
 fatal to the vegetative process : hence then carbonic 
 acid, applied to the leaves of plants, is not essential 
 to vegetation, neither is it destructive to the continu- 
 ance of that process. 
 
 45. All that has been hitherto said, applies to 
 the circumstances of plants growing naturally in air : 
 when they are placed in water, other phenomena arise, 
 from which have been drawn arguments in favour 
 of an absorption and emission of gases by leaves. 
 Dr Priestley first advanced the opinion, that plants, 
 in certain circumstances, emitted oxygen gas; and In- 
 genhousz soon after discovered, that the leaves of 
 plants, when immersed in water, and exposed to the 
 light of day, produced an air which he announced as 
 oxygen gas. But whatever may be the results of 
 these experiments on plants immersed in water, they 
 are not necessarily to be received as proof of the 
 same actions being performed by the leaves when 
 growing naturally in the air : on the contrary, it has 
 been shewn by direct experiment (S3.), that when 
 plants are confined in a given bulk of atmospheric 
 air, they gradually and completely destroy its oxy- 
 genous portion, which could not possibly happen if 
 they possessed the power of emitting oxygen gas. 
 Moreover, the experiments of Dr Ingenhousz him- 
 self teach us, that this supposed emission of oxygen 
 
58 
 
 gas does not depend so much on the power of the 
 leaves, as on the quality of the water in which they 
 are immersed ; for if the water be previously boiled, 
 little or no oxygen gas is collected. River water af- 
 fords very little gas, but pump water is the most pro- 
 ductive of all : arid Sennebter also proved, that if 
 the air be previously deprived of all its air by boil- 
 ing, the leaves of plants immersed in it do not emit 
 a particle of air *. But not only must the water 
 previously contain air, but it must contain carbonic 
 acid ; for Sennebier has shewn, that no oxygen gas 
 is yielded by leaves when plunged in water destitute 
 of carbonic acid : that the quantity of oxygen afford- 
 ed is proportional to the quantity of carbonic acid 
 which the water contains ; and that when the water 
 loses the power of affording oxygen gas, all the car- 
 bonic acid which it contained has disappeared. These 
 experiments prove, that the oxygen gas, which is se- 
 parated when the leaves of plants are immersed in 
 water, depends altogether upon the presence of car- 
 bonic acid ! 
 
 46. But surely it cannot be maintained by any 
 one, who for a moment considers the structure and 
 livingfunctions of vegetables, that the leaves of plants, 
 when, as in these experiments, they are separated 
 from the branches, and wholly immersed in water, 
 absorb and decompose this carbonic acid as a natu- 
 ral and healthy function, and afterwards emit its oxy- 
 gen and retain its carbon. Not only, in these cir- 
 cumstances, must the circulation of their fluids be 
 
 Thomson's Chemistry, vol. iv. p. 28 4. f Ibid. 
 
59 
 
 completely suspended, but the plants must be en- 
 tirely cut off from the contact of atmospheric air, 
 which has been shewn to be essential to their life ; 
 and if the separated leaf speedily dies when its upper 
 surface only (23.) is laid upon water, what reason 
 have we to conclude that its life can be preserved by 
 immersing it completely in that fluid. Even grant- 
 ing that the production of oxygen gas, in these ex- 
 periments, was effected by a natural function of the 
 leaves, no sensible advantage, depending on this 
 power, can be supposed to arise to those plants which 
 flourish in the open air ; for the carbonic acid, from 
 the decomposition of which alone the oxygen gas is 
 derived, exists in the atmosphere in a quantity in- 
 finitely too small to be productive of any good effect: 
 and we have shewn, that instead of absorbing car- 
 bonic acid from the atmosphere, plants, by their ve- 
 getation, are constantly producing it ; and instead 
 of emitting oxygen gas, they are at all times con- 
 verting it into this very acid. Neither, after all, is 
 it clear that the oxygen gas, which is thus obtained 
 by the decomposition of carbonic acid, depends on 
 any peculiar operation of the leaves ; " for light is 
 an essential agent in the decomposition, and it is pro- 
 bably by its agency, or by its entering into combi- 
 nation with the oxygen, that this substance is enabled 
 to assume the gaseous form, and to separate from 
 the carbon *." 
 
 47. That the organized structure of the leaves is 
 not at all necessary to effect the separation of air from 
 
 * Thomson's Chemistry, vol. iv. p. 285. 
 
60 
 
 the water in which they are immersed, may be infer- 
 red also from the experiments of Count Rumford, 
 who found that dried leaves, fibres of raw silk, and 
 even of glass, when placed in similar circumstances, 
 produced a like separation of air. This air varied in 
 quality, being in many cases less pure than that of 
 the atmosphere * ; and Dr Woodhouse found river 
 water to yield chiefly nitrogen gas ! This varia- 
 tion arises no doubt from the nature of the air which 
 the water contains ; for Mr Dalton remarks, that al- 
 though atmospheric air, expelled from pure water, 
 contains 38 fier cent, of oxygen, yet that, by stagna- 
 tion, it loses a part or all of it, notwithstanding its 
 constant exposition to the atmosphere : and this must 
 arise from some impurities in the water which com- 
 bine with the oxygen, since pure rain water lost 
 none of its oxygen after standing in a bottle more 
 than a year J. Hence, then, we see, that to effect the 
 separation of air from water, the organized structure 
 of the leaf is not only not necessary, but that the 
 quality of the separated air is altogether different from 
 what this supposed function of the leaves ought to 
 supply. No proof, therefore, of the absorption and 
 emission of gases, much less of oxygen gas, by the 
 natural functions of leaves, can be derived from these 
 experiments on plants immersed in water : and were 
 the experiments even more precise, they would not 
 in the least apply to the case of vegetables which 
 nourish in the open air. 
 
 * Philosophical Transactions, 1787. 
 
 f Nicholson's Journal, July 1802. 
 
 ; Manchester Memoirs, vol. i. new series. 
 
CHAP. III. 
 
 <<>F THE CHANGES INDUCED ON THE 
 AIR BY THE RESPIRATION OF INSECTS, 
 WORMS, FISHES, AND AMPHIBIOUS ANI- 
 MALS. 
 
 48. JL o the commencement of living action 
 in vegetables, the presence of water (1. 23.) has been 
 shewn to be essentially necessary, and among several 
 inferior animal beings, its operation is equally striking 
 and apparent. The ova of innumerable tribes of ani- 
 mals, some of which afterwards inhabit the air, are 
 deposited in water, and undergo their various stages 
 of evolution only while exposed to the influence of 
 that necessary fluid. Neither is its agency confined 
 to the earliest states of existence, nor to those animals 
 which i may be properly called aquatic ; for examples 
 abound, where its operation extends through every 
 period of life, and among animals which reside whol- 
 ly in the air. Snails in their shells have been thrown 
 into a drawer, and lain by for fifteen years ; but reco- 
 
62 
 
 vered action on being immersed in a bason of water *. 
 The gordius, or horse-hair eel, while in water, is in 
 incessant motion, but, if the water dry up, its move- 
 ments cease, it shrivels up, and may be kept in this 
 state for an indefinite length of time ; but place it a- 
 gain in water, it begins to move, and, in a few mi- 
 nutes, is as brisk and lively as ever. There is an a- 
 nimalcule that sometimes resides in wheat, which, 
 after lying dormant for nearly thirty years, has reco- 
 vered its vital functions, merely by moistening the 
 grain with water. The rotifer (vorticetta rotatorla\ 
 which lives in small puddles of water, often on the 
 tops of houses, shrivels up as the water evaporates, 
 till it becomes like a piece of dried parchment, in 
 which state it may be preserved for years without 
 suffering the smallest change : but on moistening it 
 with water, it resumes its pristine form, and soon 
 becomes as lively as ever. Suffer the water again to 
 evaporate, and the animal dries up as before : but 
 restore to it the moisture, and again it is brought to 
 life. In this way it has been alternately deadened and 
 revived eleven times, without any apparent exhaus- 
 tion of its vital powers : and although subjected, 
 during its torpid state, to a heat of 56 Reaumur, 
 and a cold of 19, it was equally susceptible of 
 revivification as at first f. These facts sufficient- 
 ly demonstrate the necessity of water to the com- 
 mencement of animal action. 
 
 * Philosophical Transactions, 1771. 
 
 f A ' H?crcations in Agriculture, No. x. p. 255. et seq. 
 
63 
 
 49. Equally essential to this action is the pre- 
 sence and operation of heat. The ova of myriads of 
 insects are evolved by its immediate influence; and its 
 power is not less necessary to the maintenance, than 
 to the beginning of living action. When abstract- 
 ed to a certain degree, numerous tribes of animals 
 pass into a torpid state, and again recover action as 
 the heat is restored. Caterpillars, spiders, and ants, 
 were many times in succession rendered torpid, and 
 again restored to action, in the experiments of Dr 
 Michelotti, by the alternate abstraction and commu- 
 nication of heat *. In a temperature, 1 Reaum. 
 Spallanzani found living action in snails to cease ; 
 and the same result was obtained by exposing the 
 marmot to a similar abstraction of heat t. The heart 
 moves quicker in hot than in cold animals, says Dr 
 Irvine ; and in many animals, during the severe cold 
 of winter, it does not move at all. The heart of snails, 
 which beats manifestly in summer, was found to be 
 perfectly at rest in winter ; and the same thing is ob- 
 served in many of the fly tribe. Of the same descrip- 
 tion too, are the serpent and viper tribe, frogs, 
 toads, and tortoises : even the bat, which is naturally 
 a hot animal, becomes, during the winter, as cold 
 as the surrounding medium, and its heart is perfect- 
 ly at rest J. If an egg be opened some days after 
 incubation, so that the fiunctum saliens may come in- 
 to view, according as it is exposed to heat or cold, 
 
 * Phil. Mag. June 1804. 
 
 f Memoirs on Respiration, p. 154. 334. 
 
 t Irvine's Chemical Essays, p. 201. 
 
64 
 
 says Dr Mayhow, you will perceive the corculum or 
 heart to pulsate, or to languish and cease from mo- 
 tion *. The hearts of frogs, reduced to torpidity, 
 were removed by Spallanzani, and living action 
 could still be excited in them, by the re-application 
 of a proper degree of heat : and the heart of a turtle, 
 on being put into milk-warm water, was repeatedly- 
 observed by Dr Gardiner to yield a tremulous mo- 
 tion, six or seven hours after it was removed from 
 the body, and had become much shrivelled and 
 dried. If suffered to become cold, it was insensible 
 to every stimulus ; but when again warmed in wa- 
 ter, it repeated its palpitations on being pricked with 
 a needle f These and many other facts which 
 might be adduced, sufficiently establish the necessary 
 concurrence of heat to the production of action in 
 animals : and prove likewise, that its abstraction, 
 although causing a suspension of the animal functions, 
 does not necessarily destroy the capacity of their re- 
 newal. 
 
 50. The direct effects of light in producing the vari- 
 ous colours of vegetables (25.) have been distinctly 
 proved : and the experiments and observations of 
 Mr Davy, go to shew that this agent exerts a simi- 
 lar operation on the various classes of animal beings. 
 He has observed, that the zoophyta exposed to light, 
 are uniformly brighter coloured than those which 
 have by any means been secluded from it ; and he 
 succeeded in altering the colour of two sea anemones 
 
 * Tractat. Quinq. p. 325. An. 1674. 
 j- On the Animal (Economy, p. 4 ft. 
 
65 
 
 from a dark red, to a pale pink, by secluding them 
 from light. The parts of fish, which are exposed to 
 light, exhibit various colours ; but the belly, which 
 is deprived of light, is uniformly found white in all 
 of them. The birds that inhabit the tropical coun- 
 tries, are much brighter coloured than those of the 
 north : and those parts of the feathers, which are 
 exposed to light, are almost always coloured, while 
 the parts secluded from light are generally pale or 
 white. The same observations apply to the hairs of 
 quadrupeds : and, not only are the beasts of the 
 equatorial uniformly brighter coloured than those of 
 the polar countries, but the northern animals are 
 dark coloured in summer, and white, or pale, in 
 winter *. These observations point out the influence 
 of light, in giving rise to the colour of animals ; but 
 they do not, any more than in vegetables, shew the 
 immediate necessity of it to the existence of animal 
 action. 
 
 5\. But when, by the combined agency of water 
 and heat, the animal structure is brought into a con- 
 dition fitted for exhibiting living action, the presence 
 of atmospheric air is required to enable it to con- 
 tinue this action ; for, as far as observation has ex- 
 tended, no living being can long subsist without a 
 due supply of fresh air, " In insects," says Mr Ray, 
 " there are many orifices on each side of their 
 bodies for the admission of air, which, if you stop 
 with oil or honey, the insect presently dies, and re- 
 yives no more. This, he adds, was an observation 
 
 * Contribution* to Science by Beddoes, p. 192- 
 
66 
 
 of the ancients, though the reason of it they did not 
 understand, (oleo /V///0, insect a omnia exanimantur, 
 Plin.), which was nothing but the interceding of 
 the air ; for, though you put oil upon them, if you 
 put it not upon or obstruct those orifices therewith,, 
 whereby they draw the air, they suffer nothing *." 
 Mayhow observes, that if the oil be applied only on 
 some of these orifices, the neighbouring parts im- 
 mediately become paralytic, by being deprived of 
 the nitro-aerial particles of the air, while the other 
 parts, in the meanwhile, continue sound f. Mr Der- 
 ham found, that wasps, bees, hornets, and grasshop- 
 pers, seemed dead in two minutes, when placed 
 under the exhausted receiver ; but revived in two 
 or three hours on being restored to the air, even 
 though they had remained in vacuo twenty-four 
 hours J. Of the vcrmes class, snails survive several 
 hours in the exhausted receiver : efts, or slow worms, 
 two or three days ; and leeches, five or six ((. 
 
 .j(2. The same necessity of fresh air in the water 
 in which they live, is required by aquatic animals. 
 Zoophytes, according to Mr Davy, require the pre- 
 sence of air in the water in which they grow, and 
 they act upon it like fishes . The animalcules in 
 pepper water remained in vacuo twenty-four hours : 
 and, being afterwards exposed a day or two to the 
 
 * Wisdom of God in the Creation, p. 82. 
 
 f Tractat. Quartus, cap. iv. p. 39. 
 
 J Physfco-Thedlogy, p. 8. 7th Edition. 
 
 J| Hutton's Mathematical Dictionary, article Air Pump, 
 
 Bcddocs's Contributions to Science, p. 138. 
 
67 
 
 open air, Mr Derham found some of them dead and 
 some alive *. Mr Ray remarked, that fishes cannot 
 live in water without air : they will live in a vessel 
 of water with a narrow mouth for months or years ; 
 but if the vessel be stopped, so as wholly to exclude 
 the air, or interrupt its communication with the wa- 
 ter, they will be suddenly suffocated f. Dr Priest- 
 ley confined several small fishes in a vessel, contain^ 
 ing three pints of rain water, that had been previously 
 well boiled to deprive it of its air, and they lived 
 only between three and four hours J. Mr Davy 
 introduced a large thornback into a jar containing 
 three cubic inches of water, which had been depri- 
 ved of its air by distillation through mercury : he 
 was very quiet for four minutes and a half, but 
 then began to move about, and, in seven minutes, 
 had fallen on his back, but still continued to move 
 his gills. In eleven minutes, he was motionless, and 
 when taken out, after thirteen minutes, he did not 
 recover ||. Amphibious animals, likewise, cannot 
 live without air, but its deprivation is not imme- 
 diately fatal to them. Frogs and toads bear the 
 pump for two or three hours, and a frog recovered 
 on exposure to the air, after remaining in vacua 
 seemingly dead for eleven hours . Hence we see, 
 that, to all these animals, whether inhabiting the air 
 
 * Physico-Theology, p. 8. 
 
 f Wisdom of God in the Creation, p. 81. 
 
 1 Observations on Air, vol. v. p. 139. 
 
 j| Researches concerning Nitrous Oxide, p. 367> 
 
 $ Physico-Theology, loc. cit. 
 
 E 2 
 
6'S 
 
 or the water, a constant renewal of fresh air is re- 
 quired, while the actions of life continue. What 
 then are the changes induced on atmospheric air by 
 these several classes of animal beings, whereby it is 
 rendered so essential to the maintenance of vital ac- 
 tion ? 
 
 53. For the first, and most accurate knowledge 
 we possess concerning the changes which the air 
 suffers by the respiration of insects, v/e are indebted 
 to the labours of the celebrated M. Vauquelin. The 
 experiments of this excellent chemist were made on 
 the grasshopper (gryllus viridissimus}, which is de- 
 scribed as having twenty-four stigmata, or breathing 
 pores, ranged parallel with, but exterior to, two 
 white lines, extending longitudinally on the middle 
 of the belly. In this insect they are of an oval form, 
 but they vary in shape in different insects : and it is 
 chiefly by their mediation, that the changes on the 
 air are effected. A female grasshopper was placed 
 in eight cubic inches of atmospheric air : it breath- 
 ed from fifty to fifty -five times in a minute, and li- 
 ved thirty-six hours. The air had not sensibly di- 
 minished in volume, but, when examined by the test 
 of lime-water, carbonic acid was detected, and after 
 this was removed, the remaining air still extinguish- 
 ed a taper. When many grasshoppers were put at 
 the same time into a given bulk of air, and left 
 till they died, the oxygen gas was nearly, but not 
 entirely, consumed : and phosphorus melted in the 
 residual air when heat was applied, but burned very 
 little. A male grasshopper lived eighteen hours in 
 six cubic inches of oxygen gas : its respiration was 
 oppressive, and it breathed from sixty to sixty-five 
 
69 
 
 times in a minute. The volume of air was not sen- 
 sibly diminished, but it lost y~ of its bulk by being 
 washed in an alkaline solution *. From these facts, 
 we learn, that insects, by their respiration, consume 
 the oxygenous portion of the air : that carbonic acid 
 is, at the same time, produced ; and that, when all 
 the oxygen gas has disappeared, the animal no long- 
 er survives. 
 
 54. M. Huber found, that bees very speedily die 
 when put into nitrogen gas, but that they survive in 
 a close vessel of atmospheric air, until almost the 
 last atom of its oxygen gas is consumed ! We 
 likewise confined a number of flies in a flask, con- 
 taining nine cubic inches of air, and then inverted 
 it into a tall glass of mercury. By the third day, 
 the flies were all dead, and the mercury had risen con- 
 siderably into the neck of the flask. The residual air 
 lost about ^ by agitation with lime-water, and the 
 remainder did not suffer the smallest diminution by 
 being placed in contact for two days with phospho- 
 rus. These results, therefore, agree with those ob- 
 tained by Vauquelin, and prove farther, that, by the 
 respiration of flies, the whole of the oxygen gas of 
 the air disappears, and that a bulk of carbonic acid 
 nearly equal thereto is formed. The small diminu- 
 tion of bulk also which the air suffered, is to be re- 
 garded as a necessary consequence attending the con- 
 version of oxygen gas into carbonic acid, and which, 
 as it accounts for the whole loss the air experienced, 
 
 Ann. de Chitnie, torn. xii. 
 
 t Mem. sur la Germination, &c. par M. M. Huber et Senncbier. 
 
 E3 
 
70 
 
 seems to authorise the conclusion, that, while the 
 oxygen gas had, in this case, completely disappear- 
 ed, the nitrogenous portion of the air continued un- 
 diminished, and probably unaltered. 
 
 55. M. Vauquelin proceeded next to investigate 
 the changes produced on the air by the respiration 
 of the wrmes class of animals. He confined a red 
 slug in twelve cubic inches of atmospheric air, and 
 it lived forty-eight hours. He thinks, that in this 
 animal the breathing pores are situated chiefly be- 
 hind the head. The air was not sensibly diminish- 
 ed in volume, but it extinguished candles, and copi- 
 ously precipitated lime from water. Phosphorus 
 was melted in this air, but did not suffer any com* 
 bustion or change of colour. A snail (helix fioma- 
 tia) was next put into twelve cubic inches of atmo- 
 spheric air, and lived four days. The oxygen gas 
 entirely disappeared ; for the residual nitrogen gas 
 contained not an atom of vital air, and, consequent- 
 ly, phosphorus did not burn in it at all : it contain* 
 ed, however, carbonic acid. Slugs and snails, there- 
 fore, require fresh air while in an active state, the 
 oxygen gas of which, by the function of their respi- 
 ratory organs, is made completely to disappear, and 
 a quantity of carbonic acid is produced, while the 
 nitrogenous portion of the air remains unaltered : 
 and when these changes are effected, living action 
 speedily comes to an end. So exactly do these ani- 
 mals separate the oxygenous from the nitrogenous 
 portion of the atmosphere, that M. Vauquelin sug- 
 gests the employment of them for eudiometrical pur* 
 poses *. 
 
 * Ann* de Chimie, loc. cit. 
 
71 
 
 56, These experiments of M. Vauquelin were re- 
 peated by the late Abbe Spallanzani, and nearly with 
 ihe same results. That industrious philosopher con- 
 iined a slug (limax fla<vus) in a given quantity of atmo- 
 spheric air ; and the whole of its oxygen gas entirely 
 -disappeared, for the residual air was not in the least 
 diminished by the introduction of phosphorus *. O- 
 ther living slugs also entirely consumed the oxygen 
 gas of the air, and produced carbonic acid, while the 
 nitrogen gas remained unaltered f. In other in- 
 stances, however, the whole of the oxygen gas did not 
 disappear during the life of the animal : but, whether 
 this happened or not, the nitrogen gas was, in all 
 cases, left undiminished J. When placed in pure 
 oxygen, a portion of that gas likewise disappeared, 
 and carbonic acid was, in like manner, produced |j. 
 Different species of worms were shut up in a given 
 quantity of air, and they all consumed the whole of 
 the oxygen gas it contained, and carbonic acid was 
 always produced : and when pure oxygen gas was 
 employed, more of it disappeared, and carbonic acid 
 was in proportion produced , The results of all 
 these experiments coincide completely with those of 
 M. Vauquelin, related in the preceding paragraph : 
 they prove that the oxygenous portion of the air en- 
 tirely disappears, that carbonic acid is produced, 
 and that the nitrogen gas continues unaltered. 
 
 57. Different species of snails were next submit- 
 ted to experiment by the same author. One of 
 
 * Memoirs on Respiration, p. 24-1. f Ibid. p. 2.53. 
 
 t Ibid, p. 2JS. I] Ibid. p. 253. Ibid. p. 63 70, 
 
72 
 
 these animals was confined in seven cubic inches of 
 air, inverted over mercury, for the space of six days, 
 in a temperature varying from 7 to 8 Reaum. : 
 and the bulk of air was sensibly diminished. A 
 quantity of the residual air was then introduced into 
 an eudiometrical tube, filled with mercury, so as to 
 occupy one hundred parts : it was afterwards washed 
 in lime-water, and re-introduced into the eudiome- 
 ter, on which the mercury rose to 11. Phospho- 
 rus was next inflamed in the remaining air ; and 
 when every thing was again brought back to the 
 former temperature, the mercury had risen to 1 ly *. 
 This elevation of the mercury, he adds, is not very 
 sensible when phosphorus is employed, but when e- 
 qual quantities of the residual air, and of nitrous gas 
 were mixed together, so as to occupy two hundred 
 parts, the diminution was four, five, or six of these 
 parts, which indicated the quantity of oxygen gas 
 the air contained f. From these results, he is led to 
 conclude, in opposition to M. Vauquelin (55,), that 
 the consumption of oxygen gas., by the respiration 
 of snails, is not complete. 
 
 58. With regard to this partial consumption of 
 oxygen gas by snails, it may be observed, that the 
 author found the same thing sometimes to happen 
 with slugs, which he, nevertheless, concludes gene- 
 rally to consume the whole of this portion of the at- 
 mosphere. The portion of oxygen gas unconsumed, 
 is likewise so very small, and the result, by the test 
 of phosphorus, so nearly approximates to that ob- 
 
 * Memoirs or. Respiration, p. 146. ' Ibid. p. 
 
73 
 
 tained by Vauquelin, as to render it difficult to con* 
 ceive, but for some occasional error, that they should 
 not entirely coincide. Of the probable existence of 
 such error, the great difference in the analysis, made 
 by the combustion of phosphorus, and by the test of 
 nitrous gas, furnishes strong suspicion: and it is 
 known that this latter method requires many precau- 
 tions to insure complete accuracy. Even if all these 
 had been observed, more attention to the actual sur- 
 rounding temperature was required, than seems, in 
 these experiments, to have been bestowed ; for, in 
 a small graduated tube, air is itself the most sensible 
 thermometer, and readily suffers a change of bulk 
 from the slightest variations of temperature, and, in 
 the measurement of small quantities, demands, there- 
 fore, the utmost attention and correctness. In his 
 analysis of the air which had been respired by snails, 
 M. Vauquelin could not well be deceived by the non- 
 combustion of phosphorus ; and, although this fact 
 does not disprove the assertion of Spallanzani, yet, 
 as it proves more, it is entitled to the preference ; 
 for, where the same degree of credit is attached, a posi- 
 tive must ever outweigh a negative proof. The snail 
 too, in this experiment, might have died from some 
 accidental cause, before the consumption of the oxy- 
 gen gas was complete, while, in other instances, the 
 same species of animal would entirely consume it : 
 and we find, accordingly, that Spallanzani himself, 
 in several parts of his work, asserts that snails, by 
 their respiration, consume all the oxygen gas of the 
 
 * Memoirs on frc-spiration, p. 153. 165. 172. 213. 222. 2*9. 340. 
 
71 
 
 ,59. But not only does Spallanzani dissent from 
 the conclusion of Vauquelin, as to the complete de- 
 struction of oxygen gas by the respiration of snails : 
 he contends likewise, that a portion of the nitro- 
 gen gas at the same time disappears. He placed dif- 
 ferent single snails in several equal bulks of atmo- 
 spheric air, where they remained till they ceased to 
 exhibit any signs of life. To ascertain the con- 
 sumption of nitrogen gas, he passed the residual air 
 into the eudiometrical tube through mercury, and 
 compared its bulk with that which it possessed be- 
 fore the snails were placed in it. Having then as- 
 certained the complete destruction of the oxygen 
 gas, by the test of phosphorus, he next withdrew the 
 carbonic acid by means of lime-water ; and every 
 degree of diminution beyond ~, which he considers 
 us the proportion of oxygen gas contained in atmo- 
 spheric air, was referred to the destruction of the ni- 
 trogenous portion of the air *. 
 
 60, But, in these experiments, he has overlooked 
 many circumstances which ought to have been at- 
 tended to : and hence the results differ so much as 
 to render the conclusion quite unsatisfactory. In 
 two instances, - of the oxygen gas of the air disap- 
 peared, and from five to ~ of the nitrogen : but, 
 in two others, only ^ or ^ of the former were 
 lost, and from three to ~ of the latter ! Where, 
 in one experiment, two snails were confined together, 
 the air lost ~ of its oxygen, but only ~ of its ni- 
 
 Memoirs on R"Pp!ratipn, p. 162, f Void. p. 
 
it ogen ; so that much less nitrogen gas was de- 
 stroyed in this case by two snails, than in the 
 former experiments by one. In other experiments 
 with snails, he has more than once found the whole 
 eighty parts of nitrogen gas remaining ; and the 
 helix vivifiara consumes all the oxygen gas of the 
 air without producing any increase or diminution of 
 the nitrogenous portion *. These facts, taken in 
 connection with the apparently decisive experiments 
 of Vauquelin, and those of the author himself with 
 regard to slugs, lead us to the direct conclusion, that 
 in the respiration of slugs, worms^ and snails, the 
 whole of the oxygenous portion of the atmosphere 
 completely disappears ; that carbonic acid is, in all 
 cases, produced, and that the nitrogen gas remains 
 unaltered. 
 
 6 1 . Nearly the same results were obtained by ex- 
 periments on the respiration of another species of 
 snails, whose residence is wholly in the water. Spal- 
 lanzani found, that the helix vivifiara, which inhabits 
 still rivers and pools, consumed, by its respiration, 
 the oxygen gas of the atmosphere, like snails which 
 live on the land : that this action did not go on in 
 temperatures a few degrees only above the freezing 
 point, but was very considerable in higher tempera- 
 tures : that water, deprived of its air by boiling, did 
 not support life, neither did it, when freed from its 
 oxygen gas, and standing in contact with nitrogen 
 gas : that snails, confined to the bottom of a jar of 
 water, consumed only half the quantity of air that 
 those did which were allowed to come to the surface : 
 
 * Memoirs on Respiration, p. 21 4s 301. 
 
76 
 
 that the water attracts oxygen gas to supply the place 
 of that which is consumed ; and that, when confined 
 in air only, these snails, by means of their skins, 
 consume all the oxygen gas, and produce carbonic 
 acid, without changing the quantity of nitrogen gas*, 
 62. Other aquatic animals, as muscles (niytilus e- 
 didis} were next confined in a jar, and water was 
 poured over them to the depth of an inch. They 
 soon opened the thinnest part of their shell, and, in 
 ten or fifteen minutes more, threw out a small quan- 
 tity of water : the shells were then shut again for a 
 few minutes, and this action was alternately repeat- 
 ed. When two of these muscles were placed in a 
 tube, half filled with water and half with air, in a 
 temperature of about 66 Fahrenheit, they continued 
 to live seven days, and, on the eighth day, the oxy- 
 gen gas of the air was considerably diminished. 
 With a portion of oxygen gas occupying the supe- 
 rior part of the vessels, these muscles would live nine 
 days, but when nitrogen gas was in the same way 
 employed, they died in three days. In every case, 
 whatever quantity of nitrogen gas was present, it al- 
 ways remained unchanged f. On several species of 
 marine testacca, he likewise made experiments, and 
 found that they consumed the oxygen gas of sea- 
 water, which attracts more to supply the waste ; that 
 when confined in air only, they consume all its oxy- 
 gen gas, and that they soon perish when nitrogen 
 gas only is made to rest on the water, producing no 
 change upon it J. 
 
 * Memoirs on Respiration, p 288. 296. 301. 
 
 f Ibkl. p. 304. 306. 309. J Ibid, p, 311. et scq, 
 
77 
 
 63. It has been already shewn (52.), that fishes 
 cannot live in water deprived of air, nor unless this 
 air be constantly renewed : and Dr Priestley proved, 
 by experiment, that they deteriorate the air which 
 water contains. Several minnows were confined by 
 him in a large phial of water till they died : and the 
 air being afterwards expelled from the water, was 
 examined by the test of nitrous gas, and found to 
 contain less oxygen than that in which a candle goes 
 out. He likewise impregnated water, previously de- 
 prived of its oxygen, with nitrogen and with hydro- 
 gen gases, and found, that in such water, the fishes 
 died in about an hour *. Mr Davy ascertained that 
 fishes die in a few minutes in water containing nitro- 
 gen gas, but live in that which is impregnated with 
 oxygen gas: that the proportion of this latter gas is 
 diminished by them, and carbonic acid is produced ; 
 for, on adding lime-water to that in which the fishes 
 had been confined, a cloudiness was very perceptible, 
 indicating the formation of carbonate of limef- 
 Dr Carradori has observed, that fishes are able to 
 exhaust water entirely of its oxygen, which ebulli- 
 tion is unable to effect : and that they die instant- 
 ly in water wholly deprived of oxygen J. To 
 supply the waste of oxygen gas, occasioned by the 
 respiration of fishes, water, as Scheele first remark- 
 ed, is endued with the power of attracting it in pre- 
 
 * On Air, vol. v. p. 137. et seq. 
 
 f Bcddoes's Contributions to Physical Knowledge, p. 1 3' 
 
 i Nicholson's Journal, vol. xvi, p. 76. 
 
78 
 
 ferencc to common air, and nearly in the same pro- 
 portion *. Dr Crawford observes, that if a portion 
 of atmospheric air be exposed in an inverted jar to 
 water which has had its air separated by boiling, the 
 purer part will be attracted by the water, and the 
 noxious portion will remain distinct in the vessel f. 
 The experiments of Mr Dalton, however, do not 
 allow us to suppose that water thus completely ana- 
 lyzes the air, for, according to him, if a quantity of 
 water, deprived of air, be agitated with atmospheric 
 air, the water will attract portions of each of its consti- 
 tuent parts the same as if they were presented to it 
 separately in their proper density J. From the opera- 
 tion of this distinct attractive power of water for the 
 two gases of which the atmosphere is composed, it 
 follows, that the oxygen gas, contained in water, will 
 bear a considerably larger proportion to the nitro- 
 gen gas than exists in the air of our atmosphere ; 
 and that as this oxygen gas is consumed by the re- 
 spiration of aquatic animals, more will be attracted 
 to supply its place, and to maintain the due respira- 
 bility of the air. 
 
 64. The changes induced on the air by fishes are 
 effected by the branchia or gills, which in form and 
 structure vary greatly, according to the mode of 
 life of the fish. In eels, and those fishes which live 
 in impure water, the gills are supported by boney 
 arches, and are very large in proportion to the size 
 
 * Thomson's Chemistry, vol. iv. p. 490. 
 
 f On Animal Heat, p. 145. 
 
 \ Manchester Memoirs, vol. i. new series. 
 
79 
 
 of the fish : others, as the lamprey, receive the wa- 
 ter into their gills by the mouth, and expel it through 
 several holes in their sides, while those fishes that 
 move rapidly, and make long migrations, take in the 
 water largely by the mouth, and reject it very often by 
 the gills. The gills were by the ancients considered 
 to perform an office for fishes similar to that which 
 the lungs perform for land-animals ; and Mayhow 
 conceived them to be especially constituted for se- 
 parating the air from water, whereby some vital 
 aerial property was conveyed into the mass of blood. 
 Hence it is, he adds, that fishes alternately draw in 
 and expel water, as land-animals receive and expire 
 air *. 
 
 65. But a somewhat more particular description 
 of the structure of the gills of fishes will much as- 
 sist our conception of the manner in which the re- 
 spiratory function is performed by them. In fishes, 
 the heart consists only of one auricle and one ven- 
 tricle ; and from the latter, one artery is sent off, 
 which is spent entirely on the gills. This artery 
 conveys venal blood from the heart, which, in its 
 passage through the gills, assumes a florid hue, and 
 being afterwards collected by the branchial veins 
 into one large trunk, is distributed, without the inter- 
 vention of a second heart, to all parts of the body ; 
 from which it is again brought back to the heart in 
 a venal state, to undergo the same circulation. The 
 gills, upon which the branchial artery ramifies, are 
 of great extent. In each side of the body of a skate, 
 
 * Tractat. Quinq. p. 259. 
 
80 
 
 says Dr Monro, there are four double gills, or gills 
 with two sides each, and one single gill ; or there 
 are in all eighteen sides or surfaces on which the 
 branchial artery is spread out. On each of these 
 sides, there are about fifty divisions or doublings of 
 the membrane of the gills ; and each division has 
 on each side of it one hundred and sixty subdivi- 
 sions or folds of its membrane, the length of which, 
 in a very large skate, is about one-eighth part of an 
 inch, and its breadth about one-sixteenth part : so 
 that in the whole gills there are about 1 44,OOO sub- 
 divisions or folds, the two sides of each of which are 
 equal to the sixty-fourth part of a square inch j or the 
 surface of the whole gills in a large skate is equal to 
 2.250 square inches, that is, to more than fifteen 
 square feet, which have been supposed equal to the 
 whole external surface of the human body. When, 
 after a good injection of the branchial artery, a mi- 
 croscope is applied, the whole extent of the mem- 
 brane of the gills is seen covered with a beautiful 
 network of exceedingly minute vessels ; and if dis- 
 tilled oil of turpentine, coloured with vermilion, has 
 been injected with moderate force in a living or re- 
 cently dead skate, some of the colourless parts of 
 the oil exude upon the surface of the gills *. From 
 all these facts, concerning the respiration of fishes, 
 we learn, that the oxygen gas of the air contained 
 in water, is changed (63.) into carbonic acid by the 
 medium of their gills ; and that their blood, like 
 that in the lungs of breathing animals, loses at 
 
 * Monro on the Structure, &c. of Fishes, p. 
 
81 
 
 same time its venal characters, and becomes arteriali- 
 zed. Since also, it has been shewn (63.), that fishes 
 do not live in water which contains only nitrogen gas, 
 it is reasonable to infer, that, like water-snails and 
 muscles, they produce no change upon it. 
 
 66. The amphibious class of animals, which live 
 partly in air and partly in water, exhibits great va- 
 riety in the structure of their circulating and respi- 
 ratory organs. In the frog and toad, the heart con- 
 sists of one auricle and one ventricle, as in fishes. 
 The auricle receives the venal blood from the body, 
 which passes into the ventricle, and from thence in- 
 to the aorta ; but this aorta soon divides into two 
 branches, one for the body and one for the lungs ; 
 and hence but half of their blood is, in each circula- 
 tion, exposed to the action of the air received into 
 the lungs. In the turtle, the same intention is ef- 
 fected by a different mechanism ; for, though the 
 heart of that animal consists of four cavities, yet the 
 ventricles freely communicate, and therefore the pul- 
 monary artery and aorta arise in fact from one cavi- 
 ty ; so that only half of the blood, thrown out at each 
 contraction of the ventricles, will pass through the 
 lungs, provided the areas of the two arteries corres- 
 pond. As a part only of the blood is thus sent 
 through the lungs in each circulation, it is plain that 
 a cessation of the respiratory function does not neces- 
 sarily put a stop to the circulation of the blood in these 
 animals, as it does in fishes, in the mammalia and in 
 birds ; and such animals are said to possess a jiulmo 
 arbitrarlus^ or are able to live either in water or in 
 air. The length of time which they can live without 
 respiration, has been supposed to depend on the 
 
 F 
 
$2 
 
 structure of their lungs, and the capacity of these to 
 receive and contain air. In the frog and toad, the 
 lungs consist of two large membranous bags, divided 
 into a great number of vesicles, over which the blood- 
 vessels are minutely distributed ; and, in the snake, 
 viperj and many others, the lungs are continued 
 down through the whole belly, in form of two bags. 
 Many animals also jflr ttrin ifttt\ as the otter and por- 
 poise, whose lungs are constituted like those of man, 
 can live a considerable time under water without 
 breathing : and this power is much improved by 
 habit. This is the case even in the human subject ; 
 for, while in ordinary persons, suffocation begins to 
 take place in about half a minute when the body is 
 submersed in water, those who dive for pearls, co- 
 rals, cc. are said, by long practice, to be able to 
 prolong this period to several minutes, being able to 
 keep under water as long as the seal, porpoise, and 
 the amphibia. 
 
 67. Of all the foregoing animals, therefore, which 
 by naturalists have been placed in the class amphibia, 
 none can be said to be truly amphibious, or to possess 
 the faculty of supporting life, for an indefinite length 
 of time, either in water or in air. This faculty belongs 
 only to the syren, an animal said to be furnished 
 both with lungs and gills. Something of the same 
 sort may, indeed, be attributed to the frog at diffe- 
 rent periods of its existence ; for we are told, that, 
 during the first fourteen days of its life as a tadpole, 
 it has only gills projecting like fins : that, by the 
 thirty-sixth day, these are taken into the jaws, and 
 form four rows of gills on each side, like those of 
 fishes : and that, during this time, the lungs, as in the 
 
83 
 
 foetus of the mammalia, are inert, and not called 
 into use until the animal exchanges his watery habi- 
 tation for an aerial one, when the gills gradually 
 shrink *. Hence, then, it appears, that however 
 various the structure of the lungs in this class of ani- 
 mals may be, a more or less constant supply of fresh 
 air is required to enable them to support the func- 
 tions of animal life. 
 
 68. To obtain a knowledge of the specific changes 
 which the air suffers by the respiration of the am- 
 phibia, the following experiments were instituted. 
 A toad, supported on a small hoop, was inclosed in 
 one hundred and eight cubic inches of atmospheric 
 air contained in a jar inverted over water, and stand- 
 ing in a room varying from 55 to 60 Fahren- 
 heit. He died on the fifth day. The water had ri- 
 sen considerably into the jar, and the residual air 
 was still farther diminished by agitation with lime- 
 water, which it rendered turbid. Fifty parts, after 
 being washed in lime-water, were next shaken in 
 the eudiometer with the liquid sulphuret of potassa, 
 and lost only one part of its bulk. The experiment 
 was repeated by confining another toad, in the same 
 manner, in another jar containing forty cubic inches 
 of atmospheric air, inverted over mercury. Un- 
 der the hoop which supported the animal, was 
 placed a small cup containing 1.5 cubic inch of the 
 water of potassa, which floated on the mercury. The 
 whole was then set aside in a room, of the tempe- 
 rature of 64. By the twelfth hour, the mercury had 
 
 * John Bell's Anatomy, vol. ii. p. 1S2. 
 F 2 
 
84 
 
 risen nearly half an inch into the jar, which was 
 thickly moistened with vapour, and the breathing of 
 the animal seemed rather languid : by the twenty- 
 first hour, he breathed very faintly ; and, by the 
 twenty-fourth hour, he had ceased to breathe. The 
 jar was allowed to stand some hours, at the end of 
 which time, the mercury stood about eight-tenths of 
 an inch high, and one-tenth of an inch of fluid was 
 deposited on its surface. The jar was now raised, 
 and diluted sulphuric acid being poured into the al- 
 kaline solution, excited in it a verybrisk effervescence. 
 It is inferred, therefore, from these experiments, that 
 the oxygenous portion of the air almost entirely dis- 
 appears during the respiration of these animals, after 
 which they cease to breathe ; and that a large por- 
 tion of carbonic acid is at the same time produced. 
 69. Proceeding on the supposition, that the loss 
 in the bulk of air, evinced by the ascent of the mer- 
 cury, in the last of the foregoing experiments, arose 
 from the attraction of the carbonic acid by the al- 
 kaline solution, we endeavoured to ascertain the 
 proportion which this loss of bulk bore to that of 
 the whole air originally employed. With this view, 
 a frog was procured, and placed in a jar of the ca- 
 pacity of forty cubic inches. Under the hoop which 
 supported him, about half way up the jar, was placed 
 a small cup, containing one cubic inch of the water 
 of potassa, and the jar being then filled with atmo- 
 spheric air, was inverted into a dish of mercury, and 
 kept steady by a weight pressing upon it. In the 
 room in which the animal was placed, the barometer 
 stood at 29.2 inches, and the thermometer at 61. 
 At the end of twenty-nine hours, the animal was 
 
85 
 
 \ 
 
 resting quietly on the hoop, with no appearance of 
 distress, and the mercury in the jar, when that in 
 the dish was brought to a level with it, had risen 
 six-tenths of an inch. In twenty-four hours more, 
 the frog was still alive : his respiration seemed now 
 to labour, and he rose often to the top of the jar as 
 if desirous of escaping, or of obtaining fresh air : 
 the mercury had now risen to 1.15 of an inch. 
 From this time, the difficulty of breathing continued 
 to increase, and, at the close of the fifty-ninth hour 
 from the commencement of the experiment, .after 
 having lain quiet for a considerable time, he gave a 
 convulsive struggle, and moved no more. The 
 mercury in the dish was now brought to a level with 
 that in the jar, and its height was 1.2 of an inch. 
 The barometer, at this period, was 29.8, and the 
 thermometer 65. 
 
 70. In order to examine the residual air, we 
 plunged the dish under water, which, rising into the 
 jar, displaced the mercury, and the cup, with its so- 
 lution, was then withdrawn under water, The resi- 
 dual air suffered no diminution by being shaken with 
 lime-water, nor by contact with phosphorus, but it 
 lost rather more than ^ by agitation with the liquid 
 sulphuret of potassa. The jar originally held forty 
 cubic inches, but the animal, with the hoop, cup, and 
 solution, occupied a space equal to four, so that the 
 actual bulk of air employed was thirty-six cubic 
 inches. Having placed the jar on its bottom, water, 
 to the quantity of twenty-seven cubic inches, was 
 poured in, till it reached the point to which the mer- 
 cury, during the experiment, had risen ; and this, 
 therefore, indicated the volume of residual air : it 
 
 F3 
 
86 
 
 then required nine cubic inches more of water td 
 fill the jar completely, which, consequently, was the 
 bulk of air that had disappeared. Hence, therefore, 
 we have 27 * 9 2 29 ' 8 = 27.5547, but 4 X * 5 * = .22819 
 and 27.554 .22819 = 27.32651, the corrected 
 volume of air at the close of the experiment. But 
 farther, 36 27.32651 = 8.67349, and 3 7 6 -- 9 = 
 ^- 5 ; so that the diminution of bulk which the air 
 suffered in this experiment is rather greater than r^ 
 
 O 4.o4 
 
 the proportion of oxygen gas which the atmosphere 
 contains. In a second experiment, another frog 
 lived in the same volume of air about sixty hours, 
 and the diminution which it suffered, after making 
 the necessary reductions, amounted to ~g of the 
 whole. Where the carbonic acid, formed by the 
 respiration of another frog, was suffered to remain, 
 the jar, after the death of the animal, adhered firm- 
 ly to the saucer in which it was inverted, and, when 
 cautiously elevated, the surrounding mercury rush- 
 ed in, and occupied only about one-tenth of the space 
 which it filled in the above-mentioned cases. The 
 inferences deducible from these facts, instruct us, 
 that the diminution which atmospheric air suffers by 
 the respiration of these animals, bears a near propor- 
 tion to the oxygen gas which it contains, when all 
 the carbonic acid is removed : and, as a small loss 
 ,of bulk likewise takes place when this acid is allow- 
 ed to remain, we must ascribe a part of the observed 
 diminution to the necessary loss which always ac- 
 companies the conversion of oxygen gas into carbo- 
 nic acid. 
 
87 
 
 71. It follows from the preceding series of expe- 
 riments, that the oxygenous portion of the air is 
 changed by the respiration of amphibious animals in 
 the same manner as by that of the other classes, car- 
 bonic acid, in proportion thereto, being, in all cases, 
 produced : and that when the whole, or nearly the 
 whole, of that gas is so changed, the animal no long- 
 er survives. But if the animal die when all the 
 oxygen gas is changed, and all the air that has disap- 
 peared, when the carbonic acid is removed, be oxy- 
 gen gas, then the bulk of air that remains, and is 
 unchanged, must consist wholly of nitrogen gas; 
 and, as this nitrogen gas, joined with the oxygen gas 
 that has disappeared, makes up the whole bulk of 
 air originally employed, it follows also, that, while 
 the oxygen gas of the air has diminished and suffer- 
 ed change, the nitrogenous portion has continued 
 undiminished and unaltered. 
 
 72. During all these changes operated on the air 
 contained in water by the respiratory functions of a- 
 quatic animals, the water itself seems to suffer little 
 or no alteration. Mr Carlisle took separate glasses, 
 each containing one pound of distilled water, which 
 was previously boiled to expel all its air, and then, 
 inverting them over mercury, he put into them one 
 gold fish, one frog, two leeches, and one fresh wa- 
 ter muscle. The animals were confined several days 
 in these situations, and exposed to the sun during 
 January in temperature 43 an$l 48 Fahrenheit : but 
 no air-bubbles were produced in the vessels, nor 
 was there arty sensible diminution of the water. 
 The frog died on the third day, the fish on the 
 fifth, the leeches on the eighth, and the muscle on 
 
 F 4 
 
88 
 
 the thirteenth day. This experiment was made to 
 ascertain the changes produced in water by the re- 
 spiration of aquatic animals ; but the water had not 
 undergone any chemical alteration *. 
 
 73. From this enumeration of the principal facts, 
 concerning the changes induced on atmospheric air 
 by the respiration of all these several classes of ani- 
 mals, we obtain positive evidence, that, in most cases 
 (53. et seq.)> its nitrogenous portion, as in the 
 growth of vegetables (5. 29.), continues unaltered : 
 and since, in the remaining cases, the air in degree 
 is proved to suffer the same change, and the ultimate 
 result, viz. the display of living action, is, in all re- 
 spects, the same ; it is not, we hope, exposing our- 
 selves to the charge of too hasty generalization, or of 
 resting too much on analogy, if we conclude, that, 
 in the whole view which we have hitherto taken of 
 animal respiration, the nitrogen gas of the atmosphere 
 remains unchanged. Moreover, as this gas itself 
 suffers no change, so neither does it seem to exert 
 any influence on the animal in contact with it ; for 
 Spallanzani found, that snails could live in nitrogen 
 gas twelve hours t 5 which is as long as they live 'in 
 vacua (51.) : and, in all the experiments we have 
 made in atmospheric air, the animals did not appear 
 to die from the superabundance of nitrogen gas, but 
 from the small proportion, or total absence, of oxy- 
 gen gas. 
 
 * Philosophical Transactions, 1805. 
 f Memoirs on Respiration, p. 3i7. 
 
89 
 
 74. Assuming then, as a facf, that the nitrogen 
 gas of the air neither produces nor suffers change, 
 we have next to inquire, what becomes of the oxy- 
 gen gas, which has been shewn, more or less, in all 
 cases, to disappear. Is it absorbed by any organized 
 structure of the animal adapted to the performance 
 of such an office ? No vessels fitted for such a pur- 
 pose have been yet demonstrated in the animal system. 
 The small size of the stigmata, or breathing pores, 
 in insects, renders them but little suited to be re- 
 ceptacles for containing and decomposing air : and, 
 in many of the venues class, the mucous matter with 
 which their bodies are constantly smeared over, 
 must oppose great difficulties to such an absorption. 
 In the case of aquatic animals, these difficulties are 
 still farther increased ; for the air must be first se- 
 parated from the water before it can be taken up by 
 absorption ; and, after this is effected, it is not easy 
 to conceive how the gills of fishes can be rendered 
 capable of absorbing and emitting air. It is not 
 probable, that this air is taken up in its entire state, 
 for as the nitrogen gas undergoes no change (73.), 
 its absorption can answer no obvious use, but would 
 tend rather to impede the decomposition supposed 
 to go on within the vessels, and the subsequent 
 formation and emission of the carbonic acid. If, on 
 the other hand, the air be considered to be decom- 
 posed previous to its absorption, then a new com- 
 pound must be at once formed ; and, if this be 
 brought about by the union of some substance with 
 the oxygen gas, then that gas, simply as such, can- 
 not be held to enter into the animal system. No 
 one has ever vet detected air in the animal fluids 
 
90 
 
 while in a healthy state ; and if we consider the great- 
 extent of surface, and extreme minuteness of the 
 vessels in the gills of fishes (65.), we cannot but 
 consider them as well adapted to produce an exten- 
 sive contact of surfaces, and but little fitted to ab- 
 sorb, decompose, and again emit aeriform fluids. 
 
 75. If to account for this supposed entrance of 
 air into the vascular system, the agency of chemical 
 affinity be had recourse to, by what means, we would 
 ask, can its operation be in this case explained ? No 
 sensible .or obvious principle, equal to such an ef- 
 fect, can be held to reside in the blood, since the 
 changes go on equally in all these animals, though 
 the blood be of various colours, and, in many in- 
 stances, where it is totally devoid of colour. During 
 a torpid state also, Spallanzani has shewn, that no 
 change is effected by the animal on the air * ; and 
 consequently, no oxygen gas is then attracted by the 
 blood, although, if the supposed carbon of that 
 fluid be considered to attract this gas, the union 
 ought still to proceed, because, according to the re- 
 ceived opinions, the animal system is at this period 
 surcharged with carbon. Neither can the condi- 
 tions, indispensable to the operation of chemical affi- 
 nity, be in these cases fulfilled; for the interposi- 
 tion of organized substance between the air and the 
 blood, altogether precludes that degree of absolute 
 contact, which is held to be essential to chemical ac- 
 tion. Even if the oxygen gas were attracted into 
 thi blood by the operation of chemical affinity, by 
 
 * Memoirs on Respiration, p. 334-. 
 
91 . 
 
 what power would the carbonic acid that is formed 
 be again given out by that fluid ? No chemical agent 
 either in the air or the water can be imagined equal 
 to the re-attraction of it through the organized struc- 
 ture of the animal : nor is it conceivable, how the 
 blood, by any power of its own, should be able to 
 emit it, independent of such agent. And to suppose, 
 that, by a power of chemical affinity, this gas should 
 enter into the blood, and be afterwards expelled 
 from it, as carbonic acid, by any method analogous 
 to the ordinary animal excretions, is too inconsistent 
 to be entertained for a single moment. 
 
 76. On the grounds, therefore, that the oxygen 
 gas of the air does not obtain admission into the 
 blood-vessels, either by the function of absorption, 
 or by the operation of chemical affinity, we must re- 
 ject the belief of its union with the supposed carbon 
 of the blood, to form the carbonic acid that is pro- 
 duced. Still, however, the gradual disappearance 
 of that gas, and the production of carbonic acid 
 which ensues, justify the conclusion, that in the ani- 
 mal, as well as in the vegetable, kingdom, they ob- 
 serve always a regular and progressive ratio, and 
 are, in fact, proportional to each other, which ad- 
 mits of no other solution than that of their being 
 converted into one another. To effect this conver- 
 sion, however, no other substance but the animal 
 was present, in these experiments, from which the 
 carbon could be derived ; consequently, the acid 
 must be formed by the union of carbon furnished 
 by the animal with the oxygen gas of the air, and 
 this, too, exterior to the vascular structure of the 
 animal. 
 
92 
 
 77. Those who maintain, that the carbonic acid 
 is not directly formed by the union of the oxygen 
 gas of the air with the animal carbon, but that it 
 escapes ready formed from the animal system, ought 
 to point out some other source from whence, in suf- 
 ficient quantity, the oxygen gas can be derived : to 
 tell us at the same time what becomes of the oxyge- 
 nous portion of the air that actually disappears : and 
 why the production of carbonic acid bears always so 
 constant a proportion to the loss of this oxygen gas. 
 To suppose with Spallanzani, that this acid is yield- 
 ed by the process of digestion, because some snails 
 which had been well-fed, furnished more of it than 
 others which had suffered a long abstinence, is by no 
 means proving the point ; for a snail which had 
 long fasted, yielded as much, in one instance, as 
 those which had been recently fed, and, in the other 
 examples, the starved snails fell short only in a small 
 degree. Every animal function, also, is, cateris fia- 
 ribus, carried on best in a state of health and vi- 
 gour, which again depends altogether on a due sup- 
 ply of food : hence, therefore, the debility succeed- 
 ing to abstinence, must afreet the organs of respira- 
 tion, in common with the other organs, and conse- 
 quently their power of acting so completely on the air. 
 Whatever substances, moreover, are received into 
 the animal system, suffer or produce some change : 
 but to suppose, that carbonic acid should be first 
 formed in the stomach, then taken up by the lacteal 
 vessels, and carried through the mass of blood to be 
 again thrown out by the respiratory function, simply 
 as carbonic acid, is not only without proof, but a- 
 gainst all probability. It is also to the quantity of air 
 
93 
 
 changed, and not to that of food taken in, that this 
 acid bears a proportion ; and, provided living action 
 be equally well maintained, as much air seems to be 
 required by an animal when he abstains from food 
 as when he takes it, and as much carbonic acid to 
 be produced. In as much, however, as a long ab- 
 stinence from food, debilitates the system, and af- 
 fects the production of carbon, in so much will it di- 
 minish the quantity of carbonic acid, which the ani- 
 mal is accustomed to form. 
 
 78. But the quantity of this acid, when formed 
 by the respiration of a given volume of air, does not 
 seem to exert any noxious operation on the animal 
 powers ; for Spallanzani found, that animals placed 
 in a given bulk of air, did not live longer when the 
 carbonic acid was abstracted by an alkaline solution 
 as soon as formed, than when it was suffered to re- 
 main * ; and, in all our experiments, where the car- 
 bonic acid was allowed to remain, the death of the 
 animals seems to have arisen, not from the over-pro- 
 portion of that acid, but from the diminished quan- 
 tity, or total absence, of the oxygen gas. Since, in- 
 deed, air is necessary to the continuance of living 
 action in all animals, and its nitrogenous portion ap- 
 pears to suffer no alteration, this necessity must arise 
 from its containing oxygen gas, and from the requi- 
 site changes which, in respiration, that gas is made 
 to undergo. When, therefore, the greater part, or 
 the whole of the oxygen gas of the air is so chan- 
 ged, death ought to happen ; and then accordingly, 
 
 * Memoirs on Respiration, p. 317. 
 
94 
 
 and not till then, does this event take place. It is 
 therefore to the small proportion or total absence of 
 oxygen gas, and not to the presence of carbonic acid 
 formed out of that gas, that the cessation of the ani- 
 mal functions, in all the foregoing examples, is to 
 be immediately ascribed. 
 
CHAP. IV. 
 
 OF THE CHANGES INDUCED ON THE 
 AIR BY THE RESPIRATION OF BIRDS, 
 OF QUADRUPEDS, AND OF MAN". 
 
 SECTION I. 
 
 79. JN OT only, as we have seen, is water 
 necessary to prepare the organization of vegetables, 
 and of the inferior animals, for exhibiting living 
 action, but it is required also by those which be- 
 long to the superior orders. " The whole material 
 world," says Mr Hunter, " has been very properly 
 divided into solids and fluids, these being the only es- 
 sentially different states of matter which we are able 
 to observe. From one of these states into the other, 
 matter appears to be continually passing ; but no spe- 
 cies of matter can assume a solid form without ha- 
 ving first been in a fluid state ; neither can any 
 change take place in a solid, till it be first reduced 
 to, or suspended in, a fluid. The living animal 
 
96 
 
 body is obedient to these general laws ; for all the 
 solid matter of animals has been once in a fluid state, 
 and, having passed into the solid form, becomes a 
 recipient for other fluids, out of which the solids 
 therrtselves may, in turn, be increased and reno- 
 vated *." . 
 
 80. This conversion of fluid into solid matter, 
 cannot, however, go on in the animal body without 
 the constant presence of heat, the agency of which 
 is essentially necessary to carry forward these trans- 
 formations. When, to a certain degree, heat is ab- 
 stracted from the body, its vital functions gradually 
 decline, and at length finally cease. Many animals 
 experience these effects periodically, without injury 
 to the vital organs, and the actions of life re-ap- 
 pear as the temperature of the season returns. In 
 our own climate, the hedge-hog, the bat, the dor- 
 mouse, and several birds, pass into a state of torpidi- 
 ty during the winter season ; and, in the more nor- 
 thern parts of Europe and America, the bear and al- 
 ligator do the same : from which we may conclude, 
 that a certain degree of heat is necessary to sustain 
 the actions of life in the superior, as well as in the 
 inferior animals. 
 
 81. The operation of light on the colour of ani- 
 mal bodies (50.), and probably on some of their other 
 proper ties, has been already noticed. In the human 
 subject, the colour of the skin depends on that of 
 the reticular membrane placed beneath the cuticle, 
 which assumes various colours in different parts of 
 
 Treatise on the Bipod, p. 12. 
 
97 
 
 the earth. In the native American, the inhabitant 
 of Asia, and the southern European, the colour va- 
 ries from dark copper to pale tawny ; and in the ne- 
 gro it is quite black. The inhabitants of the north- 
 ern countries, on the other hand, are white : and, 
 not only are those parts of the body, which are most 
 covered, the whitest, but a sensible difference in co- 
 lour exists in the same person at different seasons of 
 the year. These variations in colour have been held 
 to depend very much on the agency of light. 
 
 82. The necessity of atmospheric air to the con- 
 tinuance of living action in all the superior animals, 
 was long ago proved by the experiments of Mr 
 Boyle, soon after the discovery of the air-pump ; 
 and he observed farther, that the function of respi- 
 ration is quickly suspended, unless the lungs are 
 furnished with a regular supply of fresh air *. " A- 
 nimals, whose hearts have two ventricles and no 
 foramen ovale, says Mr Derham, as birds, dogs, cats, 
 and mice, die under the action of the air-pump in 
 less than half a minute, counting from the very first 
 exsuction, especially in a mall receiver t" In cor- 
 roboration of these facts, we may mention an expe- 
 riment, exhibited before the Royal Society, by the 
 celebrated Dr Hooke. He cut away the ribs, dia- 
 phragm, and pericardium of a dog, whereby the 
 lungs and heart were brought into view ; and then, 
 dividing the windpipe, he introduced into it the 
 nozzle of a pair of double bellows, and made, at the 
 
 * Boyle's Works, vol. i. p. 99. 
 f Physico-Theology, p. 8. 
 
98 
 
 same time, several small punctures through the outer 
 coat of the lungs. By blowing in a stream of fresh 
 air, which continued to escape through the small 
 apertures made in the lungs, he was enabled to keep 
 those organs fully distended. As long as he sup- 
 plied the lungs with air, the actions of life continued, 
 and the heart beat very regularly ; but, on intermit- 
 ting the supply, the dog would immediately fall into 
 dying convulsive fits, and revive again as soon as 
 the lungs were filled with a stream of fresh air. The 
 circulation through the lungs continued both during 
 their distended and collapsed state, and as well when 
 they were kept at rest, as during a state of motion : 
 whence he concluded, that neither the motion of the 
 lungs, nor the cessation of their motion, nor the 
 stopping of the circulation of the blood through them, 
 was the immediate cause of death, but the want of 
 a sufficient supply of fresh air *. By the researches 
 of later philosophers, it has been proved, that the 
 air, in all animals, serves the same uses"; to fit it for 
 which, it undergoes the same changes. In our in- 
 quiry into the nature of these changes, the manner 
 in which they are effected, and the uses which" they 
 are found to serve, we shall confine ourselves chiefly 
 to the facts which take place in human respiration, 
 not only on account of their greater interest, but 
 also because they have occupied more research, and, 
 having been more frequently submitted to experi- 
 ment, are in some respects better ascertained. 
 
 * Lowthorpe's Abrid. Phil. Trans, vol. iii. p. 6G. 
 
99 
 
 83. When atmospheric air is respired by man and 
 by other animals, it undergoes two remarkable chan- 
 ges : its bulk is diminished, and its qualities are altered. 
 This diminution of bulk was early noticed by Boyle, 
 who estimated it at about of the air employed. 
 May how, whose genius enabled him to anticipate so 
 many important discoveries of modern chemistry, 
 confined an animal in a glass-vessel inverted over 
 water, and, by the aid of a syphon, brought the 
 water on the inside of the vessel to a level with that 
 on its outside. Having then marked the height of 
 the water by pieces of paper affixed to the vessel, he 
 observed its gradual rise as the animal continued to 
 breathe : and then comparing the space occupied by 
 the air at the commencement of the experiment, 
 with that which it possessed when the animal ceased 
 to breathe, he found that it was reduced about j^ 
 part of its bulk *. In the experiments of Dr Hales, 
 the degree of diminution varied from ~ to ~- of the 
 whole air employed f : and in those of M. Lavoisier 
 from i to ij part J ; with which the results of Dr 
 Goodwyn's experiments on his own respiration nearly 
 coincide ||. Dr Priestley confined a mouse in a jar 
 containing a given quantity of air, which was invert- 
 ed over mercury : the animal \vas suffered to remain 
 two or three days after he had died, in which time 
 there was no sensible diminution of the air, but 
 
 * Tractat. Ouinq. p. 104. 
 
 f Statical Essays, vol. i. p. 238 vol. ii. p. 320. 
 
 t Mero.-Acad. 1777 and 1780. 
 
 [! Connexion of Life with Respiration, p. 51. 
 
 G2 
 
100 
 
 on passing lime-water into the jar, the air was 
 diminished ^ part of its bulk ; and v/hen, in a sub- 
 sequent experiment, the residual air was agitated in 
 water, it was reduced between one-fifth and one- 
 sixth of the whole *. Dr Crawford found also, 
 that when the experiment was made over mercury, 
 the diminution was not sensible ; but that, if water 
 of potassa was added to the residual air, it became 
 mild, and the air was diminished in the same de- 
 gree as if the experiment had been made over water, 
 or nearly one-fifth of its bulk f. These variations 
 in the results arise, no doubt, from the more or less 
 complete attraction of the carbonic acid by the fluids 
 over which the experiments were made ; and, from 
 the whole of them, we may collect, that, when mer- 
 cury is employed, which has no attraction for car- 
 bonic acid, the diminution is hardly sensible ; but 
 that when this acid is completely abstracted by an 
 alkaline fluid, the loss of bulk amounts nearly to 
 one-fifth of the whole air employed. This inference 
 corresponds very exactly with the facts which occur 
 in vegetation, and in the respiration of the inferior 
 animals. 
 
 84. But experiments of this nature, although they 
 shew the extent to which the destruction of the oxy- 
 gen gas, contained in a given quantity of air, may, 
 by the process of respiration, be made to proceed, 
 yet they do not apply to the ordinary circumstances 
 in which that function is carried on ; for the air of 
 
 * Observations on Air, vol. v. p. 112. et scq. 
 | On Animal Heat, p. 116. 
 
101 
 
 the vessels in which the animals were confined, 
 must, by repeated breathing, have become less and 
 less fit for respiration, and was therefore gradually 
 declining from that state in which it is usually in- 
 spired. We have seen, that several of the inferior 
 animals will live in a given quantity of air until its 
 oxygen gas is completely (54. et seq.) consumed : 
 but those of the superior orders do not bear this to- 
 tal privation. Birds die in air confined by lime-wa- 
 ter, before they have consumed two-thirds of its 
 oxygen gas : and a mouse and guinea pig expire 
 when about three-fourths of this gas have disappear- 
 ed, although the carbonic acid be withdrawn *, 
 Spallanzani observes likewise, that birds and quadru- 
 peds consume not more than ^~ of the oxygen gas 
 of the air, and sometimes only 17, 16, or ^, and 
 then die, even although the carbonic acid be remo- 
 ved t Lavoisier found, that by repeatedly with- 
 drawing the animal from the vessel when he be- 
 gan to sicken, and re-introducing him after he 
 had revived, he could be made to consume almost 
 the whole of the oxygenous portion of the air J. la 
 all the foregoing experiments, it may be doubted, 
 whether the same actual diminution in bulk takes 
 place, as would have occurred if the same volume 
 of air had been submitted to successive respirations 
 in the open atmosphere. It is only with the latter 
 
 * Higgins's Minutes of a Society, p. 158? 
 f Memoirs on Respiration, p. 318. 
 J Mem, Acad. 1783. 
 
 G 3 
 
102 
 
 kind of diminution, viz. that which takes place in 
 natural respiration, that we are at present concern- 
 ed ; and as this cannot be determined by experi- 
 ments made on brutes, we must resort to the facts 
 which have been ascertained by those instituted on 
 the respiration of man. 
 
 85. A knowledge, however, of the diminution of 
 bulk which the air, during respiration, suffers, im- 
 plies a previous determination of the quantity ordi- 
 narily inspired. To ascertain this point, many modes 
 of experiment have been adopted, and the conclu- 
 sions which have been drawn from them very widely 
 differ. Borelli estimated the bulk of air taken in at 
 a single inspiration, at 15 cubic inches * ; Mr Kite 
 from 12 to 17 t ; Dr Goodwyn at 14 { ; Mr Davy 
 from 13 to 17 ; and Drs Jurin, Hales, Haller and 
 Sauvages, at 4O cubic inches. With the conclusion 
 of these latter authors the experiments of Dr Men- 
 zies nearly coincide, and as the methods which he 
 adopted seem less liable to objection than those of 
 any other author, it may not be improper shortly to 
 give the detail of them. He procured an allantoid, 
 and fixed to it a machine consisting of two pretty 
 large tubes, joined at right angles, nearly in the 
 form of a common brass cock. One end of the 
 horizontal tube was connected with the allantoid, 
 and the other received into the mouth, while the 
 upright tube, which rose from its centre, communi- 
 
 * De Motu Animal. 
 
 j- Essay on Apparent Death, p. 24. 
 
 J Connexion of Life with Respiration, p 28. et seq. 
 
 Researches, p. 410. & 433. 
 
103 
 
 "cated with the atmosphere. The tubes were large, 
 and valves, made out of an allantoid, were affixed to 
 the end of the upright tube, and to that attached to 
 the allantoid, so that the air, when expelled from 
 the lungs, should not escape into the atmosphere, 
 nor return from the allantoid, after having once en- 
 tered it. Precautions were taken also, by covering 
 the mouth and nostrils, to prevent any air from pass- 
 ing in or out of the lungs, except by the tubes above 
 mentioned. Things being thus prepared, he began 
 to respire, and did not remove his mouth from the 
 tube till he had filled the allantoid, taking care to 
 stop his nostrils during expiration. The allantoid 
 was filled, in repeated trials, by about 56 expira- 
 tions, as natural as possible ; and as its capacity was 
 240O cubic inches, the average bu.lk of air thrown 
 out of the lungs by each expiration, was 42.8 cubic 
 inches. He then fixed another allantoid, whose ca- 
 pacity had been previously ascertained, to the end 
 of the upright tube ; and having filled it with at- 
 mospheric air, he inspired the air from one allantoid 
 and expired it into the other, and the quantities were 
 found to be nearly the same. Several persons of the 
 middle size repeated this experiment with nearly the 
 same result ; the difference being scarcely ver more 
 than one or two cubic inches. By another mode of 
 experiment, first proposed by Boerhaave, of plun- 
 ging a man into a tub of water up to his chin, and 
 judging of the dilatation of the lungs from the as- 
 cent and descent of the water, he obtained, by seve- 
 ral trials, nearly the same results ; and when these 
 same men were made to breathe from and into the 
 allantoids, in the manner above described, the cor- 
 
104 
 
 respondence by the two methods was almost com- 
 plete *. As there seems no obvious source of in- 
 accuracy in the processes here employed, and their 
 results so remarkably coincide ; and as they present 
 the average bulk deduced from 56 respirations, we 
 may conclude, says Dr Bostockf, that 40 cubic 
 inches is the quantity of air employed in an ordinary 
 act of respiration J . 
 
 86. The difficulty in arriving, by experiment, at 
 certain conclusions respecting the volume of air ta- 
 ken into the lungs in each inspiration, may arise from 
 a difference in the state or capacity of those organs 
 in different individuals ; from the relative vigour or 
 debility of the muscular powers carrying on the re- 
 spiratory function ; from the circumstances in which 
 the animal is placed ; the composition of the air it- 
 self; or the manner in which it is breathed. In 
 many modes of experiment also, the friction be- 
 tween the air and apparatus employed, or the resis- 
 tance which this latter may create to the ordinary 
 process, will greatly vary the result : and considerable 
 errors must likewise have arisen from the variation 
 in bulk, occasioned by the change of temperature, 
 which the air, during'its respiration, suffers ; from the 
 difficulty of breathing in a natural manner when the 
 
 * Menzies on Respiration, p. 21. et seq. 
 
 f- On Respiration, p. 34-. 
 
 J Besides the respectable authorities mentioned in the text, 
 Dr Bostock quotes the names of Blumenbach, Chaptal, Bell, 
 Fontana, and Richerand, as estimating the bulk of a single inspi- 
 ration at between 30 and 4<0 cubic inches of air. 
 
105 
 
 inind is directing that process ; and from the em- 
 barrassments opposed to the natural action of the 
 respiratory organs by the contrivances adapted to 
 them. 
 
 87. It will not be denied, that the size and capa- 
 city of the chest must, in a certain degree, regulate 
 the quantity of air which is taken into or expelled 
 from it ; and since respiration is neither wholly a 
 voluntary nor an involuntary act, but within certain, 
 limits partakes of the nature of both, and is carried 
 on by the exertion of muscular powers, the bulk of 
 respired air must vary also, either from an altera- 
 tion in the action of these powers, or from a change 
 in the will of the agent who exerts them. This 
 may be illustrated by considering the different quan- 
 tities of air taken into the lungs in different states of 
 natural and forced respiration. Dr Goodwyn, sup- 
 posing a person at death to make a complete expira- 
 tion, endeavoured to ascertain the bulk of air then 
 remaining in the lungs, which he estimated at 109 
 cubic inches *. This estimate he fornfed by measuring 
 the capacity of the chest, in subjects who had died a 
 natural death by disease, previous to which the ex- 
 piratory powers must have been much weakened, and 
 unable, in consequence, to expel so much air as 
 when in a state of health and vigour ; and in such 
 cases, therefore, expiration might be final without 
 being complete. Mr Cruickshank observes, accord- 
 ingly, that the lungs in the dead body, (though ex- 
 piration is the last action of life), always retain more 
 
 Connexion of Life with Respiration, p. 27* 
 
106 
 
 air than is given out at several expirations*. By a very 
 different mode of experiment, we find Mr Davy to 
 conclude that his lungs, after a forced expiration, 
 contain only 32 cubic inches of air, when it is re- 
 duced to the temperature of 55, but which, by the 
 heat of the lungs, and saturation with moisture, are 
 increased to 41 cubic inches; and, after a natural 
 expiration, they contained 118 cubic inchest ; so that 
 the difference between the two states of natural and 
 forced expiration is 77, which is somewhat more than 
 Dr Menzies allows, who remarked that many men, 
 after an ordinary expiration, could still expel from their 
 lungs 70 cubic inches of air{. Mr Davy adds, that his 
 estimate of 1 1 8 cubic inches, as the capacity of the 
 lungs after natural expiration, agrees very well with 
 that of Dr Goodwyn, who makes it about 1O9{; and, 
 on the supposition that the general debility which 
 precedes the ordinary extinction of life, so weakens 
 the expiratory muscles, as to disable them from ma- 
 king so complete an expulsion of the air, as they can 
 effect when in health and vigour, the agreement is 
 very striking ; for nearly the same quantity of air 
 would, in that case, remain in the lungs at the pe 
 riod of natural death, as after that of ordinary expi- 
 ration. 
 
 88. Dr Bostock conceives, that Dr Goodwyn's 
 estimate of 109 cubic inches of air remaining in the 
 lungs after complete expiration, is not very remote 
 
 * On Insensible Perspiration, p. 97. 
 
 f Researches, p. 409. & 4-10. 
 
 J Dissertation on Respiration, p. 31. Ibid. p. 411. 
 
107 
 
 from the truth : and he objects to Mr Davy's mode 
 of ascertaining the residual air of the lungs after a 
 forced expiration, from a supposition that the hy- 
 drogen gas which he inspired for that purpose was 
 not, in consequence of its low specific gravity, uni- 
 formly diffused through all the cavities of the lungs : 
 and therefore, that the proportions of the gas dis- 
 charged could furnish no accurate estimate of those 
 which were retained *. But Mr Dalton has shewn, 
 that hydrogen gas and atmospheric air intermix, 
 when the former is kept in a phial above the latter, 
 and communicating only by the small tube of a to- 
 bacco-pipe ; and both in a state of rest f How 
 much more readily then may this be expected to 
 take place, where the gases are exposed to so large 
 a surface, such great agitation, and increased tem- 
 perature, as they must have been in the experiments 
 of Mr Davy. Neither is the small quantity of air, 
 which Mr Davy assigns, so incompatible, as Dr 
 Bostock supposss, with the anatomical structure of 
 the thorax ; for if we call to mind the space which 
 the heart and the lungs occupy, and recollect, that, 
 under a violent exertion, the chest is made to con- 
 tract in every direction, and more especially by the 
 ascent of the diaphragm nearly ,to the fourth or fifth 
 rib, there is no difficulty in imagining the quantity 
 of air in the lungs, in such circumstances, to be near- 
 ly that which Mr Davy's experiments assign. 
 
 * Essay on Respiration, pp. 17. 25. 
 f Manchester Memoirs, vol. i. new series. 
 
108 
 
 89. From a review, therefore, of all the facts and 
 experiments above stated, we venture to draw the 
 following conclusions, as approaching nearest to the 
 truth. First, then, according to Mr Davy, the lungs 
 contain, after a forced expiration, a bulk of air equal 
 to about 41 cubic inches ; and according to the 
 same author and Dr Goodwyn, they contain, after 
 a natural expiration, from 109 to 118 cubic inches: 
 therefore the state of forced is to that of natural ex- 
 piration as 41 to 118. Secondly, according to Dr 
 Menzies, 40 cubic inches of air are received into the 
 lungs at each ordinary inspiration : therefore the 
 state of natural expiration to that of natural inspira- 
 tion will be as 118 to 158. Mr Davy found like- 
 wise, that by a forced expiration after a forced in- 
 spiration, he could expel from his lungs 19O cubic 
 inches of air, and Dr Menzies often found it to a- 
 mount to 200 inches : therefore the state of great- 
 est exhaustion of the lungs is to that of greatest re- 
 pletion, as 41 to 231. But the 41 cubic inches of 
 air, when inspired at temperature 55, occupied a 
 bulk equal only to 32 ; and therefore, by the same 
 rule of proportion, 19O cubic inches, inspired at the 
 same temperature, will be increased to 241.5 : con- 
 sequently, the greatest diminution of the capacity of 
 the chest to its greatest expansion will be as 41 to 
 241, in the case of Mr Davy. But these numbers 
 must be considered as indicating proportions only, 
 the absolute quantities being different in different 
 persons *. These facts decidedly shew how much 
 
 * At the time of making these experiments, Mr Davy states 
 
109 
 
 the volume of air in the lungs will at all times de- 
 pend on the relative capacity of those organs, on the 
 more or less vigorous state of the expiratory powers, 
 and on the degree of voluntary exertion with which 
 the function may be performed. 
 
 90. The circumstances in which the animal may 
 happen to be placed, will render this variation still 
 more striking. Thus, from the experiments of Mr 
 Kite and Mr Coleman, we learn, that in the act of 
 drowning, animals are able to expel almost all the 
 air which their lungs contain, by which those organs 
 are brought into a state of collapse *. Dr Good- 
 wyn, on the other hand, found, that in three execu- 
 ted persons, the lungs were expanded almost to their 
 utmost extent, containing 250, 262, and 272 cubic 
 inches of air t : and Mr Coleman observes, that 
 when, previous to their suspension, he secured the 
 trachea of animals by a ligature at the instant an 
 inspiration was made, in less than four minutes they 
 ceased to struggle, though the whole of the air was 
 confined within the lungs, and no obstruction to the 
 passage of the blood existed from their collapse *. 
 Dr Baillie also has often observed the lungs filling 
 the chest, and distended with air and mucus, in per- 
 sons who have died asthmatic : so that to die and to 
 
 his chest to have been narrow, not exceeding in circumference 
 29 inches. (Researches, p. 410.). 
 
 * Kite on Apparent Death, p. 27. 29. 
 
 Coleman on Suspended Respiration, p. 7. et seq, 
 
 f Goodvvyn's Essay, p. 25. 
 
 On Suspended Respiration, p. 111138. 
 
expire are by no means synonymous terms, an ob- 
 servation long since made by Mayhow, who remark- 
 ed, that if air be drawn into the lungs, and the 
 mouth and nostrils afterwards closed, " quamvis in- 
 flati maneant pulmones, mori tamen necesse erit, 
 quia non licet expirare *. If indeed we reflect, that 
 during submersion in water no fresh air can enter 
 into the lungs, but that all which they contain may 
 freely escape ; and if we consider, that before sus- 
 pension by the neck in the human subject, a deep 
 inspiration, under the influence of fear, as Dr Good- 
 \vyn observes, is made, and that no air can after- 
 wards pass out, if the cord completely close up the 
 trachea; it is reasonable to expect, that this -varia- 
 tion in the bulk of air contained in the lungs should 
 obtain, under the very different circumstances in 
 which respiration is brought to a stand. 
 
 91. How much the composition of the air itself, 
 and the manner in which it is breathed, will vary the 
 bulk of residual air in the lungs, we may collect 
 from the experiments of various authors. Dr Hales 
 moistened a bladder, and fixed to it a fosset, both 
 of which would contain 74 cubic inches of air. Ha- 
 ving blown up the bladder, he put the small end of 
 the fosset into his mouth, and, at the same time, 
 pinched his nostrils close, that no air might escape 
 through them, and he then breathed to and fro the 
 air contained in the bladder. In less than half a mi- 
 nute, he found a considerable difficulty of breathing, 
 and was forced after that to draw his breath very 
 
 Tractat. Quinque, p. 300. 
 
Ill 
 
 fast : and at the end of the experiment, the suffo- 
 cating uneasiness was so great as to oblige him to 
 take away the bladder from his mouth. Towards 
 the end of the minute, the bladder was become so 
 flaccid that he could not blow it above half full, 
 with the greatest expiration that he could make *. 
 When also Mr Davy respired atmospheric air in a 
 natural manner, he took in, he says, only 13 cubic 
 inches and expelled 12.7, so that only about ^ 
 part of the original bulk was retained : when he 
 made one respiration of 10O cubic inches of air, the 
 diminution was to 99, or |~ : when, after a com- 
 plete exhaustion of his lungs, he respired 141 cubic 
 inches of air, once only for one-fourth of a minute, 
 they were reduced to 1 39, or ~ nearly : and when 
 161 cubic inches were breathed for about a minute, 
 their bulk was diminished to 152, or ~t ; in every 
 case, the diminution augmenting with the repetition 
 of the respiration, and consequent impurity of the 
 air, and distress of the respiratory organs. So like- 
 wise, when Dr Henderson breathed from and into 
 the gasometer GOO cubic inches of air for four mi- 
 nutes, they were reduced to 570, or lost ^ of their 
 bulk ; and he adds, that he held on respiring until 
 the sense of oppression about the chest obliged him 
 to desist |. These distressing symptoms, brought 
 on by the repeated breathing of the same quantity 
 
 * Statical Essays, vol. i. p. 288. 
 f Researches, p. 431, 4-32, 433. 435, 
 Nicholson's Journal, May 1 804-. 
 
112 
 
 of air, were felt in a still greater degree by Mr Kite ; 
 for on respiring 591 cubic inches of atmospheric air 
 from and into a bladder, he experienced, in one mi- 
 nute, great anxiety at the breast, which in half a 
 minute more became intolerable : his face swelled, 
 became black, and felt excessively hot, and sparks 
 of fire danced before him : loss of sight, giddiness 
 and confusion of the senses succeeded, and at the 
 end of little more than two minutes, he fell back 
 into a chair. He was relieved by fresh air, but 
 remained confused and giddy *. The amount of 
 the diminution of respired air, says Professor Pfaff, 
 depends not only on the time during which a given 
 volume of air is respired, but principally on the 
 magnitude of the volume of air itself : it must be 
 proportionally less the greater the quantity inspired. 
 He breathed 144 cubic inches of air once only in 
 the time of ten or twelve seconds, and the diminu- 
 tion was four cubic inches, or i of the primitive vo- 
 lume : when he respired the same volume of air 
 twice, during twenty seconds, it lost eight cubic 
 inches, or ^ : and when it was thrice respired, du- 
 ring thirty seconds, the diminution amounted to 
 twelve cubic inches, or ~ of the primitive volume f. 
 Now, in all these cases, the volume of air respired 
 was precisely the same, and could not, therefore, 
 affect the ratio of diminution : but as the times were 
 doubled and tripled, so nearly were the degrees of 
 diminution. But the more frequently the same air 
 
 * Essay on Apparent Death, p. 25. 
 f Nicholson's Journal, December 1805. 
 
113 
 
 is breathed, the more unfit does it become for respi- 
 ration : and to this change of composition, more 
 than to the time, or the magnitude of the volume 
 of air, is the increased degree of diminution to be 
 ascribed, 
 
 92. This will perhaps appear more striking, if 
 we attend to what happens in respiring nitrous oxide, 
 which is composed of the same elements as atmo- 
 spheric air, but contains a much larger proportion 
 of oxygen. After exhausting his lungs, Mr Davy 
 inspired 108 cubic inches of this gas, which, when 
 expired, were reduced to 99, or had lost ~ of their 
 bulk. When he made two respirations of the same 
 quantity of the oxide, the diminution was to 95, or 
 about one-eighth : and when he respired 102 cubic 
 inches of nitrous oxide, mixed with -^ of common 
 air, for half a minute, the volume of air, after the 
 seventh expiration, was reduced to 62, or had suf- 
 fered a loss equal to -^ *. Hence it appears, that 
 in the natural respiration of atmospheric air, only a 
 small diminution (85.) of its bulk takes place : that 
 this diminution increases as the air becomes vitiated 
 (91.) by repeated respirations, or is breathed in a 
 preternatural manner : and that when a gas of the 
 same elementary materials, but combined in very 
 different proportions, is substituted into the place of 
 pure atmospheric air, the diminution increases in a 
 tenfold degree. Now, the repeated breathing of the 
 same atmospheric air, has been shewn to bring on 
 
 Researches, p. 39-1-. 416. 
 H 
 
114 
 
 the most distressful symptoms, and at length an ut- 
 ter inability to continue respiration ; and Mr Davy 
 tells us, that, after a voluntary exhaustion of his 
 lungs, he could respire the nitrous oxide with accu- 
 racy, when stooping, for about half a minute, but, 
 even then, strong sensations were produced, with 
 fulness about the head rather alarming : that if 
 the respiration extended to three-fourths of a mi- 
 nute, he could not rely on the accuracy of any ex- 
 periment ; and that the determination of blood to 
 the head became, in- less than a minute, so great, as 
 often to deprive him of voluntary power over the 
 muscles of his mouth *. But respiration is a func- 
 tion carried on by the exertion of muscular powers, 
 in a great degree obedient to the will (87.) ; and 
 the quantity of residual air in the lungs in preterna- 
 tural respiration, will, at all times, be much influen- 
 ced by the manner in which the will exerts itself, 
 and the degree in which the muscles are able to act. 
 When, therefore, the power of the will over the mus- 
 cles is in any degree diminished, or is wholly lost, or 
 the muscles themselves are much weakened, a pro- 
 portional derangement will take place in the respira- 
 tory function ; and as, in the natural condition of 
 the body, expiration is subsequent to inspiration, 
 the ability to inspire will last longer than the ability 
 to expire : consequently, the cessation of the process 
 is brought about by a failure in the expiratory 
 powers. But if the expiratory powers are unable to 
 expel the air from the lungs, it must remain in. 
 
 Researches, p. 392. 
 
115 
 
 those organs ; and hence we see 1 , in all the forego- 
 ing examples, that the diminution in the volume of 
 expired air was greater in proportion as the respira- 
 tory organs suffered distress or oppression, and a- 
 mounted even to more than one-third of the air in- 
 spired when all voluntary powers ceased. 
 
 93. By Mr Davy, however, and many others, the 
 difference in bulk between the volume of nitrous 
 oxide inspired, and that which is expired, is consi- 
 dered to arise in every case from a " rapid absorp- 
 tion of this elastic fluid by venal blood through the 
 moist coats of the pulmonary veins * :" and he also 
 thinks it " reasonable to suppose, that the whole 
 compound atmospheric air, passing through the 
 moist coats of the vessels, is first dissolved by the 
 serum of the venal blood, and, in its condensed 
 state, decomposed by the affinity of the red particles 
 for its oxygen ; the greater part of the nitrogen be- 
 ing liberated unaltered, but a minute portion of it 
 possibly remaining condensed in the serum and coa- 
 gulable lymph, and passing with them into the left 
 chamber of the heart f." It happens, rather unfor- 
 tunately for this opinion, that, in the natural respi- 
 ration of atmospheric air, a very small difference 
 exists between the inspired and expired volumes, 
 though the powers of absorption, if such there be, 
 must then be acting in their greatest vigour ; while 
 under an almost total exhaustion of muscular and 
 vital power, this absorption is considered to take 
 place in an extraordinary degree. But the subject 
 
 * Researcher,, p. 396. Ibid. p. 4-47. 
 
 H2 
 
116 
 
 of the absorption of elastic fluids in the human 
 lungs is of so much importance in itself, and has so 
 much divided the opinions of physiologists, as to de- 
 mand from us a more distinct and detailed discus- 
 sion. 
 
 94. When then the air, received into our lungs, 
 is said to obtain admission into the blood-vessels by 
 a process of absorption, it may be proper, in the 
 first place, to inquire into the structure of those or^ 
 gans, at least so far as to ascertain by what means 
 such a process takes place. The lungs, one of which 
 occupies each cavity of the chest, are composed prin- 
 cipally of air-cells and blood-vessels, connected 
 through their whole extent by intervening cellular 
 membrane. The trachea, or windpipe, on its arri- 
 val in the chest, divides into a right and a left 
 branch, which branches again subdivide into smaller 
 ones called bronchia, and these into others still more 
 minute, until at length they lose their cartilaginous 
 texture, become membranous, and expand into a 
 cellular structure, which fills at all times the cavity 
 of the chest. " The cells composing this structure, 
 are purely membranous, of an irregular figure, com- 
 pressed and closely connected, and have a free com- 
 munication with each other. Between the different 
 lobes, lobules, and cells of the lungs, a large quan- 
 tity of common cellular substance, destitute of fat, 
 is interposed, which unites and strengthens them ; 
 but the cells have no communication with this sub- 
 stance ; for, when air is blown into it, the lobules 
 are compressed, but when the air is blown in through 
 a branch of the trachea, the cells are again dis- 
 tended, and the lobules recover their former dimen- 
 
117 
 
 sions *." The pulmonary artery, which conveys the 
 venal blood from the right side of the heart, divides 
 into two branches, which are dispersed through the 
 substance of the lungs ; and its smaller branches, 
 running in the common cellular substance, become 
 at length inconceivably minute, forming at last a 
 /ilexus, or fine net-work, upon the proper cells .: and 
 they terminate afterwards, partly into exhalent ves- 
 sels, and partly into corresponding branches of the 
 pulmonary veins,. These veins, by frequent anasto- 
 mosis, diminishing in number and increasing in size, 
 form at last four large trunks, which finally deliver 
 the blood into the left side of the heart. From this 
 description, it is manifest, that, between the air con- 
 tained in the cells, and the blood flowing through 
 the vessels, are interposed the coats both of the cells 
 and vessels. When, therefore, air is said to enter 
 into the blood from the cells of the lungs, it must, 
 in some way, be conveyed through the coats of these 
 cells and blood-vessels. After what manner, there- 
 fore, is it able to effect a passage ? 
 
 95. Every anatomist will allow, that the surface 
 of the cells of the lungs, like every other surface of 
 the body, is duly furnished with absorbent vessels, 
 of which not only the ordinary absorption of fluids 
 carried on by this surface, but the frequent remo- 
 vals of morbid matter from the bronchial cells, sup- 
 ply abundant proof. Mr Cruickshank has frequent- 
 ly seen the absorbents of the lungs turgid with blood 
 in cases of hcemoptoe, which blood they had ab- 
 
 * Fyfe's Compendium of Anatomy, vol. ii. p. MS, 
 H 3 
 
118 
 
 sorbed from the air-cells instead of their transparent 
 fluid *. Does the air also, which is supposed to 
 pass out of the cells of the lungs into the blood-ves- 
 sels by a process of absorption, take the route of 
 these absorbent vessels ? To this question we reply, 
 in the language of Haller, that the fineness of those 
 vessels, the mucus perpetually smearing the surface 
 of the cells, the elastic nature of air itself, and its 
 repulsion by water, so that it neither penetrates moist 
 paper, cloth, nor skin, all demonstrate that no air 
 by this route gets into the blood |. If, indeed, air 
 were taken in by the absorbents, it must, as Dr 
 Goodwyn observes, take the route of those vessels, 
 and, by passing directly to the right side of the 
 heart, change the colour of the blood there ; which, 
 however, does not happen J : nor, when air was 
 forced down the windpipe of a dog, in the experi- 
 ments of Dr Hales, was it able to pass into the pul- 
 monary artery or veins ||. 
 
 96. If, then, no proof exist of the passage of air 
 into the blood by the ordinary course of the absorb- 
 ent vessels, the only other mode of effecting this 
 purpose that has been hitherto suggested, is the 
 power of chemical affinity. What then are the che- 
 mical affinities subsisting between venal blood and 
 atmospheric air ? About the middle of the 17th cen- 
 tury, Dr Lower observed, that the upper surface of 
 
 * On the Absorbents, p. 42. 
 
 f Prim. Lineae, par. 306. 
 
 J Connexion of Life with Respiration, p. 
 
 H Statical Essays, vol. ii. p. 276. 
 
119 
 
 venal blood, received into a vessel, acquired a scar- 
 let colour by exposure to the air : that if this sur- 
 face was removed, the subjacent one was soon chan- 
 ged to the same colour : that if the cake of blood, 
 after being allowed to settle in the vessel, was in- 
 verted, its exterior and upper surface speedily also 
 assumed a florid hue : and, lastly, that if venal blood 
 was shaken in a vessel, so that the air thoroughly 
 intermixed with it, it became entirely florid *. These 
 opinions were afterwards held by Sig. Fracassati and 
 Dr Slare, the latter of whom observes, that the 
 blood thrown up by a rupture of the capillary ves- 
 sels of the lungs, is frothy and of a scarlet colour ; 
 the first of which effects he attributes to the inter- 
 mixture of air, and the latter to its tinging power f. 
 Mr Hewson employed similar arguments to prove, 
 that the florid colour, acquired by venal blood on 
 exposure, was produced by the contact of the air : 
 and, by injecting air into the jugular vein of a rab- 
 bit, he found that it there also rendered the blood 
 florid J. M. Cigna not only confirmed the forego- 
 ing facts, but proved also that the change of co- 
 lour in this fluid did not take place when the blood 
 was covered with oil or placed in vacuo ; and Dr 
 Priestley ascertained, that not only by common air, 
 but more especially by oxygen gas, this florid colour 
 was produced on the black crassamentum of blood (|. 
 
 * Tract, de Corde, p. 178. An. 1669. 
 f Lowthorpe's Abrid. Phil. Trans, vol. Hi. p. 235. 
 J Hewson on the Blood, p. 9. 
 '] Priestley on Air, vol. iii. p. 66. 
 H4 
 
120 
 
 97. In effecting these remarkable alterations in 
 the colour of the blood, the air itself, at the same 
 time, suffers material changes. Dr Priestley found, 
 that in Jwenty-four hours oxygen gas was so far de- 
 praved by being in contact with venal blood, that 
 one measure of it and two of nitrous gas occupied 
 the space of a measure and a half, whereas, at the 
 beginning of the experiment, they occupied the space 
 of no more than half a measure *. Dr Goodwyn 
 confined venal blood under a jar of oxygen gas in- 
 verted in mercury, and repeatedly observed that the 
 change of colour was always very sudden, and, after 
 several minutes, the mercury ascended two or three 
 lines ; from which he concluded that a small portion 
 of the air had disappeared |. The precise change, 
 however, which the air underwent, seems first to 
 have been observed by Dr Girtanner, who placed 
 six ounces of venal blood in a jar of oxygen gas in- 
 verted in mercury : the blood presently assumed a 
 florid colour : the air was somewhat diminished in 
 bulk, and contained a portion of carbonic acid, 
 which was attracted by lime-water J. Dr Bostock 
 observes also, that a diminution of oxygen and 
 production of carbonic acid take place when a piece 
 of crassamentum is placed in a jar filled with oxy- 
 gen gas ||. The same production of carbonic acid 
 we found to occur when blood is placed in contact 
 
 * Priestley on Air, vol. iii. p. 75. 
 f Goodwyn's Essay, p. 61. 
 
 f Memoirs on Irritability in Bcddoes' Obs. on Calculus, &c. 
 p. 219. 
 
 !| On Respiration, p. 227. 
 
121 
 
 with atmospheric air. A quantity of this fluid was 
 received into a cup, and confined in a jar of air in- 
 verted in water, a glass of lime-water having been 
 previously placed in the cup. The internal surface 
 of the jar was soon bedewed with moisture, and a 
 pellicle began to form on the lime-water, which in 
 a few hours was increased to a thick crust of carbo- 
 nate of lime. The crassamentum was then removed, 
 and a fresh glass of lime-water was placed in the se- 
 rum, which in thirty-six hours had acquired a crust 
 like the former, and the water had risen considera- 
 bly into the jar. In another experiment, where the 
 serum was placed for twenty-four hours in a jar of 
 air inverted in mercury, the residual air rendered 
 lime-water milky, and the remainder had lost a part 
 of its oxygen. A similar production of carbonic 
 acid seems to have occurred, when, with a small di- 
 minution of the gas, a slight change of colour was 
 produced on venal blood by placing it in contact 
 with nitrous oxide, in the experiments of Mr Davy : 
 for when a solution of strontian was admitted to the 
 oxide, it became slightly clouded, and, with the di- 
 minution of bulk that followed, minute portions of 
 carbonic acid and nitrogen gas were produced *. 
 Hence then we learn, that when venal blood is ex- 
 posed to the contact of atmospheric air, of oxygen 
 gas, or of nitrous oxide, it presently assumes a florid 
 colour, and, at the same time, the volume of air is 
 somewhat diminished, and a portion of carbonic 
 acid is produced. 
 
 Researches, p. V ST7, 380, 387. 
 
122 
 
 98. Does then the carbonic acid, which is here 
 met with, proceed ready formed from the blood, or 
 is it in part formed by the decomposition of the air ? 
 No one has yet proved -that any aeriform fluid, much 
 less that carbonic acid, exists naturally in the blood ; 
 and if this be true, no such aerial acid can be ex- 
 pected to issue from it. The carbonic acid also, is 
 not formed by blood when it is confined in nitrogen 
 gas ; neither does the colour of the blood, in that 
 case, undergo any sensible change : but this acid is 
 formed by blood, either in oxygen gas, in nitrous 
 oxide, or in atmospheric air, all of which are dete- 
 riorated thereby ; whence it follows, that without 
 the presence of oxygen gas, the blood is unable to 
 form carbonic acid, and that this acid, therefore, is, 
 in part, formed out of that gas. If the oxygen gas 
 that disappears do not contribute to form the carbo- 
 nic acid [hat is produced, in what other manner can 
 its loss be accounted for ? or from what other source 
 than the oxygen gas of the air, in contact with the 
 blood, >can that ingredient of the acid be derived ? 
 Those who suppose the carbonic acid to be furnish- 
 ed by the blood, independent of the air employed, 
 must likewise suppose that the nitrogen gas is fur- 
 nished by it also ; for the experiments of Mr Davy 
 teach us, that a portion of that gas, as well as of 
 carbonic acid, is always present when nitrous oxide 
 is decomposed, which renders it probable that the 
 same thing likewise occurs when air is changed by 
 venal blood. But in what manner the blood should 
 be able to furnish nitrogen gas, it is not easy to con- 
 ceive, since no affinity exists between that gas and 
 
123 
 
 venal blood*. We infer, therefore, from these 
 facts, that atmospheric air is decomposed by being 
 placed in contact with venal blood, its oxygenous 
 portion being in part converted into carbonic acid, 
 and a quantity of its nitrogen being, in consequence, 
 left free. 
 
 99. But, supposing the air to be thus decomposed by 
 the blood, it still remains a question, whether it has 
 been first attracted by that fluid, then decomposed, 
 and afterwards in part expelled ; or, whether the de- 
 composition has been effected without such previous 
 attraction and intermixture of air. The only evi- 
 dence of this supposed attraction seems to be the small 
 diminution of bulk which the air in all cases suf- 
 fers ; but this cannot be considered as a proof of the 
 attraction of the air ; for it is a necessary conse- 
 quence of that conversion of oxygen gas into carbo- 
 nic acid which has been shewn (11.) to take place 
 when these substances are brought into contact. Even 
 granting to the blood this power of attracting air, 
 or its oxygenous portion, it is not easy to conceive 
 why it should so readily lose it, and again give out 
 this air in the form of carbonic acid. No change 
 of quality in the blood, nor any variation of tempe- 
 rature, can have taken place sufficient to alter so ra- 
 pidly its affinity for these substances : and it cannot 
 proceed from a want of affinity between the blood 
 and the carbonic acid that is formed ; for that acid 
 suffers a greater diminution, either than oxygen gas 
 or atmospheric air, by being placed in contact with 
 
 * Davy's Researches, p. 37.-". 
 
124 
 
 blood. We incline, therefore, to the opinion, that 
 neither the air nor its oxygen gas is attracted by, 
 and diffused through the blood, as happens with se- 
 veral gases when placed in contact with certain fluids: 
 but that the air is decomposed, and its oxygen gas 
 changed into carbonic acid, without entering into 
 the substance of that fluid. 
 
 100. But, for the formation of this acid, the blood 
 must supply carbon, since no other substance was 
 present from which it could be derived r and it is well 
 known also, that carbon enters largely into the com- 
 position of that fluid ; and our experiments (97.) 
 prove, that it exists as well in the serous as in the 
 more solid parts. By some it may be objected, that 
 because carbonic acid is formed directly by the com- 
 bustion of charcoal, it cannot be produced at so low 
 a temperature as exists in these experiments. To 
 this we can reply only by an appeal to the general 
 facts exhibited through the whole course of our in- 
 quiry, by which it appears, that both by the living 
 functions of vegetables and animals, and by the de- 
 composition of animal and vegetable matter, this acid 
 is, in like manner, formed at temperatures equally 
 low. Even those who consider this acid to have 
 proceeded ready formed from the blood, cannot at- 
 tribute its production to the operation of heat ; for 
 in the animal body, the temperature of the blood 
 seldom exceeds 1OO a degree of heat incompetent 
 to form carbonic acid by any process analogous to 
 combustion. The combination of many bodies is, 
 indeed, greatly accelerated by being exposed to very 
 high temperatures ; but this surely does not set aside 
 the fact of their spontaneous union at temperatures 
 
125 
 
 much more low. From this review of the effects 
 which take place between the blood and air, we con- 
 clude, that the chemical phenomena which arise when 
 these substances are placed in contact, do not prove 
 an attraction and diffusion of air through the blood ; 
 but shew only that a reciprocal action takes place, 
 by which a new product is formed : no inference, 
 therefore, in favour of an attraction of air by the 
 blood in the lungs, can be drawn from the recipro- 
 cal action which they exert on each other out of the 
 body. 
 
 101. To the operation of chemical affinity also, a 
 degree of absolute contact is required, which may, 
 and does exist between air and venal blood out or, 
 the body ; but the intervention of the coats of the 
 cells and blood-vessels altogether forbids this neces- 
 sary condition in the lungs. The supposition, that 
 the coats of these vessels and cells are so thin, that, 
 when moist, they allow the air, or its oxygen gas, to 
 pervade them, is wholly gratuitous, and in opposition 
 to the results (94.) of direct experiment : and the be- 
 lief, that certain pores exist, through which elastic 
 fluids may permeate, is equally unsupported by ana- 
 tomical fact ; for the terminations of the absorbent 
 and exhalent vessels are the only orifices which are 
 known to open on the surface of the bronchial cells. 
 If, indeed, air did permeate the bronchial cells through 
 any supposed pores, it would more readily pass into 
 the cellular substance which connects them together 
 than into the pulmonary vessels, and thus would 
 create, at all times, an emphysematous state of those 
 organs ; but this is never known to be the case, for 
 these cells are impermeable by air. It is, lastly, ex- 
 
126 
 
 tremely difficult to conceive how the same air, which 
 is so readily confined in membranous substances out 
 of the body, should, with such perfect freedom, pass 
 to and fro through a much more complicated struc- 
 ture within it. It has indeed been said, that, when 
 any gas is confined in a bladder, it will permeate its 
 coats and escape, while atmospheric air, passing at 
 the same time through these coats, will supply its 
 place. Allowing this to be the case, it will not be 
 denied, that many days, or even weeks, are requi- 
 red to accomplish this operation : and it bears, there- 
 fore, no sort of analogy to that rapid attraction and 
 expulsion of air which is supposed to go on through 
 the cells and blood-vessels of the lungs. It should 
 be remembered also, that the bladder, when remo- 
 ved from the body, soon loses its living properties, 
 by which its power of resisting the passage of fluids 
 may be diminished ; for we know that the bile, a 
 much denser fluid than air, is during life perfectly 
 retained within the gall bladder ; but a short time 
 after death, its colouring matter often escapes, and 
 gives to the surrounding viscera a yellow tinge. 
 Neither can any thing, necessarily residing in the 
 venal blood, be held sufficient to account for this 
 supposed attraction of air ; for Girtanner found, 
 that arterial blood produced the same changes *, and 
 the like occur (97.) when serum only is employed ; 
 and even if, by superior ailinity, the blood did attract 
 air through the coats of the cells and vessels of the 
 Jungs* in what way shall we account for its so ra- 
 
 Mi'ir.oirs on Irritability, p. 228. 23 J. 
 
127 
 
 pidly losing this superiority, and again giving out 
 nearly the whole of this air, through these same 
 blood-vessels and cells ? 
 
 102. But if, either by the function of absorption, 
 or by the operation of chemical affinity, air did enter 
 into the blood, we may surely with reason demand 
 some proof of its presence ; yet, says Haller, " Nul- 
 ia unquam in vivo calido animali bulla aeris in san- 
 guine visa est *." This opinion is confirmed by the 
 direct experiments of Dr D. Darwin ; for having in- 
 closed a portion of the jugular vein of a sheep be- 
 tween two ligatures, it was cut out, stripped of its 
 adhering cellular membrane, and then thrown into a 
 glass of water of temperature 100, standing under 
 the receiver of an air-pump. It at once sank to the 
 bottom, and did not rise when the air was exhaust- 
 ed ; nor, when afterwards taken out, wiped dry, and 
 laid on the floor of the receiver, did it exhibit any 
 swelling under the exhaustion of the vessel. The 
 experiment was repeated with a similar result on a 
 portion of the vena cava of a swine f. 
 
 103. Neither do the effects resulting from the ad- 
 mixture of aeriform fluids with the blood, favour the 
 notion of the entrance of air into that fluid. " Ani- 
 mal, cui aer in sanguinem inflatur," says Haller, 
 6C perit certo et velociter ; neque quidquam satis certi 
 est in sanguinis venarum pulmonalium aucto ru- 
 bore J." This assertion is confirmed likewise by 
 
 * Prim. Lin. par. 306. 
 
 f Philosophical Transactions, vol* Ixiv. p. 
 
 .(: Prim. Lines, loc. cit. 
 
128 
 
 direct experiment. When Dr Girtanner injected 
 oxygen gas into the jugular vein of a dog, he cried 
 dreadfully, breathed quick, and died in three mi- 
 nutes : when nitrogen gas was thrown in, death 
 happened in 20 seconds *. Air, says M. Bichat, 
 thrown into the vascular system, quickly brings on 
 agitation, convulsions, and death f. By forcing air 
 through the windpipe into the lungs with a syringe, 
 and confining it there, he has made it to enter into 
 the blood-vessels, which immediately brings on agi- 
 tation and exertion in the animal ; and if an artery 
 in the leg or foot be now opened, the blood will 
 spring out frothy, and full of bubbles of air. If 
 hydrogen gas has been used, the bubbles may be in- 
 flamed ; and when this frothy blood has flowed 30 se- 
 conds, the actions of life cease, and cannot again be 
 restored, even although fresh air be supplied J. 
 
 104. As the chief arguments in favour of the en- 
 trance of air into the blood, have been drawn from 
 the experiments of Mr Davy in his excellent " Re- 
 searches" into the nature and respiration of nitrous 
 oxide, it may, perhaps, be required that we should 
 notice the leading proofs which he adduces in favour 
 of it. Mr D. had found that hydrogen gas, when 
 inspired, only mingles with the air present in the 
 lungs, and is again thrown out unaltered with a 
 portion of the residual air ||. The bulk of this resi- 
 dual air, reduced to the temperature of 5J, he es- 
 
 * Memoirs on Irritability, p. 221. 223. 
 
 f Rechcrches sur la Vie et la Mort, p. 179. Ibid. p. 303. 
 
 I! Researches, p. 39<). 
 
129 
 
 timates at 32 cubic inches, being composed of nitrogen 
 23, carbonic acid 4.1, and oxygen gas 4.9 *. When 
 he breathed the nitrous oxide, the proportions of these 
 residual gases appeared nearly the same as when hy- 
 drogen gas was respired ; which led him to conclude, 
 that no portion of the nitrogenous or oxygenous 
 gases was produced by the decomposition of the oxide 
 in the lungs t But a reference to the results of Mr 
 Davy's experiments will shew, that the carbonic acid 
 in the lungs and air-holder, which, before breathing 
 the oxide, was estimated at 4. 1 , was afterwards increa- 
 sed to 5.2 cubic inches ; and that the oxygen gas also 
 was increased from 5.6 to 6. 1 . In another experiment, 
 the carbonic acid was increased, after breathing the 
 oxide, from 4.1 to 6.8, and the oxygen gas, at the 
 same time, from 5.5 to 6.3 ; and Mr Davy admits, 
 " that the quantity of carbonic acid and oxygen is ra- 
 ther greater than that which existed in the experi- 
 ments on hydrogen J." Finding, indeed, the bulk of 
 these two gases in the lun s and air-holder to be on- 
 ly 9 cubic inches when hydrogen was respired, and 
 that it amounted to 13.1 when nitrous oxide was 
 breathed, and, denying that the acid is formed by 
 the immediate decomposition of the nitrous oxide it- 
 self, Mr Davy is led to believe, that " it is wholly 
 or partially liberated from the venal blood through 
 the moist coats of the vessels ;" a supposition, a- 
 gainst which it has, we trust, been already suffi- 
 ciently argued. 
 
 * Researches, p. 409. f Ibid. p. 41 4-. 
 
 t Ibid. p. 413. 415. Ibid. p. 420. 422. 
 
 I 
 
130 
 
 . Together with this increase of oxygen gas 
 and carbonic acid, beyond what the residual air in 
 the lungs would supply, there was a considerable 
 increase also of nitrogen gas, which corresponds 
 with what takes place when this oxide is exposed to 
 blood out of the body ; for, besides the carbonic a- 
 cid produced, there are always present small por- 
 tions of nitrogen gas*. Since, therefore, the same 
 products are obtained when this oxide is breathed, 
 as when it is exposed to blood out of the body, it 
 may be inferred that they are effected by similar 
 means. These means cannot, in the latter case, be 
 absorption in the sense we apply that term to the li- 
 ving human body ; because the blood speedily be- 
 comes an inert mass> bearing no analogy in its pro- 
 perties to the absorbent function in the lungs : and, 
 even if chemical attraction were allowed to ope- 
 rate between this oxide and blood out of the body, 
 this will not apply to the circumstances hi which 
 they are respectively present in the lungs ; because 
 the intervention of cells and blood-vessels wholly 
 forbids that degree of actual contact, which is essen- 
 tial to chemical action. It is therefore only on the 
 supposition of a decotnfusititin taking place, that these 
 contradictory results can be reconciled. 
 
 106. Against the idea of such decomposition in 
 the lungs, Mr Davy urges, that " it is difficult to 
 suppose how nitrous oxide, which requires the tem- 
 perature of ignition for its decomposition by the 
 most inflammable bodies, should be partially ab- 
 
 * Researches, p. 387. 
 
131 
 
 sorbed, and partially decompounded at 98% by a fluid 
 possessed apparently of uniform attractions *." To 
 this it may be replied, that the difficulty of decom- 
 posing a substance by some bodies, in certain cir- 
 cumstances, at a high temperature, is no proof that 
 it cannot be decomposed, by other bodies, in other 
 circumstances, at a low temperature. This oxide, 
 Mr Davy believes, to be in some manner decom- 
 posed by the blood during its circulation with that 
 iiuid : but, does the temperature of the blood ex- 
 ceed that of the lungs ? Even out of the body, and 
 in the ordinary temperature of the atmosphere, the 
 experiments of Mr Davy (97.) evince, that this 
 oxide can be partially decomposed by venal blood. 
 
 107- According to the earliest and the latest ex- 
 periments of M. Lavoisier on the respiration of at- 
 mospheric air, its nitrogen gas was considered to be 
 in no respect affected by that function ; in which 
 conclusion Goodwyn, Menzies, and most other au- 
 thors, have acquiesced : and we have seen that this 
 gas is not affected by the growth of vegetables, 
 (5. 29.), nor by the respiration (73.) of the inferior 
 animals. Dr Priestley, however, at one time sup- 
 posed, that the oxygen gas passing the membrane of 
 the lungs, carried with it some part of the nitrogen 
 with which it was previously combined ; but, on the 
 suggestion of Sir Charles Blagden, he afterwards 
 thought it more probable that the deficiency of ni- 
 trogen gas was owing to the greater proportion of it 
 existing in the lungs after the process than before f. 
 
 * Researches, p. 415. 
 
 -\- Experiments on Air abridged, vol. iii. p. 380. 
 I 2 
 
132 
 
 Mr Davy, from the results of his experiments, has 
 been led to revive this opinion concerning the en- 
 trance of nitrogen into the blood. In the respiration 
 of atmospheric air, he calculates that 5.2 cubic inches 
 of nitrogen disappear every minute * : and when 
 nitrous oxide is breathed, he supposes that " an im- 
 mense quantity of this substance is taken into the 
 blood ; and that the part of it not expended in new 
 combinations, during living action, is liberated in the 
 aeriform state by the exhalents, or through the moist 
 coats of the veins f." But why the blood should so 
 much more powerfully attract nitrogen from nitrous 
 oxide than from atmospheric air, is enough of it- 
 self to beget doubts of the truth of Mr Davy's con- 
 clusion, more especially if we consider, that " nitro- 
 gen and oxygen exist, perhaps, in this oxide in the 
 most intimate union which those substances are ca- 
 pable of assuming J." The only direct evidence in 
 favour of this opinion, is the loss in bulk which ni- 
 trous oxide suffers in respiration. For this we have 
 endeavoured to assign an adequate cause (9 1 . et seq.); 
 and a reference to Mr Davy's experiments (92.) will 
 shew, that when he breathed this oxide once only, 
 the loss in bulk was one-twelfth ; when twice, the 
 loss was about one-eighth ; and when seven times, 
 the loss amounted to more than one-third ; thus re- 
 gularly increasing with the repeated respiration of a 
 noxious gas, and consequent debility of the expira- 
 tory powers, just as takes place in the repeated 
 breathing of the same volume of atmospheric air. 
 
 * Researches, p. 434. f Ibid. p. 415. f Ibid, p. 328. 
 
133 
 
 108. Mr Davy, however, taking for granted that 
 immense quantities of nitrogen gas enter into the 
 blood, observes, that " this being true, the quan- 
 tity of nitrogen produced in respiration, ought to be 
 increased in proportion as a greater quantity of ni r 
 trous oxide enters into combination with the blood *." 
 This he attempts to prove by experiment. Having 
 exhausted his lungs, he inspired the oxide out of a 
 silk bag, containing eight quarts of that gas, and thus 
 made nine respirations. The gas of the first expi- 
 ration was not preserved, but that of the seconc} 
 gave 29 of the oxide, and 1 7 parts nitrogen gas 5 
 the third was as 22 to 8 ; the fifth, as 27 to 6 ; the 
 seventh, as 23 to 7 ; and the ninth was as 26 to 1 1. 
 vSo far, therefore, from the nitrogen gas increasing 
 in proportion to the supposed combination of the 
 oxide with the blood, it observes nearly a decrea- 
 sing ratio ; and Mr Davy accordingly admits, that 
 the " results of these experiments are not so con- 
 clusive as could be wished f." He nevertheless 
 goes on to say, that " if any portion of nitrous 
 oxide were decomposed immediately by the red parr 
 tides of the blood, one should conjecture that the 
 quantity of nitrogen produced, ought to be greater 
 during the first inspirations, before these particles 
 became fully combined with condensed oxygen j." 
 And what is the fact from the experiments just re^ 
 lated ? Why, that in the second expiration, the ni- 
 trogen gas was 1 7 parts ; in the third only 8 ; and 
 in the fifth but 6 ; evidently showing that the greater 
 
 Researches, p, 4-16. f Ibid - P- 418 - t Ibid P- 4<19 
 
134 
 
 quantity of this gas did appear during the first in- 
 spirations ; and not after the supposed combination 
 of oxygen with the red particles of the blood. 
 
 109. Mr Davy has breathed the nitrous oxide, 
 in a state of purity, for four minutes and a half, 
 and some have respired k five minutes ; and he 
 states the proportion of oxide absorbed to be as 
 great in the last, as in the first inspirations, the con- 
 sumption, by the same individual, being nearly in 
 the ratio of the time of respiration *. He thinks a- 
 bout a pint, or 30 cubic inches, to be the ordinary 
 range of consumption in different individuals, which, 
 he says, is not far from two cubic inches, or about 
 one grain every second ; or in one minute 120 cu- 
 bic inches, or 60 grains f. This quantity amounts, 
 in five minutes, to 600 cubic inches, or 300 grains ; 
 and in one hour, to 720O cubic inches, or 360O 
 grains ; equal to more than 31 gallons measure, or se- 
 ven ounces and a half troy weight, if it were possible 
 so long to continue respiring, and the consumption 
 were nearly in the ratio of the time of respiration. 
 But taking the proportion only for five minutes, 
 and allowing all that Mr Davy would deduct as li- 
 berated again from the blood in the same space of 
 time, it would amount to a bulk and weight alto- 
 gether inconsistent with what we know to be the 
 state and condition of the sanguiferous system ; and 
 the inference, if followed out, might therefore, in 
 the form of a reductio ad absurdum, be employed a- 
 gainst Mr Davy's opinion. 
 
 * Researches, p. 425. f Ibid. p. 427. 
 
135 
 
 i 1O. The idea, that " the absorption of nitrous 
 oxide by venal blood, is owing to a simple solution 
 of the gas in that fluid, analogous to its solution in 
 water or alkohol *, will by no means obviate the 
 objection to such immense quantities of it entering 
 into the blood ; for it appears, that but a very small 
 diminution of bulk takes place (96.) when this oxide 
 is placed in actual contact with venal blood j and when 
 again it " is carried through the pulmonary veins 
 and left chamber of the heart to the arteries," and 
 made to undergo decomposition, in order that " its 
 oxygen may be chiefly expended in living .action," 
 and its " nitrogen be partially consumed in new 
 combinations f, will not then the oxygen gas be in 
 contact with a fluid, over which, out of the body, it 
 does not "perceptibly diminish j," and the nitrogen 
 gas exist in one over which it ** possesses no power of 
 action, and with which it is incapable of combi- 
 ning || ?" The effects resulting from such admix- 
 tures of aeriform fluids with the blood in the living 
 body, have already (103.) been distinctly noticed. 
 If it be said, that this gas and atmospheric air are 
 received into the blood in a peculiarly modified 
 state as to their gaseous form, then they cannot be 
 supposed to pass entire (93.) through the moist 
 coats of the cells and vessels, but must suffer a pre- 
 vious decomposition in the lungs. If, on the other 
 hand, they do pass entire into that fluid, and if, 
 from its incapacity of combining with the blood, 
 
 * Researches, p. 378. f Ibid. p. 419. 
 
 J Ibid p. 381. I! Ibid, p, 375. 
 
136 
 
 " the greater part of the nitrogen gas (93.) be again 
 liberated unaltered," why cannot we discover some 
 traces (102.) of its existence in that fluid ? or why 
 should its presence be so rapidly fatal, as experiment 
 (1O3.) proves it to be ? 
 
 111. But the supposed stimulant effects of this 
 oxide have been assigned as an argument for its be- 
 ing in some way contained in the blood. In many 
 cases, however, these effects do not follow the inha- 
 lation of that gas : and, in other instances, common 
 air, breathed under the impression of its being this 
 oxide, has produced them. Similar effects likewise 
 are attributed by some authors to the inhalation of 
 carbonic acid ; and Dr Percival quotes M. Beaumc 
 as relating the history of a man who was recovered 
 from apparent death, produced by exposure to the 
 foul air of a cellar, who asserted that he had felt 
 neither pain nor oppression, but that, at the point of 
 time when he was losing his senses, he experienced 
 a delightful kind of delirium. This account, adds 
 Dr Percival, receives confirmation from the testimo- 
 ny of Dr Heberden, who says, that he had seen an 
 instance in which the fumes of charcoal brought 
 on the same delirium as intoxicating vegetables pro- 
 duce. The Abbe Fontana likewise breathed a cer- 
 tain portion of hydrogen gas, not only without in- 
 convenience, but with unusual pleasure : he had a 
 facility in dilating the breast, and never felt an e- 
 qually agreeable sensation, even when he inhaled 
 the purest oxygen gas *. From these facts, it ap- 
 
 Manchester Memoirs, vol. ii. p. 491, 492. 
 
137 
 
 pears, that the inhalation of any gas, unfit to carry 
 on respiration, will, in some persons, produce effects 
 precisely similar to those which have been ascribed 
 to an absorption of nitrous oxide ; and, on the other 
 hand, that no such stimulant effects are produced in 
 other persons by the inhalation and supposed ab- 
 sorption of that gas. In no respect, therefore, can 
 we trust to the effects, which succeed to the respira- 
 tion of this oxide, as affording any decisive proof of 
 its absorption and commixture with the blood. 
 
 112. It is, as we have seen, the opinion of Mr Da- 
 vy, and of some other authors, that air passes entire 
 into the blood-vessels, and is decomposed during the 
 circulation of that fluid. Now, the gases expired 
 are, and must be admitted to be, formed out of those 
 previously inspired, and, in natural breathing, the 
 air suffers but a small diminution (85.) in bulk ; 
 and the time of an ordinary inspiration occupies 
 from ~ to -^ part of a minute. To suppose the 
 inspired air to enter through the moist coats of the 
 cells and blood-vessels, su. er decomposition in the 
 blood, and be again returned through these vessels 
 and cells, with so small a difference in bulk, and in 
 so short a time, seems not only without proof, but 
 against all probability, especially if we consider the 
 extreme minuteness of the pulmonary vessels, and 
 the great rapidity with which the blood is transmit- 
 ted through them. If indeed, the reciprocal chan- 
 ges which take place between the blood and the air 
 were effected during the circulation of that fluid, 
 and it were only after that period the expired airs 
 could be rendered, we ought not to expect them for 
 nearly three minutes ; for suppose one ounce and a 
 
138 
 
 half of blood to be propelled at each contraction of 
 the heart, and each contraction to occupy ~ of a 
 minute, and the whole circulating mass to be about 
 324 ounces, then a complete circulation is perform- 
 ed in the time of 216 pulsations, that is, in three 
 minutes. The almost instantaneous return of the 
 inspired airs, however, is totally adverse to this fact : 
 and if this be true of one respiration, it must be true 
 of all, since the power acting, and the substances 
 acted upon, preserve always the same relative state 
 and circumstances, and the actions are performed 
 not in time only, but in succession also ; and thus, 
 as one cannot begin until the other is ended, each is 
 only a repetition of the same event, which, being 
 operated by like powers, and in like circumstances, 
 must afford invariably a like result. 
 
 1 13. If then there be no proof that air, either by 
 absorption, or by chemical affinity, enters into the 
 blood-vessels of the lungs, the sensible effects which 
 it is there known to produce on the blood, must, in 
 some other way, be accounted for. The celebrated 
 Lower, not only first remarked the scarlet colour 
 which black blood acquired by exposure to the air, 
 but he first proved likewise, that the same change 
 was produced by the air received into the lungs. In 
 combating the opinion of Willis, that the florid co- 
 lour of the blood was derived from an innate fire 
 kept up in the heart, he asserted, that, if this were 
 the case, it should, from their similarity of structure, 
 be effected by the right, as well as by the left, side 
 of the heart, whereas the blood sent into the lungs 
 from the right heart was dark and venal. He pass- 
 ed a ligature also round the trachea of an animal, so 
 
139 
 
 as effectually to exclude the air, and then opening 
 an artery in the neck, he found that the blood which 
 issued from it was black : and he farther remarked, 
 that in a strangulated animal, where the entrance of 
 air was precluded, the blood in the left side of the 
 heart was like to venal blood ; but if, before it coa- 
 gulated, it was forced through the lungs, and air 
 was .at the same time blown into those organs, it a- 
 gain became florid, after the death of the animal, 
 in the same manner as when he was alive *. Mr 
 Hunter, by means of a double bellows, was enabled 
 by one action to exhaust the lungs of their air, and 
 by another to pass, at the same time, fresh air into 
 those organs, without mixing the two together ; and 
 having introduced the nozzle of his bellows into the 
 trachea of a dog, whose heart and lungs had been 
 previously brought into view, he found, that, while 
 he continued the artificial breathing, the heart con- 
 tinued to act as before, but more frequently : if, 
 however, he stopped the motion of the bellows, the 
 contractions of the heart gradually became weaker, 
 and at length ceased. On recurring to the artificial 
 breathing, the heart again began to move, and be- 
 came at length as strong as at first. This process 
 he repeated ten times, stopping from five to ten mi- 
 nutes between each ; and he observed, that every 
 time he left off working the bellows, the heart be- 
 came extremely turgid with blood, and that the 
 blood in the left side was as dark as that in the 
 right, which was not the case when the bellows were 
 
 *Tractat. de Corde, p. 
 
140 
 
 working *. Dr Goodwyn repeated these experi- 
 ments, both with common air and with oxygen gas, 
 on dogs, and on lizards and toads, whose lungs con- 
 sist only of a transparent bladder ; and he observed 
 the same changes of colour, according as the lungs 
 were supplied with or deprived of fresh air f. This 
 suddenness of effect seems wholly inconsistent with 
 the supposed entrance of air into the blood, and its 
 subsequent decomposition during the circulation of 
 that fluid ; and favours the probability of its being 
 actually decomposed in the lungs. 
 
 114. Not only, however, in the lungs, but in 
 the bronchial cells pf those organs does this de- 
 composition appear to happen, of which the vast ex- 
 tent of their surface is a strong presumptive proof. 
 Dr Hales estimates the capacity of each cell, in the 
 lungs of a calf, at the ~ part of an inch diameter, 
 and the surface of all the cells together at 4O,000 
 square inches, to which, if 1635 square inches be 
 added as the estimated surface of the bronchia, the 
 sum of the surface of the whole lungs in that animal 
 will be 41635 square inches, equal to nineteen times 
 the surface of a man's body, which is computed at 
 fifteen square feet {. Dr Keil estimates the diame- 
 ter of each cell in the human lungs at ~ part of an 
 inch, and its superficies at 0.01256: the whole 
 number of cells he calculates at 17,441,860, which, 
 multiplied by the superficies of each, makes the sum 
 
 * Observations on the Animal (Economy, 2d edit. p. 134. 
 f Goodwyn's Essay, p. 58. 62. 
 | Statical Essays, vol. i. p. 241. 
 
141 
 
 of the superficies of all the cells, independent of that 
 of the bronchia, equal to 2 1 907 square inches, or 
 more than ten times greater than that of the whole 
 body *. The irregularity in the shape and size of the 
 cells, and the want of adequate means correctly to 
 determine their number, render an accurate estimate 
 of their entire surface extremely difficult, if not al- 
 together impossible : it is sufficient for our purpose 
 to have shewn, that they exceed, in extent, by many 
 times, the surface of the human body. The ramifi- 
 cations of the pulmonary artery and veins, running 
 over these cells, must be minute and extensive, in 
 proportion : and " by this admirable contrivance," 
 says Dr Hales, " the blood is spread out into a vast 
 expanse, commensurate to a very large surface of 
 air, from which it is divided by thin partitions, so very 
 thin as thereby to bring the blood and air within the 
 sphere of each other's attraction f." Such a struc- 
 ture, how favourable soever it may be to an extensive 
 contact of surfaces, is but little fitted to promote a 
 mixture of substances by absorption. 
 
 SECTION II. 
 
 115. IN the former section, we have endeavoured 
 to shew the necessity of atmospheric air to the con- 
 tinuance of living action in the superior animals, 
 
 Tentam. Med. Phys. p. 80. 
 Statical Essays, vol. i. p. 247- 
 
142 
 
 and have noticed the diminution of bulk which it 
 suffers in respiration : we have attempted to ascer- 
 tain the volume of air ordinarily inspired by man, 
 and to appreciate the relative proportions which ex- 
 ist in the lungs in the different states of those or- 
 gans : we have laboured to prove, that the actual 
 bulk of air in the lungs must, and does vary, accord- 
 ing to the various conditions of the system, to the 
 circumstances in which respiration is carried on, the 
 composition of the air itself, and the mode in which 
 it is breathed ; deducing from the whole, that the 
 bulk of residual air in the lungs, at any period of 
 life or of death, must be calculated always with re- 
 ference to these considerations. We have spoken 
 generally against the entrance of any part of the in- 
 spired air into the blood-vessels of the lungs, by the 
 function of absorption, as unsupported by anatomi- 
 cal fact ; and have held its supposed attraction by the 
 Mood through the coats of those vessels as inconsis- 
 tent with the acknowledged laws of chemical affinity : 
 we have denied that any proof exists of air being, at 
 any time, present in healthy blood, and have demon- 
 strated its fatal effects whenever it gains admission 
 into thaf fluid : we have examined the experiments 
 and arguments of Mr Davy, in favour of an absorp- 
 tion of aeriform fluids by the blood, and have found 
 them wholly insufficient, and leading to conclusions 
 which are completely at variance with the established 
 laws of the animal system. Lastly, we have adduced 
 arguments against the possibility of the inspired air 
 being decomposed during the circulation of the 
 blood ; and have assigned reasons in proof of its ac- 
 tual decomposition in the lungs, and even in the 
 
143 
 
 bronchial cells of those organs. This leads us, 
 therefore, in the next place, to speak of the nature 
 and extent of this decomposition ; in other words, 
 of the changes in quality which the air of our atmo- 
 sphere experiences in the lungs during its respira- 
 tion. 
 
 116. It has been already shewn (96. et seq.}, that 
 when the dark crassamentum of venal blood is 
 placed in contact with atmospheric air, or with oxy- 
 gen gas, it speedily assumes a florid colour : that 
 the bulk of air is, in a small degree, diminished : 
 that a portion of the oxygen disappears, and a quan- 
 tity of carbonic acid is produced : and these effects 
 have all been considered to arise from the combina- 
 tion of the oxygenous portion of the air with carbon 
 supplied by the blood. In the living body, the ve- 
 nal blood experiences the same change of colour 
 (113.) when atmospheric air, or oxygen gas, is're- 
 ceived into the lungs : the air itself also is somewhat 
 diminished in bulk : a portion of its oxygen gas, as 
 we shall hereafter see, disappears : and that carbo- 
 nic acid is, in like manner, produced, was first pro- 
 ved by the experiments of Dr Black. " So early 
 as the year 1757," says this distinguished philoso- 
 pher, " I convinced myself that the change produ- 
 ced on wholesome air by breathing it, consists chief- 
 ly, if not solely, in the conversion of fiart of it into 
 fixed air : for I found, that by blowing through a 
 pipe into lime-water, or a solution of caustic alkali, 
 the lime was precipitated, and the alkali was render- 
 ed mild *." At a later period, Mr Bewley detect- 
 
 * Black's Lectures by Robison, vol. ii. p. 87. 
 
144 
 
 ed the formation of carbonic acid in respiration by a 
 method somewhat similar : he found, that on breath- 
 ing through an infusion of litmus, the same change 
 to a red colour was produced in it, as when it was 
 exposed to the action of fixed air ; and when, by 
 adding a few drops of the water of potassa, the blue 
 colour was restored to the infusion, it could again 
 be made to dissappear by supersaturating it with 
 the acid expired from the lungs *. 
 
 117. The particular substance which constituted 
 the wholesome part of atmospheric air, was not, how- 
 ever, known to Dr Black at the time his experi- 
 ments were made : and long before the compound 
 nature of the atmosphere was ascertained, it had 
 been supposed by many philosophers, that, to use 
 the language of Bishop Berkeley, " there was no 
 such thing as a pure simple element of air. There 
 is," he adds, " some one quality or ingredient in 
 the air on which life more immediately and prin- 
 cipally depends. What that is, though men are 
 not agreed, yet it is agreed it must be the same 
 thing that supports the vital and the common 
 fiame ; it being found, that when air, by often 
 breathing in it, is become unfit for the one, it 
 \\ill no longer serve for the other. This quality of 
 the air is necessary both to vegetables and animals, 
 whether terrestrial or aquatic ; neither beasts, insects, 
 birds, nor fishes, being able to subsist without air : 
 and when air is deprived of this ingredient, it be- 
 cometh unfit to maintain either life or flame, even 
 
 * Priestley on Air, vol. v. p. 383. 
 
145 
 
 though it should retain its elasticity *." Dr Hooke 
 asserted, that this ingredient or substance inherent 
 in, and mixed with the air, is like, if not the very 
 same with that which is fixed in saltpetre, by which, 
 during combustion, inflammable bodies are dissol- 
 ved f. The same opinion was afterwards held by 
 Willis, Lower, and Mayhow, all of whom likewise 
 considered the nitrous quality of the air to act an 
 important part in respiration. The last author, in 
 particular, made experiments precisely similar to 
 those which have lately been brought forward, to 
 prove, that both by the burning of a candle and 
 other combustible bodies, and by the respiration of 
 animals, the nitro-aerial particles of the air were ex- 
 hausted, whereby the volume of air was diminished, 
 and the residual air was unable afterwards to sup- 
 port either life or flame J. The exhibition, how- 
 ever, of this peculiar, or nitro-ae'rial, part of the air 
 in a distinct and separate form, we owe to the ge- 
 nius of Dr Priestley, who, by the discovery of pure 
 or dephlogisticated air, in the month of August 
 1774, first made us acquainted with the true com- 
 position of the atmosphere. This great discovery, 
 and the use which he made of it, enabled this cele- 
 brated philosopher to propose the first consistent 
 explanation of the phenomena of respiration that 
 had ever been offered to the public ; and, although 
 the theoretical opinions on which that explanation 
 
 * Sins, par. 14-3. et seq. 2d Edition, 
 f Micrographia, p. 103. An. 1665. 
 t Tractat. Quinque, p. 98. et seq. 
 
 K 
 
146 
 
 was partly founded, no longer exist, yet it should 
 never be forgotten that his experiments and discove- 
 ries first pointed out the true path of investigation ; 
 and have contributed, in a pre-eminent degree, to 
 advance our knowledge of this most important 
 function. The cause of the unfitness of air, beyond 
 a certain extent, to support life and flame, he proved 
 to arise from the destruction of its pure part, or 
 what has since been called its oxygen gas ; and he 
 concluded, that, by the several processes of respira- 
 tion, combustion, and calcination, it underwent pre- 
 cisely the same changes *. M. Lavoisier, in the 
 following year, investigated these changes with great- 
 er accuracy. He confirmed the fact of the disap- 
 pearance of oxygen gas during respiration ; and add- 
 ed, that the residual air of this process differed 
 from that left after the calcination of metals in con- 
 taining a quantity of carbonic acid t ; thus verifying 
 also the fact discovered more than twenty years be- 
 fore by Dr Black. 
 
 118. Whence then is this carbonic acid, which is 
 formed in respiration, derived ? In the several process- 
 es of germination, of vegetation, and of respiration in 
 the inferior classes of animals, it has been shewn, that y 
 as carbonic acid is formed, the oxygenous portion of 
 the air gradually and completely disappears ; and that 
 the acid produced bears always a constant proportion 
 to the oxygen gas lost. We have now to shew, that 
 
 * Philosophical Transactions, 1776. 
 f Mem. Acad. 1777, 
 
147 
 
 the same things take place in the respiration of the 
 superior animals, and of man. Mr Davy confined 
 a small mouse in a jar, inverted over mercury, and 
 containing fifteen cubic inches of atmospheric air, 
 previously deprived of its carbonic acid by long ex- 
 posure to the water of potassa. In 55 minutes, the 
 animal was taken out apparently dying,- and of the 
 quantity of oxygen gas originally present in the jar, 
 2.6 cubic inches had disappeared, and two of car- 
 bonic acid were produced. In another experiment, 
 where a mouse was confined in 15.5 cubic inches of 
 the same air, 2.7 of its oxygen gas, in nearly the 
 same time, had disappeared ; and 2.1 of carbonic 
 acid were formed *. Mr Davy himself breathed 
 141 cubic inches of air one-fourth of a minute, ma- 
 king one inspiration and one expiration, seven or 
 eight different times : and he found, by analysing 
 the respired air, that the quantity of oxygen gas lost 
 in each respiration, was from five to six cubic inch- 
 es, and that of carbonic acid produced from five to 
 5.5 cubic inches f. Dr Henderson, on examining 
 10O parts of the air which he had respired four mi- 
 nutes, found, in one experiment, that of the ^ 
 
 of oxygen gas originally present, ^ had disappear- 
 ed, and j~ of carbonic acid were produced : in 
 another, ^ of the oxygen gas were lost, and its 
 place was supplied by of carbonic acid : and 
 when, in a third experiment, the loss of oxygen gas 
 fell between the foregoing numbers, or was ~, so 
 
 * Researches, p. 437. 443. f Ibid. p. 43.1, 
 
 K2 
 
148 
 
 likewise did the production of carbonic acid, beinp; 
 in that case ^ *. This reciprocal increase and di- 
 minution of these two gases could spring only out 
 of some necessary connexion betwixt them : and 
 the variations occurring so regularly, and in such 
 small quantities, are inconsistent with the supposi- 
 tion that the oxygen gas, which forms the carbonic 
 acid of respiration, is derived from any other source 
 than that of the inspired air. If there were any such 
 source, why should not this acid be expired, in a 
 quantity beyond what the residual air of the lungs 
 will supply (104.), when hydrogen gas is breathed ; 
 or why should it appear only when air containing 
 oxygen gas is respired, and in proportion always to 
 the disappearance of that gas ? Why also should 
 this acid be produced, and life for a time be sustain- 
 ed, when animals are confined in pure oxygen gas : 
 and why should death speedily take place and no 
 acid be formed (beyond what the residual air of 
 the lungs will supply), when they are placed wholly 
 in nitrogen gas ? These facts decisively shew, that 
 the oxygen gas, which composes, in part, the car- 
 bonic acid formed in respiration, is derived from 
 the inspired air alone : and that the production of 
 this acid bears always a constant proportion to the 
 loss of oxygen gas. 
 
 119. But, to constitute this acid, carbon, its 
 other ingredient, must be supplied, which, in the 
 inferior animals, has been shewn (76.) necessarily 
 to proceed from the animal system. In the forego- 
 
 * Nicholson's Journal, May 180K 
 

 149 
 
 ing experiments, also, where carbonic acid was form- 
 ed when the animals were confined in jars of pure 
 atmospheric air inverted over mercury, the animals 
 must have furnished the carbon, since no other sub- 
 stance was present which could afford it. This car- 
 bon is likewise given out by the blood of animals 
 (100.) after it is withdrawn from the body, in a state 
 .capable of uniting with oxygen gas : and, it is gene- 
 rally admitted, that the animal fluids possess the same 
 power of affording carbon, while retained in the li- 
 ving system ; to the union of which substance, with 
 the oxygen gas of the inspired air, the formation of 
 the carbonic acid of respiration has been ascribed. 
 
 120. Concerning the manner, however, and the 
 place in which the union of these two substances is 
 brought about in the living body, opinions have 
 greatly varied. By some, the carbon of the venal 
 blood has been supposed to attract the oxygen gas 
 of the air through the coats of the pulmonary cells 
 and vessels, and, by uniting with a portion of it, to 
 form the carbonic acid, which is again returned im- 
 mediately through the coats of these same vessels 
 and cells. According to others, the oxygen gas 
 enters into the blood-vessels, where it loosely com- 
 bines with the blood in the capillaries of the lungs, 
 and performs a circulation with it : during this cir- 
 culation, a part of the oxygen unites with the carbon 
 of the blood, so as to form an oxide of that sub- 
 stance, which, on the return of this fluid to the lungs 
 in a venal state, is, by the acquisition of more oxy- 
 gen, transformed into carbonic acid, and afterwards 
 expelled through the coats of these same capillary 
 vessels. Others, again, imagine the air to enter in- 
 
 K 3 
 
150 
 
 to the blood-vessels in its entire state, and to be dis- 
 solved and afterwards suffer decomposition during 
 the circulation of that fluid : while these changes are 
 going on, a part of the oxygen combines with the 
 carbon of the blood, and forms carbonic acid, which 
 is liberated through the moist coats of the cells and 
 vessels, when the blood returns to the lungs. In 
 all these hypotheses, it is taken for granted, that 
 the carbonic acid is in some way formed by the union 
 of the carbon of the blood with the oxygen gas of the 
 inspired air ; and the chief difference arises from 
 this union being held by some to take place only in 
 the capillaries of the lungs, and by others, through 
 the whole course of the vascular system. But if, as 
 we contend, no part of the inspired air gains ad- 
 mission into the blood-vessels of the lungs, neither 
 of these opinions can be any longer maintained ; 
 for no oxygen gas can, on such grounds, be held to 
 enter into the blood to unite with its carbon, neither 
 could the acid, which it is supposed to form, be af- 
 terwards expelled from that fluid, 
 
 12 K The objection stated above, applies, how r 
 ever, in part only to the opinion of Mr Aberncthy, 
 who considers the carbonic acid of respiration to be 
 derived, not from the oxygenous part of the inspired 
 air, but to be simply exhaled from the pulmonary 
 vessels *. As this opinion rests on the belief that 
 the quantity of oxygen gas that disappears in respi- 
 ration is not sufficient to account for the bulk of car- 
 bonic acid produced, it can no longer be maintained. 
 
 Essays, Surgical, &c. p. 146. 
 
151 
 
 if it be shewn that the volume of that gas which is 
 lost, actually exceeds that of the .carbonic acid 
 which is formed. , Now, this has been amply done 
 In the experiments (118.) already given: and, in- 
 deed, the excess of oxygen lost, was so apparent 
 and so constant, as to lead Lavoisier, and others 
 after him, to conclude, that it was employed to form 
 a part of the water expelled from the lungs, by uni- 
 ting with hydrogen supposed to reside in the blood, 
 just as water and carbonic acid are formed by the 
 combustion of wax and many other substances. It 
 is, moreover, incumbent on those who hold this opi- 
 nion concerning the production of carbonic acid, to 
 find out some adequate source from whence the oxy- 
 gen gas, forming the expired carbonic acid, can be 
 derived ; to explain to us, at the same time, what 
 becomes of the inspired oxygen gas which actually 
 disappears ; and why the acid produced bears always 
 so constant a proportion to the disappearance of that 
 gas. 
 
 122. Taking for granted, then, that the carbonic 
 acid expired, is formed by the union of the inspired 
 oxygen gas with carbon furnished by the animal 
 system, and that the gas lost exceeds in volume the 
 quantity of acid produced, we proceed next to in- 
 vestigate the amount of this difference and its cause. 
 From the experiments of Doctors Priestley and 
 Crawford (S3.), it appears, that when animals are 
 confined in air inverted over mercury till they die, 
 little or no diminution of the bulk of gas takes 
 place: those of M. Lavoisier, however, furnish a 
 different result. He confined a guinea pig in a jar, 
 containing 248 cubic inches of gas, consisting prin- 
 
 K4 * 
 
152 
 
 cipally of oxygen, which was inverted over 
 ry : in an hour and a quarter, the animal breathed 
 with much difficulty, and, being removed, the air 
 was examined, and found to be diminished in bulk 
 by eight cubic inches : 4O cubic inches more were 
 attracted by the water of potassa, and consequently 
 consisted of carbonic acid; from which it appears, 
 that the volume of acid produced, was one-sixth less 
 than that of oxygen gas which had disappeared *. 
 In a second experiment, another animal of the same 
 species was confined an hour and a half in a cubic 
 foot, or 1728 cubic inches, of oxygen gas, inverted 
 over mercury : the gas suffered a diminution of 55 
 cubic inches, and 229.5 cubic inches more were at- 
 tracted by the water of potassa, leaving a residue of 
 pure oxygen "h hence, therefore, the whole loss which 
 the oxygen gas experienced, amounted to 284.5, 
 while the quantity of acid produced was only 229.5, 
 or was ^ less in volume than the oxygen which 
 had disappeared. In these experiments, however, 
 the air breathed was not natural to the lungs, and 
 the confinement of the animal was prolonged until 
 he breathed with difficulty, which, on the principles 
 already stated (92.), would necessarily increase the 
 volume of residual air in the lungs, and render the 
 apparent diminution greater than it ought to be. 
 In the experiments made by Mr Davy on his own 
 respiration, the oxygen gas that disappears every 
 minute, is estimated at 31.6 cubic inches, and the 
 
 * Mem. A cad. 178O. 
 
 y Ann. de Chimie, t. v, p. 261. et seq. 
 
153 
 
 carbonic acid produced at 26.6, a difference in vo- 
 lume equal to ~ 2 * : and Dr Bostock concludes, 
 that in 24 hours, a man consumes somewhat more 
 than 26 cubic feet of oxygen gas, and produces 
 about 22 cubic feet of carbonic acidf, so that the. 
 bulk of the acid falls short of that of the oxygen 
 eras by ~ of the whole. It follows from all the 
 
 fc> J 6.3 
 
 foregoing facts, that the volume of carbonic acid, 
 produced by the respiration of animals, is, in all 
 cases, from one-fifth to ~ less than that of the 
 
 f D.5 
 
 oxygen gas which has disappeared during that pro- 
 cess. 
 
 123. From what cause then does this loss of bulk 
 proceed ? It cannot arise from an absorption or at- 
 traction of the oxygen gas, and subsequent combi- 
 nation of it with the blood ; for both of these cir- 
 cumstances are forbidden by the actual structure of 
 the lungs. But we have seen, that, by the conver- 
 sion of oxygen gas into carbonic acid, in the process 
 of combustion, a diminution of its bulk (11.)? a - 
 mounting nearly to one-seventh, takes place : and 
 we have traced this diminution, in a less degree, 
 when the same change is produced on oxygen gas 
 by the growth of seeds and of plants, by the respi- 
 ration of the inferior animals, and also by its being 
 placed in contact with blood. It is true, that the 
 degree of this diminution varies, in these several 
 examples, from ^- 7 to -- of the volume of oxygen 
 
 * Researches, p. 434-. 
 
 f Essay on Respiration, p. 99- 
 
154 
 
 gas employed, but this variation does not destroy 
 our belief in the general fact : and if we consider 
 that these calculations have been formed from expe- 
 riments where the authors had no such circumstance 
 in their contemplation ; that the correct observation 
 of the fact itself requires an attention to many cir- 
 cumstances of much delicacy and difficulty ; and 
 that the composition of carbonic acid is not yet de- 
 termined with such rigour as to leave no room for 
 doubt, we must be satisfied with such an approxi- 
 mation towards the truth, as our present knowledge 
 entitles us to make. In conformity, therefore, with 
 these views, we venture to conclude, that oxygen 
 gas, by its conversion into carbonic acid, suffers in 
 respiration, as well as in combustion, a diminution 
 of about one-seventh of its bulk : and since this ne- 
 cessary diminution accounts nearly (122.) for the 
 whole difference in volume between the oxygen gas 
 lost, and the carbonic acid produced in respiration, 
 we may farther conclude, that the whole of this oxy- 
 gen ga 3 has been employed to form the carbonic acid 
 in question. 
 
 1 24. But, notwithstanding this diminution of vo- 
 lume, which the oxygen gas thus experiences by 
 uniting with carbon, the weight of the compound 
 that is formed, is, at the same time, increased. 
 MM. Lavoisier and Seguin estimated the weight of 
 oxygen gas, consumed by a man in 24 hours, at 
 15661.66 grains: and that of carbonic acid produ- 
 ced in the same space of time, at 17720.89 grains * : 
 
 Bostock on Respiration, p. 83. 8G- 
 
155 
 
 so that the weight of the carbonic acid exceeded that 
 of the oxygen gas by 2059.23 grains. This excess 
 must be attributed to the addition of carbon (119.) 
 derived from the animal system ; and which makes 
 it to constitute ^ of the weight of the acid that is 
 formed. In the experiments of Mr Davy (122.), 
 the volume of oxygen gas that disappeared every 
 minute, was 31.6 cubic inches, and that of carbonic 
 acid produced 26.6 inches. But one cubic inch of oxy- 
 gen gas weighs 0.3474 of a grain, and therefore 31.6 
 cubic inches will weigh 10.97784 grains : and again, 
 one cubic inch of carbonic acid weighs 0.467 of a 
 grain, and therefore 26.6 inches will weigh 1 2.4222 
 grains : so that the weight of the carb -nic acid ex- 
 ceeds that of the oxygen gas by 1.4444 grains, or 
 the carbon, as before, constitutes ~ of the com- 
 
 o.o 
 
 pound. From these facts, it would se^ m, that the 
 proportion of carbon, which composes the carbonic 
 acid of respiration, is much less than that which 
 forms the same acid in combustion, which, as we 
 before remarked (13.), was estimated by Guyton at 
 one-fifth. But the whole subject is, at present, sur- 
 rounded with so many difficulties, that nothing more 
 can be expected than an approximation to the truth ; 
 and we must content ourselves, therefore, with sta- 
 ting generally, that, although the carbonic acid 
 formed in respiration be less in bulk than the oxy- 
 gen gas which disappears, yet that it much exceeds 
 it in weight ; and that this excess is derived from 
 the carbon supplied by the animal system. 
 
 125. As the conversion of oxygen gas into car- 
 bonic acid is at all times going on in the lungs, du- 
 
156 
 
 ring the continuance of the respiratory process, it is 
 next in order to examine how far the necessary dinv- 
 nution of bulk (123.) which attends that conversion, 
 will go to explain the absolute loss which the air, du- 
 ring its respiration, suffers. The amount of this loss 
 has been already stated to have been very variously- 
 estimated : and the cause of this variation in experi- 
 ments made on animals (83.) confined in jars of air, 
 has been explained. It is our present object to in- 
 quire into that degree of diminution which takes 
 place in air that has only once passed through the 
 lungs, and where all the circumstances of the expe- 
 riment resemble those which occur in the ordinary 
 process of respiration. Dr Goodwyn states, that 
 the volume of air, taken into the lungs at a single 
 inspiration, loses ~, or sometimes only ~ of its 
 bulk, when expelled from those organs by the next 
 succeeding expiration *. Dr Menzies, taking the a- 
 verage amount of the loss which the air suffered by 
 56 successive respirations, observes, that the volumes 
 of air, received into and expelled from the lungs, 
 were nearly the same t Several attempts were 
 made by Mr Davy to estimate the degree of diminu- 
 tion experienced by the air in a single respiration ; 
 and it has been shewn that his determinations great- 
 ly vary (91.) according to the manner in which the 
 experiment was made. In the only instance where 
 all the circumstances were perfectly natural, the j 3 
 cubic inches of air which he inspired, lost 0.3 of a 
 
 * Connexion of Life with Respiration, p. 51. 
 f Essay on Respiration, p. 22. 
 
157 
 
 cubic inch, or ~r P a , rt f their bulk **. Dr Bostock, 
 on the other hand, concludes, that the air, by a 
 single respiration, loses only -^ part of its bulk t 
 Amid such contradictory results, it is not to be ex- 
 pected that a conclusion can be drawn which shall 
 truly express the amount of the diminution in que- 
 stion : and indeed, from a consideration of the 
 powers which govern respiration, and the various 
 circumstances which sensibly affect that process, we 
 cannot but consider the actual loss of bulk which 
 the air suffers by a single respiration, as in its nature 
 extremely difficult, if not impossible, to determine. 
 All, therefore, that we can at present venture to as- 
 sert, is, that the difference between the volumes of 
 air inspired and expired, in natural respiration, is ex- 
 tremely small : and to this we may add our belief, 
 that the necessary loss of bulk which oxygen gas 
 suffers, by its conversion into carbonic acid, in that 
 process, may be made sufficient to account for it. 
 
 126. Several attempts have been made to ascer- 
 tain the absolute quantity of oxygen gas consumed 
 in a given time by respiration : but many circum- 
 stances concur to render this a matter of very diffi- 
 cult determination. Dr Menzies, from his experi- 
 ments, was led to conclude, that, in respiration, 36 
 cubic inches of oxygen gas were, every minute, con- 
 verted into carbonic acid ; and that thus 51840 cu- 
 bic inches of that gas, equal to 17625.6 grains, were 
 
 * Researches, p. 433. 
 
 t Essay on Respiration, p. 99210. 
 
158 
 
 daily consumed. MM. Lavoisier and Seguin esti- 
 mated the mean consumption of oxygen gas, every 
 24 hours, at 46037.38 cubic inches, or 15661.66 
 grains : and the same illustrious philosopher, on re- 
 peating his experiments in the following year, ob- 
 tained nearly the same results, an account of which 
 he was employed in drawing up, when cut off* by 
 the murderous tyranny of the French government. 
 Upon the estimate of Mr Davy, that 31.6 cubic 
 inches of oxygen gas are consumed every minute, 
 the amount, in 24 hours, will be 45504 inches, a 
 quantity equal to 15471.36 grains. This estimate, 
 says Dr Bostock, coincides nearly with that of La- 
 voisier, though it was obtained by a different pro- 
 cess, and by the use of a different apparatus : we 
 may therefore conclude, he adds, that between 45 
 and 46,OOO cubic inches, or about 15500 grainy 
 2 Ibs. 8 oz. troy, is th,e average quantity of oxygen 
 consumed in 24 hours by the respiration of an ordi- 
 nary man*. 
 
 127. Notwithstanding, however, the necessity of 
 oxygen gas to the continuance of respiration, and 
 the great quantity of it that is thus daily consumed, 
 many facts tend to prove, that, by the very consti- 
 tution of that function, a necessary limit is placed 
 to its consumption : and that this limit is determined, 
 not by the purity of the air employed, but by some 
 circumstances inherent in the animal system. It has 
 beeji already observed (14. 40.), that the growth of 
 vegetables is retarded by a great superabundance of 
 
 ' K^ay on Respiration, p. 84-, 
 
159 
 
 oxygen ; and that, although insects will live a con- 
 siderable time in this gas, yet their breathing be- 
 comes oppressive, and they die (3.) long before 
 the whole of it is consumed. There can be little 
 doubt but that the other classes of inferior animals 
 would, under the same circumstances, suffer in the 
 same manner. In the experiment also made by 
 Lavoisier on the guinea pig (122.), the animal is 
 said to have breathed with much difficulty, although 
 not more than one-fifth of the oxygen gas was consu- 
 med : but some experiments of the same author, at 
 a later period, seem in opposition to this fact. In 
 comparing together the phenomena of combustion 
 and respiration, he observes, that much more com- 
 bustible matter is consumed in a given time in vital 
 air, than in that of the atmosphere, but that the 
 same circumstance does not hold in respiration : 
 for whether animals respire oxygen gas in its pure 
 state, or mixed with a proportion, more or less con- 
 siderable, of nitrogen gas, the quantity of oxygen 
 which they consume is always the same. If a gui- 
 nea pig, he adds, be kept for several days in oxygen 
 gas, or in a mixture composed of fifteen parts ni- 
 trogen and one of oxygen, preserving constantly 
 these proportions, the animal in both cases conti- 
 nues in his natural state : his respiration and circu- 
 lation do not sensibly appear to be either accelerated 
 or retarded : his temperature remains the same, ar^d 
 he has only, when the proportion of nitrogen ga's is 
 too great, a slight disposition to drowsiness *, 
 
 * Mem. Acad. 1789. 
 
160 
 
 128. The results of Mr Davy's experience, how- 
 ever, do not correspond with theso conclusions of 
 Lavoisier. He introduced a mouse into a jar con- 
 taining an atmosphere composed of 10.5 cubic inch- 
 es of oxygen, and three inches of nitrogen gas. In 
 half an hour, the animal appeared to suffer much, 
 and, in about an hour, lay down on his side, as if 
 dying : in an hour and a quarter he was withdrawn 
 from the jar alive, but motionless. The residual 
 air, on being analysed, was found to have lost only 
 2.1 cubic inches of its oxygen gas, and consequent- 
 ly 8.4 inches of that gas still remained. Another 
 mouse, which was put at the same time into a jar 
 containing 15.5 cubic inches of atmospheric air, was 
 taken out through the mercury alive, but unable to 
 stand, in 5O minutes : and on analysing the residual 
 air, 2.7 cubic inches of its oxygen were consumed. 
 Hence it appears, that the mouse in atmospheric air 
 consumed nearly one-third more of oxygen in 5() 
 minutes, than the other mouse did in an hour and a 
 quarter, when placed in a jar containing so large a 
 portion of oxygen *. The results of these experi- 
 ments on mice are corroborated by those made by 
 Mr Davy on his own respiration ; for he found, 
 that he consumed much less oxygen gas when he 
 respired it pure, than when, for the same length of 
 time, he breathed atmospheric air ; and the quantity 
 of carbonic acid formed in the first case, was but 
 little more than half that obtained by the respiration, 
 for the same time, of atmospheric air t- These ex- 
 
 Davy's Researches, p. 4 1 f Ibid. p. 442. 
 
161 
 
 periments differ greatly therefore from those of La- 
 voisier as to the effects produced by the respiration 
 of oxygen on the animal system ; for, while the lat- 
 ter philosopher informs us, that this gas may be re- 
 spired for many days without inconvenience, Mr 
 Davy has shewn that the animal dies long before the 
 whole of it is consumed. Trusting, therefore, to 
 the accuracy of Mr Davy's experiments, as in all 
 respects supported by analogy, we infer, that an ex- 
 cess of oxygen gas in the air that is breathed, is not 
 suited to the due maintenance of the respiratory 
 function : and, on the other hand, the oppressive 
 symptoms which the respiration of impure air occa- 
 sions, as well as the results of Lavoisier's experi- 
 ments (127.X m wn ich nitrogen superabounded, 
 equally instruct us, that a deficiency of this gas is 
 alike unsuited to it. Consequently, we may con- 
 clude, that the atmosphere, as it is naturally com- 
 posed, is best adapted to the economy of the animal 
 system ; but, that this system is, at the same time, 
 so constituted as to be able to bear great variations 
 in the composition of the air without immediate injury 
 to the powers of animal life. 
 
 129. When, however, this variation proceeds to 
 a certain extent, the air is no longer capable of sup- 
 porting vital action ; but different animals, when 
 confined in given volumes of air, possess the power 
 of prolonging this action in very different degrees. 
 Thus we have seen (53. et seq.), that insects, worms, 
 fishes, and the amphibia, live until all the oxygen 
 gas of the air is nearly or entirely consumed ; while 
 birds die in a given quantity of air before they have 
 consumed two-thirds of its oxygen (84.), and a 
 
 L 
 
162 
 
 mouse and guinea pig expire when about three- 
 fourths of this gas have disappeared. Dr Priestley 
 observed, that if a mouse can stand the first shock 
 of being put into impure air, or has been habituated 
 to it by degrees, he will live a considerable time in 
 air in which other mice will instantaneously die *. 
 The following case, related by the late Dr Percival, 
 seems to prove that a similar observation may be ex- 
 tended to the human subject. An unfortunate man 
 descended into a coal-pit 9O feet in depth, soon af- 
 ter which the sides of the pit fell in. No assistance 
 could be rendered him till the afternoon of the se- 
 venth day, at which time he was found sensible and 
 spoke to his companions : his hands and feet were 
 extremely cold, and no pulse could be felt at the 
 wrist : he slept when not aroused to take nourish- 
 ment, and lived a few hours after being taken out 
 of the pit. The compass of the cavity which he had 
 dug, and in which he was laid on his belly when his 
 companions reached him, was three yards in length 
 and two in breadth, and the stratum of coal was two 
 feet thick* No circulation of air could possibly 
 have taken place through this cavity ; and the foul- 
 ness of the vapours which issued from it, prevented 
 any one; for some time, from venturing to his assist- 
 ance f. Now, the capacity of a cavity, nine feet 
 long, by six wide and two deep, will be equal 
 to 108 cubic feet ; from which, if we deduct three 
 feet, as the space occupied by his body, the actual 
 
 * Experiments on Air abridged, vol. iii. p. 257. 
 f Manchester Memoirs, vol. ii. p. 467. 
 
163 
 
 quantity of air which the cavity contained will be 
 equal to 105 cubic feet. If we suppose this air to 
 have been at first similar in composition to that of 
 the atmosphere, containing ^ of oxygen gas, the 
 quantity of that gas may be taken at 23 cubic feet, 
 by which the function of respiration was supported 
 for more than seven days. But, in ordinary respira- 
 tion, the daily consumption of oxygen ( 1 22. ) a ounts 
 to 26 cubic feet ; so that, in this case, animal life was 
 protracted for seven days on much less than one-se- 
 venth of the usual quantity of that gas. When, 
 however, death does happen to animals confined in 
 a given volume of air, it must arise either from the 
 noxious operation of the nitrogen gas that is always 
 present, or from that of the carbonic acid, which is 
 formed ; or it must proceed from the deficiency, or 
 total absence, of oxygen gas. Now, although nitro- 
 gen gas do not of itself support life, yet we have 
 no evidence that it exerts any injurious effect on the 
 animal system. In vegetation (5. 29.), and in the 
 respiration of the inferior animals (73.), it has been 
 shewn to be wholly inactive ; and when, in the ex- 
 periment of Lavoisier (127.)> it constituted || of 
 the air employed, a degree of drowsiness only seems 
 to have been induced by it. That it is entirely pas- 
 sive, is still farther confirmed by an experiment of 
 Lavoisier, who found that hydrogen gas, mixed in 
 due proportion with oxygen, would serve the pur- 
 poses of respiration as well as the air of the atmo- 
 sphere. We have no proof that nitrogen is able to 
 enter the vessels so as to produce any direct opera- 
 tion on the blood, an effect which is still farther 
 
 L2 
 
164 
 
 forbidden by its incapacity of uniting with that flui<L 
 We may therefore conclude, that nitrogen gas, when 
 respired, neither suffers any change itself, nor pro- 
 duces any direct operation on the animal system. 
 
 130. The only other gas to which the death of 
 animals, in these circumstances, can be ascribed, is 
 carbonic acid, which, however, when formed by 
 respiration, does not seem destructive to animal life. 
 Dr Goodwyn observes, that when the same air is 
 breathed several times, so as to increase the quan- 
 tity of carbonic acid, its noxious operation is to be 
 attributed not to the presence of this acid, but to 
 the deficiency or absence of oxygen gas * ; and 
 when Spallanzani, by means of an alkaline sub- 
 stance, abstracted this acid as soon as formed by the 
 respiration of birds and quadrupeds, he did not find 
 that they lived longer in a given bulk of air than 
 when it was suffered to remain t In the forego- 
 ing case also, related by Dr Percival, the carbonic 
 acid formed by respiration must have been retained 
 in the cavity, and yet no destructive effect seems to 
 have followed from it. Dr Higgins observes like- 
 wise, that debility, convulsions, and death, follow 
 the successive diminution of the oxygen gas of the 
 air in respiration, long before the whole of that gas 
 is consumed, although the carbonic acid that is ge- 
 nerated be, in the mean time, carefully withdrawn J . 
 Indeed, we might in this, as in former examples, be 
 
 * Connexion of Life with Respiration, p. 66. 
 
 f Memoirs on Respiration, p. 318. 
 
 J Higgius's Minutes of a Society, p. 160. 
 
165 
 
 led to suppose, that neither the carbonic acid formed 
 in respiration, nor the nitrogen gas employed in that 
 process, would exert any positively destructive ope- 
 ration on the animal powers, since both of them 
 must, at all times, necessarily be present in the sys- 
 tem ; and seeing, moreover, that the abstraction of 
 oxygen gas alone is sufficient to account for the fa- 
 tal effects which ensue, it must be deemed unneces- 
 sary to resort to the supposed agency of any subor- 
 dinate cause. 
 
 L3 
 
CHAP. V. 
 
 OF THE SOURCE OF THE CARBON IN VE- 
 GETABLES AND ANIMALS BY WHICH 
 THE CHANGES IN THE AIR ARE EF- 
 FECTED. 
 
 i 
 
 131. IN the preceding chapters, it has been 
 generally concluded, that the oxygenous portion of 
 the air is converted into carbonic acid by the pro- 
 cesses of germination and vegetation, and by the re- 
 spiration of animals : we must therefore presume 
 that seeds, and plants, and animals, in some way, 
 furnish the carbon whereby the acid^n question can 
 be formed. To inquire into the immediate source 
 of this carbon is the object of our present investiga- 
 tion. 
 
 1 32. It appears, from the results of experiments al- 
 ready related (1.), that if seeds, in a dry state, be ex- 
 posed to atmospheric air, they neither suffer any 
 change themselves, nor produce any on the air in con- 
 tact with them : but that if they be confined in water, 
 
167 
 
 tihey gradually suffer decomposition, forming car- 
 bonic acid and carburetted hydrogen gases ; and the 
 faculty of germinating is then destroyed. If, far- 
 ther, they be first steeped in water, and then placed 
 in nitrogen gas (5.), carbonic acid is, in like man- 
 ner, formed ; and the same acid is likewise produced 
 when they are simply plunged into a tube of mer- 
 cury without the aid of any elastic fluid at all ; but, 
 in neither of these cases, do they exhibit any sign of 
 germination. Whatever, in these experiments, may 
 be the source from whence the oxygen gas forming 
 the acid is derived, it must be granted that the seed, 
 in every case, furnished the carbon. But this acid 
 is likewise produced when steeped seeds are made to 
 germinate in atmospheric air (6.), or in oxygen gas, 
 where nothing also but the seed is present from 
 which the carbon can be derived : hence therefore, 
 it follows, that the seed, during its germination, as 
 well as under decomposition, must .be able to supply 
 carbon. 
 
 133. Does then the carbon, which thus contri- 
 butes to the formation of the acid, issue directly 
 from the seed, and unite with the oxygen gas of the 
 air exterior to its substance ? or does this oxygen 
 gas previously enter into the seed, and, combining 
 with its carbon, again escape from it in the form of 
 carbonic acid ? If the view which we have taken of 
 the structure of seeds, and of the manner in which 
 the air is affected by their growth^ be just, this latter 
 supposition .cannot be entertained : and, from the 
 facts stated in the foregoing paragraph, it is suffi- 
 ciently clear, that, in many cases, the entrance of 
 oxygen gas into the seed is by no means necessary 
 
 L4 
 
168 
 
 to abstract its carbon ; for carbonic acid was formed 
 by seeds where none of that gas previously existed. 
 From the following facts, it will be seen also, that 
 the seed, during its germination, is able to yield car- 
 bon, without the supposed agency of oxygen gas to 
 abstract it. M. Huber observed that seeds did 
 not germinate in air, which had been previously in- 
 jured by the respiration of bees, although a quanti- 
 ty of oxygen gas was added to it sufficient to carry 
 on germination, if mixed with pure nitrogen gas : 
 at first, he attributed this failure to the presence of 
 the carbonic acid which remained in the residual 
 air, but when he carefully removed this acid by 
 first washing the residual air in lime-water, he still 
 found, that although fresh portions of oxygen were 
 added, no germination took place, until after a very 
 considerable quantity of that gas had been supplied. 
 He found precisely the same thing to happen when 
 he employed the residual air that had served for 
 successive germinations, since fresh seeds would not 
 grow in such air until it was mixed with a quantity 
 of oxygen much more than sufficient to carry on 
 germination in pure nitrogen gas. Reflecting on 
 these circumstances, he conceived, that as seeds yield 
 carbon to unite with the oxygen gas of the air du- 
 ring their germination, that substance might also com- 
 bine with its nitrogenous portion : and when thus sa- 
 turated with carbon, the nitrogen might not favour 
 germination if mixed only with its usual quanti- 
 ty of oxygen, because the oxygen gas, having a 
 greater affinity for carbon than the nitrogen, would 
 carry it off from the latter to form carbonic acid, 
 and consequently not leave sufficient oxygen to sup- 
 
169 
 
 port germination. This supposition was rendered 
 the more probable, because he found, that when oxy- 
 gen gas was mixed with nitrogen thus saturated 
 with carbon, carbonic acid was formed, although 
 the two gases had, previous to their mixture, been 
 carefully washed in lime-water *. 
 
 1 34. That the carbon of the seed had really esca- 
 ped, and combined with the nitrogen gas in the ex- 
 periments above stated, is farther evinced by what 
 follows. M. Huber prepared an artificial atmosphere 
 in proportions similar to that of common air, but in 
 which hydrogen was substituted for nitrogen gas : 
 and in this atmosphere he caused successive quanti- 
 ties of lettuce-seeds to germinate, till they ceased 
 any longer to grow. He then carefully washed this 
 mixture of gases in lime-water, until it produced not 
 the slightest discolouration ; and, when all the car- 
 bonic acid was by this means removed, he added to 
 the residue a portion of oxygen, previously deprived 
 of every particle of carbonic acid, and inflamed the 
 mixture in Volta's eudiometer over lime-water. At 
 the moment of inflammation, the lime-water was 
 rendered very turbid, and a great precipitation of 
 carbonate of lime took place. As no carbonic acid 
 existed in the two gases previous to their inflamma- 
 tion, its production must have arisen from that pro- 
 cess ; and we must therefore admit, that the carbon 
 which had combined with the hydrogen during ger- 
 mination was again separated from it, and had uni- 
 ted with the oxygen gas to form the carbonic acid 
 
 * Mem. sur la Germination, p. 41. et seq. 
 
no 
 
 in question. If, after being thus prepared, the twx 
 gases, instead of being inflamed, were placed together 
 over lime-water, carbonic acid was, in like manner, 
 formed ; but the same result, which in one case was 
 effected in an instant, required, in the other, a much 
 longer time to produce. It is farther remarked, 
 that pure hydrogen gas burns with a lively white 
 flame, but that the gas which has been employed as 
 an atmosphere in germination, yields a tinge more 
 or less blue, although it may have been washed in 
 lime-water. From these facts, says Sennebier, it 
 must be concluded, that hydrogen gas, which has 
 been used in germination, becomes charged with 
 carbon, and that this carbon can have been furnish- 
 ed only by the operation of germination or vegeta- 
 tion, which produce upon this gas the same effect. 
 It is farther proved by these facts, he adds, that 
 growing seeds and plants are not the direct sources 
 of the carbonic acid found in the atmosphere in 
 which they have lived ; but that this acid is a com- 
 bination of the carbon which escapes from these bo- 
 dies with the oxygen gas of the air *. The decom- 
 position of carburetted hydrogen in the foregoing ex- 
 periments of M. Huber, accords with the phenome- 
 na which take place when it is placed in contact 
 wkh oxymuriatic acid gas : for all the varieties of 
 carburetted hydrogen gases, if mixed in due propor- 
 tion with this acid, are converted into water, carbonic 
 acid, and carbonic oxide gas; and if, instead of allow- 
 ing the mixture to remain at rest, the electric spark 
 
 * Sur la Germination, p. 86. et sen. 
 
171 
 
 be taken through it, a slight explosion follows, with 
 a production of carbonic acid and water *. 
 
 135. To these facts, tending to prove the emis- 
 sion of carbon from seeds, it may perhaps be ob- 
 jected, that this substance exists in so fixed a state, 
 that it cannot pass off in the attenuated form, which, 
 in these cases, is required ; but this objection ap- 
 plies equally to every case where carbonic acid is 
 spontaneously formed. Those who suppose the 
 oxygen gas to enter into the seed, and, by the power 
 of chemical affinity, to abstract its carbon, must 
 likewise admit, that this carbon is raised into a ga- 
 seous out of a fixed state, or no carbonic acid could 
 be formed. This power of assuming the gaseous 
 form, differs not only according to the circumstan- 
 ces in which the carbon may be placed, but also ac- 
 cording to the state of combination in which it ex- 
 ists. Thus, in its pure state of diamond, a very in- 
 tense heat is required to effect the union of carbon 
 with oxygen gas : in the example of plumbago, a 
 lower heat will suffice ; and in that of charcoal, a 
 much lower still. Not only, however, by these high 
 degrees of heat, but at the ordinary temperature of 
 the atmosphere, may this carbon be separated from 
 its combination with various substances. Mr Cruick- 
 shank observed, that the gas which arises from the 
 slow decomposition of humid vegetable matter, is si- 
 milar to that obtained by the decomposition of cam- 
 phor or aether by heat t *. and many examples have 
 
 * Murray's System of Chemistry, vol. ii. p. 385, 
 f Ibid. vol. ii. p. 384, 
 
172 
 
 been given, in which carbonic acid and carburetted 
 hydrogen gases were formed by moistened seeds, at 
 the ordinary temperature of the atmosphere. Nor 
 is even the decomposition of the seed necessary to 
 extricate its carbon ; for M. Vauquelin found, that 
 moistened seeds, which had yielded carbonic acid in 
 hydrogen gas, would afterwards grow when placed 
 in the open air *. From these facts it follows, that 
 the carbon of vegetable bodies may be made to 
 unite with oxygen gas at a low temperature, as well 
 as at a high one ; and that this matter may be given 
 out by seeds antecedent to their decomposition, as 
 well as under that process. 
 
 136. It is more difficult to ascertain the form 
 which carbon assumes, and the means by which, in 
 these various examples, it is brought into that form 
 in which it exists at the momerit of its union with 
 oxygen gas. In the processes where heat is employ, 
 ed, its particles are perhaps so changed or attenua- 
 ted by the operation of that subtile fluid, as to be 
 rendered capable of combining with the oxygen gas 
 presented to them ; but no such operation of heat 
 can be going on where the carbon is made to unite 
 with that gas at the ordinary temperature of the at- 
 mosphere. Some other means, therefore, must be 
 sought, which shall be sufficient to reduce the car- 
 bon to the state required for such a combination. 
 Now, we have seen, that the carbon of the seed 
 does not combine with the oxygen gas of the air, so 
 long as the seed continues in a dry state ; but when 
 
 * Phil. Mag. vol. xxv. p. 223. 
 
173 
 
 it is duly moistened, this carbon, in the same tempe- 
 rature, and under the same circumstances, is yielded 
 by the seed, and combines either with oxygen, ni- 
 trogen, or hydrogen gas. No other agent but hu- 
 midity, therefore, being introduced, to which this 
 variation in effect can be ascribed, we are led 
 to attribute it to that cause ; and to believe that 
 carbon, as it exists in seeds, may be so acted on or 
 dissolved by the imbibed water, as to be capable of 
 passing off from the seed with its exhaled moisture, 
 which thus becomes the proper vehicle of it. It is 
 well known, that both seeds and plants contain a 
 large portion of carbon, and it is extremely probable 
 that they derive it from a state of solution in water ; 
 for M. Giobert found, that plants did not grow well 
 in a mixture of simple earths until he moistened his 
 soil with water from a dunghill. Now it is certain, 
 from the experiments of Hassenfratz, says Dr 
 Thomson, that this water contains carbon j for 
 when evaporated, it constantly leaves behind a resi- 
 duum of charcoal. All those manures likewise 
 which act with efficacy and celerity, contain carbon 
 in such a state of combination, that it is soluble in 
 water ; and the efficacy of the manure is proportio- 
 nal to the quantity of carbon so soluble *. 
 
 137. If then it be granted, that the carbon of de- 
 composing vegetables is largely soluble in water, there 
 seems no reason why the carbon of the seed, which 
 gradually diminishes (20.) during germination, 
 should not be soluble in it also : and since the wa- 
 
 * Thomson's Chemistry, vol. iv. p. 253. 256. 
 
174 
 
 rer imbibed is constantly escaping from the seed, 
 there is reason to believe, that it is the proper men- 
 struum and vehicle of this carbon. Hence, in the 
 foregoing experiments of M. Huber (133-4.), the 
 carbon furnished by the seed united first with the 
 oxygen gas, until that gas became saturated, and af- 
 terwards combined with the nitrogen or hydrogen, 
 whose affinity for it is not so strong as that of oxy-.. 
 gen *. Even after combining with hydrogen, car- 
 bon will again leave it to unite with oxygen, either 
 when the two gases are inflamed together by electri- 
 city, or are suffered for a few hours to remain at 
 rest. If, however, fresh portions of oxygen be suc- 
 cessively supplied, the carbon, afforded by the seed, 
 seems to unite with it alone, and germination regu- 
 larly proceeds. There does not, in these examples, 
 appear to be any ground for supposing the oxygen 
 gas to be first received into the seed in order to 
 combine with its carbon, and to be afterwards ex- 
 pelled from it in the form of carbonic acid ; for no 
 known powers of the seed are capable of performing 
 such an office, and such a supposition is the less ne- 
 cessary, since the seed is able to emit carbon with- 
 out it. If also, from the mere fact of their combi- 
 nation, the oxygen gas be considered to enter into 
 the seed, and abstract its carbon, the hydrogen gas 
 must be held to do the same ; and the same thing 
 may likewise be affirmed of nitrogen gas, with 
 wkich (133.) the carbon of the seed will combine. 
 Moreover, the seed affords carbon through every 
 
 * Murray's System of Chemistry, vol. ii. p. 311. 
 
175 
 
 stage of its decomposition, where we must suppose 
 it to escape, like the other elements of the body, 
 from the dissolution of those affinities by which they 
 were held together. From the whole, therefore, we 
 are led to conclude, that the carbon of the seed, by 
 the action of the imbibed water, is reduced to a state 
 capable of combining with oxygen gas at the ordi- 
 nary temperature of the atmosphere, and that k 
 passes off from the seed with the moisture exhaled 
 from it. 
 
 138. If any cause obstruct the emission of this 
 water and carbon, germination is either partially or 
 wholly prevented, as the following experiments will 
 prove. Equal numbers of steeped peas were sup- 
 ported by two hoops in two equal jars of atmosphe- 
 ric air ; but the peas of one jar were previously 
 dipped in oil. Both jars were then inverted over 
 mercury, and placed in a room beside each other. 
 The oiled peas in a few days had sprouted about 
 one-third of an inch, while the radicles of the others 
 were more than an inch in length. The air of the 
 jar in which the oiled peas had been confined was 
 diminished but little by agitation with lime water ; 
 while that of the other jar rendered the lime water 
 quite milky, and lost nearly one-fifth of its bulk. 
 The same results were obtained when varnish was 
 applied over the peas instead of oil, very little car- 
 bonic acid being formed, and the residual air still 
 retaining ~ of oxygen ; while the residual air of the 
 jar in which other peas had for the same time been 
 growing, when freed from carbonic acid, contained 
 no oxygen gas at all. In like manner, if the im- 
 bibed water be too rapidly abstracted, the growth of 
 
176 
 
 the peas is checked, and the air in proportion sur- 
 fers but little change. A jar containing some steep- 
 ed peas, supported on a hoop, was, as before, in- 
 verted over mercury ; but within it was previously 
 placed a small glass cup, rilled with powdered lime, 
 which floated on the mercury. The jar, during the 
 whole experiment, continued dry ; the peas sprout- 
 ed but little, and shrunk nearly to the same size as 
 when in their dry state ; and the lime greatly in- 
 creased in bulk. The residual air, when examined, 
 suffered no diminution by being shaken with lime 
 water ; but it still contained a large portion of oxy- 
 gen gas ; while the air of another jar, placed in si- 
 milar circumstances, except that it held no lime, 
 had in the same time lost the whole of its oxygenous 
 portion : the peas also were nearly double their ori- 
 ginal sizo, and their radicles 1.2 inch long. Hence, 
 therefore, it is clear, that whatever obstructs the 
 emission of moisture and carbon from the seed, or 
 too rapidly draws off the moisture, necessarily puts 
 a stop to those reciprocal changes between the seed 
 and the air, so essential to its future growth. 
 
 139. That the carbon which issues from the seed 
 during its germination, is given out in union with 
 the moisture previously imbibed, will, perhaps, ap- 
 pear more evident from what we have next to offer 
 concerning the carbon furnished by plants during 
 their vegetation. It has been shewn (3O.), that the 
 presence of oxygen gas is essential to the growth of 
 plants, and that, by the upper surfaces of their 
 leaves (26.), it is converted into carbonic acid : it 
 has been also shewn (39.), that the carbon, which 
 effects this conversion, proceeds neither from the 
 
177 
 
 soil, nor from decayed leaves, but from the living 
 leaf itself. In what manner, then, is the living leaf 
 enabled to exercise this important function ? The 
 sap of plants, which is chiefly taken up by the roots, 
 passes through the vessels of the stem to the surface 
 and extremities of the leaf, where it suffers conside- 
 rable changes (36. ), and is then again returned by 
 the vessels of the footstalk to the branches and stem. 
 By this flow of the sap, the function of perspiration 
 in the leaf is supported, and when, from any cause, 
 it diminishes, vegetation seems, in a great degree, to 
 be suspended. This happer.s to many plants during 
 a cold rain, as well as through the night ; and the 
 leaves, in consequence, assume a change of form 
 and condition, which some have supposed to resem- 
 ble a state of sleep ; and in winter, we know, with 
 the cessation of the flow of the sap, vegetation whol- 
 ly ceases. But the plant, during all these times, is 
 in contact with the atmosphere, and the cessation of 
 the usual vegetative changes cannot, therefore, be 
 attributed to the want of oxygen gas : they must, 
 consequently, proceed from some alteration in the 
 state of the plant itself, which alteration consists in 
 the diminished or suppressed flow of the sap, by 
 which all the vegetable secretions are suspended. 
 
 140. If, farther, the leaves of a plant be separated 
 from the stem, they retain life for a considerable 
 time : but the circulation of fluids through them 
 must then of necessity be cut off. Dr Woodhouse 
 exposed handfulls of the leaves of twelve different 
 plants separately to the light of the sun in 40 ounce 
 measures of air for four hours, and its purity was 
 found to be neither increased nor diminished. Af- 
 
 M 
 
ITS 
 
 ter they had remained sixteen hours in the air, no ef- 
 fect was produced on it. The leaves were fresh ga- 
 thered, and no decay could be observed in any part of 
 them *. Now, as the only difference in the state of 
 these leaves consisted in their removal from the 
 plant, and consequent cessation of the flow of the 
 sap, by which the usual changes on the air were pre- 
 vented, it is to that cessation arising from such re- 
 moval, and not to any other alteration in the struc- 
 ture or condition of the leaves, that this prevention 
 of change is to be ascribed. But this flow of the 
 sap is a proof of the existence of living action in 
 plants, and by its means all the vegetable secretions- 
 are carried on : whence it follows, that as the leaves 
 act upon the air, and consequently emit carbon only 
 while this flow continues, its emission is the result 
 thereof, and consequently of a living action also. 
 
 141. By smearing over the upper surface of the 
 leaves with varnish or oil, a mechanical obstruction 
 is furnished to the emission of carbon, and the plant 
 dies (26.) just as when the circulation itself is cut off, 
 little or no change, it is believed, being then produ- 
 ced on the oxygen gas of the air. Since then the 
 leaves alone effect changes on the air, and these on- 
 ly while attached to the stem ; and since these 
 changes cease, either when the circulation of the sap 
 is suspended or cut off, or a mechanical obstruction 
 is furnished to the escape of the carhon ; we must 
 conclude .that the carbon, thus emitted by the leaves, 
 is in truth a vegetable excretion, dependent, like 
 
 Nicholson's Journal, July 1 802, p. 159. 
 
179 
 
 others, on the due circulation of the fluids of the 
 plant, and varying as that circulation varies. As we 
 observe this carbon, however, only in combination 
 with oxygen gas, its precise form or mode of exist- 
 ence cannot as yet be ascertained ; but the fact of 
 its always accompanying the exhalation of moisture 
 from the leaf, leads to the opinion, that it is not only 
 coincident with that excretion, but is carried on 
 likewise by the same structure ; the exhaled moisture, 
 as in the case of the seed, serving as the proper vehi- 
 cle of it* It appears also from the facts already stated 
 (134.), that carbon is afforded by the plant, as well 
 as by the seed, where no oxygen gas was present to 
 unite with it ; which shews that its emission depends 
 immediately on the power of the plant, and not on 
 any supposed chemical agency of the air. It may 
 be added, that the experiments of Dr Woodhouse 
 demonstrate likewise, that leaves as well as seeds, 
 under decomposition, form carbonic acid * : but this 
 we notice for the sake of discrimination only, since 
 we are now considering the changes which atmo- 
 spheric air undergoes from the agency of certain li- 
 ving processes alone. 
 
 142. Ascending next, in the order of our inquiry, 
 into the animal kingdom, we find, that, to insects 
 and worms, the presence of oxygen gas is essential 
 to the continuance of living action (53. et seq.), 
 and that they gradually, and, in many instances, 
 completely convert this gas into carbonic acid. No 
 other substance but the animal being present in the 
 
 * Nicholson's Journal, June 1802, p, 159. 
 M 2 
 
180 
 
 experiment by which the carbon could be supplied, 
 we must refer its production to that source. The 
 formation of this acid was found also by Spallanzani 
 to depend very much on temperature ; for snails con- 
 sumed more oxygen, and died in equal bulks of air, 
 much sooner when the temperature was mild, than 
 when it was severe * \ and, by gradually reducing 
 the temperature, he at length reached that point 
 where the air in which the snails were confined, un- 
 derwent no change at all ; which point he found to 
 be at zero t The circulation all this time was gra- 
 dually declining, and at 1 Reaum. the pulsations 
 altogether ceased, the heart remained perfectly at 
 rest,and the whitish fluid usually flowing in the vessels 
 was in a state of stagnation {. Nearly similar results 
 were obtained from another species of snails in a 
 hybernating state. In these, the covering or opercle 
 with which they close up their shell in winter, effec- 
 tually excludes the air on that side, neither does it 
 penetrate their shells elsewhere ; for a small glass 
 tube, filled with mercury, being inserted into this 
 covering, and then inverted, sustained a column of 
 28 inches of that fluid, which corresponded to the 
 height of the mercury in an adjacent barometer. 
 When, however, the covering was punctured so as 
 to admit the air, it instantly forced down the mer- 
 cury. The small portion of air contained in these 
 shells for several months during the torpid state of 
 the animals, was found in several trials to be of like 
 
 * Memoirs on Respiration, p. 150. 
 | Ibid. p. 152. \ Ibid, p. 151. 
 
181 
 
 purity with that of the atmosphere * ; so that when 
 living action had ceased in these animals, no change 
 was produced on the air in contact with them. 
 
 143. Nor are these effects peculiar to those ani- 
 mals which change the air by the mediation of their 
 skin : they are common also to those which breathe 
 by lungs. A marmot that was cold to the touch, 
 and so closely rolled up as to be tossed about and 
 irritated in every shape without exhibiting the slight- 
 est indication of life, was exposed in a recipient of 
 atmospheric air, inverted over mercury, to a tempe- 
 rature -12 Reaumur. He continued motionless 
 three hours and a half, during which time the mer- 
 cury in the jar remained stationary : and the air, on 
 being analysed, was found in every respect similar 
 to that of the room in which the experiment was 
 made f- The same animal was then transferred in- 
 to ajar of carbonic acid at temperature 12.5 Reau- 
 mur, where he remained four hours without shewing 
 the least sign of motion, or suffering the smallest in- 
 jury ; but a rat, put at the same time into the jar, 
 died almost instantly ; and when, in a second expe- 
 riment, the temperature was raised to zero, the mar- 
 mot also began to breathe, and then speedily died in 
 a jar of this gas. These facts prove, not only that the 
 carbon, which decomposes the air, is in every case fur- 
 nished by the animal, but that its emission depends 
 wholly on the state of the circulating fluids ; for, as 
 the circulation increased, declined, or ceased, so like- 
 wise did the emission of carbon, and consequent pro- 
 
 * Memoirs, p. 197. 203. f Ibid. p. 333. et seq. 
 
 M3 
 
182 
 
 duction of carbonic acid : the oxygen gas could not 
 vary the result, being in all cases equally abundant. 
 144. Does then this emission of carbon proceed 
 from the fluids circulating in the larger vessels of 
 the animal ? Or is it given out from the exhalent 
 structure of the body ? That these fluids may con- 
 tain carbon as a constituent element, we do not 
 mean to deny : but, that this substance can be emit- 
 ted through that organized structure which carries 
 on the respiratory function, may, we think, be just- 
 ly disputed. This structure has been shewn to em- 
 brace a large portion of the surface of the bodies of 
 insects, which again is occupied only by the termi- 
 nations of exhalent vessels, whose office it is to emit 
 fluids, previously separated from the blood, while 
 the blood itself usually circulates in the deeper seat- 
 ed vessels, or even at the centre of the animal. 
 When also the usual changes on the air are suspend- 
 ed, by placing the animal in a cold medium, the car- 
 bon might be expected to accumulate in the blood, 
 if that were the source from which it is directly de- 
 rived, and carbonic acid to be at that time most 
 abundantly produced : but the preceding facts evince, 
 that it is not to the supposed accumulation of this 
 substance in the blood, but to the state and vigour 
 of the circulating fluids, that the production of this 
 acid is to be ascribed. We have therefore no proof 
 that carbon exists in the mass of blood in a state ca- 
 pable of attracting air, or of being attracted by it 
 through the organized structure of the animal : nor 
 even if these attractions were allowed to take place, 
 do the phenomena at all correspond with what, un- 
 der such a supposition, ought to happen. 
 
183 
 
 145. If, then, the carbon in question be not emit- 
 ted from the blood while yet in the larger vessels, 
 nor after it has ceased to be in motion, it must be 
 given out by that fluid while in circulation, and after 
 it has entered into the minuter vessels : and thus it 
 becomes an animal excretion, derived, like other ex- 
 cretions, from the blood, and emitted, like them, by 
 some appropriate structure from the surface of the 
 body. Hence any cause, as cold, checking the cir- 
 culation, restrains the production of this carbon ; or, 
 although the circulation be not checked, the emis- 
 sion of this substance is prevented by smearing the 
 bodies of insects (51.) over with unctuous matter, 
 which in consequence causes their death. All that 
 has now been said of insects and snails, applies 
 equally to the marmot and other torpid animals ; for 
 the emission of carbon in all, is obedient to the same 
 laws, except that, in the former case, it is given out 
 by the exhalent surface of the body, while, in the 
 latter, it proceeds from the exhalent structure of the 
 lungs. For these reasons, we consider the emission 
 of carbon in these animals to be truly an excretion 
 dependent on the due circulation of their blood, and 
 partaking of all its variations. The experiments of 
 Spallanzani, it may be observed, prove likewise, that 
 under decomposition, animal as well as vegetable 
 substances form carbonic acid in air where no oxy- 
 gen gas was originally present *, a fact with which 
 the experiments of Priestley had long before made 
 us acquainted f. 
 
 * Memoirs on Respiration, p. 346. 
 f Priestley on Air, vol. iii. p. 34?0. 
 M4 
 
184 
 
 146. To the life of water-snails, muscles, fishes, 
 and the amphibia, a supply of fresh air has been 
 shewn to be equally necessary, its oxygenous portion 
 being, in every instance, more or less completely 
 consumed and changed into carbonic acid ; the car- 
 bon for which purpose could be derived only from 
 the animal system. We have not, in this instance, 
 the evidence of experiment to prove that the carbon 
 was emitted by the living power of the animal, that 
 is, only while its circulation continued : nor, although 
 some of these animals are known to suffer torpidity, 
 can we experimentally shew that they then produce 
 no change on the air contained in the water in which 
 they are placed. In every view of the subject, how- 
 ever, analogy is so strongly in our favour, that we 
 cannot hesitate to admit both suppositions to be true : 
 and the termination of the branchial artery in exha- 
 lent orifices (65.) on the gills of fishes, would seem 
 to point it out as the structure by which, in that 
 class of animals, the carbon, in combination with the 
 exhaled fluid, escapes. At all events, the blood of 
 a very large number of these animals being devoid of 
 colour, is at least a proof that carbon does not cause 
 the blackness of that fluid ; or, if it still be contend- 
 ed that it does, then blood, which is white, and may 
 be presumed therefore to contain no carbon, can ef- 
 fect the same changes on the air, as that which owes 
 its blackness to this substance. 
 
 147. Analogous to the emission of carbonic mat- 
 ter from the surface of the bodies of snails (145.), 
 seems to be that which is carried on by the external 
 and internal surfaces of the human body. The 
 Count de Milly first observed small bubbles of air 
 
185 
 
 to form on the surface of the body when it was immer- 
 sed in the warm bath : and, by means of a funnel 
 and inverted glass jar, he collected, in a few hours, 
 a considerable quantity of it, which, being examined 
 by M. Lavoisier, was found to precipitate lime-wa- 
 ter, and to suffer no diminution from the admixture 
 of nitrous gas *. Mr Cruickshank observed, in re- 
 peated trials, that the air of the glass- vessel in which 
 his hand and foot had been some time confined, ren- 
 dered lime-water turbid, and that a lighted taper 
 burned very dimly in it f. Mr Abernethy exposed 
 his hand for five hours to seven ounce measures of 
 atmospheric air, contained in a jar inverted over 
 mercury : the quantity of air diminished half an 
 ounce, and an ounce more of it rapidly disappeared 
 by agitation with lime-water, and the lime was preci- 
 pitated : the remaining air being examined by the 
 test of nitrous gas, was found to contain nearly one- 
 sixth less of oxygenous gas, than it did before the 
 experiment. In another similar experiment, conti- 
 nued for nine hours, more than an ounce measure 
 of carbonic acid was produced, and the remaining 
 air contained one-fourth less of oxygen than before |. 
 M. Jurine fixed well-dried bottles under his arm- 
 pits and round his waist, and, after they had re- 
 mained an hour in these situations, the air in them 
 was much vitiated : and on different occasions was 
 found to be diminished in bulk, and to contain from 
 
 * Berlin Transactions for 1777, p. 35. 
 
 f- On Insensible Perspiration, p. 81. & 82, 
 
 J Essays, Surgical and Physiological, p. 118. 
 
186 
 
 from four and a half to ^ of carbonic acid. The 
 air confined under the bed-clothes where different per- 
 sons had slept, he found likewise to contain from 
 four to of carbonic acid. In the stomach also, 
 carbonic acid is formed : and from that organ the 
 proportion of oxygen gas diminishes progressively 
 downwards, the carbonic acid varies in quantity, and 
 the nitrogen gas is uniformly increased *. These 
 experiments prove, that carbonic acid is produced 
 wheresoever the surfaces of the living body are 
 brought into contact with atmospheric air. 
 
 148. Whence, then, does this carbonic acid pro- 
 ceed ? Dr Ingenhousz, finding that small bubbles of 
 air still escaped from the surface of the body, even 
 although he first carefully removed those which na- 
 turally adhered to it, was led to consider them as an 
 aerial transpiration from the skin ! But when Dr 
 Priestley employed rain water, from which the air 
 had been completely expelled by boiling, and detach- 
 ed also the adhering bubbles from his arm, he was 
 never able to collect a single bubble of air {. Dr 
 Pearson also found, that there was no appearance of 
 aerial bubbles on the surface of the cuticle during 
 bathing in warm water that had been previously 
 boiled, so as to expel the air usually mixed 
 with it. And M. Jurine, after remaining in the 
 bath for hours together, in temperatures varying 
 
 * Mem. de la Societe de Med. torn. x. 
 
 f Exper. sur les Vegetaux, torn. i. p. 152. 
 
 J Experiments and Observations, vol. v. p. 103. 
 
 Hunter's Observations on Animal (Economy, p. 168, 
 
187 
 
 from 70 to 100, never could obtain a single bub- 
 ble of air from his skin. These facts correspond 
 precisely with those already adduced concerning the 
 separation of air from water (45. 47.) by vegetable 
 bodies and other fibrous substances. They prove 
 decisively, that the human body, like other solid bo- 
 dies, has the power of separating the air naturally 
 contained in water : but that when the water con- 
 tains no air, none is separated by immersing the bo- 
 dy in it. It matters not what the substance is, says 
 Mr Hunter, if it be but warmer than the water ; 
 for a piece of iron heated to 150, and immersed in 
 water at 70, will cause the water to part with its 
 air ; but if the iron be ten degrees colder than the 
 water, little or no air will be separated *. The 
 small hairs distributed over the body facilitate, in all 
 probability, this separation of air ; for Mr Abernethy 
 observed, that, on introducing his hand and arm in- 
 to a jar of water at the temperature of .60, every 
 pore seemed covered with a little spherule of air, 
 which, on agitating the water, was detached, and 
 quickly rose to the top of the vessel ; and when the wa- 
 ter was changed at each repetition of the experiment, 
 a much greater quantity of air was collected, but in 
 moderately warm water he scarcely procured any 
 airf The air separated in experiments of this na- 
 ture was, according to Dr Priestley, just that mix- 
 ture of carbonic acid and partially vitiated air which 
 pump-water generally abounds with : and Dr Klapp, 
 
 * On the Animal (Economy, p. 168. 
 f Essays, Surgical, &c. p. 115, 
 
188 
 
 who has lately repeated these experiments in Ameri- 
 ca, found> that the gases collected by holding the 
 arm in an inverted glass-vessel filled v/ith water, 
 differed in no considerable degree from those which 
 had been previously obtained from the water. These 
 facts altogether forbid our drawing any conclusion 
 in favour of an aerial transpiration by the skin, from 
 the mere circumstance of air being separated by the 
 body when it is immersed in certain kinds of water, 
 149. But Mr Abernethy supports the opinion of 
 an aeriform perspiration by the skin on other grounds, 
 maintaining, that carbonic acid is given out as well 
 when the hand is plunged in a jar of mercury, or is 
 immersed in any kind of air, as when it is placed in 
 contact with oxygen gas *. After detaching the ad- 
 hering air from the surface of his hand, by moving 
 it for ten minutes in mercury, he introduced it with- 
 in a jar filled with that fluid, the thermometer 
 standing at about 6O. Minute air bubbles rose at 
 first to the top of the fluid, but more slowly after- 
 wards ; and in an hour, when the hand was with- 
 drawn, the air collected was equal in bulk to one 
 scruple of water. In sixteen hours, it equalled half 
 an ounce ; and of this, two-thirds were attracted by 
 lime water, and the remainder did not diminish on 
 mixture with nitrous gas : whence he concludes, 
 that it consisted of carbonic acid and nitrogen gas^ 
 and that these gases were perspired by the skin. 
 The small quantity of air separated in this experi- 
 ment, during sixteen hours, is itself a proof that this 
 
 * Essays, Surgical, &c. p. 121, 122, 
 
189 
 
 supposed aerial perspiration is very inconsiderable ; 
 and the air obtained is to be considered not as the 
 result of one, but of many experiments ; for we are 
 told, that after the first hour the hand was with- 
 drawn ; and there can be little doubt but that this 
 was several times repeated before the experiment 
 was closed. By so frequent an introduction of the 
 hand, it is very probable that small quantities of ad- 
 hering air would be repeatedly earned in, which is 
 confirmed by the observation of Mr Abernethy, that 
 the separation of the air was greatest on first intro- 
 ducing the hand *. He farther says, that when in 
 temperatures between 60 and 7O, he kept his hand 
 five hours in seven ounce measures of nitrogen gas, 
 confined in a jar over mercury, there appeared no 
 difference in the quantity of the air ; and on passing 
 lime-water into the jar, rather more than one ounce 
 of carbonic acid was produced, when no oxygen was 
 present. The same results, nearly, were obtained, 
 when hydrogen and other gases were employed f. 
 
 15O. These experiments of Mr Abernethy have 
 lately been repeated in Philadelphia by Dr Klapp, in 
 the presence of Dr Woodhouse, Professor of Che- 
 mistry in that university ; and with completely dif- 
 ferent results. This gentleman carefully separated 
 all the adhering air from his hand and wrist, by mo- 
 ving them in different directions under the surface 
 of the mercury, for ten minutes, as Mr Abernethy 
 had done : they were then introduced into the in- 
 verted glass-vessel, filled with the same fluid, at tern- 
 
 Essays, Surgical, &c. p. 112. f Ibid - P- 
 
190 
 
 perature 73, and retained for more than three 
 hours. During all this time, not a bubble, either 
 of carbonic acid or nitrogen gas, was seen to ema- 
 nate from the skin ; and the experiment was a se- 
 cond time repeated with precisely the same result. 
 He next immersed his hand and wrist in very pure 
 lime-water, of the temperature of 70, and held 
 them in that situation, in different experiments, for 
 one and two hours, during which times no carbonate 
 of lime was formed ; but, on the contrary, the lime- 
 water, when the hand was withdrawn, was as trans- 
 parent as before it was introduced. He next placed 
 his hand and wrist, after separating all adhering air 
 from their surface, in four ounce measures of pure 
 hydrogen gas, confined in a vessel inverted over 
 mercury, and kept them in this situation for three 
 hours. The air in the vessel was then examined in 
 the presence of Professor Woodhouse : its volume 
 was not in any degree diminished ; neither, on the 
 addition of lime water, could any trace of carbonic 
 acid be discerned in it *. In addition to these ex- 
 periments, disproving the existence of an aeriform 
 function in the skin, it may be observed, that if such 
 a function did really exist, the bulk of air ought to 
 be increased, \vlvn carbonic acid is formed by placing 
 the arm in a jar of atmospheric air ; but, according 
 to Mr Abernethy himself, the air was considerably 
 diminished. All the arguments already urged against 
 the absorption of aeriform fluids, forbid us to suppose 
 that the oxygen gas which disappears, is taken up 
 
 Inaugural Dissertation, Philadelphia, 1805, p. 21. ct scq. 
 
191 
 
 by a natural function of the skin : and the apparent- 
 ly decisive experiments of Dr Klapp, prove, that no 
 such absorption of oxygen is necessary, because no 
 emission of carbonic acid, or of any other aeriform 
 fluid, can take place from that organ. The produc- 
 tion of carbonic acid in the stomach, with the dimi- 
 nution of oxygen, and progressive increase of nitro- 
 gen gas in the intestines, observed by M. Jurine, 
 appear necessarily to arise from the longer time in 
 which the air remained in contact with the body, 
 and the greater degree of decomposition which it in 
 consequence underwent. 
 
 151. If, then, there be no proof that oxygen gas 
 enters into the body through the skin, and if it be 
 also allowed that no carbonic acid can be emitted 
 from that organ, it must be granted, that the acid 
 actually produced (14?.)> is formed exterior to the 
 skin itself; and the disappearance of the oxygen 
 gas, in proportion to the production of this acid, 
 leads at once to the opinion that it is formed in part 
 out of that gas. M. Jurine found accordingly, that 
 as much acid was formed out of a given bulk of 
 air, when his arm was confined in it for one hour, 
 as when the experiment was continued for three or 
 four hours, which, it appears to us, could have a- 
 risen only from all the oxygen gas having within the 
 first hour been changed. For the formation of this 
 acid, however, carbon, its other ingredient, must be 
 in some way supplied ; and as, in these experiments, 
 no other substance but the skin was in contact with 
 the, air, from which it could be derived, we must 
 ascribe the production of the acid to the spontane- 
 ous union of carbon, furnished by the skin, with 
 
192 
 
 the oxygen gas of the air ; and this carbon we con- 
 ceive to be given out by its exhalent function. 
 
 152. In vegetables, and in the inferior animals, it 
 has been shewn, that carbonic matter is yielded only 
 while the circulation of their fluids continues: and 
 there are some facts related by M. Jurine which ren- 
 der it very probable, that the same thing holds true 
 concerning the carbon which is furnished by the hu- 
 man skin. He found, that in given times, the great- 
 est quantity of carbonic acid was formed by the skin 
 of a person in the vigour of life, next by a boy of 
 ten years of age, and least of all by a man of 70 ; 
 and that its production depended much, likewise, on 
 the season of the year, and the vigour of the cuta- 
 neous organs. All these facts seem necessarily to fol- 
 low from the opinion that this carbonic matter is 
 truly an excretion, depending altogether on the dis- 
 tribution of the animal fluids, and subject to ail their 
 variations. Whether, as in other animals, the air is 
 no longer decomposed by the skin when the circu- 
 lation has wholly ceased, we have not the means of 
 deciding ; but the results of the foregoing experi- 
 ments of Jurine, lead us to suppose that such is ac- 
 tually the fact. Even at the period in which Mr 
 Cruickshank wrote, it is probable, that, had the 
 composition of carbonic acid been then determined, 
 he would have formed a similar opinion concerning 
 the source of the carbon : for, in referring to his 
 experiments, he observes, " that (admitting the 
 common theory of fixed air and phlogiston) some- 
 thing passed off with the vapour of insensible per- 
 spiration by the skin, which rendered air fixed, and 
 
193 
 
 heavier than atmospheric air *." By wearing a 
 fleecy hosiery vest also next the skin for a month at 
 a time in the hottest parts of summer, he collected 
 an oily substance which accumulated on the nap : 
 this substance he exposed to a red heat in a silver 
 spoon, and, when burnt, it left behind a black pow- 
 der resembling in every thing the powder of char- 
 coal. " This all seems to prove," he adds, " that 
 phlogiston" (by which he can here mean nothing 
 else than carbon) " is emitted from the pores of the 
 skin f ." From the establishment of this power in 
 the skin, we obtain, besides, this additional and im- 
 portant fact, that carbonic acid may be formed out 
 of the oxygen gas of the atmosphere, when brought 
 into contact with the human body, without the neces- 
 sity or even the possibility of believing that the car- 
 bon entering into its composition, is derived from 
 the venal blood ; for the exhalent vessels of the skin 
 are all of arterial structure, and the fluids which 
 they contain and emit, exhibit nothing of the colour 
 or the properties of venal blood. 
 
 153. The changes effected on the air by the eggs 
 of birds, may still farther elucidate this subject. 
 The shell of the egg is formed of carbonate of lime, 
 with a small portion of phosphate, united to an or- 
 ganic tissue. Spallanzani found it to be full of holes, 
 through which pass the extremities of very minute 
 vessels, coming off from the chorion, or membrane 
 with which the shell is internally lined : these vessels 
 terminate by small orifices on the surface of the 
 
 On Insensible Perspiration, p. 82. f Ibid. p. 
 
 N 
 
194 
 
 shell, forming a fine and transparent net-work. A 
 hen's egg was placed in three cubic inches of atmo- 
 spheric air, inverted over mercury: on examining 
 the air at the end of the fourth day, it was some- 
 what diminished in bulk, its nitrogenous portion re- 
 mained unaltered, a large quantity of its oxygen had 
 disappeared, and a portion of carbonic acid was pro- 
 duced. This diminution of the oxygen could not 
 be owing to any supposed attraction of it by the 
 substance of the egg ; for the shell, when deprived of 
 its contents, was found to produce a similar effect on 
 the air *. As the carbonic acid in this, as well as in 
 the former instances, was formed when the air and 
 the egg were in contact, and neither of them sepa- 
 rately could produce it, it is evident that the air must 
 have furnished the gas, while the egg supplied the 
 carbon. It is known that an exhalation is constant- 
 ly going on from the egg, by which it is daily be- 
 coming lighter ; and the more rapidly this exhalation 
 proceeds, the sooner does the egg decay. The egg 
 of a sparrow, weighing 29 grains, was exposed on a 
 table in a room varying from 60 to 66 Fahrenheit. 
 By the third day, it had lost one grain : by the fifth 
 day, two grains : by the tenth, three grains : by the 
 seventeenth, four and a half grains : and by the twen- 
 ty-sixth day, six grains: when broken, its contents 
 were very putrid. A fresh hen's egg, weighing 13 
 drachms, and exposed in the same room from June 
 15. to July the 30th, lost in weight 2 drachms 2O 
 grains : while another egg, equally exposed, but 
 
 * . Memoirs on Respiration, p. 237. ct seq. 
 
195 
 
 smeared over with oil, lost, in the same time, only 
 45 grains. If also eggs be varnished over, or pla- 
 ced under the exhausted receiver, they do not cor- 
 rupt : and Bomare mentions the circumstance of 
 three eggs being found hi the wall of a church, 
 which, by being protected from the air, were quite 
 fresh after the lapse of 300 years *. These facts 
 prove, that a reciprocal action is going on at all 
 times between the carbon of the egg and the oxygen 
 gas of the air, by which carbonic acid is produced ; 
 but which action may, at any time, be interrupted, 
 either by closing the pores of the shell, or by ab- 
 stracting the pure part of the air. The loss of 
 weight which eggs suffer, arises, in all probability, 
 from the escape of their more watery parts, which 
 pass off through the processes of the chorion descri- 
 bed above ; and, as the same causes which obstruct 
 the exhalation of moisture, seem also to check the 
 emission of carbon, it is reasonable to suppose, that 
 they both proceed from the same individual struc- 
 ture. 
 
 154. We have, in the next and last place, to in- 
 quire whence issues the carbon which unites with 
 the oxygen gas of the air to form carbonic acid, in 
 the respiration of man and other warm blooded ani- 
 mals. For the most part, this carbon has been sup- 
 posed to be derived from the venal blood, either in 
 the course of its transmission through the pulmona- 
 ry vessels of the lungs, or during its circulation 
 through the capillaries of the system: and to the 
 
 * Encyclop. Brit. art. Physiology, 
 N 2 
 
196 
 
 presence of this substance in the blood, the black 
 colour of that fluid has been attributed. Formerly, 
 indeed, this blackness of colour was ascribed to the 
 agency of phlogiston, or of hydrogen : but as the 
 existence of the former principle is now altogether 
 denied, and the latter substance can be shewn to re- 
 side in the blood only as a constituent part, as it does 
 in the other fluids and solids which are of various co- 
 lours, we must reject the opinion of its sufficiency 
 to account for the black colour of the blood. Be- 
 cause also red blood became black when placed in 
 nitrogen gas, by some, that gas has been held to be 
 the cause of its blackness ; but, according to Mr 
 Davy, no affinity subsists between nitrogen gas and 
 blood *. The same effect takes place also, when 
 blood is exposed to the contact of other gases, and 
 even in 'vacuo without the agency of any gas at all. 
 Dismissing then these opinions concerning the cause 
 of the black colour of the blood, let us next inquire 
 how far it may, with justice, be attributed to the 
 presence of carbon. 
 
 155. That carbon is a constituent element of ani- 
 mal substances, is generally allowed ; and that it ex r 
 ists in the blood, and is yielded by it to form carbo- 
 nic acid, have likewise been admitted : but that, to 
 this carbon, the black colour of the blood is, or can 
 be, owing, has not hitherto been distinctly proved. 
 It remains to-be shewn in what state this carbon ex- 
 ists, and by what agency it can produce this colour ; 
 for in its pure state of diamond, nothing is farther 
 
 Researches, p. 375. 
 
197 
 
 removed from blackness than earbon. The disap- 
 pearance of the black colour of the blood, as carbo- 
 nic acid is formed in respiration, is no proof of such 
 blood losing its colour, in consequence of yielding 
 carbon ; for we have seen, that, by living vegetables, 
 by animals, and by the human skin itself, this acid 
 is equally formed where no black blood is present 
 to supply carbon. Dr Lower also relates the case 
 of a young woman bled in the foot, whose blood 
 was as white as milk, and yet she was in good health : 
 and, in two other cases, the blood drawn from a 
 vein in the arm, was of the same complexion *. The 
 loss of the black colour of the blood too may be 
 owing to the acquisition, rather than to' the escape, 
 of some colouring matter ; or it may arise from 
 some change in the properties of the blood, which 
 shall vary its power of reflecting the rays of light. 
 Neither because carbonic acid is formed by exposing 
 black blood to the air, can we admit this as a proof, 
 that its blackness arose from the presence of car- 
 bon ; for Dr Girtanner found, that this acid was 
 formed equally by arterial blood (101.), which is 
 red : and we have seen it to be produced by the se- 
 rum (97.)? as we ^ as by the crassamentum of the 
 blood. Other agents also, which are totally inde- 
 pendent of carbon, are capable of producing this 
 black colour in the blood. Every one knows, that 
 a part exposed to great cold, speedily becomes black : 
 and Dr Crawford found, that, on immersing a dog, 
 whose temperature was 100, in water at 45, the 
 
 * Lowthorpe's Abrid. Phil. Trans, vol. iii. p. 239, 
 
 N3 
 
108 
 
 blood drawn from his jugular vein, was the darkest 
 he had ever seen *. Lastly, we have endeavoured to 
 prove, that no gases either exist in the blood, or can 
 be transmitted through the cellular and vascular 
 structure interposed between the air and that fluid in 
 the lung? ; consequently, no oxygen gas can enter 
 into the blood to unite with its supposed carbon, 
 nor,, if such an union did take place, could the car- 
 bonic acid be afterwards expelled from that fluid. 
 
 1)56. If, then, the carbon, which unites with the 
 oxygen gas of the inspired air during respiration, 
 come not directly from the blood flowing through 
 the pulmonary vessels, it must in some other way be 
 supplied by the animal system* It is generally ad- 
 mitted, that the cellular surface of the lungs is fur- 
 nished with exhalent vessels, like every other surface 
 of the body : and indeed, the excessive quantity of 
 fluid which is sometimes poured out by these vessels 
 into the bronchial cells, is not unfrequently the cause 
 of severe disease. This exhalation of fluid from the 
 lungs, contrary to what is the case in other exhalent 
 surfaces, seems to be chiefly supplied by the watery 
 parts of the venal blood ; for the pulmonary artery 
 in the lungs, not only terminates into veins, but, like 
 other arteries, and like the branchial artery in fishes 
 (65.), has an exhalent termination also. Dr Thrus- 
 ton injected a coloured liquor into the pulmonary 
 artery of a sheep, and the fluid partly passed into the 
 veins, and in part also escaped by the trachea f ; 
 
 * On Animal Heat, p. 310. 
 
 f De usu Respirationis, p. 48. an* 1671. 
 
199 
 
 and Dr Hales having filled the lungs with air, after 
 pouring water into the pulmonary artery of a calf, 
 observed the water to flow so freely into the bron- 
 chial cells, as to run out at the windpipe ; but the 
 vessels by which it escaped, were not large enough 
 to permit the colouring particles of the blood to 
 pass *. In like manner, injections will pass from 
 the pulmonary veins into the bronchia ; so that the 
 exhalent function in the lungs is carried on by the 
 whole system of pulmonary vessels. And, indeed, 
 if we reflect how much larger a portion of water the 
 venal blood (from the termination of the trunk of 
 the lymphatic system into the left subclavian vein at 
 only a short distance from the heart) must possess 
 in the lungs, than after it has passed those organs 
 and circulated through the whole body, we cannot 
 but admire this deviation from the ordinary laws of 
 the exhalent system, directed, as it is, to continue 
 and preserve, in sufficient abundance, an excretion 
 apparently so essential to the existence of living ac- 
 tion. 
 
 157. These exhalent vessels of the lungs, like 
 those of the skin and intestines, appear to be endued 
 with a power, not only of exhaling water, but like- 
 wise of emitting carbon ; for water and carbonic 
 acid are expelled from the lungs in respiration, in 
 the same manner as they are produced by the skin 
 (147.) when in contact with atmospheric air. As, 
 therefore, the products of respiration and perspira- 
 
 * Statical Essays, vol. ii. p. 72, 
 N4 
 
200 
 
 tion are in kind precisely similar, we are justified in 
 ascribing their formation to similar laws : and, since 
 it seems to have been demonstrated by direct expe- 
 riment (L5O.), that no transpiration of aeriform fluids 
 takes place through the skin, we may presume that 
 none is able to be carried on through the cells and 
 blood-vessels of the lungs. Not only is the cellular 
 surface of rhe lungs furnished with absorbent and 
 exhalent vessels like that of the skin, but it is sup- 
 plied from within by the same blood, and exposed 
 from without to the same atmospheric air. It has 
 been shewn also, that the colourless fluids of various 
 animals, are able to effect the same change on the 
 air, as that which is produced by the blood : and that 
 the serum of the blood itself, (which is especially 
 destined to supply- the exhalent function), produces 
 on the air the same identical change as it experien- 
 ces in the lungs : all which circumstances strongly 
 incline us to suppose, that the function of the lungs 
 resembles in kind that of the skin. The proofs 
 likewise already adduced, that the carbon furnished 
 by vegetables (14O.), and by the inferior animals, as 
 well those which respire by the skin (142.), as those 
 which breathe by lungs (J43.), depends wholly on 
 the due circulation of their fluids, and is, consequent- 
 ly, the result of a living action, are strong presump- 
 tive evidence, that the same law obtains in the supe- 
 rior animals, and in man : and seem to authorise the 
 conclusion, that the carbon supplied in human respir 
 ration, is truly an animal excretion, carried on by 
 the exhalent vessels of the lungs ; and therefore, 
 that it primarily depends, like other excretions, on 
 
201 
 
 the due circulation and distribution of the blood, 
 and is more or less affected by all its variations *. 
 
 158. In the inferior animals we have seen (142.3.) 
 that the production of carbonic acid, and consequently 
 of carbon, is very much influenced by external tempe- 
 rature, as this more or less affects the circulation of 
 the blood : and even in the human subject, it has been 
 shewn (152.), that the same causes possess a decided 
 
 * Long since this opinion concerning the source of the carbon 
 furnished in animal respiration, was entertained by the author, 
 his friend Dr Nugent has pointed out to him a passage in a 
 French writer, wherein a similar notion is distinctly stated. The 
 author, M. Caron, in opposing the opinion of Dr Goodwyn con- 
 cerning the entrance of oxygen gas into the blood-vessels, and 
 the emission of carbonic acid from them, has these words : " Je 
 vais demander ici a nos chymistes modernes, si, pour qu'il se 
 forme de Pair fixe dans la respiration, il ne suffit pas que Pair 
 vital puisse se charger, se souler de Phumeur qui sort des vaisseaux 
 exhalans des poumons : pour moi, je crois que cet effet pent 
 s'operer tout bonnement de cette maniere, sans que Pair vital 
 communique en quelque chose avec le sang : Je suis d'autant plus 
 fpnde a le croire, que Goodwyn, ainsi que tous les chymistes mo- 
 dernes, avouent qu'ils ne connoissent pas encore la route que 
 Poxigene peut prendre pour y parvenir, ni par quelle vertu jl 
 peut agir sur kii." In a subsequent paragraph also, he asks, 
 whether, instead of saying with Goodwyn, that a certain quanti- 
 ty of oxygen gas is separated from the inspired air in the lungs 
 by respiratidn, and a certain quantity of carbonic acid substituted 
 in its place, it would not be better to say : " Une certaine 
 quantite de gaz oxigene constituante Pair atmospherique, se 
 charge dans les poumons d'une certaine quantite de Phumeur de 
 la respiration, qui le metamorphose, et en change tellement la na- 
 ture, que Poxigene n'est plus reconnoisable, et qu'il est devenu 
 fl'acide carbon i que *." 
 
 * Recherches Critiques, par J. C. F, Caron, p. 29, 51. an, 1798, 
 
202 x 
 
 / 
 
 influence over the formation of this acid by the skin, 
 M. Jurine not only proved the greater consumption 
 *>f oxygen gas by the skin during a vigorous action 
 of the cutaneous organs, but likewise shewed, that 
 increase of temperature, muscular exertion, and di- 
 gestion,, had similar effects on the products of respi- 
 latioa. These experiments were followed out with 
 greater accuracy by MM. Lavoisier and Seguin, who 
 found, that a man at rest, with an empty stomach, 
 and in a temperature of 82, consumed in an hour 
 121O cubic inches of oxygen gas ; but that during 
 digestion, the consumption amounted to 18 or 190O 
 inches. With an empty stomach, and during vio- 
 lent exercise, 3200 inches were consumed in an 
 hour ; and when the same exertion was used after 
 taking food, the quantity was increased to 460O cu- 
 bic inches *. The difference in the results of these 
 experiments, is so very great, as to lead to a suspi- 
 cion, that some considerable error exists ; but, at all 
 events, they seem to prove, that whatever, in a cer- 
 tain degree, accelerates the circulation of the blood, 
 increases the consumption of oxygen gas in the 
 hrn'gs, and as this gas, in natural respiration, dis- 
 appears only in consequence of its union with car- 
 bon, it follows, that the carbonic matter must be 
 proportionally increased, which is a necessary conse- 
 quence of its being considered an animal excretion, 
 immediately dependent on the motion of the blood. 
 1\59. But, granting unto the several classes of ve- 
 getables and animals which have passed under our re- 
 
 * Mem. de 1'Acacl. de Scien. 1789,^790. 
 
view, and to the surfaces of the human lungs and skin, 
 the power of emitting carbon, it follows,: since this e- 
 mission is carried on through the whole period of living 
 action, and is essential to the continuance of it, that 
 some ulterior source must be provided, from whence 
 its supply may be duly maintained. In seeds, this 
 carbon forms a considerable part of their substance,. 
 and> during the whole period of germination, its pro- 
 portion is constantly diminishing (20.), by uniting: 
 with the oxygen gas of the air. Before this supply is 
 exhausted, roots are sent off from the seed into the 
 earth, by which it draws in nutritious matter,, a large 
 portion of which is considered to be carbon ; and this 
 carbonic matter undergoing the ordinary changes iiL 
 the assimilating organs, is, in part, like other excre- 
 ted substances, thrown off by that expansion of the 
 seed which now forms the plant, and flourishes in: 
 the open atmosphere. So long as water and heat 
 are duly supplied, so long does living action conti- 
 nue, of which the absorption of carbonic matter by 
 the roots of plants, and its expulsion by the leaves, 
 are natural and necessary consequences ; but the va- 
 rious intermediate changes which it is made to un- 
 dergo, we do not pretend to explain : they belong, 
 in fact, to another branch of inquiry, and cannot be 
 understood until the functions of nutrition and se- 
 cretion be more fully ascertained. 
 
 160. In the inferior animals, carbon is likewise a 
 necessary constituent substance; and certain of the 
 laws by which it is expelled from them, we have at- 
 tempted to ascertain. By them too, the means of 
 acquiring it, must be as constant as its expulsion, 
 during living action, has been shewn to be j and 
 
204 
 
 from no other source than through the organs of 
 digestion and secretion can it be conceived to be de- 
 rived. But our knowledge of the theory of these 
 functions in animals, as in vegetables, is extremely 
 limited and imperfect : and while it so continues, no 
 rational explanation of this matter can be expected 
 
 or obtained. 
 
 * 
 
 161. Nearly the same remarks maybe made con- 
 cerning the primary and original source of carbon in 
 the superior animals. To the organs of digestion, 
 assimilation, and secretion alone, are we enabled to 
 trace it ; but the mode in which it is reduced to that 
 state in which it is afterwards expelled by the surfa- 
 ces of the lungs and skin, involves a knowledge of 
 the nature and qualities of our food, of the various 
 and successive changes which it is made to undergo 
 in the system, and of its distribution by the blood to 
 the different organs of secretion, according to the se- 
 veral uses which it is afterwards destined to answer : 
 concerning all of which subjects, we have of late 
 succeeded in getting rid of much error and absurdi- 
 ty, but have not, in any instance, attained to com* 
 plete knowledge. 
 
CHAP. VI 
 
 OF THE PHENOMENA WHICH ARISE FROM 
 THE CHANGES INDUCED ON THE AIR 
 BY THE LIVING FUNCTIONS OF VEGE- 
 TABLES AND ANIMALS. 
 
 162. IAAVINO, in the foregoing chapters, 
 endeavoured to trace the immediate source of the 
 carbon furnished by vegetables and animals in the 
 several processes above described, and the changes 
 which it effects in" the air that surrounds them, we 
 are prepared next to inquire into the phenomena 
 which arise from, and always accompany these 
 changes. Our conception of these phenomena will 
 be much aided by premising a few fundamental facts 
 regarding the doctrine of heat with which modern 
 chemistry has made us acquainted. 
 
 163. The sensation of heat which we experience 
 from certain bodies, is known to depend on the ope- 
 ration of a power to which chemists have applied 
 
206 
 
 the term caloric ; and the same power, when it en- 
 ters into bodies, not only raises their temperature, 
 but almost universally expands them : hence the sen- 
 sation of heat, temperature, and expansion, are con- 
 sidered as effects, of which caloric is the immediate 
 cause *. The quantities of caloric contained in ho- 
 mogeneous bodies are proportioned to their tempe- 
 ratures and quantities of matter ; but, in other cases, 
 it is established as a general law, that different bodies 
 in equal quantities, whether estimated by weight or 
 volume, contain unequal quantities of caloric f. This 
 property or power in bodies to contain very different 
 quantities of caloric, has, by some, been termed the 
 capacity of bodies for heat, while others express the 
 fact more simply, by employing the phrase specific 
 caloric to denote the relative quantities of heat which 
 different bodies contain. 
 
 164. The greater the quantity of caloric that en- 
 ters into bodies, the more do they become expand- 
 ed ; and if this expansion proceed to a certain ex- 
 
 * This distinction between heat, and the effects which it pro- 
 duces, did not escape the observation of the ancients. Theo- 
 phrastus, iu his book De Jgne, says Bishop Berkeley, distinguish- 
 eth between heat and fire. The first he considers as a principle 
 or cause, not that which appeareth to sense as a passion or acci- 
 dent existing in a subject, and which is in truth the effect of that 
 unseen principle. This invisible fire, he adds, is present in all 
 parts of the earth and firmament, though, perhaps, latent and un- 
 observed, till some accident produceth it into act, and renders it 
 vigible in its effects *. 
 
 f Murray's System of Chemistry, vol. i. p. 350, 
 
 * Sim, par, 157. 17G. 
 
207 
 
 tent, solids are converted into fluids, and fluids into 
 vapours and airs. The rarity and specific caloric of 
 bodies, however, though intimately connected, are 
 by no means proportional to each other. Solid bo. 
 dies contain less caloric than fluids, and fluids less 
 than airs ; and, when a change of form in these bo- 
 dies takes place, caloric either enters into, or is gi- 
 ven out by them. Dr Black, whose admirable dis- 
 coveries relating to heat, form so brilliant a period 
 in the history of modern chemistry, found, that ar 
 quantity of heat, equal to 140 of Fahrenheit, was 
 required to convert ice into water, although the wa- 
 ter still continued at the temperature of 32 : and to 
 raise water into vapour required 800 more, although 
 the temperature of the steam did not exceed that of 
 boiling water. As the caloric employed in producing 
 these changes of form, did not sensibly raise the 
 temperature of bodies, he called it latent heat : and 
 he shewed that this latent heat was again given out 
 when the vapour was reduced to water, or the wa- 
 ter passed into the state of ice. The permanently 
 elastic fluids are subject to the same laws ; for when 
 on being disengaged from a combination, they as- 
 sume a gaseous form, a large quantity of caloric, in 
 a latent state, unites with them, which is again libe- 
 rated, whenever, by compression, or by chemical 
 combination, they are reduced to a denser state. 
 
 165. Many experiments have been made to disco- 
 ver the comparative quantities of heat, or the speci- 
 fic caloric which, at a given temperature, equal 
 weights of various solid, liquid, and aeriform bodies 
 contain. Taking the specific caloric of water as 
 1.0000, Dr Crawford found that of arterial blood 
 
208* 
 
 1.0300 ; and that of venal blood 0.8928. Among 
 elastic fluids, the specific heat of oxygen gas, com- 
 pared with that of water, is as 4.749O to 1 .OOOO : of 
 nitrogen, the specific heat is 0.7936 : of atmospheric 
 air, 1.7900: and of carbonic acid gas, 1.6454*. 
 This high specific heat of oxygen gas, is productive 
 of very important effects in the operations both of 
 nature and art. The power of this gas to supply 
 heat, says M. Berthollet, is well known, and there 
 is no substance which suffers so much of it to escape 
 by the changes of its constitution. In the forma- 
 tion of water by the combustion of oxygen and hy- 
 drogen gases, the greater part of the heat might at 
 first be supposed to proceed from the condensation 
 of the latter ; but solid bodies, as phosphorus, occa- 
 sion the greatest emission of heat by combining with 
 a given quantity of oxygen ; and when water is de- 
 composed by sulphuric acid and iron, much heat is 
 given out, and nevertheless all the hydrogen of the 
 water resumes its elastic form. This heat therefore 
 must, he adds, have proceeded from a change in the 
 state of the oxygen of the water, which gives only a 
 small part of that which it yields, by combining with 
 the iron, to enable the hydrogen to assume the gase- 
 ous form t- We now proceed to apply these facts 
 to the illustration of our present subject. 
 
 1 66. Through the whole of our inquiry it has ap- 
 peared, that the oxygen gas of the air is converted 
 into carbonic acid by the living processes which have 
 
 * Crawford on Animal Heat, 
 f Chemical Statics, vol. ii. p. 15. 
 
209 
 
 fallen under our examination ; and since, from what 
 has just been stated, the specific caloric of this acid 
 is found to be very little more than one-third of that 
 which the oxygen gas itself previously contained, it 
 necessarily follows, that a large quantity of caloric 
 is liberated whenever this conversion of gases takes 
 place. For this office it appears, from the observa- 
 tions of Berthollet, that oxygen gas is peculiarly 
 fitted, since no other substance gives out so much 
 caloric by the changes of constitution which it suf- 
 fers : and its powers, in the particular instance of 
 change which we are now considering, may perhaps 
 be still farther aided by the degree of condensation 
 which we have seen it in all cases to undergo. Let 
 us then inquire, w r hether the presence of caloric be 
 manifest in the several living processes above men- 
 tioned, and how far it may be deemed to arise from 
 the changes effected on the oxygenous portion of 
 the air ; and first of all in the instance of germina- 
 tion. 
 
 167. Some peas, which had been steeped in water 
 36 hours, were placed in a small glass-jar standing 
 in a room at temperature 63. In this situation they 
 remained till signs of germination appeared, when 
 the bulb of a very delicate thermometer was plun- 
 ged into the midst of them, which had previously 
 been compared with another standing beside it. The 
 standard thermometer continued stationary for more 
 than an hour ; but the one that was plunged among 
 the peas, although at first 0.4 below the former, 
 rose in 20 minutes O.2 above it, and, in 4O minutes, 
 was 0.4 above it ; but at the end of half an hour 
 more, it had again fallen to 0.2 of superiority only, 
 
 O 
 
210 
 
 In a second trial, after turning the peas out of the 
 jar, and again replacing them with a quantity o 
 fresh air, the difference indicated by the thermome- 
 ter plunged in the peas, amounted, in half an hour, 
 to one degree, but, at the end of another half hour, 
 k was only 0.5. The amount of this difference 
 must be expected to vary according to the more or 
 less active manner in which the process is going on ; 
 but the result seems to prove, that, during the pro- 
 cess of germination, a production of temperature ac- 
 tually takes place- The process of malting affords 
 us an example of the same thing. This is conduct* 
 ed by exciting germination to a certain extent in the 
 seed ; and, during the process, a considerable pro- 
 duction of temperature takes place ; so great, indeed, 
 that in certain circumstances, grain improperly kept 
 has even taken fire. From this fact we may con- 
 clude, that during the germination of seeds in the 
 earth, a production of temperature likewise takes 
 place *. 
 
 168. From what source then is this increase of 
 temperature derived ? It cannot proceed from any 
 kind of action in the seed of a mechanical nature, 
 for its structure is not fitted to produce it : neither 
 can it be derived by communication from an exter- 
 nal source, for the surrounding bodies are all cold- 
 er, and are constantly drawing off the heat. Far- 
 ther, it occurs only while germination is going on ; 
 but during that process, the oxygen gas of the air is 
 constantly changing (8. 9.) into carbonic acid by uni- 
 ting with carbon (20.) furnished by the seed. In 
 
 * Thomson's Chemistry, vol. iv. p. 238. 
 
211 
 
 suffering this change, however, the greater portion 
 of its specific caloric is necessarily yielded (166.) 
 by the oxygen gas, and a rise of temperature in con- 
 sequence takes place ; and hence, therefore, we must 
 ascribe the production of temperature which occurs 
 in germination, to the decomposition of the oxyge- 
 nous portion of the air, by the agency of the living 
 seed. 
 
 169. As the same changes are produced on the 
 oxygen gas of the air by the growth of plants (30. 
 et seq.), as by the germination of seeds, the same 
 extrication of its heat may reasonably be expected 
 to follow. The sudden thaw of snow lying on grass, 
 while that on the adjoining gravel continues unthaw- 
 ed ; the fact of the moisture of dead sticks freezing, 
 while growing twigs are not at all affected ; and of 
 herbaceous plants resisting often degrees of cold 
 which freeze large bodies of water, all, says Sir 
 Charles Blagden, seem to shew that vegetables pos- 
 sess a power of generating heat *. Mr Hunter also 
 found the leaves of plants to resist freezing longer 
 than water. A growing fir-shoot, and the leaf of a 
 bean, were laid on a cold mixture of the tempera- 
 ture of 28, and in some minutes they had thawed 
 the surface on which they lay. This did not arise 
 from the greater warmth of the leaves when first ap- 
 plied, for, on removing the fir-shoot to another part, 
 the same effect \yas produced f By applying like- 
 wise thermometers to the internal parts of vegetables, 
 he discovered them to possess a temperature above 
 
 * Philosophical Transactions, 1775, f Ibid. 
 
 02 
 
212 
 
 that of the atmosphere, but less than that of cold- 
 blooded animals. When the atmosphere was below 
 .56, the temperature of the tree was always above 
 it ; but when the weather was warm, the heat of the 
 tree was several degrees lower *. 
 
 170. But the most remarkable and decisive facts, 
 relating to the temperature of vegetables, are con- 
 tained in the following account of M. Hubert, con- 
 cerning the heat given out by the sfiadices of the 
 arum cordifolltim during the process of fecundation. 
 This plant grows in Madagascar and the Isle of 
 France : its flowers exhale a strong and rather plea- 
 sant odour. About sun-rise, M. Hubert tied five 
 spadices, which had unfolded during the night, round 
 the bulb of a thermometer, and the mercury rose to 
 44 \ while another instrument of comparison, at 
 six o'clock in the evening, was only 19. At eight 
 next morning, the standard thermometer was only 
 21, and that used in the experiment had fallen to 
 42 ; and by nine at night had sunk to 28, while 
 the first remained at 21. The next day, at nine in 
 the morning, the thermometer of experiment follow- 
 ed the ordinary course. These trials were repeated 
 seven or eight times with similar results. When 
 surrounded with very fine spadices, the mercury rose 
 to 45, but reached only to 42 with the smallest ; 
 and, in one instance, where the thermometer was 
 surrounded by twelve flowers, the maximum of heat 
 was 49.5. The male parts of six spadices raised 
 the thermometer to 41% while the same number of 
 the female parts of the flowers raised it only to 2&* 
 
 Encyclop, Brit. art. Physiology. 
 
or 3O. No variation in the results occurred, whe- 
 ther the experiments were repeated in a dry room, 
 or under the shade of thick and humid trees. 
 
 171. M. Hubert next endeavoured to ascertain 
 the part of the spadix to which this increase of tem- 
 perature was owing. He had found that the medul- 
 la, or pith, raised the thermometer, when plunged 
 into it, in the same way as the exterior surfaces had 
 done ; but reflecting that the heat of the pith might 
 arise only from the exterior surface, he removed this 
 latter from four spadices without touching the me- 
 dulla. These medullas were then tied round a ther- 
 mometer, which, at sun-rise, was at 17, but no sign 
 of increased temperature occurred during 24 hours, 
 and the uncovered spadices withered towards the mid- 
 dle of the day. At the same time, he tied the remo- 
 ved surfaces of the spadices round the bulb of ano- 
 ther thermometer, and it rose to 39. This he re- 
 peated several times, which convinced him, that this 
 singular faculty is possessed by the exterior surfaces 
 of the spadices, and within the thickness of ~ of an 
 inch at most. If the spadices of the plant were di- 
 vided some time before the development of this 
 heat, the fluid that escaped from the divided portions 
 was colourless, which is not the case when the heat 
 has been previously given out. 
 
 172. Lastly, M. Hubert made experiments to dis- 
 cover the circumstances necessary to the production 
 of this great increase of temperature. He found 
 that if the spadices were closely coveivd with a cloth 
 dipped in olive oil, grease, or tallow, no increase of 
 temperature took place : and that if, at their highest 
 temperature, they were plunged into cold ivater, 
 
214 
 
 their heat quickly disappeared, but again returned, 
 in 25 or 30 minutes, on their being withdrawn. 
 When they were covered with oil or honey, the pro- 
 duction of heat in the spadkes was suspended for 
 about an hour; and a spadix covered with starch 
 gave no indication of increased temperature till its 
 covering dned and fell off in small portions. Other 
 species of the same genus were found to possess si- 
 milar properties of producing heat ; and it is con- 
 cluded, that this property is confined to the outer 
 surface of the spadix, that it is independent of light, 
 but that the contact of air is necessary to its produc- 
 tion. It was moreover proved, that the air in which 
 th se fiowers had grown, had suffered considerable 
 changes : for it extinguished a lighted taper, and a 
 chick was suffocated in a closed jar in which several 
 fpadices had remained five hours, but recovered on 
 b ing withdrawn *. Th< se facts, in connection with 
 those before related, sufficiently prove that vegetables 
 possess, during their growth, a temperature above 
 that of the ambient air, and that the contact of this 
 air is essential to its production ; and, since it has 
 been shewn, that the oxygen gas of the air is con- 
 verted (38 ) into carbonic acid, by carbon exhaled 
 (141.) by the living plant, whereby the greater part 
 of the specific caloric (166.) of that gas is disenga- 
 ged, it is to this change of composition in the air, and 
 consequent extrication of its heat, that the increased 
 temperature observed in plants, during their vegetar 
 tion, is to be ascribed. 
 
 * Voyage dans les Isles dt-s mers d'Afiique, par M. Bory de St 
 Vincent. 
 
215 
 
 173. We proceed next to speak of the heat of 
 those animals, which, from the low temperature they 
 possess, have been denominated cold-blooded, and 
 this we shall do in the same order in which they 
 have been already considered. With regard to in- 
 sects, Dr Martine observes, that the whole tribe is 
 commonly brought under the class of cold animals. 
 Caterpillars have but a small degree or heat, about a 
 division or two, above that of the air they live in 5 
 but the heat of a swarm of bees raises a thermome- 
 ter buried among them, above 97, a degree of heat 
 nothing inferior to our own *. Some insects, as flies 
 and wasps, can sustain a loss of heat without losing 
 life, but a bee cannot f. Coleopterous insects be- 
 come torpid at 34 : at 36, they move slowly and 
 with difficulty, and, at a lower temperature, their 
 muscles cease to be irritable J. During their state 
 of dormancy, the chrysalides of many insects may 
 be frozen without destroying their power of recover- 
 ing action . 
 
 174. In the class vermes, Spallanzani observes, 
 that when a snail or slug is insulated in a jar of at- 
 mospheric air, a thermometer placed in the jar will 
 continue stationary ; but when several are confined 
 together, the mercury rises one-tenth, one-seventh, 
 and even one-fifth of a degree, and in oxygen gas 
 one-third of a degree ; from which he concludes, 
 that snails and slugs, in decomposing oxygen gas, 
 
 * Martine on Thermometers, p. 140, 141. 
 
 f Hunter's Observations on Animal (Economy, p. 108. 
 
 $ Carlisle Phil. Trans. 1805, p. 25. Ibid. p. 
 
 04 
 
216 
 
 give out caloric enough to be sensible to the ther- 
 mometer *. This experiment we repeated, by con- 
 fining several snails in a pint jar of air, from the top 
 of which a small thermometer was suspended, and 
 at the bottom a glass of lime-water was placed. 4- 
 film of carbonate of lime soon overspread the lime- 
 water, the inside of the jar was dimmed by moisture, 
 and the mercury in the thermometer rose at the 
 same time nearly one degree. Dr Martine says, that 
 from the result of several trials which he made, 
 snails were about two degrees warmer than the air ! 
 Mr Hunter found the lungs of snails 38, when the 
 atmosphere was 34 ; and, in other instances, snails 
 were six and seven degrees above the atmosphere, 
 when it was so low as 30. Earth worms he found 
 58.5, when the atmosphere was 56 ; and, in other 
 trials, the worms exceeded by four, leeches by three, 
 and slugs by four degrees the temperature of the 
 ambient air {. The temperature of a snail, which 
 was 44, sank, on exposure to a cold mixture, down 
 to 31, and then froze; and several leeches froze 
 likewise when reduced to 31 . In all these expe- 
 riments, the animals, when thawed, were found to be 
 d ad ; but Mr Carlisle says, that the garden snail 
 may be frozen, during its state of dormancy, with- 
 out destroying its muscular irritability (j. 
 
 * Memoirs on Respiration, p. 255. 258. 
 
 f On Thermometers, p. 141. 
 
 f Treatise on the Blood, p. 298. et seq. 
 
 Observations on the Animal (Economy, p. 105. 
 
 (j Philosophical Transactions, 1805, p. 18. 
 
217 
 
 1 75. The class of fishes possesses also a temperature 
 above that of the medium they inhabit. In floun- 
 ders, whitings, cod-fish, and haddocks, the tempera- 
 ture, according to Dr Martine, was scarcely a degree 
 more than that of the water they were swimming in, 
 even when that was so low as 41. Nor are the 
 red-blooded fishes much warmer than the white ones; 
 for trouts were but 62, when the river water 
 they were swimming in was 61; and carp and 
 eels hardly exceeded the heat of the water they 
 inhabited *. Mr Hunter, however, found the mer- 
 cury in the thermometer to stand at 69 in the 
 stomach of a carp, when the water in the pond 
 from which he was taken, was only 65.5. That 
 fishes part with this excess of heat to the cold- 
 er bodies which surround them, we learn also from 
 the experiments of the same author. He put two 
 carp into a glass-vessel with common river water, 
 and then placed the vessel in a freezing mixture : 
 and, as the water in the vessel did not freeze fast e- 
 nough, snow was added so as to render the whole 
 thick. The snow round the carp melted, and more 
 being added, it melted also, till at last he grew tired 
 of putting it in, and left them to freeze by the 
 joint operation of the freezing mixture, and the 
 natural cold of the atmosphere. At length they 
 froze, but did not, when thawed, recover life with 
 flexibility f ; but, that both snakes and fishes, after 
 being frozen, have still retained so much of life, as 
 
 * On Thermometers, p. 141. 
 
 f Observations OH the Animal (Economy, p. 89. 
 
-218 
 
 when thawed to recover vital action, is a fact, says 
 Mr Hunter, so well attested, that we are bound to 
 believe it. 
 
 176. Amphibious animals exhibit a great variety 
 in the structure of the respiratory organs, and, con- 
 sequently, in the degrees of animal heat. Frogs and 
 land tortoises possess a temperature about five de- 
 grees higher than that of the medium they inhabit, ac- 
 cording to Dr Martine. The same may be said of 
 sea tortoises, toads, vipers, and all the serpent kind, 
 all of whom have lungs of the same fabric, and the 
 same cold constitution of body *. Mr Hunter ob- 
 served, that the frog and toad were about four or 
 five degrees warmer than the atmosphere when it 
 was at 35 or 36 : and that some hours after 
 ck'ath, they gradually fell down to the temperature 
 of the surrounding air f. This difference of tempe- 
 rature appears to increase in a warmer atmosphere : 
 for Mr Carlisle kept three frogs for many days 
 in an equable atmosphere of 54, and their sto- 
 machs preserved a temperature of 62 J. In an 
 atmosphere of 58, Mr Hunter found the ther- 
 momter, introduced into the stomach of a healthy 
 viper, to stand at 68 ; but, after the animal was put 
 into a pan, and the pan into a cold mixture of 10, 
 where it remained about ten minutes, the heat was 
 reduced to 37, and in twenty minutes more to 31, 
 nor did it sink lower : its tail now began to freeze, 
 
 * Essay on Thermometers, p. 142. 
 f Treatise on the Blood, p. 298. 
 Philosophical Transactions, 180. 
 
219 
 
 and the animal was very weak. A frog also, whose 
 temperature was 44, when put into a cold mixture, 
 soon fell down to 31; and beyond this point it was 
 not possible to lessen the heat without destroying 
 the animal *. A toad being placed in cold water 
 just deep enough not to cover his mouth, the whole 
 was put into a cold mixture between IO and 15. 
 The water froze around the toad, and, as it were, 
 closed him in, but he did not die, and therefore was 
 not frozen. Why the animals, mentioned in these 
 experiments, died before they were frozen, while 
 those which are exposed to the atmosphere in very 
 cold climates do not die, is a point which Mr Hunter 
 does not pretend to determine ; not knowing the 
 difference, he says, between the effects of a natural 
 and artificial cold f 
 
 177. The experiments of Mr Hunter farther 
 prove, that the temperature of most of the foregoing 
 animals not only falls rapidly in a colder medium, 
 but that it rises more quickly in a warmer one than 
 that of those which possess a higher standard tempe- 
 rature. In the stomach of a frog, the thermometer 
 rose from 45 to 49 : the animal was then placed 
 in an atmosphere made warm by heated water, 
 where it remained for twenty minutes, and, upon in- 
 troducing the thermometer again into the stomach, 
 the mercury rose to 64 J. A healthy viper was put 
 into an atmosphere of 108, and, in seven minutes, 
 the heat of the animal, both in the stomach and 
 
 * Observations on Animal (Economy, p. 104'. 
 f Ibid. p. 89, 90. t Ibid. p. 90. 
 
220 
 
 anus, was found to be 92.5, beyond which it could 
 not be raised in the above heat. An eel, very weak, 
 whose heat was 44, which was nearly that of the 
 atmosphere, was put into water heated to 65, for 
 fifteen minutes j and upon examination, it was of 
 the same degree of heat with the water. The heat 
 of a tench was, in ten minutes, raised from 41 to 
 55 both in the stomach and rectum, by being put 
 into water at 65. He found also, that a living and 
 a dead tench, and a living and a dead eel, put toge- 
 ther into warm water, received heat equally fast : 
 and when they were exposed to cold, both the living 
 and the dead admitted the cold likewise with equal 
 quickness *. Hence, therefore, the animal heat, in 
 all the classes of animals hitherto mentioned, whether 
 they inhabit the air or the water, seems to follow 
 nearly that of the medium in which they are placed ; 
 and their standard temperature cannot, in conse- 
 quence, be restricted to any fixed point, but must be 
 considered always in relation to that of their sur- 
 rounding medium, 
 
 178. Notwithstanding, however, the low degree 
 of heat which these several classes of animals pos- 
 sess, hardly, in some instances, exceeding that of 
 the medium in which they live, yet this small excess 
 is a proof that they possess within themselves a 
 power of producing heat. The loss of heat which 
 insects (173.) suffer under cold, the fall of tempera- 
 ture which many of the vermes class (174.) undergo 
 from the same cause, the melting of snow (175.) by 
 
 * Observations on Anirral (Economy, p. 104, 105. 
 
221 
 
 the heat of fishes, and the decline of animal heat 
 which the amphibia (176.), when exposed to great 
 cold, experience, all demonstrate, that the surround- 
 ing medium, whether it be air or water, is constant- 
 ly drawing off their heat, which renders necessary 
 as constant a reproduction of it. 
 
 179. By what process, then, or from what source, 
 is this superiority of temperature derived ? It does 
 not, says Mr Hunter, depend on the motion of the 
 blood, as some have supposed, because it likewise 
 belongs to animals which have no circulation : neither 
 can it be said to depend on the nervous system, for 
 it is found in animals which have neither brain nor 
 nerves. * It is probable, he adds, that it arises from 
 some principle, so connected with life, that it can 
 and does act independently of circulation, sensation r 
 and volition ; and is that power which preserves and 
 regulates the internal machine *. This supposition 
 in no degree removes the difficulty. Of the princi- 
 ple of life, or that power which enables organized 
 bodies to exhibit living action, we can form some 
 idea, although we know not its nature, just as the 
 astronomer speaks of gravitation without pretending 
 to define what it is ; but of another principle con- 
 nected with life, and producing sensible effects in the 
 system, independently of all the animal functions, 
 we certainly know nothing ; and, fortunately for 
 science, the admission of such imaginary principles, 
 or agents, is now banished from all chaste philoso- 
 phy. Admitting also, with Mr Hunter, the insuffi- 
 
 * Observations on the Animal (Economy, p. 9L 
 
224 
 
 those quadrupeds also which mostly inhabit the 
 ters, the temperature is quite as high. Dr Martine 
 found the heat of the skin of a sea-calf to be near 
 1O2, and, in the cavity of the abdomen, it was about 
 a division higher *. Such, too, is the case in the 
 cetaceous order of animals, the temperature of the 
 whale and porpoise being as high as that of land ani- 
 mals. In all these animals, the organs of respira- 
 tion and circulation, in their general structure and 
 actions, resemble nearly those of man, with such 
 difference only as the peculiar configuration and 
 modes of life of the animal necessarily introduce. 
 
 1 82. But in birds, which have been stated to pos- 
 sess the highest temperature of all animals, the lungs 
 are differently constituted, and are much larger in 
 proportion to the animal than in other cases. In- 
 stead of lying loosely in the chest, and yielding to its 
 alternate contractions and dilatations as in the mam- 
 malia class, they adhere to the thin transparent mem- 
 brane which covers their lower surface, and performs 
 the office of a diaphragm : besides this attachment, 
 they are also connected to the ribs and sides of the 
 vertebrae. In the larger cavities of the body, and in 
 the interstices about the breast and axilla, are placed 
 air-bags of different sizes, some of which communi- 
 cate immediately with each other, and all may be 
 said to have a communication by means of the lungs. 
 The bones also, which, in other animals, are filled 
 with medullary matter, are, in birds, receptacles for 
 air. Some of these, as the sternum, ribs, and verte- 
 
 * On Thermometers, p. 146. 
 
225 
 
 brae, have their internal substance divided into in- 
 numerable cells ; while others, as the shoulder and 
 thigh bones, are hollowed out into one large canal, 
 with sometimes a few bony columns running across at 
 the extremities ; and, at that end of the bones next to 
 the trunk of the bird, several holes or openings are 
 placed. There are openings in the lungs, by which 
 the air they receive is transmitted to other parts ; 
 and the diaphragm also is perforated with holes of a 
 considerable size, to 'each of which is joined a dis- 
 tinct membranous bag, which, being continued 
 through the whole of the abdomen, is retained in its 
 proper situation, by being attached to the back or 
 sides of that cavity. Each bag receives air from the 
 lungs through its respective opening, and such bags 
 extend over the whole abdomen. At their superior 
 part, the lungs communicate with the large cells 
 of a loose net-work, situated on the anterior part of 
 the breast ; and when these are distended with air, 
 the size of the part is considerably increased, as is 
 plainly seen in the turkey-cock, the pouting pigeon, 
 and in the breast of the goose when she cackles. 
 These cells in the breast communicate with others in 
 the axilla, which again communicate with the os hu- 
 meri, by small openings near the head of that bone. 
 The posterior edges of the lungs open into the cells 
 of the bodies of the vertebras, into those of the ribs, 
 the canal of the spinal marrow, and into the cells of 
 the bones of the pelvis, from which parts the air 
 finds a passage into the cavity of the thigh bone. 
 Thus, the cells, situated in the soft parts and in the 
 bones of birds, can be furnished with air through 
 the medium of the lungs. 
 
 P 
 
226 
 
 183. Mr Hunter, from whom the foregoing de 
 scription of the respiratory organs of birds is taken, 
 made several experiments upon the breathing of these 
 animals. He found, by making an opening into the 
 belly of a cock, and introducing a silver canula, pre- 
 viously to tying up the trachea, that the animal breath- 
 ed by this opening, and might have lived, but for an 
 inflammation of the bowels supervening, which, by 
 adhesion, cut off the communication with the air. 
 He next cut the wing through the os humeri, and, 
 tying up the trachea, found that the ait passed to and 
 from the lungs by the canal in this bone. The same 
 experiment was made with the os femoris of a 
 young hawk, and was attended with nearly the like 
 success : but the difficulty of breathing was greater 
 than in the former case, and the animal soon died *, 
 From the great size and peculiar communications of 
 the respiratory organs of birds, it may be presumed, 
 that, in proportion to their bulk, they respire a very 
 large quantity of air ; and accordingly, it has been 
 found by experiment, that, in a given time, they 
 consume more oxygen than other animals of the 
 same size, and therefore die sooner in a given vo- 
 lume of air. With regard to their temperature, Dr 
 Marrine found them to be warmer than quadrupeds 
 by three or four degrees : for the thermometer be- 
 ing lodged in the groin of ducks, geese, hens, 
 pigeons, partridges, swallows, &c. the mercury was 
 raised to 103, 104, 105, 106, and 107 : and, in 
 
 * Observations on the Animal (Economy, p. 8L 
 
227 
 
 a hen hatching eggs to 108 *. Mr Hunter also 
 found the heat of the hen to raise the mercury from 
 103 to 104, when introduced within the rectum ! 
 184. But the degree of heat in animals varies, not 
 only at different times under a change of circum- 
 stances, but also in different parts of the same ani- 
 mal, where all the circumstances continue the same. 
 Some examples of this fact have already been inci- 
 dentally mentioned as occurring in different animals. 
 Under the tongue of a man, Mr Hunter found the 
 degree of heat to be 97 : at one inch within the u- 
 rethra, 92 : at two inches 93 : at four inches 94 : 
 and at the bulb 97. The heat of the rectum in the 
 same man was 98.5. In the mouse, when the at- 
 mosphere was 60, the mercury in the thermometer 
 stood in the pelvis at 96.8, and at the diaphragm 
 99. In the rectum of a dog, the temperature was 
 10O.5 : in the liver, 10O.7 : and in the right ven- 
 tricle of the heart and stomach, it was exactly 101 J. 
 The temperature of a horse, killed by dividing the 
 spinal marrow and large blood-vessels, was, in the 
 colon, 98 : in the stomach 101 : and in the spleen 
 103, according to Mr Carlisle, when the atmosphere 
 was at 30. The urine of the same animal was 97, 
 and the flowing blood 103. The water flowing 
 from a tapped person was 101, but, at the surface, 
 the temperature was 96, the atmosphere at the time 
 being 43 . From these facts, it is plain, that eve- 
 
 * On Thermometers, p. 147. 
 f Observations an the Animal (Economy, p. 103. 
 t Ibid. p. 94. et scq. 
 Philosophical Transactions, 1805, p. 22. 
 P2 
 
228 
 
 ry part of an animal is not of the same degree of 
 heat : and that the more interior and vital parts pos- 
 sess the highest temperature. 
 
 185. We have seen that animals which ordinarily 
 pbssess a degree of heat but little above that of their 
 surrounding medium, very readily adapt their tem- 
 perature (177.) to all the variations of that medium. 
 In man, however, and other warm-blooded animals, 
 the system is able to bear very great changes of tem- 
 perature without a corresponding change in the de- 
 gree of animal heat. Our sensations, indeed, often 
 apprize us of very slight alterations in the tempera- 
 ture of surrounding bodies, when no perceptible dif- 
 ference, ascertainable by the thermometer, exists. 
 These sensations again, are not only influenced by 
 the general healthy condition of the body, but by 
 the habits with regard to heat and cold which we 
 have been accustomed to indulge. Thus persons 
 who clothe themselves warmly, or live generally in 
 uniform temperatures, are affected by slight varia- 
 tions, which others of hardier habits totally disre- 
 gard : and those parts of the body which are com- 
 monly exposed to the irregularities of the season, arc 
 less susceptible of the sensations of heat and cold 
 than such as are more protected from them. The 
 sensation also of heat or of cold, when presenjt in 
 any part of the body, is not only in a great degree 
 independent of the actual temperature of that part, 
 but depends immediately on the previous state or 
 condition of the part itself with regard to sensation ; 
 for the same substance will feel hot or Cold when 
 applied to a part, according as that part previously 
 possessed the sensation of heat or cold relatively to 
 
that substance. If, for example, the hands be great- 
 iy cooled, and then plunged into fresh pump-wa- 
 ter, the water will communicate the sensation of 
 heat, while the same water, at another time, when 
 the hands were previously warmed, would impress 
 the sensation of cold. In like manner, we are often 
 not conscious of any feeling of cold in our hands or 
 feet, but if we apply them to a less exposed and 
 warmer part of the body, the feeling of cold is im- 
 pressed on that part. The same fallacy with regard 
 to the actual temperature of the body, as indicated 
 by our sensations, exists, when those sensations are 
 connected with internal causes ; for a general feel- 
 ing of cold not unfrequently extends over the body, 
 where no variation in its actual temperature, or in 
 that of the surrounding bodies, has occurred. " In 
 the beginning of an ague fit, when I was all shiver- 
 ing and under a great sense of cold," says Dr Mar- 
 tine, " my skin was two or three degrees warmer 
 than in a natural healthy state V 
 
 186. But the actual temperature of the human 
 body, is not only in a great degree independent of 
 sensation, but also of the variations in temperature 
 which the surrounding medium suffers. Governor 
 Ellis, in the year 1758, observed, that a person re- 
 maining in a medium higher in temperature than 
 that of his own body, preserved, nevertheless, his 
 natural standard heat. Dr Fordyce went successive- 
 ly into three rooms heated to 90, 11Q, and 120. 
 He staid in the first room five minutes, which gently 
 
 * Qn Thermometers* p. 150. 
 
 P3 
 
230 
 
 sweated him : in that of 1 10% the perspiration was 
 more profuse, and streams of water *ran down his 
 body : in ten minutes more, he entered the room of 
 120, and after staying there twenty minutes, the 
 mercury of the thermometer which he held in his 
 hand, and placed under his tongue, stood just at 
 10O, and his urine was of the same temperature. 
 His pulse was 145, the veins on the surface of the 
 body were much enlarged, the external circulation 
 greatly increased, and an universal redness and 
 strong sense of heat diffused itself over the whole 
 body. He afterwards remained fifteen minutes in a 
 room heated to 139, and the temperature of liis 
 body under the tongue, in his hand, and urine, did 
 not exceed 100. Sir Joseph Banks, Sir Charles 
 Blagden, and Dr Solander, wont afterwards into 
 rooms heated to 21 2\ the heat of boiling water, 
 where they remained several minutes. The feeling 
 communicated by the air was unpleasant, but easily 
 borne : and respiration was little affected, except 
 in the want of that refreshing coolness which the in- 
 spiration of cool air imparts. If they breathed on 
 the thermometer, it sank several degrees ; and eve- 
 ry expiration was cool to the nostrils, previously 
 scorched by the hot inspired air. The body, to the 
 touch of the ringers, felt cold as a corpse, and the 
 actual heat of the skin, and under the tongue, was 
 98. Sir Joseph Banks alone sweated profusely. 
 In a subsequent experiment, Sir Charles Blagden re- 
 mained eight minutes in a room heated to 260 : the 
 air felt hot, but did not give pain, and the sweating 
 was not profuse : the sensible heat of the body va- 
 ried but little. For seven minutes, the breathing 
 
231 
 
 continued perfectly natural, but anxiety and oppres- 
 sion then came on : the pulse was 144. When the 
 clothes were stripped off, the air at 212 was more 
 disagreeable for five or six minutes, until a profuse 
 perspiration breaking out, gave instant ease : the 
 breathing during this experiment was not oppressed, 
 partly because the pulse was eight beats less in a mi- 
 nute, and partly because the experiment was made 
 with an empty stomach, and the former with a full 
 one *. Dr Dobson also went into a stove heated to 
 224, and felt no oppressive sensation of heat, al- 
 though every metal about him became speedily hot : 
 a porter remained twenty minutes in the stove when 
 heated to 210, and, on leaving it, the pulse beat 
 164 in a minute, and the animal temperature rose 
 only to 101.5 f. M. Tillet has observed, that girls 
 accustomed to attend an oven, have hprne, for ten 
 minutes, a heat equal to 280 Fahrenheit J. 
 
 187. Other animals, when exposed to intense 
 heats, exhibit similar phenomena. A bitch, of a mo- 
 derate size, was put into a room heated to 220 ; in 
 ten minutes, she panted and held out her tongue, 
 but shewed no other sign of distress. After remain^ 
 ing in the room half an hour, when the heat had ri- 
 sen to 2S6, the basket in which she had been con- 
 fined, was opened, and its bottom was found wet 
 with saliva. The thermometer being placed in her 
 flank, fell to 110, only nine degrees above the na- 
 tural standard heat : and when turned into the cool 
 
 * Philosophical Transactions, 1775. f Ibid. 
 
 Mem. Acad. 1761% 
 
 P4 
 
232 
 
 air, she was as brisk and lively as ever, and appa- 
 rently not in the least injured by the heat.- That the 
 heat of these rooms, was truly indicated by the ther- 
 mometer, was proved by the effects produced on 
 inanimate matter ; for an egg, placed on a tin plate, 
 was, in twenty minutes, roasted quite hard : in 
 thirty-three minutes, a beef-steak was roasted dry, 
 and when the heated air was directed in a stream 
 through a bellows, it was roasted in about thir- 
 teen minutes *. On the other hand, the human 
 body, and that of other animals, will bear great de- 
 grees of cold, without any actual diminution of its 
 ordinary standard heat ; and thus the temperature 
 of man is the same nearly in winter as in summer 
 in an inhabitant of the frigid as in one of the torrid 
 zone. 
 
 188. Since, then, in every climate, the human 
 body, and that of the superior animals, support a 
 standard temperature, which varies but little under 
 every vicissitude of heat and cold, consistent with 
 the due performance of the animal functions, there 
 must exist in man, and in other animals, appropriate 
 organs by which this temperature can be at all 
 times sustained : but no living powers of the animal 
 system appear to be sufficient for this purpose, in- 
 dependent of the concurring aid of external agents. 
 No supposed attrition between the contiguous soft 
 parts of the animal ; no friction between the vessels 
 and the globules of the blood ; no action of the solid 
 parts upon one another ; no circumstances arising 
 
 * Philosophical Transactions, loc. cit. 
 
238 
 
 out of digestion or fermentation in the living body ; 
 no imagined combustion of phosphorus in the blood ; 
 nor liberation of the phlogistic, or any other princi- 
 ple, through the system, can be received as suffi- 
 cient to account for the uniform height and steadi- 
 ness of this temperature. As, therefore, the animal 
 system, by virtue of its own powers, is unable with- 
 in itself to produce this high degree of heat, to what 
 external agent shall we have recourse, and to what 
 organs shall we refer the production of that elevated 
 temperature, which, in all animals, we have seen to 
 have place ? 
 
 189. In our review df the temperature of the in- 
 ferior animals, it ha$ been observed, that insects, 
 worms, and fishes, which have no respiratory struc- 
 ture similar to that of the lungs ; and the amphibia, 
 the surface of whose lungs, in proportion to that of 
 the body, is comparatively small, and whose blood, 
 at each circulation, is but partially exposed to the 
 influence of the air, possess a degree of heat but 
 little above that of the medium in which they 
 live : while the mammalia class has a temperature 
 considerably higher, and birds, whose lungs bear 
 the largest proportion to their bodies, a're the warm- 
 est of all animals. The observation of these facts 
 led naturally to the opinion, that the temperature of 
 animals was immediately connected with the function 
 of the respiratory organs ; and we have endeavoured 
 to prove, that the small excess of temperature, which 
 not only the inferior animals, but which vegetables 
 also possess, is actually derived from the decompo- 
 sition of the air by these several classes of beings, 
 so long as living action continues. No explanation, 
 
234 
 
 however, of the mode in which the air contributes 
 to sustain animal heat, was attempted, till after the 
 great discovery of latent heat by Dr Black. That 
 excellent philosopher having already proved, that the 
 change effected in the air by respiration, consisted 
 in the formation of carbonic acid (116.), similar to 
 what happens in many examples of combustion, as- 
 cribed the production of animal heat to the decom- 
 position of the air in the lungs, by which its latent 
 heat was rendered sensible, in the same manner as 
 it is given out in combustion. The blood, in its 
 passage through the lungs, had, he conceived, its 
 temperature by thrs means raised ; and thus was 
 rendered capable of communicating heat to all parts 
 of the body, in the course of its circulation through 
 the system. To this it was objected by Dr Cullen, 
 that, if true, the temperature of the body ought to 
 be greatest in the lungs, and to diminish gradually 
 as the distance from the lungs increases, which is 
 not according to fact. This difficulty was removed 
 by the ingenuity of Dr Crawford, who, by a happy 
 extension of Dr Black's doctrine, maintained, that 
 the heat, liberated by the decomposition of the air 
 in the lungs, passed into the blood, and existed in 
 that fluid in the form of latent, or, what is now term- 
 ed, specific heat, in consequence of which its tempe- 
 rature was not raised ; and that this heat, by other 
 chemical changes, was given out by the blood in a 
 sensible form during its circulation. 
 
 190. In what manner, then, does the air, breath- 
 ed by the superior animals, give out its heat, to sup- 
 port that high degree of temperature above the sur- 
 rouna'ing medium, which they all possess ? We 
 
235 
 
 have seen reason to conclude, that the inspired air 
 is decomposed in the bronchial cells (114.) of the 
 lungs, and that all its oxygenous portion which dis- 
 appears, is converted (123.) into carbonic acid, by 
 carbon emitted from the exhalent surface (157.) of 
 those organs. During this gradual conversion of 
 the oxygen gas, a quantity of specific caloric (166.), 
 much greater than what is necessary to maintain the 
 elasticity of the carbonic acid that is formed, is ne- 
 cessarily set free ; and to this excess of heat, thus 
 constantly liberated in the lungs, by the decompo- 
 sition of the air, do we look as the source of that 
 superiority of temperature, above the surrounding 
 medium, which man and other animals, under every 
 vicissitude of climate, are enabled to exhibit and 
 maintain. 
 
 191. But if a quantity of caloric be thus con- 
 stantly disengaged in the lungs, it may be expected 
 that the blood, in its transmission through those or- 
 gans, should acquire a certain portion of it. To as- 
 certain this point, Dr Crawford, pursuing the dis- 
 cpveries of Drs Black and Irvine, mixed together 
 certain quantities of water, at the temperature of 
 53, with separate portions of arterial and venal 
 blood ; and then measuring the heat of the mixture, 
 at different successive periods, till coagulation took 
 place, he found, that the water containing arterial 
 blood preserved a superiority of temperature over 
 that mixed with venal blood ; and, from the re- 
 sults of several trials, he concluded, that the specific 
 heat of the arterial blood of a dog, was to that of 
 fhe venal as 114 to 100, and that of a sheep as 115 
 
236 
 
 to 100, or as IH to 10*. These results derive 
 confirmation from the experiments of Mr Coleman, 
 who, in order to discover the relative specific heat 
 of arterial and venal blood, \yhile yet retained in the 
 system, strangled a cat, and immediately opened its 
 chest, while the blood in the left ventricle was still 
 florid. He then introduced a thermometer, through 
 an opening in the pericardium on each side of the 
 heart, and it stood at 98 : in the left ventricle, the 
 temperature was only 97, and in the right ventricle 
 it was nearly 99. In fifteen minutes, however, in- 
 stead of the right ventricle possessing two degrees 
 of heat more than the left, it was found to have four 
 degrees less. Mr Astley Cooper repeated this ex- 
 periment in different ways, and found invariably, 
 that although the venal blood was superior in tern- 
 perature at first, yet before coagulation was com- 
 plete, the arterial became from three to six degrees 
 warmer t- These facts afford clear and decisive 
 proof, that the specific heat of the arterial blood ex- 
 ceeds that of the venal, and demonstrate, likewise, 
 that this excess is obtained during the passage of 
 that fluid through the lungs. 
 
 192. Admitting the lungs, then, to be the organs 
 in which, by a decomposition of tne air, the blood, 
 as it passes through them, obtains its heat, it is next 
 required to shew the sufficiency of this decomposi- 
 tion, to supply heat enough for the maintenance of 
 that superiority of temperature, which the wanner 
 
 * On Animal Heat, p. 279. 
 
 | Coleman on Suspended Respiration, p. 42. et seq. 
 
237 
 
 blooded animals possess. It is now very generally 
 allowed, that the caloric given out during the com- 
 bustion of bodies, is, like that obtained in respira- 
 tion, derived from the changes which the oxygen 
 gas of the air is made to undergo. When, there- 
 fore, equal quantities of oxygen gas are' converted 
 into carbonic acid by combustion and by respiration, 
 equal quantities of caloric may be expected to be 
 set free ; and this opinion, too, Dr Crawford at* 
 tempted by experiment to establish. He found, 
 'that 100 ounce measures of oxygen gas, changed 
 into carbonic acid by the respiration of a guinea-pig, 
 shut up in a close vessel, communicated to 31 lb, 
 7 oz. troy of water, surrounding that vessel, 17.3 
 of heat ; while the same quantity of oxygen gas, 
 converted into the same acid by the burning of 
 charcoal, communicated to a like quantity of water 
 19.3 of heat; and he concluded, therefore, that, 
 somewhat more heat was produced by the combus- 
 tion of charcoal, than by the respiration of an ani- 
 mal *. M. Lavoisier, likewise, made experiments 
 of a similar nature, but he substituted the melting of 
 ice, as a measure of the comparative heat given out 
 by respiration and combustion, for the raising of the 
 temperature of water ; and the conclusion which he 
 drew, when equal quantities of oxygen gas were 
 changed into carbonic acid by the respiration of a 
 guinea-pig and the combustion of charcoal, made 
 the quantity of heat produced by the animal, exceed 
 that by the combustion of charcoal in the propor- 
 
 * On Animal Heat, p. 351. 
 
238 
 
 rion of 13 to 10.3*. Although, therefore, the re- 
 sults, obtained by these philosophers, do not com- 
 pletely coincide, yet in experiments of so much dif- 
 ficulty and delicacy, the small difference that ex- 
 ists is not sufficient to invalidate the general con- 
 clusion, That, " when equal quantities of oxygen 
 gas are converted into carbonic acid by animal re- 
 spiration, and by the combustion of carbon, nearly 
 equal quantities of caloric are set free f." 
 
 193. But farther, as the quantity of heat produ- 
 ced, when a given bulk of carbonic acid is formed by 
 the combustion of charcoal, has been demonstrated 
 by Lavoisier J ; and as the quantity of air usually 
 respired (85.) has been ascertained, it is evident, 
 that the quantity of heat liberated in the lungs, in 
 any given time, could also be found out, were we 
 able to estimate the quantity of carbonic acid contain- 
 ed in air which has beon once breathed. Thus, says 
 Dr Menzies, if the bulk of air commonly inspired be 
 estimated at 40 cubic inches, and the number of 
 respirations at 1 8 in a minute, the volume of air in- 
 spired every minute will be 72O cubic inches ; of 
 
 which ~, or 158.4 cubic inches, consist of oxygen 
 
 gas, the only part of the air changed by respiration. 
 But of the air thus inspired, only, or two cubic 
 inches, are changed in each respiration, which gives 
 for the amount of carbonic acid, formed every mi- 
 
 * Mem. de 1'Acad. dcs Scicn. 1783. 
 
 f Crawford on Animal Heat, p. 353. 
 
 J Kcrr's Lavoisier's Elements of Chemistry, 4th edit, p. 113, 
 
239 
 
 nute, 36 cubic inches, or 51840 cubic inches in the 
 space of a day, equal in weight to 3.9697 lb. troy. 
 But for every pound of carbonic acid formed by 
 the combustion of charcoal, a quantity of heat is 
 given out, according to Lavoisier, sufficient to melt 
 27.02024 lb. of ice : therefore, 3.9697 lb. of this 
 acid, produced daily in respiration, will furnish heat 
 enough to melt 107.2 lb. of ice ; for 27.02024 x 
 3.9697 107.2622. Of this quantity of heat, 
 however, a portion, adds Dr Menzies, is carried off 
 in the form of sensible heat with the air that is ex- 
 pired, and another portion is rendered latent by 
 combining with the vapour that issues from the 
 lungs. These two portions, from experiment, be 
 calculates to amount to heat sufficient to melt 
 32.9833 lb. of ice ; which sum, subtracted from 
 107.2622, leaves a remainder of 74.2789 lb. as the* 
 quantity of ice capable of being dissolved by the 
 heat daily set free, by the decomposition of the air, 
 in the lungs of an ordinary man *. How admirably 
 the blood in the lungs is disposed to receive this 
 heat, the vast extent of the cellular surface (114.) 
 of those organs, and the decomposition of the air, 
 and consequent extrication of its heat, going on 
 over every point of that surface, furnish abundant 
 proof. 
 
 194. The inferences drawn from the foregoing 
 experiments and calculations appear to be just, if 
 the premises on which they are founded be in all 
 respects correct : but neither the quantity of air ta- 
 
 Menzics on Respiration, p. 50. et seq-, 
 
210 
 
 ken in at each inspiration, nor the portion of it that 
 is changed, nor the number of respirations made in 
 a minute, are yet finally agreed on ; and, indeed, 
 from various causes, they must be subject always to 
 much variation. Greater accuracy, also, in the ex- 
 periments of Crawford and Lavoisier, as to the com- 
 parative quantities of heat, produced by respiration 
 and by combustion, is desirable ; and in those of 
 Dr Menzies, the abstraction of heat by the air and 
 vapour expelled from the lungs, appears to be much 
 overrated. In their present state, therefore, these 
 inferences must be considered only as approximations 
 to the truth ; but the particulars on which they rest, 
 seem to embrace all the necessary considerations, if 
 in every instance they were accurately determined. 
 At any rate, it may, we think, be safely held, that in 
 the lungs of animals, sufficient air is decomposed to 
 furnish a large quantity of heat ; and the fact, that 
 the temperature of all animals is in proportion to 
 the relative capacity which their respiratory organs 
 bear to the body, and to the quantity of air which 
 they breathe in a given time, appears to justify the 
 conclusion, that animal temperature is derived chief- 
 ly, if riot entirely, from this source alone. 
 
 195. But we have seen, that the oxygen gas of 
 the air is converted into carbonic acid by the human 
 skin (147.) and intestines, as well as by the lungs ; 
 and in many of the inferior animals, their superiori- 
 ty of temperature is sustained by the changes indu- 
 ced on the air through the agency of the skin alone. 
 As, therefore, the skin, by the same powers, changes 
 the air after the same manner as the Jungs, it is rea- 
 sonable to infer, that phenomena similar in kind. 
 
241 
 
 result therefrom ; and that as insects and worms, by 
 means of their skins, preserve a superiority of tem- 
 perature over the medium in which they live, so 
 likewise those surfaces of the human body which 
 communicate directly with the air, derive from this 
 source a degree of heat also, by which means they 
 become, in some measure, auxiliary to the lungs *. 
 196. Besides this production of heat, as the prin- 
 cipal phenomenon arising from the change which 
 the air suffers in respiration, it has been supposed, 
 that water also is formed by an union of a portion of 
 the inspired oxygen gas with hydrogen residing in the 
 blood. But Mr Davy remarks, that there are no 
 reasons for supposing any residual atmospheric oxy- 
 gen to be immediately combined with fixed or na- 
 scent hydrogen, or hydro-carbonate, in the venal 
 blood at 98 ; and, consequently, none for believing 
 that water is immediately formed in respiration f ; 
 and Dr Bostock observes, that the discharge of hy- 
 drogen from the blood has been admitted without 
 
 * This power in the skin to produce heat, naturally occurre4 
 to trie author from the view which he had taken of the effects 
 produced on the air by that organ. From information which he 
 has since obtained, he learns, that the same opinion was, many" 
 years ago, taught by Mr Allen in the lectures on physiology 
 which he delivered in Edinburgh j and a similar doctrine has been 
 pointed out to him in the " System of Chemical Knowledge" by 
 M. Fourcroy, where, however, it is obscured by the language of 
 that imperfect analogy which chemical physiologists never fail to 
 institute between respiration and combustion. 
 
 f Researches, p. 423. 
 
 Q 
 
242 
 
 sufficient evidence*, and conceives, with Mr Davy, 
 that the pulmonary exhalation proceeds from a se- 
 cretion carried on by the vessels of the lungs. More- 
 over, we have endeavoured to shew, that no oxygen 
 gas can enter into the blood-vessels, but that the 
 whole of it that disappears is decomposed in the 
 lungs, and is actually expended in forming the car- 
 bonic acid (123.) which is there produced : so that 
 none remains for the formation of water by uniting 
 with the supposed hydrogen of the blood. Even if 
 oxygen gas did enter into the blood, and hydrogen 
 also existed in that fluid, we have no proof that their 
 affinity for each other is sufficient to form water ; 
 for, out of the body, actual ignition, or a great de- 
 gree of compression, is required to reduce them to 
 a fluid form. 
 
 197. If we consider the vast extent of the cellular 
 surface (1 14.) of the lungs, and the similarity which 
 in its structure and functions (157.) it bears to that 
 of the external surface of the body, we are naturally 
 led to the belief, that the pulmonary excretion, like 
 every other, and especially like that from the skin, 
 is carried on by an appropriate structure, and accord- 
 ing to the ordinary laws of the exhalent system. 
 That the great bulk of fluid exhaled from the lungs 
 is derived from this source, none will venture to deny. 
 Why then should we imagine another mode of produc- 
 tion for this fluid, which is so totally at variance with 
 it? At least, the insufficiency of the ordinary function to 
 
 * On Respiration, p. 129. 
 
243 
 
 furnish all the water exhaled, ought first to be clear- 
 ly proved. The daily loss of weight which the whole 
 body experiences by the escape of perspirable matter, 
 is estimated by Haller at about 60 oz. in the warmer 
 climates, and from 56 to 30 in more temperate 
 climes *. Of this latter quantity, Dr Hales calcu- 
 lated the loss by the pulmonary exhalation to be 
 about 6 oz. f ; kvt the more accurate trials of Lavoi- 
 sier and Seguin make it to amount to about one-third 
 of the whole. The mean loss sustained by perspira- 
 tion was, according to them, 2 Ib. 13 oz., of which 
 the pulmonary discharge was 15 oz. and the cuta- 
 neous 1 Ib. 14 oz. J. When, therefore, we bear in 
 mind, that the surface of the bronchial cells is ten 
 times greater (114.) than that of the external skin, 
 and is, like it, duly furnished with exhalent vessels, 
 we are so far from seeing the necessity of resorting 
 to the supposition that water is formed chemically 
 in the lungs, that we can more readily imagine the 
 pulmonary exhalation to be rated in these estimates 
 below \vhat the extent of its exhalent surface may 
 be considered able to supply. 
 
 198. Rejecting, therefore, the opinirn, that any 
 water is formed by the chemical union of oxygen 
 and hydrogen in the lungs or blood-vessels, it is not 
 within our plan (which professes to treat only of the 
 phenomena which arise out of the changes the air 
 suffers) to inquire farther into the laws of its pro- 
 
 * Elements Physio!, torn. v. p. 58, 
 i Vegetable Statics, vol. i. p, 10, 
 Mem. de PA cad. 1790. 
 
244 
 
 auction. One consequence, however, which seems 
 to follow from considering the aqueous vapour to be 
 wholly emitted by the exhalents of the lungs, we 
 cannot omit to notice ; for, on this supposition, there 
 seems no reason to think that any part of the caloric 
 liberated in the lungs by the decomposition of the 
 air, is, as Dr Menzies imagined, employed to raise 
 that fluid/ into vapour. The power of the skin, 
 \vhen its heat is many degrees below that of the 
 lungs, is sufficient to emit its excretion in a vapori- 
 fic form, nor is there any reason why the exhalents 
 of the lungs should not, without the aid of a fresh 
 portion of caloric, emit vapour also. In fact, both 
 the external -and internal surfaces of the body always 
 do so ; and it is only when the exhalation is exces- 
 sive that the insensible perspiration is condensed in- 
 to a fluid form. 
 
 HERE; for the present, the author closes his in- 
 quiry. After the review which has now been taken 
 of the changes induced by living vegetables arid ani- 
 mals on the air, it Was originally his intention to 
 have entered on 'an investigation of the effects 
 which these changes produce upon them. Had he 
 pursued the subject, it was his design, by a strict ap- 
 plication of the preceding facts, to have attempted the 
 illustration of certain phenomena which occur in ger- 
 mination and vegetation ; and likewise in the evolu- 
 
245 
 
 tion, continuance, and suspension of living action in 
 the inferior classes of animals. The phenomena of 
 incubation, and the evolution of viviparous animals, 
 would next have engaged his attention ; and these 
 would have been succeeded by an endeavour to ex- 
 plain the phenomena of respiration, as connected 
 with the appearances and properties of the blood, 
 and the distribution and maintenance of animal heat. 
 A short view of some pathological states of the sys- 
 tem, arising immediately out of the preceding discus- 
 sions, would then have concluded the whole. 
 
 This enumeration, it is obvious, comprehends 
 some of the most curious and important problems 
 in physiology j and many of them are of such a na- 
 ture as not to admit, in the present state of science, 
 of completely satisfactory proof. All, therefore, 
 that could be hoped for, would be a probable ex- 
 planation, essentially different, indeed, from all the 
 theories which have been hitherto proposed, but 
 perfectly consistent with the view already ta- 
 ken, and supported and illustrated by the aid of 
 such additional facts and experiments as a farther 
 prosecution of the subject would necessarily intro- 
 duce. 
 
 As, however, the whole of the author's reason- 
 ing would be founded on the principles which, in 
 this publication, he has attempted to establish, it is 
 evident that the truth or falsity of the principles 
 themselves, ought first to be ascertained. With 
 this view, he submits them to the judgment of the 
 public in their present form. If they shall be fa- 
 vourably received, and the author be in consequence 
 encouraged to proceed, he will endeavour to exe- 
 
246 
 
 cute the remaining part of his plan in the best manner 
 that he is able. If, on the contrary, they shall be 
 shewn to be fallacious, he will at once desist from 
 the farther prosecution of the inquiry : and will thus 
 be spared the mortification of adding to the perplexi- 
 ty o a subject which he was unable to explain. 
 
 TINIS. 
 
 Abernethy & Walker, Printer*, 
 Old Bank Close.