EH ^1 LESSONS IN CHEMICAL Pfi'i'ti HEMICAL HILOSOPHY. BY JOHN HOWARD APPLETON, A.M., // Professor of Chemistry in Brown University, AUTHOR OF BEGINNER'S HANDBOOK OF CHEMISTRY," " THE YOUNG CHEMIST," "QUALITATIVE CHEMICAL ANALYSIS," "QUANTI- TATIVE CHEMICAL ANALYSIS." FOURTH EDITION. SILVER, BURDETT & CO., PUBLISHERS, NEW YORK . . . BOSTON . . . CHICAGO. 1897. BIOLOGY LIBRARY - PROFESSOR APPLETOfl'S WORKS ON CHEMISTRY, ' I.'! THE BEGINNER'S HANDBOOK OF CHEMISTRY: Price $1.00. This is ah introduction to ..he study of Chemistry, suitable for general readers. It treats chiefly the npv-m.etals, these being generally found to furnish the best material for an ele- ,'rAeiJtalry' ):our ; ) as the unit of compari- son of velocity and calling it i, and substituting for m and m' the ratios of their respective molecular weights, i and 16, we obtain, v- 4. CERTAIN GENERAL LAWS OF MATTER. 8 1 This result means that when the velocity of the oxygen molecule is called i, the velocity of the hydrogen molecule is 4. Now this is in fact the rate of motion of the molecules as proved by Graham. And the result obtained from the course of reasoning here pursued has involved but one supposition; namely, that the two equal balloons, or in fact any two equal volumes of the gases, under like conditions of temperature and pressure, contain equal numbers of molecules. The correct result attained contributes materially to place the hypothe- sis of Avogadro upon a mathematical foundation, CHAPTER VIII. CERTAIN FORMS OF ENERGY CLOSELY CONNECTED WITH CHEMICAL CHANGES. HEAT AND ELECTRICITY. IT has been remarked that " a chemical operation pre- sents two aspects to the investigator ; it involves a change in the form or distribution of matter and a change in the form or distribution of energy." Two forms of energy are especially involved in chem- ical changes : they are heat and electricity. These subjects belong in a certain sense to the de- partment of physics, yet by their sources, uses, and effects, they are so closely connected with chemistry that they admit of brief discussion here. HEAT. The invisible agency by whose transfer sensations of warmth and cold are produced, is itself called heat. Two kinds of heat may be distinguished : 1. Absorbed heat is that which resides in a hot body, often remaining in it for a considerable time. It is transferred to another body, mainly by contact. 2. Radiant heat is heat in the act of passing with great velocity (about 190,000 miles per second) through space, whether vacuous or otherwise ; radiant heat may 82 CERTAIN FORMS OF ENERGY. 83 be either dark heat or luminous heat (the latter form being commonly known as light). Theories of the Nature of Heat. The dynamical the- ory of heat, and that now generally accepted, supposes that all matter as well as all space is pervaded by an extremely delicate and elastic medium called the ether. This theory regards (i) absorbed heat as a vibration of the molecules of matter ; (2) radiant heat as an undula- tory movement in the ether. Heat as Motion. That heat is not a form of matter appears to be shown by a variety of facts. For exam- ple, neither does addition of heat to a body increase its mass, nor does loss of heat by a body diminish that mass. The quantity of heat in a given system is capable of indefinite increase ; again, it can be destroyed as material substances cannot. That heat is a form of energy in other words, of motion appears to be suggested by a multitude of phenomena. Of these a few may be mentioned. FIRST. The general quantitative relations between heat and motion are very simple. A given amount of mechanical motion may be changed into a certain definite amount of heat and no more ; and on the other hand, a given amount of heat is capable of generating only a certain fixed amount of motion. Some of the contrivances ordinarily used to effect such interchange are imperfect and involve large losses during the transformations; these, how- ever, are not losses of the total amount of energy, but only of that partic- ular form of it which the appliance or machine may be intended to afford. Hence the strength of the argument is not impaired. SECOND. The sources of heat are well explained by this view. They are friction, percussion, chemical action, the sun. (The internal heat of the earth need not be discussed here.) 8 4 CERTAIN FORMS OF ENERGY. Friction and percussion involve a diminution of mass-motion or its entire quenching. But these have not been destroyed ; they appear to have been merely transformed into minute molecular motions. Chemical action evolves heat when certain substances combine. The true source of this heat appears to be that molecular percussion or atomic bombardment which is sustained when a myriad of atoms of one kind clash into combination with a myriad of another kind. The sun gives out an enormous amount of heat. Only a minute fractional part of it is received by this earth. (But this is a large amount as compared with man's ordinary means of producing energy.) But this tremendous and continual transfer does not appear to diminish the weight of the giver nor to increase that of the receiver. Again, it appears more rational to be- lieve that the incredible velocity of radiant heat is associated with a progressive flow of energy rather than' with an actual trans- portation of matter. THIRD. The effects of heat are best explained by this view. The principal of these effects are the following.: (a) tem- perature, () expansion and contraction, (c) change of state, ( O ^3 n3 O u SiS g S *1 -5 o "S o THE ATTRACTIONS OF MOLECULES. 107 * .a JS M -2 v 6 S ^ FOURTH SYSTEM. RHOMBIC OR ORTHORHOMBIC. Three rectangular axes, no two equal. ALLIED FORMS. Right rectangular prism. Right rhombic prism. Right rectangular octahedron. Right rhombic octahe- dron. S 37 " " sulphur dioxide gas (SO 2 ) " 68,861 " " ammonia gas (NH 3 ) " 1,049,600 " 136 THE ATTRACTIONS OF MOLECULES. The very large amounts in several of these cases are believed to be due to the definite chemical unio;i of the gases with the water to form new compounds. It is a fact worthy of mention that molten silver has the power of draw- ing oxygen from the air and dissolving it in a quantity equal to twenty FIG. 94. Apparatus for illustrating diffusion of gases. If a heavier gas is placed in the lower flask and a lighter gas is placed in the upper flask, and the stop-cocks are opened, it is found experimentally that the lighter gas diffuses rapidly downward into the other, and that the heavier gas diffuses upward (although more slowly) into the lighter. times its own volume. When the silver solidifies, this gas is violently expelled. (The same principle is manifested by water; upon freezing, it expels the oxygen and nitrogen it previously dissolved from the air.) (F) ADHESION BETWEEN GASES AND GASES. The extraordinary tendency of gases to intermingle and interdiffuse has already been discussed under the THE ATTRACTIONS OF MOLECULES. 137 title of diffusion of gases. This tendency is so strong that it overcomes the greatest differences of specific gravity. These phenomena are not mainly due to adhesion, how- ever, though there are grounds for believing that there is such a thing as gaseous adhesion. Thus Regnault has shown that when a liquid evaporates in the air, more vapor rises than when it evaporates into the same volume of vacuous space. The tendency of gases to intermingle seems to be mainly a development of their tension or expansive power. This phenomenon is due to that motion within the mass which the molecules of all kinds of matter even the most rigid possess. But the molecules of the gaseous form of matter are almost uninfluenced by cohesion. Hence they manifest this intermolecular motion to the most striking degree. And so when gases themselves are compared, it can be proved that the molecules of the lightest ones move with the greatest rapidity. Of .course the ample spaces between the molecules of a gas offer great opportunities for the entrance of the molecules of another gas. The Terrestrial Atmosphere. The atmosphere of our globe affords a splendid example of gaseous diffusion constantly at work on a large scale. i. The atmospheric air consists mainly of a mixture of oxygen gas and nitrogen gas, in the following propor- tions : COMPOSITION OF ATMOSPHERIC AIR. By volume. By weight. Oxygen 20.9 per cent. 23.1 per cent. Nitrogen 79.1 76.9 " 100 100 FIG. 95. Balance for showing that certain gases are heavier than the atmosphere. The one jar contains atmospheric air. When a heavier gas is poured into the other jar, the needle of the balance is boldly deflected. THE ATTRACTIONS OF MOLECULES. 139 Now any given measure of oxygen gas is sixteen times as heavy as the same measure of the standard gas, hydro- gen ; but nitrogen gas is only fourteen times as heavy as hydrogen. Yet in our atmosphere the heavier oxy- gen does not settle out, but remains thor- oughly intermingled with the nitrogen. 2. The respiration of living animals and the burning of all our chief fuels are constantly casting into the atmos- phere immense quantities of a heavy gas, carbon dioxide (CO 2 ). This gas is twenty-two times as heavy as the stand- ard gas, hydrogen. Of course, therefore, it is much heavier than the oxygen or the nitrogen of the atmospheric air ; it does not settle out from the air, how- ever, but promptly intermingles with it and remains intermingled. NOTE I. On the density of atmospheric air. The air contains minute amounts of a multitude of gases, but oxygen and nitrogen so largely predominate that only these need be taken into the account here. The density of the air is somewhere between the densities, 1 6 and 14, of its two chief constituents : it is about 14.4. FIG. 96. Reg- nault's method of suspending, from the balance-pan, a globe containing a gas to be weighed. A globe of similar volume is also suspended from the other pan. i volume of oxygen gas, weighing 1 6 units .... 1 6. units. 4 volumes of nitrogen gas, each weighing 14 units. . 56. " 5 volumes of mixture (air) will weigh 72. " I volume of air will weigh 14.4 " NOTE II. On the density of carbon dioxide gas (CO 2 ). By actual weighing, in comparison with an equal volume of the standard gas, hydrogen, this gas has been found to have the density 22; i.e. to weigh, bulk for bulk, 22 times as much as hydrogen. The density may be computed from the molecular weight as follows : I4O THE ATTRACTIONS OF MOLECULES. Formula of a Molecule of Carbon Dioxide Gas Weight of one atom of carbon ...... 12 microcriths. " " two atoms of oxygen (i6x 2) ... 32 " " " one molecule of carbon dioxide . . 44 " " " one molecule of hydrogen, H 2 (1X2) 2 " Hence a molecule of carbon dioxide weighs twenty-two times as much as a molecule of hydrogen. But all gaseous molecules have the same size; hence, any volume of carbon dioxide weighs twenty-two times as much as the same volume of hydrogen. NOTE III. Of course, in weighing atmospheric air and other gases, pressure and temperature must be considered. The pressure must be measured by some form of barometer. The temperature must be measured by some form of thermometer. CHAPTER XIII. THE ATTRACTION OF ATOMS. CHEMICAL AFFINITY. CHEMICAL affinity is an agency which acts at in- sensibly small distances, and tends to produce combina- tions of certain atoms and molecules of matter into groups of a precisely determinate kind. The characteristics of this agency cannot be described in a few words. To it are referred a multitude of phenomena, displaying under different circumstances the greatest variety of action. Such differences are for example : As to the original quantity and intensity of the activity itself. As to the conditions under which its active powers are displayed. As to the methods by which it works. As to the sphere of activity extremely narrow in a certain sense and extremely wide in another. As to the results accomplished by it. The Conditions favoring Chemical Change. I. This force manifests its chief activity between atoms or mole- cules of different kinds. Thus, an atom of hydrogen has affinity for another atom of hydrogen, and the two may unite to form a 141 142 THE ATTRACTION OF ATOMS. molecule of hydrogen, expressible by H H, also written H2. Again, an atom of chlorine has affinity for another atom of chlorine, and these two may unite to form a molecule of chlorine expressible by the formula Cl Cl, or C1 2 . But when a molecule of hydrogen is brought in contact with a molecule of chlorine, the two generally suffer decomposition, so that a rearrangement may take place and two new molecules of hydrochloric acid (HC1) may be produced. This chemical change may be ex- pressed by the following equation : H 2 + Cl, 2HC1 One molecule of One molecule of Two molecules of Hydrogen, Chlorine, Hydrochloric acid, 2 7 1 73 parts by weight. parts by weight. parts by weight. ~~73 73 Evidently the atom of chlorine has more affinity for an atom of hydrogen than for another atom of chlorine. And an atom of hydrogen has more affinity for an atom of chlorine than for another atom of hydrogen. 2. It is manifested between different substances with very different, though definite, degrees of force. Thus the metal gold and the metal iron oxidize (that is, com- bine with oxygen) with different degrees of ease ; but it is always the iron that oxidizes the easier. 3. Certain physical conditions are of great importance in connection with chemical action. When physical conditions are favorable,* chemical action proceeds with great vigor ; when they are unfavorable, the same pro- cesses sometimes fail to advance at all, or they may be even reversed. Under unfavorable conditions chemical affinity appears either not to exist or to be dormant. THE ATTRACTION OF ATOMS. 143 The following are some of the physical conditions which determine chemical changes changes that may have as their prominent features either the building up or the breaking down of molecules : () The Liquid Condition. Some substances that chemically unite when mixed as solutions, manifest no affinity when they are mingled in the solid form. Thus, solid tartaric acid and solid hydro-sodic carbonate when mingled manifest no change. When water is added, however, each* solid dissolves, and a chemical change at once ensues, hydro-sodic tartrate, carbon dioxide, and water being formed. The chemical change may be expressed as follows : + 2 HNaCO , One molecule of Two molecules of Tartaric acid, Hydro-sodium carbonate, 150 168 parts by weight. parts by weight. 3i8 2C0 2 + Two molecules of (H 2 Na 2 )0 4 -(C 4 H 2 2 ) One molecule of + 2H 2 Two molecules of Carbon dioxide, Hydro-sodium tartrate, Water, 88 194 36 parts by weight. parts by weight. parts by weight. 318 The equation indicates that water is actually formed by the operation; it appears evident, therefore, that the water which acted as the solvent was not demanded in the building up of the molecules produced, but did, in fact, act as a favoring physical agent. It appears to be proved, however, that certain solid bodies, finely pul- verized, thoroughly mixed, and then subjected to great pressure, produce new compounds as the result of the pressure (and not of the heat attend- ant). The quantities of the compounds changed appear to increase with the duration of the pressure and its amount, as well as the fineness and thoroughness of intermingling of the powders. Thus, in a certain experiment, mixtures of dry, pure precipitated barium sulphate and sodium carbonate were subjected to a pressure of six thousand atmospheres under varying conditions of temperature and duration of the pressure. Afterward the product was tested. After a single compression the amount of barium carbonate produced was about one per cent; the solid 144 THE ATTRACTION OF ATOMS. block produced was pulverized and compressed again, when five per cent of barium carbonate was produced; further treatment brought it up to eleven per cent. It has been concluded that 1. A sort of diffusion takes place in solid bodies. 2. Matter assumes under pressure a condition relative to the volume it is obliged to occupy. 3. For the solid state, as for the gaseous, there is a critical tempera- ture above or below which changes by simple pressure are no longer possible. () Heat. Many substances, when practically in contact with each other, do not combine chemically unless the whole or a portion of the mass is raised to some definite point of temperature. When this point is reached, union at once commences. The process of combustion of ordinary fuels affords an appropriate illus- tration. If a portion of a mass of coal is heated in the air to the point at which union with oxygen takes place, the phenomena of combustion (a form of chemical union) are witnessed. The chemical change initiated may be expressed in part as follows : C0 2 One molecule of Carbon dioxide, 44 parts by weight. 44 44 It is an interesting fact that generally the combustion of the first por- tions of the coal evolves, by the act of chemical union, sufficient heat to raise yet other portions to the igniting point. This process, repeated, ena- bles the operation to proceed from portion to portion so long as the supply of carbon and oxygen are kept up unless, indeed, some unfavorable physical condition is allowed to supervene. There are numerous other examples known, in which chemical action is stimulated by an amount of heat insufficient to produce light. In fact, addition of heat is the method oftenest used for developing or arousing chemical affinity. Thermolysis and Dissociation. Another, and at first seemingly inconsistent chemical effect of heat, ought to be mentioned here. It has c + 2 One atom of One molecule of Carbon, Oxygen, 12 32 parts by weight. parts by weight. THE ATTRACTION OF ATOMS. 145 already been pointed out that addition of heat expands material bodies, and even changes solids and liquids to the gaseous form. (See p. 43.) These effects are believed to be essentially associated with a motion of the particles of the body, such that the molecules are moved farther and far- ther apart, and even beyond the range of influence of those cohesive forces FIG. 97. Henri St. Clair Deville, distinguished French chemist, noted for his discoveries in the chemistry of high temperatures, dissociation, for example. that bind them into solid and liquid masses. It would be quite consistent with this view if still greater addition of heat were found to be sufficient to drive even atoms apart from each other, and so to place them beyond the minute distances within which the force of chemical affinity is exerted. This would result in a decomposition of compound molecules and a lessen- 146 THE ATTRACTION OF ATOMS. ing of the number of atoms capable of existing together in elementary molecules. Now the experiments of Deville and others fully confirm these sugges- tions. It is, in fact, proved that certain substances, as water, for example, may be decomposed into their elements by influence of high temperature alone. In this, and some similar cases, the elements may reunite when the temperature of the mixture falls slightly. This kind of temporary decom- position is called dissociation. It may be added that light and electricity, as well as heat, are in some cases capable of accomplishing it. In another class of cases, of which ammonia gas (NH 3 ) may serve as an example, the molecule is permanently broken up; that is, its elementary substances do not, by fall of temperature, rejoin to produce the original compound. In such cases the operation is called thermolysis. It is also observed that certain elementary substances, as sulphur, for example, manifest a gradual lessening of their relative vapor densities as they are raised to higher and higher temperatures. This lessening of vapor density is accepted as an indication that the molecules contain fewer and fewer atoms; that is, undergo dissociation. The methods of Victor Meyer and others have directed attention to this subject. Heat often produces a modification of the relative chemical attractions of bodies. Thus, at ordinary temperatures, sulphuric acid is capable of displacing boric acid from its salts in solutions. At high temperatures the red heat, for example the chemical affinities are reversed : boric acid displaces sulphuric acid. In a few cases chemical decomposition is producible by mechanical means, as, for example, in certain explosive compounds; but it is proba- ble that the mechanical is not always the immediate cause. In the familiar cases where mechanical percussion produces decomposition of certain explosives, evidently the heat generated by the percussion is the true (c) Light. This agent, as usually produced by luminous bodies, is by no means a homogeneous one; the prism shows it to be divisible into thousands of kinds of energy, characterized by greater or less differences. The white light, as emitted by most of its sources, has at least three classes of rays, luminous rays of various colors, non-luminous chemical rays, non- luminous heat rays. The non-luminous chemical rays, called also actinic rays, have a specific power of determining the chemical union of certain elements and the chemical decomposition of certain compounds. Thus chlorine gas and hydrogen gas, when mixed in a dark room, do THE ATTRACTION OF ATOMS. 147 not readily unite; when such a mixture is exposed to sunlight, almost instantaneous combination ensues. The decomposing influence of certain rays of light is displayed in the photographic print, the substance decomposed being argentic chloride. (dQ Electricity. The influence of electricity in connection with chemical action is manifested in at least four different forms. FIRST. Operations coming under the head of electrolysis. Electricity of low tension, such as that produced by the galvanic bat- tery, is capable of most important influences on chemical compounds. In FIG. 98. Apparatus showing how an electric current may precipitate a metal from its solution. processes of electro-plating with copper, nickel, silver, gold, and other metals, it sets in motion an invisible current by which atoms of metal are driven away from the metallic plate called the anode, then into the mole- cules of acid or metallic salt dissolved in the plating-bath, and thence upon the surface of the object to be plated, called the cathode. The deposition of the metal is merely incidental; the molecular transfer is the important feature. Metals may be dissolved by this method if desired. Again, non- metals may be made to combine, or, if combined, may be separated in like fashion by the current. SECOND. When a current, in the form of an electric arc, passes through compounds or through mixtures of elementary substances, chemical changes often occur. One of the most marked illustrations of this kind of influ- 148 THE ATTRACTION OF ATOMS. ence is in the direct union of carbon and hydrogen whereby the substance known as acetylene (C 2 H 2 ) is formed. One of the most important features of interest in connection with this operation is the fact that acetylene rep- resents a starting-point for the synthesis, or building up, of organic com- pounds directly from their elements. THIRD. Another form of action is by the influence of the electric spark from a Ruhmkorff coil, a Holtz machine, or similar appliance. In this way distinct chemical action is stimulated over a limited field. The field, how- ever, may be widened by continuance of the electric discharge. FIG. 99. Apparatus for decomposing water into its components, hydrogen and oxygen, by means of a galvanic current generated by a Bunsen battery. Such an electric discharge, in the form of sparks flowing from platinum terminals through dry atmospheric air, gives rise to a direct union of the oxygen and nitrogen. Brown fumes of N 2 O 4 or NO 2 are thus formed. These with water may form nitric acid (HNO 3 ). Probably lightning discharges form nitric acid, in this way, and thus contribute to the available nitrogen of the soil. FOURTH. The silent electric discharge produces certain marked effects, of which the most noteworthy is the change of ordinary oxygen into ozone. In all these cases the action may be twofold. There is the true electric influence, and, especially in cases of the second and third methods already referred to, there is the additional influence of the heat connected with the arc or with the luminous discharge. H THE ATTRACTION OF ATOMS. 0) Vital Processes of the Higher and Lower Living Beings. The vital powers of the higher orders of animal and -vegetable beings have most marked influence upon chemical action. Thus vast numbers of com- pounds are recognized as existing in living animals and plants that have not yet been produced without the intervention of vital force. A few crys- FlG. 101. Ruhmkorff, the celebrated manufacturer of electric instruments, and inventor or the Ruhmkorff coil. tallizable substances, ordinarily the products of living organisms, have lately been produced by circuitous chemical operations without intervention of life. Doubtless others will be in future. Certain processes of acetic, butyric, and other fermentations, purely chemical in their nature, have been shown to be due to the presence and action of microbes, and not to go on in their absence. THE ATTRACTION OF ATOMS. I$I Organic and Inorganic Compounds. Chemists long ago recog- nized certain differences between the substances found in distinctly animal and vegetable matters, on the one hand, and the substances found in min- eral matters, on the other between those things which constitute organ- isms like animals and plants, as opposed to non-living substances like clay, iron-rust, alum, saltpetre, etc. Animal matters and vegetable matters are the products of bodies pos- sessing organs. Organs are parts having specific functions. Thus the stomach is an organ possessing the function of digestion, and the lungs are FIG. 102. The Bunsen battery, F, energizes the Ruhmkorff coil, E, affording a series of electric sparks in the flask A . The flask contains a mixture of nitrogen and oxygen in a very dry condition. The sparks lead the two gases to combine. organs possessing the function of respiration. Again, the leaves, the flowers, the seeds, the roots, of plants, are separate organs, and they possess special and very different functions of the living vegetable to which they belong. Accordingly, substances derived from vegetables and animals are called organic. Non-living objects, as rocks and other mineral and earthy substances, do not possess organs, and they have long been called inorganic. This division of matters into organic and inorganic was formerly thought an essential one ; it is not now considered so. It is now known that the chemical changes of living animals and plants are governed by the same laws 3& those prevailing in the changes of rocks and other lifeless forms of matter. 152 THE ATTRACTION OF ATOMS. Grounds for this Division. Chemistry is still, however, commonly divided into the two great departments, inorganic chemistry and organic chemistry; but this division is recognized as a matter of convenience mainly. FIG. 103. Yeast plant, illustrating the formation of additional cells by fission. Three reasons, which may be mentioned, why the distinction is still maintained, are : FIRST. The number of organic compounds is very great. FIG. 104. Granules of wheat starch, as FIG. 105. Granules of potato starch, seen under the microscope, showing cellular as seen under the microscope, showing cellular structure. structure. SECOND. These compounds perform varied and important offices in connection with human beings in their growth and nourishment in health, and in their treatment in illness. THIRD. The processes of analysis and the methods of investigation in THE ATTRACTION OF ATOMS. 153 organic compounds are slightly different, as a whole, from those that serve for the study of inorganic. Definitions of Organic Chemistry. The inorganic and the or- ganic worlds are, however, so closely allied in some respects, and certain of the substances of the one have such close and natural affiliations with those of the other, that it is often found difficult to determine where shall be placed the line of demarcation between these two great natural groups. In fact, chemists have not found the definition incidentally introduced in a preceding paragraph sufficiently distinct. To make it more so, organic chemistry has been sometimes called the chemistry of the carbon compounds. It has sometimes been called the chemistry of the FIG. 106. Granules of corn starch as seen under the microscope, showing cellular structure. hydrocarbons. Again, the following still more rigid and scientific state- ment is often employed : organic chemistry includes those compounds in which the atoms of carbon are directly united either with other atoms of carbon, or with atoms of hydrogen, or with atoms of nitrogen^ Two Classes of Organic Compounds. There is one distinction between the classes of organic compounds themselves that ought not to be omitted here. The members of the organic family differ very much in their properties, according as they are crystalline or cellular. Crystalline organic compounds, of which cane sugar may be taken as a familiar and suitable example, are numerous. These compounds are closely allied in some respects to inorganic compounds. They do not seem to have so 1 From the Author's work, " Beginner's Handbook of Chemistry.' 154 THE ATTRACTION OF ATOMS. close a relation to the vital processes as might at first be supposed. But those organic compounds that are cellular, such, for example, as the different varieties of starch, the fibre of wood, and the fibre of lean meat, are much removed from inorganic bodies, and seem to bear a peculiar and close relation to the vital forces. In general, cellular organic compounds are called organized; while the non-cellular organic compounds are called non-orga n ized. Compound bodies then are divided from a certain point of view into two great classes, inorganic and organic. The organic are again divided into two classes, organized and non-organized. CHAPTER XIV. THE ATTRACTION OF ATOMS (.continued}. THE CHEMICAL WORK OF MICRO-ORGANISMS. IT is difficult to form a proper conception of the vast amount of chemical work accomplished by those ex- FIG. 107. Autograph letter of Louis Pasteur, being an order for board for hydrophobia patients undergoing treatment at the Pasteur Institute. ceedingly minute parasitic plants called micro-organisms, microbes, bacteria, etc. Of late years, following the work of Pasteur and Koch, many observers have indus- 155 1 5 6 THE ATTRACTION OF ATOMS. triously studied this subject. As a result, microbes or bacteria (the word bacterium is used in a general as well as in a special sense) are now recognized as of high im- portance in chemistry. FIG. 108. Dr. Robert Koch, celebrated investigator in the field of bacteriology. These organisms are exceedingly numerous as varieties and yet more as individuals. They are most widely dif- fused. They exist in air (though not largely in sea air nor even in the air of large sewers), in water (though in THE ATTRACTION OF ATOMS. !$/ varying quantities and kinds), in the soil (though in most cases, not at great depths). They effect many kinds of decomposition of mole- cules a work essentially chemical. Some of it is of industrial interest in manufactures and in agriculture ; some of it leads to the wonderful fermentative and putrefactive processes of the world ; some of it goes on as the chief factor in diseases of the higher animals, and indeed in the normal digestive operations of them. FIG. 109. Microscopic infusoria such as are found in stagnant natural water. (Thus it is observed that not all microbes are pathog- enic : some are distinctly beneficial to living animals.) General Description of Microbes. The organisms in question consist of very minute cells often not greater in length than one ten- thousandth of an inch. Yet, when floating in the air, they cannot pass through a small plug of loose cotton fibres. In form they vary very much. The common forms are the globular (micrococcus), the keyhole-shaped (like two spheres in contact, or partly run together) , the rod-shaped (bacillus form), the comma-shaped, the spiral shaped. 158 THE ATTRACTION OF ATOMS. The cells are often grouped in a tolerably definite way in filaments or chains; sometimes they are gathered in great irregular masses. A given cell usually consists of a sac of mycoprotein enclosing homo- geneous protoplasm. FIG. no. , mycoderma vim; bb, mycoderma acetl (earlier stage of development) ; c c, mycoderma acetl (advanced stage of development) . Classification and Nomenclature of Bacteria. Micro-organ- isms have been divided into two sections, (i) the Endosporea and (2) the Arthrosporea. The former consists of but one genus, sporobacterium, which has four recognized species. The latter (Arthrosporea) has two genera, bacterium and micrococcus. THE ATTRACTION OF ATOMS. 159 The genus bacterium is the more numerous, having at least twenty-five distinct species, among which are the bacteria found in the human body in the diseases of consumption, pneumonia, cholera, diarrhoea, typhoid fever, and glanders; also the forms which are found in foul ponds and sewage : it also includes the vinegar ferment, which converts ethylic alcohol into acetic acid. FIG. in. Mycoderma aceti, or mother of vinegar, as seen (enlarged 500 diameters) under the microscope. The genus micrococcus has eight species now enumerated, among which are the bacteria of small-pox, erysipelas, scarlet fever, and others. Growth of Microbes. The cells of micro-organisms are capable of extremely rapid multiplication generally i6o THE ATTRACTION OF ATOMS. by fission or some modification of it. Sometimes fission takes a course whereby forms like spores are produced. By such processes a single bacterium cell may multiply in twenty-four hours to more than a billion individuals like itself. FIG. 112. Apparatus used in Pasteur's laboratory in Paris ; boiler (sterilizer) heated by gas, for destroying microbes by steam under high pressure; oven for culture of certain microbes at a definite temperature ; oven for sterilizing tubes and flasks by hot air. The number of microbes in some kinds of food is very great. Thus, it has been computed that in the case of certain kinds of Swiss cheese one pound of the article possesses a microbian population greater than the human population of the terrestrial globe. FIG. 113. Large oven used in Pasteur's laboratory for the culture of certain selected microbes. (It is provided with an automatic gas regulator.) 1 62 THE ATTRACTION OF ATOMS. Micro-organisms are influenced in their growth by prevailing conditions; some conditions are highly favorable, some unfavorable. I. They need a certain temperature, varying in particular cases (100 F. is generally favorable) . They are best killed by very high temperatures FIG. 114. Sterilizing apparatus. It is for the purpose of destroying bacteria in such substances as may be placed in the flasks. The apparatus consists essentially of a strong vessel, made tight, and provided with a safety valve and steam gauge. It has jackets to prevent loss of heat and condensation of steam. Upon applying a strong gas flame underneath the vessel, the water in the vessel is raised to the boiling-point. High pres- sure steam is produced. The flasks are therefore subjected to a temperature sufficient to destroy any microbes in them. The flasks are then withdrawn all at once by means of the caster. The plugs of sterilized cotton in the necks of the flasks prevent subsequent access of microbes from the air. (212 F. and upward). Some, however, succumb at even moderately low temperatures (50 F. and downward). THE ATTRACTION OF ATOMS. 163 2. They flourish best in presence of moisture. A small amount will serve. Moderately dry dust containing them, when put under proper con- ditions with moisture, shows life by the multiplication of the varieties present. But thorough desiccation is fatal. FIG. 115. Louis Pasteur, celebrated for his studies in the diseases produced by micro-organisms. 3. Some of them need atmospheric air; some flourish best in absence of it. As bacteria are destitute of chlorophyll, they do not obtain nutrition by decomposing carbon dioxide of the air under influence of sunlight. 4. They must have suitable pabulum. Farinaceous matters are good; albuminoid or other nitrogeneous substances are very favorable; meat extract is excellent. THE ATTRACTION OF ATOMS. 1 6$ Some bacteria attack only dead organized tissues; others attack and disorganize tissues of living beings. Some obtain nitrogen from as simple compounds as ammonia gas (NH 3 ) ; others require albuminoid compounds. Some obtain carbon from substances as simple as acetic acid; others from tartaric acid; others require as complex molecules as sugar or glycerin; yet others can take carbon, as well as nitrogen, only from proteids. Thus it appears that, like animals, microbes are dependent upon the complex molecules formed by highly organized beings. 5. They are not materially interfered with by certain chemical sub- stances, even by some that are poisonous to the higher animals. But some chemical substances they do not successfully tolerate. Such are the so-called germicides or antiseptics. Corrosive sublimate (mercuric chloride, HgCl 2 ) is one of the most fatal to them, and hence it is largely used. The following are also more or less unfavorable to their growth : chlorine, zinc chloride, zinc sulphate, blue vitriol (cupric sulphate), green vitriol or copperas (ferrous sulphate), sulphur dioxide, boric acid, ethyl alcohol, carbolic acid (phenyl alcohol), and even common salt. In some cases chloroform suspends their powers temporarily. In many cases the growth of bacteria is arrested by products of their own formation; these, when they are sufficiently accumulated, seem to poison the bacteria even though their pabulum is not exhausted and con- ditions otherwise favorable to them are maintained. 6. They must be free from the interference of inimical microbes. Often a large number of one kind of micro-organisms will successfully combat a smaller number of another kind. But in some cases one species of bacterium works only in connection with a second species. The first prepares, by its chemical activity, the pabulum essential for the life of the second. Thus, in the fermentation of starch, a certain microbe commences the work, changing starch to ethyl alcohol; then another takes up the process and converts the ethyl alcohol into acetic acid. Results of Bacterial Action. The life processes of microbes are, in a general way, comparable with those of other living organisms. They are simpler in some respects, because these minute plants are themselves so simply organized. Their enormous numbers, however, 1 66 THE ATTRACTION OF ATOMS. enable them to produce, in the aggregate, great quanti- ties of such compounds as they are able to form. The general tendencies of bacterial growth involve a breaking-down of complex molecules into somewhat sim- pler ones, although this is not the invariable result. Microbes produce certain special compounds as has already been suggested. 1. They accomplish the following important transformations: Cane sugar to ethyl alcohol ; Glycerin to ethyl alcohol, and thence to butyl alcohol ; Cane sugar to gum or mannite ; Grape sugar or milk sugar or glycerin to lactic acid and butyric acid; Urea to ammonittm carbonate ; Hippuric acid to benzoic acid ; Albumens to ptomaines ; Nitrogenous matters to nitrates. 2. They produce certain groups of substances possessed of general properties, of which the following may be noted : Substances having marked agreeable or disagreeable odors; Substances having brilliant colors; Substances called, in general, ptomaines having eminently poison- ous properties, as tyrotoxicon in milk and cheese. The ptomaines just referred to are alkaloids of a highly poisonous char- acter, generally resulting from a morbid decomposition of albuminoids under the influence of microbes. The leucomaines are analogous poisonous alkaloids, but they are pro- duced by the ordinary physiological processes of the higher animals, and thus are capable of being decomposed and excreted under the normal action of the appropriate organs, of which, apparently, the liver is the most effective. It was formerly held that the morbid conditions recognized in animals affected by certain contagious and infectious diseases were due directly to the specific microbes present. At present the abnormal action of the organ- ism is referred rather to the poisonous ptomaines produced by the microbes. Usefulness of Bacteria in the Organic World. One of the most marked features in the life-processes of the THE ATTRACTION OF ATOMS. l6/ higher animals and plants is the circulation of certain atoms. That is, there seems to be a definite and rather small stock of certain useful elements, like nitrogen, phosphorus (and to these may be added, with less force, potassium and even carbon), which are in a continual state of transfer. This " stock " is absorbed from the soil by living plants ; it is then absorbed by living animals. The bacteria assist the process of animal digestion, whereby the vegetable molecules are altered. Upon the death of animals the current stock returns to the soil, thence to be employed by a new set of growing plants, and later by a new population of living animals. Without microbes the " stock " would be withdrawn from circulation in living animals or vegetables, and locked up inactive in dead bodies. Upon the death of the animal, the microbes set up those processes of putrefaction and decay, whereby the stable molecules in the dead bodies become available for the food of grow- ing plants. CHAPTER XV. THE ATTRACTION OF ATOMS (continued}. MODES OF CHEMICAL ACTION. As a result of the operation of chemical affinity, molecules are changed in a variety of ways. The following are some of the principal ones : 1. Elementary or compound molecules may directly combine : Zn + Clj = ZnCl, One atom of One molecule of One molecule of Zinc, Chlorine, Zinc chloride, 65 71 136 parts by weight. parts by weight. parts by weight. 136 136 2. An element or group of elements may displace another element or group : 2HC1 + Zn ZnCla + H 2 Two molecules of One atom of One molecule of One molecule of Hydrochloric acid, Zinc, Zinc chloride, Hydrogen, 73 6 5 136 2 parts by weight. parts by weight. parts by weight. parts by weight. 138 138 But in some cases the displacement may be by gradual stages. Thus marsh gas (CH 4 ) may have its hydrogen replaced by chlorine, atom by atom, until all is removed. 168 THE ATTRACTION OF ATOMS. 169 Thus the following compounds may be progressively formed : CH 3 C1, CH 2 C1 2 , CHC1 3 , CC1, 3. An element or group of elements in one molecule may exchange places with an element or group of ele- ments in another molecule : CuS0 4 + One molecule of Cupric sulphate, 159 parts by weight. Ba(N0 3 ) 2 One molecule of Barium nitrate, 261 parts by weight. = BaSO 4 + One molecule of Barium sulphate, 233 parts by weight. Cu(N0 3 ). 2 One molecule of Cupric nitrate, 187 parts by weight. 420 420 4. There may be a rearrangement of elements or groups of elements within single molecules of a sub- stance : (NH 4 )O(CN) changes spontaneously into Ope molecule of Ammonium cyanate, 60 parts by weight. 60- 5. There may be a direct decomposition of a certain molecule into others of a different kind : 2 HO may be decomposed into 2 H 2 + O. 2 Two molecules of Two molecules of One molecule of Water, Hydrogen, Oxygen, 3 6 4 32 parts by weight. parts by weight. parts by weight. 36~~ "IT" I7O THE ATTRACTION* ''O'F 1 ' ATOMS. The Sphere of Chemical Action. The sphere of chemical action is evidently .that of the individual mole- cule ; as a result of- chemical,, change, molecules change their components. -This 1 sphere is a very limited one when looked at with reference to the minuteness of a single molecule. It is one of very wide range when it is remembered that all material substances are made up of molecules, and that the character .of the molecules determines the character of the mass. Chemical change, therefore, is most fundamental, altering substances in their ultimate recesses. Changing the molecules in which the identity of substances reside, it changes the identity of masses themselves. Thus all the kingdoms of nature owe to- chemical action the variety of sub- stances produced in their normal or abnormal growth, while geologic and cosmic changes involve chemical action and reaction on the largest scale. The Results of Chemical Action. The effects pro- duced by chemical change are recognized as of the most striking kind ; and this is true both in natural and in artificial prdcesses. . . Some of 'the principal effects noticed are changes of physical condition, as a substance originally solid or liquid or gaseous, at a given temperature, may change to another of these conditions ; changes of color, odor, taste, or other physiologic or toxic effect ; change of volume: thus sometimes chemical action draws atoms closer together. As already stated (p. 78), two volumes of hydrogen gas and one volume of oxygen gas, when chemically actuated, unite to form a new substance (water-vapor), occupying only two volumes altogether. THE ATTRACTION OF ATOMS. I? I Sometimes there is no reduction, but rather expansion : thus gunpowder, a solid, experiences chemical change when slightly heated, and produces an immense volume, of gas. Sometimes neither expansion nor contraction, takes place. t t . Sometimes chemical affinity produces such violent or bizarre effects that there can be no question that it is in active exercise. In other cases, where two or more sub- stances might be supposed to undergo chemical change, the evidences are so slight as to make the very exist- ence of the chemical action difficult to substantiate. > :* Among all these various, and in many cases inexplica- ble results, two principles are constantly recognized, the indestructibility of matter and the indestructibility of energy. .; General Laws of Chemical Action. The following are a few general laws relating to the results of chemical action : The Law of Insolubility. When there are brought together solutions that contain several elements such as would, if united, form a compound that is ordinarily insoluble in the liquid present, this insoluble compound will usually be formed and will appear as. a precipitate. This law is subject to certain limitations, yet it is of sufficiently wide application to sometimes enable the chemist to predict the formation of a given substance that may never have been produced before in that par- ticular way. This law finds illustration in the following equations : HC1 + Ag(N0 3 ) = AgCl + H(N0 3 ) ; NaCl + Ag(N0 3 ) - AgCl + Na(NQ 3 ) ; XC1 +AgY =AgCl 1/2 THE ATTRACTION OF ATOMS. The Law of Volatility. When there are brought together substances whose reaction can produce a gas or a substance that is volatile at the temperature of the experiment, such volatile or gaseous substance generally will be formed, and will be liberated with effervescence. The Indestructibility of Matter. The amounts of weighable matter taking part in a chemical change are definite ; and the sum of the masses of the products is always equal to the sum of the masses of the factors. The Indestructibility of Energy. The amounts of en- ergy involved in chemical changes are definite. When the elements of a chemical compound are drawn apart, a certain amount of energy is usually absorbed. When the same elements come together to form a compound, a certain amount of energy is evolved. Now the amounts of energy in these two cases are equal. It is true that the energy absorbed or evolved in such cases may vary in kind. It may be the energy of heat or that of light or that of electricity in some of its modi- fications, or it may be some combination of these. But, in any event, the facts sustain the doctrine called the conservation of energy, which involves the view that it is impossible for us to create or to destroy energy, just as it is impossible for us to create or to destroy matter. All that we can do is to change the particular form which the energy shall, for the time being, assume. Criteria of Chemical Action. It has already been stated as a fundamental principle that natural phe- nomena arrange themselves in series in which the individual members differ from their immediate neigh- bors by minute and sometimes almost indistinguishable THE ATTRACTION OF ATOMS. 173 details. Thus it may be expected that the drawing of distinct lines of division will often be impracticable. This statement is applicable to those different kinds of action called chemical action and physical action. While a multitude of operations are readily recognized as manifesting distinct evidences of chemical change, and at a distance from these may be produced changes that are referable distinctly to cohesive and physical forces, there are between these extremes phenomena in which the definite signs of the one or the other kind of action become less and less marked, or entirely fade away. Thus, on a given occasion an observer may be reason- ably in doubt whether certain intermingled or adjacent substances undergo or do not undergo what is properly described as chemical change. It is therefore desirable to consider systematically the evidences upon which a decision must be reached. The following may be accepted as a convenient rule : Gain a thorough acquaintance with all the characteristics that generally attend undoubted chemical changes ; then, in a doubtful case, observe whether one or several single characteristics are distinctly evident, and whether one or several characteristics can be recognized, if only in a feeble and rudimentary degree. The following are the chief evidences of well-marked chemical action : (a) The generation of certain physical forces, as heat, light, electricity. These are important indications, since appliances have been produced capable of detecting very minute amounts of heat and of electrical dis- turbance. () The production of new molecules. These may be recognized as follows : i. They possess new chemical composition; that is, they contain either 174 THE ATTRACTION OF ATOMS. new elements or else they contain the original elements in new propor- tions by weight (and in case of gaseous elementary substances, by volume as well). This evidence involves the important law of definite proportions. (See p. 72.) 2. They possess properties differing more or less distinctly from those of the original elementary or compound molecules which, in the case in question, are supposed to have been subject to chemical change. The changes to be looked for are in the following features : The color; degree of opacity; refracting power for light; . The taste; physiologic and toxic properties; ., The conducting power for heat, light, and electricity; The density of the substance in the solid, liquid, or gaseous condition; The melting and boiling points; The degree of solubility in solvents. CHAPTER XVI. THE ATTRACTION OF ATOMS (continued).. THERMO-CHEMISTRY. Introduction. It has long been recognized that many chemical changes give rise to an evolution of heat. Sometimes the amount evolved is very large. Indeed, practically all man's artificial .heat is the product of chemical combination. It is also well known that the amount of heat evolved by the combustion or other chemical reaction of a certain weight of one substance is very different from that given out by a corresponding chemical reaction of an equal weight of another substance. At a given moment of time any material substance or thing possesses certain internal and external relations of parts, and contents of heat and other forces, the sum total of which may be called its condition. Now any change whatever of its condition implies either some alteration of the arrangement of its parts externally or internally, or some alteration by increase or decrease of the amount of its forces. In either case the change requires for its initiation the application of some ex- ternal force, which may, perhaps, be small in amount. But the change, when once it has been instituted, either absorbs or liberates a large amount of energy of some kind, generally that of heat. -.. \ 175 1/6 THE ATTRACTION OF ATOMS. Reactions of the sort referred to are sometimes classed as direct, or exothermic (those in which heat is evolved), and indirect, or endothermic (those in which heat is absorbed). It is worthy of note that the amount of heat involved by the union of substances in an exothermic reaction is exactly equal to the amount of heat that would be required to subsequently decompose the substance produced by such operation. The facts here stated seem to be merely forms of the general law of nature, that changes in the arrangement of material substances cannot be accomplished without the aid of force. In other words, to bring about a certain change, heat or other force must be supplied, and the energy appears to be somehow taken in by the compound. In the reversal of the same operation the energy taken in is given out : then force is evolved, as heat or in some other form, equivalent in amount to that originally absorbed. Within a few years efforts have been made to learn and state with exactness the amounts of heat afforded by all the more prominent chemical actions. The exact studies of Alexander Naumann, Julius Thomsen, Marcellin Berthelot, and others rank among the classical scientific researches of this era. The whole subject has also been carefully reviewed in recent treatises by Pattison Muir and others. Laws of Thermo-Dynamics. First Law. There is a definite quantitative relation between the amount of work done and the quantity of heat produced or destroyed. Second Law. If all the heat in any body or system of bodies is at the same temperature, no mechanical work can be obtained from that body or system except by bringing it into contact with another body at a lower temperature. The important principle stated by Clerk Maxwell may with propriety be presented here : " The total energy of any material system is a quan- tity which can neither be increased nor diminished by any action between parts of the system, though it may THE ATTRACTION OF ATOMS. I// be transformed into any of the forms of which energy is susceptible." The Law of Maximum Work. Beithelot States this law as follows : " Every chemical change accomplished without the addition of energy from without tends to the formation of that body or system of bodies, the production of which is accompanied by the evolution of the maximum quan- tity of heat." Muir makes a critical examination of this statement, and thinks that Berthelot has fallen into error. Thus consider at the outset the suggestion that any chemical change can be accomplished without the addition of energy from without. Initial outward influence seems necessary to change the condition of any portions of matter not in actual process of change. Thermal Units. The amount of heat absorbed or evolved in chemical operations is usually represented in thermal units called calories. Thomsen uses the water calory, and his unit of heat is the amount of heat necessary to raise one gramme or one kilogramme of water through one degree measured in the neighborhood of the eighteenth to the twentieth degree of the centigrade thermometer. Berthelot prefers to use water at zero degrees centigrade. In some cases an ice calory is used, the heat being measured by the amount of ice that may be changed from the solid to the liquid form without rise of tempera- ture (one ice calory is equal to 80.025 water calories). It should be noted, also, that sometimes the so-called large calories (C) are used, and sometimes small calo- ries (c). The large calory relates to the kilogramme of water, the small calory to the gramme of water. 1 7 8 THE ATTRACTION OF ATOMS. The heat of combustion of an element is sometimes defined as the amount of heat evolved by the perfect combustion of one gramme or one kilogramme of the substance. Sometimes the heat of combustion means the quantity of heat produced by the chemical change of a number of grammes represented in a certain reaction of the substance. In accordance with the first definition, the heat of combustion of hydrogen is the amount of heat produced by burning one gramme of it. In accordance with the second definition, the heat of combustion of hydrogen is the amount of heat pro- duced by burning two grammes of it (representing one molecule of hydrogen) in accordance with the equation H 2 + O = H a O. Calorimeters. In the ex- periments of thermo-chemistry several different kinds of ap- pliances have to be used. The forms of chemical change are so varied as to the sub- stances taking part, as to the substances ultimately produced, and as to the conditions attend- ing the progress from the one set of substances to the other, that a thermal study of these of apparatus specially adapted calorimeters are : First, an inte- e.g. of glass or of platinum, in FIG. 117. Simple form of calor- imeter. The vessel E represents a box placed on a felt or wooden sup- port, and provided in its interior with a non-conducting wreath for thermal insolation. The thermom- eter registers the heat generated in the process of a chemical reaction. changes demands forms to the different cases. The essential parts of rior vessel of some sort, THE ATTRACTION OF ATOMS. 1/9 which the chemical operation proceeds. Second, one or more delicate and accurate thermometers, to be used in FIG. 118. One of Berthclot's calorimeters, constructed for observing the heat gen- erated by mixing chemical substances in solution. The outer jackets are for the purpose of preventing heat from passing from the room into the apparatus or from the apparatus outward. The beaker in which the experiment is performed is placed upon pointed sup* ports for thermal insolation. A stirrer, a, is provided to diffuse throughout the solution the heat generated. A delicate thermometer suspended in the solution registers the temperature. measuring the rise of temperature of the operation. Third, several protecting coatings or chambers, such i8o THE ATTRACTION OF ATOMS. as vessels of water or of air, covered with felt. Fourth, mechanical stirrers to agitate the water of the outer chamber so that the heat absorbed may be evenly dis- FIG. 119. Calorimeter devised by Berthelot for experiments upon the union of cer- tain gases. The products of chemical union are collected in the spiral 5" and the bulb R. The heat generated is absorbed in the liquid surrounding the spiral. The thermometer registers the heat produced. The vessel containing the spiral is thermally isolated by the several jackets placed about it. THE ATTRACTION OF ATOMS. l8l tributed. Fifth, in some cases the room in which the experiments are performed is most carefully maintained at a uniform temperature. Sixth, sometimes the ex- periments have a duration of several days, so that the water employed as a protective coating (and the air of the room also) may acquire a uniform tempera- ture. I. One general form of apparatus is suited to experiments on the relations of solids with water or other liquids. Under this head come such cases as the solution of many salts in water, the reaction between water solutions of acids and water solutions of alkalies, and the like. II. Another kind of apparatus is necessary in the study of combustion processes. Such are those in which solids, liquids, or gases are burned in oxygen gas, or are made to unite by a process analogous to oxygen com- bustion with sulphur, chlorine, bromine, or other substance. III. Another form may be necessary in the study of the violent changes involved in the action of chemical agents on certain organic matters, as, for example, of nitric acid on sugar. IV. Some chemical operations may demand apparatus for their indi- vidual treatment, as, for instance, when two gases act on each other at ordinary temperature : the union of nitrogen dioxide and oxygen affords an illustration. Again, the action of certain substances on others proceeds slowly, and special apparatus may be needed to deal with such changes : the action of oxygen on a solution of sodic thiosulphate (sodic hypo- sulphite) affords an illustration. Difficulties Experienced. Exact determinations of the amounts of heat evolved or absorbed by chemical opera- tions are attended with certain difficulties : FIRST. There are the mechanical difficulties associ- ated with the construction and use of the apparatus required. SECOND. In certain distinctly connected series of chemical changes heat is both absorbed and evolved. The computation is thereby complicated. 1 82 THE ATTRACTION OF ATOMS. The chemical union of hydrogen and chlorine would be a simple one if it were properly expressed by the equation H + Cl = HC1. In this expres- sion a simple and direct union of two atoms is described. But the true change taking place when hydrogen and chlorine unite is believed to be expressed by the equation H 2 + CLj = 2 HC1. This expres- sion describes something more complicated than the foregoing. It ex presses at least three operations; viz. a decomposition of a molecule of hydrogen, a decomposition of a molecule of chlorine, the union of an atom of hydrogen with an atom of chlorine (this particular operation being twice repeated). 'Now it may be safely assumed that the decomposition of the hydrogen molecule and the decomposition of the chlorine molecule both absorb heat, while the union of the atoms of hydrogen with the atoms of chlorine evolves heat. Evidently, then, the total amount of heat observed in such an operation represents a remainder equal to the excess of the evolved over the absorbed heat. H'=H-(h'+V). H f represents net observed evolution of heat. . 77 represents amount of heat evolved by actual chemical union of the substances. h represents amount of heat absorbed by decomposition of one of the molecules involved. h' represents amount of heat absorbed by decomposition of the other molecule involved. THIRD. In many cases of chemical change the heat actually generated may be partly expended in raising the temperature of the solids, liquids, or gases produced. Of course any such rise of temperature as is observed in the experiment must therefore be subjected to correction because of the different specific heats of the substances present. Thus the experimenter in thermo-chemistry must make a careful study of specific heat, and the results of thermo-chemistry cannot be relied upon unless correct results in specific heat are employed. FOURTH. In many cases the final result is complicated because the substances first produced dissolve in the water present, absorbing or evolving heat by this opera- tion. THE ATTRACTION OF ATOMS. 183 Range of the Subject. - - Evidently the field of thermo- chemistry is a wide one. As a method of observation it has been brought to bear upon many diverse forms of action. Thus it has been applied to the study of the following classes of subjects : FIRST. The amounts of heat of formation of certain binary compounds, as the compounds of hydrogen, chlorine, bromine, iodine, sulphur, oxygen, and other elements by various methods. These include energetic changes in which various substances, elementary or compound, unite with oxygen or other elements by a process analogous to combustion. SECOND. The heats afforded by neutralization of alkalies, like soda, potash, and others by acids. THIRD. The heats of solution of solids, the dilution of liquids, and hydration generally. FOURTH. The phenomena of dissociation, and the so- called abnormal vapor densities. FIFTH. Certain allotropic and isomeric substances. Thus there has been made an examination of the differ- ences in the thermal value of the combustion of different kinds of sulphur, different kinds of phosphorus, different kinds of carbon, different kinds of silicon, as well as of different isomeric compounds of the organic series, with a view of detecting, impossible, the differences of molec- ular structure of these elements and compounds. Results. While it is not possible as yet to state many distinct laws as to the heat of chemical union, it has been noticed that certain operations of a similar chemical type involve approximately equal amounts of 184 THE ATTRACTION OF ATOMS. heat, even when the particular substances taking part are all different. The amount of heat produced when certain substances are dissolved in water affords important information as to the condition of such substances in solution. Thus it is well known that upon mingling sulphuric anhydride (SO 3 ) with water, heat is produced. A critical study of this operation shows plainly that a large proportion of the heat afforded is produced by the addition of the first molecule of water, and a large proportion is also produced by the addition of the second molecule of water. Muir says that there can be little or no doubt that the various results point to the formation in aqueous solutions of sulphuric anhydride (SO 3 ), of one definite hydrate having the formula H 2 SO 4 , and not of other hydrates. The same general results are obtained by the study of the solution of other acids. " The difference commonly expressed in the terms water of constitution and water of crystallization is evidently, so far as may be judged from thermo-chemical data, strictly a difference of degree and not of kind." A few examples of the numerical results may be pre- sented here. 1 (a) Union of Elements. H + Cl - HC1 + 22,000 cal. H + Br=-~HBr + 8,440 " H +1 -HI - 6,040 " H 2 +O = H 2 O + 68,360 " H 2 +S =H,S + 4,740 " S + O 2 = SO 2 +71,080 " * (b) Union of Compounds with Water. H 2 SO 4 + H 2 O =(H 2 O -H 2 SO 4 ) +6,379 cal. KOH - 3 H 2 O + 197 H 2 O = (KOH 200 H 2 O) + 2,75 1 " 1 Muir, Elements of Thermal Chemistry. THE ATTRACTION OF ATOMS. 185 (c) Union of Acids and Bases. 2 HC1 -f Na 2 O = 2 NaCl + H 2 O + 27,480 cal. 2 HI + Na 2 O = 2NaI + H 2 O + 27,360 " In this last set of equations the similarity of the number of calories evolved at once suggests similarity in the operations in question. This similarity is noticeable in many other cases among elements and com- pounds of the same family group. Indeed, Thomsen classifies the acids in a general way into sets some- what as follows : FIRST. Acids whose heats of neutralization are about 20,000 cal. Examples are Nitrous acid ..... . ........ HNO 2 , Hypochlorous acid ........ . . HC1O, Carbonic acid ............ H 2 CO 3 , Metaboric acid ............ H. 2 B a O 4 . SECOND. Acids whose heats of neutralization are about 25,000 cal. Examples are Chromic acid ............ H 2 CrO 4 , Succinic acid ............ C 4 H 6 O 5 . THIRD. Acids whose heats of neutralization are about 27,000 cal. Examples are Hydrochloric acid ....... .... HC1, Hydrobromic acid ........... HBr, Hydriodic acid . . ......... HI, Nitric acid ..... ........ HNO 3 , Chloric acid ............. HC1O 3 , Bromic acid ............. HBrO 3 , lodic acid ............. HIO 3 , Formic acid .... ......... HOCHO, Acetic acid ............. Most of the acids examined by Thomsen belong to this group. 1 86 THE ATTRACTION OF ATOMS. FOURTH. Acids whose heats of neutralization are greater than 27,000 cal. (generally from 28,000 to 32,500 cal.)- Examples are Hydrofluoric acid HF, Sulphurous acid H 2 SO 3 , Sulphuric acid H 2 SO 4 , Selenic acid H 2 SeO 4 , Metaphosphoric acid HPOj, Phosphorous acid H 3 PO 3 , Oxalic acid H,O 2 (C 2 O 2 ). CHAPTER XVII. THE ATTRACTION OF ATOMS (continued}. THEORIES OF THE NATURE OF CHEMICAL ATTRACTION. THE force whatever may be its nature that leads substances to undergo chemical changes is often called chemical affinity. This force is capable of overcoming a certain amount of resistance ; again, a certain amount of force is necessary to undo its work. It also bears definite quantitative relations to other forces, such as heat, light, and electricity : definite amounts of chemical energy are necessary to the production of unit amounts any one of them, and definite amounts of one of them are necessary to the production of a unit of chemical energy. A very large amount of information has been secured as to its ways of working, etc., but no entirely satisfac- tory explanation of its nature has yet been offered. Views as to the nature of chemical attraction have changed from time to time as one phase of thought or another has been dominant. They have in a marked manner reflected the spirit of the time as aroused by some great discovery. i. Early Views. The general opinion of the alche mists was that somehow or other substances of like kind or origin tend to combine. This view is evidently inadequate the more, in that 187 1 88 THE ATTRACTION OF ATOMS. it is now known that the most active chemical unions are between substances that are in many ways most unlike. 2. Newton's Theory. Newton's discovery of the uni- versal attraction of bodies was naturally and easily ex- tended to the minute particles of matter, and chemical attraction was then held to be one form of general attraction. This view was advanced by Newton, and later was supported by Berthollet. These philosophers considered, however, that the universal tendency of bodies towards each other was somewhat modified by the minuteness of the particles between which chemical changes are capable of taking place. 3. The Theory of a Special Force. Chemical attrac- tion has been viewed as a unique kind of force. This theory, widely accepted during the past hundred years, is that atoms and molecules are gifted by the Creator with certain specific tendencies to unite, and that with certain fixed degrees of force. This supposes the pos- session by atoms of an inherent property called chemical affinity. The name applies in a general way to an un- explained energy somehow residing in the atom. 4. The Electrical Theory. Chemical attraction has been thought to be a phase of electrical energy. Davy, Dumas, Becquerel, Ampere, Berzelius, Gmelin, and others have held some form of electrical theory of chemical action. The general notion has been that atoms and molecules are naturally or may be artificially charged with varying amounts and kinds of electricity. By reason of their condition in this respect they are mutually attracted or repelled, and with varying degrees THE ATTRACTION OF ATOMS. 189 of force, somewhat as electrified masses of matter are. This theory derives support from certain important and well-defined facts. For bodies artificially electrified often manifest thereby stronger chemical attractions ; FIG. 120. Sir Humphry Davy, Bart. Born in Penzance, England, December 17 1778; died in Geneva, Switzerland, May 29, 1829. " Davy, when not yet thirty-two years old, occupied, in the opinion of all those who could judge of such labors, the first rank among the chemists of this or any other age." again, chemical action yields as a product a definite quantity of electricity. 5. The Theory of Motion. Williamson's theory is that chemical attraction is a form of motion. This view FIG, 121, Andre*-Marie Ampere. Born at Lyons, January 20, 1775; died at Marseilles, June 10, 1836. (The portrait is from a statue erected at Lyons.) THE ATTRACTION OF ATOMS. IQI accepts the modern idea of constant atomic and molec- ular movement. It suggests that in atoms of all or- dinary molecules a rapid but regulated interchange is going on, so that in certain cases a given atom may be continually moving from one molecule into another. A transfer of this kind could not be easily detected among molecules of the same kind, but among molecules of different kinds it would effect just such changes as are recognized in many chemical operations. Thus two or more molecules of hydrochloric acid (HC1,HC1) might make an interchange of atoms without any easily appre- ciable alteration of properties of the substance. But when a molecule of argentic nitrate and a molecule of hydrochloric acid are brought into contact, the inter- change might produce two new molecules possessing properties easily recognized as different from those of the original two, Ag(NO 3 ) + HC1 AgCl + HNO 3 Silver nitrate + Hydrochloric acid = Silver chloride + Nitric acid. This theory, moreover, not only explains such simple operations as that just referred to ; it is capable of affording an adequate reason for certain of the more obscure phenomena of chemical change. Comment on these Theories. Chemical attraction must be looked upon as a force having in it something of general powers and something of highly specialized ones. Thus any theory of it ought to include the notion that all substances tend to come toward each other, for appar- ently all chemical substances will combine there is merely a difference in the strength of this tendency in dif- ferent cases. And so if large bodies of matter gravitate THE ATTRACTION OF ATOMS. toward each other, why not molecules and even atoms ? Where shall be drawn the line at which gravitative force ceases ? Further, it must be admitted that substances are found to possess certain natural qualities inexplicable ones. What is this but a declara- tion that they are gifted, at their original creation, with specific powers? Again, the close connection of electricity with chemical force is indubitable. The modern notion of constant movement of all forms of matter applies with peculiar appropriateness to atoms and molecules, and seems to be inseparable from any idea of so intimate a change as the chemical. There is no impropriety in considering chemical attraction as a complex rather than a simple form of force. Certainly the rich variety of its modes of action and of its results must sustain such a view. Thus each of the theories stated contains truths. The acute observers and thinkers who have held them could net have been entirely misled. Each theory singly is merely inadequate. Probably in one that is adequate there must be combined the truth included in each of those stated and more, too, as chemistry advances. What is wanted, then, is a compact statement, sufficiently comprehen- sive to embrace in harmonious union the various principles known to be involved in chemical change. CHAPTER XVIII. ATOMIC WEIGHT. METHOD OF WORK AND METHOD OF DESCRIPTION. Introduction. Whatever theory an individual may hold with respect to the existence of atoms, in the most distinct import of that word, it cannot be reason- ably questioned that substances combine chemically in accordance with certain approximately fixed proportions, and that these proportions may be expressed in tolerably exact numerical form. As a matter of fact, chemists assign to each elemen- tary (and compound) substance a certain representative number. Such numbers are. certainly combining num- bers. They are probably much more. For elements, they represent at least an approach to atomic weights, and for compounds, at least an approach to molecular weights. They are, in fact, compact, single expres- sions, of the best form now known, embracing at once in harmonious union, mass ratios, volume ratios, vapor densities of elements, vapor densities of compounds, specific heats of elements, specific heats of compounds, substitution powers of elements and compounds, and even other relations besides. " It is true it may be questioned whether there is an absolute uniformity in the mass of every ultimate atom of one and the same chemical element. Probably atomic weights merely represent a mean value around which the 193 194 ATOMIC WEIGHT. actual atomic weights of the atoms vary within certain narrow limits. When, therefore, it is said, e.g. that the atomic weight of calcium is 40, the actual fact may well be that whilst the majority of the calcium atoms really have the atomic weight of 40, some are represented by 39.9 or 40.1, a smaller number by 39.8 or 40.2, and so on. The properties which we perceive in any element are thus the mean of a number of atoms differing among themselves very slightly, but still not identical." * In case of numbers accepted as atomic weights, the rule in this discus- sion is as follows : When the number involves a fractional part, this part is so modified that quantities less than .05 are rejected, while quantities equal to or greater than .05 are counted as I of the next higher denomination. Practical Importance of Atomic "Weights. The num- bers adopted as atomic weights have unquestioned practical value. They serve as a basis for the calcula- tions of the chemist in his analytical processes, and also as a foundation for the work of all the great chemical manufacturing industries of the world. Their value depends in part upon the invariability of the numerical laws of combination through all chemical mutations. Labor expended in securing Atomic Weights. The numbers adopted have so great an importance that chemists have devoted their highest skill and their most assiduous labor to the exact ascertainment of them. (Examples may be found in the work of Berzelius on many elements, that of Stas on many elements, es- pecially silver, of Crookes on thallium, Mallet on alu- minium and gold, Cooke on antimony, and Rayleigh, Cooke, and others on oxygen.) The different elements afford different kinds of information : Thus the atomic weight adopted to-day for one element may be worthy of far greater confidence than that adopted for another. l W. Crookes. ATOMIC WEIGHT. '95 And again, while the atomic weight ultimately accepted in a given case ought to be one which harmonizes with the entire body of chemical and physical knowledge, it must be expected that there should remain a few FIG. 122. Jons Jakob Berzelius. Born in East Gothland (in Sweden), August 20, 1799; died August 7, 1848. exceptional cases incapable of immediate explanation. It is acknowledged to be a matter of no slight difficulty to fix upon the atomic weight as dis- tinguished from some simple multiple or fraction of it : in some cases, owing to insufficient data, it is at present impossible. 196 ATOMIC WEIGHT. In any event the work demands two distinct kinds of operations : first, the experimental part, involving numerous tests and analyses ; second, a work that is even higher and more difficult ; i.e. the reasoning part the drawing of the proper inferences from the body of experimental facts accumulated. Since Dalton's first attempt to determine atomic weights these constants have assumed, little by little, an increasing interest. At present the effort to secure the most exact numerical expressions for them is con- sidered by chemists a work of the highest importance. Methods of Determination. In determining the atomic weight of a given element, a connected series of steps must be taken. The following is a brief outline of them : First Step. Adopt a suitable unit for the system. Second Step. Fix a basis upon which shall be selected the compounds (of the element sought} to be studied. Third Step. Proceed to make gravimetric analyses of the selected compounds. From these discover directly a combining number for the element. Fourth Step. Make choice of an atomic weight for the element from the various multiples or submultiples of the combining number discovered. In doing this, be guided by certain facts combined and applied in accord^ ance with definite principles. Thus, learn The vapor density of the element ; The vapor density of its compounds ; The volume composition of the compounds ; ATOMIC WEIGHT. 1 97 The specific heat of the element ; Any other suitable data. Fifth Step. Confirm the foregoing choice as fully as possible. In doing this, employ as many chemical and physical facts as possible. With this in view, study the element and its compounds in connection with molec- ular formulas. These involve a consideration of Molecular grouping ; Specific heats of compounds ; The boiling-points of compounds ; The crystalline forms ; Such other relationships as may be useful. Sixth Step. Bring all the atomic weights obtained into one table and arrange them in an appropriate order. This has been attempted by Newlands, Mendeldeff, 1 and many others. The two investigators mentioned have been among the most successful. NOTE. It may be noted here that it is the gravimetric analyses that give the results that are numerically capable of the highest degree of accu- racy. Indeed, they afford the only decisive foundation. The numbers they afford are certainly combining numbers. On the other hand , the various vapor densities and the various specific heats are mainly valuable as guiding in the choice between the several multiples of a number already learned. The Method of Discussion. It is plain that the subject in hand is an extended one. There is some difficulty even in the selection of a method of presenting it. Several ways are open. It is not well to follow here the exact historical course of the subject, for this has been marked by tentative and even erroneous views. However instructive these may be to the experienced chemist, they can only embar- rass the beginner. 1 Transliteration of Russian words : Nature, xli. 396 ; xlii. 6, 77, 316. 198 ATOMIC WEIGHT. It seems preferable to carry the student over a course directly leading to what is now accepted as truth; but the course to be selected should accord with the natural progress and be shaped by a sound pedagogic method. In the following discussion certain numbers are very quickly adopted. This is not improper. But it should not be forgotten that, as already intimated, the historical progress of chemistry has involved a much more circuitous route to reach these numbers than the discussion suggests. CHAPTER XIX. ATOMIC WEIGHT (continued}. FIRST STEP: A UNIT ADOPTED. THE subject involves the search for certain numbers. Now it must be remembered that all numerical expres- sions of mixed mathematics involve the use either ex- pressed or implied of some unit ; and often the unit chosen depends more upon convenience than upon any other consideration. These statements apply to atomic weights. Some unit has to be selected. At the present day the one almost universally accepted is the mass of a single atom of hydrogen, a mass extremely small, but one that it is possible (though not necessary) to express in terms of every-day weights. It is believed that the weight of a single atom of hydrogen is equal to 35 grammes divided by IO 23 , an amount about equal to one and one quarter ounces divided by one million million million million. This minute weight has received the special name microcrith. When, therefore, it is said that the atomic weight of oxygen is 16, the meaning is that a single atom of oxygen weighs 1 6 microcriths. A Different Unit might be used. It must be remem- bered that this selection of the atom of hydrogen as the standard is a matter of convenience partly, being based on the fact that no other atom has so low a mass. The mass of any other element might be used if it were found to be more convenient. 199 2OO ATOMIC WEIGHT. Dalton used hydrogen in his first table of atomic weights. Subsequently Berzelius and others recom- mended using oxygen as the standard, calling its weight IOO. 1 This latter system, however, was found to have the objection of affording in many cases numbers too large for convenience. Thus, when the atomic weight of oxygen equals 100, the atomic weight of uranium becomes about 1494. Certain practical objections have been urged recently against the em- ployment of the atom of hydrogen as the unit with the atomic weight, I. Thus it is very difficult to decide upon the exact ratio of the weight of the hydrogen atom to that of the oxygen atom. A number of ratios have been obtained as a result of extremely careful investigation. Thus it has been placed as low as H : O : : 1 : 15.869, and as high as H : O : : I : 1 6.010. But since oxygen is the starting-point for the determination of the atomic weights of a great many other elements, any error in the adopted ratio of H : O is transferred to nearly all the other atomic weights. Thus, if O = 15.869, then the atomic weight of uranium = 237.14. If O = 16.010, the atomic weight of uranium = 239.25. To avoid this difficulty it has been proposed, while using hydrogen as the nominal basis of the system, to use oxygen as the practical basis; to call the atomic weight of oxygen 16, and then to hereafter determine what the exact atomic weight of hydrogen is. At present it would be about 1.0025; but & m ig nt be expected to be slightly modified from time to time as the ratio of the combining portions of oxygen and hydrogen is deter- mined with greater and greater exactness. Meanwhile, changes in this ratio would not necessarily alter the entire scheme of atomic weights. This proposition, to adopt oxygen with atomic weight 16 as the unit, has received the approval of many eminent chemists. SECOND STEP: SELECTION OF THE COMPOUNDS AND THE PROCESSES TO BE EMPLOYED. In a certain sense every chemical compound (and every chemical process) is capable of contributing some- 1 Meyer, L., and Seubert, K., American Chemical Journal, vii. 96. ATOMIC WEIGHT. 2OI thing to the knowledge of atomic weights ; and, in fact, the study of a large number of them is necessary. But certain ones are far more serviceable than others. The compounds must be selected, then, with definite principles in mind, and they should conform to as many as possible of the following requisites : 1. They should be such as can be prepared in a form possessing a high degree of purity. Moreover, in purifying the materials used for analysis the so-called "fractional" methods should be employed. 2. As many different compounds as practicable should be tested. This helps the analyst to avoid " constant " sources of error. 3. They should be such as are capable of having their composition determined with a high degree of exactness. With this in view the analyses should require as simple processes as possible. " Improvements made of late in manipulative methods and apparatus have tended to reduce very much the magnitude of what are commonly called ' fortuitous ' errors in quantitative determinations of matter, and to increase greatly the accuracy. No one nowadays would undertake the determination of an atomic weight of one of the better-known elements without taking such elaborate precautions as must practically insure pretty close concordance of results when obtained by the same method applied in the same hands. But such mere close agreement is not alone sufficient." 4. They should be such as possess a molecular con- dition that is simple and can be distinctly ascertained. Thus it is well to employ compounds of only two elements. Again, if a given pair of elements forms a series of compounds, that one containing the smallest amount of the element under consideration is most likely to contain one atom of it. Again, compounds that are gaseous at ordinary tem- 2O2 ATOMIC WEIGHT. peratures or that can be easily vaporized, are employed. (See p. 37.) 5. In reactions depended upon, only such other elements should be concerned as may be counted among those of which the atomic weights are already known with the nearest approach to exactness. Thus, (a) Compounds with hydrogen are preferred when they are practicable ; for hydrogen has generally been adopted as the basis of the system, and so involves little or no error. Indeed, there are four compounds of hydrogen so well suited to this purpose that they have been called the type compounds of modern chemis- try; they are Hydrochloric acid (hydrogen and chlorine) ; Water (hydrogen and oxygen) ; Ammonia gas (hydrogen and nitrogen) ; Marsh gas (hydrogen and carbon). () In case hydrogen compounds are impracticable, then the com- pounds selected should, if possible, be such as contain certain atoms that have been compared directly and quantitatively with hydrogen. The majority of the elements do not form hydrogen compounds, so that, in fact, recourse is oftener had to oxygen compounds and chlorine compounds. All the elements (except fluorine, and that combines with hydrogen) form oxides, and oxygen itself has been compared with hydrogen with a high degree of accuracy. Again, chlorine combines with a great many metals, and the atomic weight of chlorine has been accurately determined. If oxygen is adopted (with the number 16) as the basis of the system, then compounds of oxygen will be selected, even in preference to com- pounds of hydrogen. 6. Further, it is desirable that as few other elements as possible the assumed atomic weights of which will have to be taken into account shall be involved in each single reaction depended upon. 7. In selecting different processes to be applied to the determination of the atomic weight of a given element, it is desirable that not the same, but as many ATOMIC WEIGHT. 2C>3 different other elements as possible, shall be concerned in the several reactions. Study of the Reactions. Careful preliminary study is required as to the general effect of each reaction involved, and as to how it may be influenced by the conditions of the experiment. For it has been learned more and more of late that many reactions perhaps it should rather be said all reactions which have been generally supposed to be of the simplest nature, are, in reality, complex. As many different and independent processes as can be devised (reasonably free from apparent sources of error) should be employed. Each process employed should be as simple as possible, both in the kind of chemical changes involved as well as in liability to manipulative Comparison of Results. In comparison of results, careful con- sideration should be given as to the probable influence of each kind of experiment; i.e. whether it tends on the whole to yield higher results or lower results than the truth. 1 l Mallet, J. W., American Chemical Journal, xii. 82. CHAPTER XX. ATOMIC WEIGHT (continued}. THIRD STEP: THE EXPERIMENTAL WORK FOR SECURING A FEW ATOMIC WEIGHTS. THIS stage is a very extended one. It involves, in fact, all the experimental work done and recorded up to the time of forming conclusions. Now chemical work has been done varying in extent and accuracy upon nearly if not all substances known to civilized nations. But of course, for the purpose of this discus- sion, only a few of the results can be referred to. I. A STUDY OF CHLORINE AND ITS AFFILIATED ELEMENTS, BROMINE AND IODINE (ALSO SODIUM, POTASSIUM, AND SILVER). - (a) Gravimetric Composition of Certain Compounds. Experimental Facts. A study of certain compounds of chlorine, bromine, and iodine has afforded a series of facts which are stated in the following table : 204 ATOMIC WEIGHT. 205 FIRST TABLE. APPROXIMATE PERCENTAGE COMPOSITION OF TWELVE COMPOUNDS. EXPERIMENTAL RESULTS. Chloride of Hydrogen. Bromide of Hydrogen. Iodide of Hydrogen. Hydrogen, 2.75 p. ct. Hydrogen, 1.24 p. ct. Hydrogen, .79 p. ct. Chlorine, 97.25 " Bromine, 98.76 " Iodine, 99.21 " IOO.OO IOO.OO IOO.OO Chloride of Sodium. Bromide of Sodium. Iodide of Sodium. Sodium, 39.38 p. ct. Sodium, 22.37 P- ct - Sodium, 15.37 p. ct Chlorine, 60.62 " Bromine, 77.63 " Iodine, 84.63 " 100.00 Chloride of Potassium. Potassium, 52.42 p. ct. Chlorine, 47.58 " 100.00 Chloride of Silver. Silver, 75.26 p. ct. Chlorine, 24.74 " Bromide of Potassium. Potassium, 32.83 p. ct. Bromine, 67.17 " 100.00 Bromide of Silver. Silver, 57-44 p. ct. Bromine, 42.56 " 100.00 Iodide of Potassium. Potassium, 23.55 P- ct - Iodine, 76.45 " IOO.OO Iodide of Silver. Silver, 45.97 p. ct. Iodine, 54.03 " IOO.OO 100.00 Experimental Results of the First Table stated differently. A consideration of the direct results given in Table I leads to the detec- tion of the following facts : 1. The numbers fall into series. 2. In each of the series of chlorides, bromides, and iodides, the chlorine is smaller in amount than the bromine, and the bromine is smaller in amount than the iodine. 3. When the amounts of hydrogen, sodium, potassium, and silver are compared, it is seen that their quantities are in the order stated; hydrogen being in smallest amount, and silver in largest. 4. If the three hydrogen compounds are compared on the basis of one part of hydrogen, the hydrogen series of compounds shows the following composition : 2O6 ATOMIC WEIGHT. SECOND TABLE. HYDROGEN COMPOUNDS. EXPERIMENTAL RESULTS. Hydrochloric Acid. Hydrobromic Acid. Hydriodic Acid. Hydrogen, I. Hydrogen, I. Hydrogen, I. Chlorine, 35.4 Bromine, 79.8 Iodine, 126.6 36.4 80.8 127.6 5. If trial is made with the numbers obtained in Table 2 (viz. 35.4 for chlorine, 79.8 for bromine, and 126.6 for iodine) in the other compounds under consideration, the following very remarkable results are obtained : THIRD TABLE. SODIUM COMPOUNDS. EXPERIMENTAL RESULTS. Sodium Chloride. Sodium Bromide. Sodium Iodide. Sodium, Chlorine, 23- 354 58.4 Sodium, Bromine, 23- 79-8 Sodium, Iodine, 23- 126.6 102.8 149.6 FOURTH TABLE. POTASSIUM COMPOUNDS. EXPERIMENTAL RESULTS. Potassium Chloride. Potassium Bromide. Potassium Iodide. Potassium, 39. Potassium, 39. Potassium, 39. Chlorine, 35.4 Bromine, 79.8 Iodine, 126.6 74.4 1 1 8.8 165.6 FIFTH TABLE. SILVER COMPOUNDS. EXPERIMENTAL RESULTS. Silver Chloride. Silver Bromide. Silver Iodide. Silver, 107.7 Silver, 107.7 Silver, 107.7 Chlorine, 35.4 Bromine, 79.8 Iodine, 126.6 I43-J I87-5 2 34.3 ATOMIC WEIGHT. 207 The results are brought together in the following table : - SIXTH TABLE. EXPERIMENTAL RESULTS. CHLORIDES. BROMIDES. IODIDES. Hydrogen Chloride. Hydrogen Bromide. Hydrogen Iodide. Per cent. Hydrogen, 2.75 Chlorine, 97-25 Ratio, i. 35-4 3^4 Per cent. Hydrogen, 1.24 Bromine, 98.76 100.00 Ratio, i. 79.8 8o78 Per cent. Hydrogen, .79 Iodine, 99.21 100.00 Ratio. 126.6 127.6 100.00 Sodium Chloride. Sodium Bromide. Sodium Iodide. Per cent. Sodium, 39-38 Chlorine, 60.62 Ratio. 23- 35-4 Per cent. Sodium, 22.37 Bromine, 77.63 Ratio. 23- 79.8 Per cent. Sodium, I 5-37 Iodine, 84.63 Ratio. 23- 126.6 100.00 58-4 100.00 102.8 100.00 149.6 Potassium Chloride. Potassium Bromide* Potassium Iodide. Per cent. Potassium, 52.42 Chlorine, 47-58 Ratio. 39- 35-4 74-4 Per cent. Potassium, 32.83 Bromine, 67.17 Ratio. 39- 79-8 Per cent. Potassium, 23.55 Iodine, 76.45 Ratio. 39- 126.6 165.6 100.00 100.00 118.8 100.00 Silver Chloride. Silver Bromide. Silver Iodide. Per cent. Silver, 75-26 Chlorine, 24.74 100.00 Ratio. 107.7 35-4 Per cent. Silver, 57.44 Bromine, 42.56 Ratio. 107.7 79.8 Per cent. Silver, 45.97 Iodine, 54-3 100.00 Ratio. 107.7 126.6 234-3 i43-i 100.00 187-5 2O8 ATOMIC WEIGHT. Inference I . Evidently, then, the following numbers have some im- portant fundamental meaning : NUMBERS WORTHY OF CONSIDERATION. Hydrogen, H, adopted as i. ( Chlorine, Cl, found to be, . . 35.4 \ Bromine, Br, " " " 79.8 I Iodine, I, " " " . . 126.6 f Sodium, Na, " ""....' 23. ^ Potassium, K, " " " 39. I Silver, Ag, " " " 107.7 (It may be noted here, as a fact, that subsequent study and comparison of all results accessible confirm the opinion that these numbers are impor- tant, and are probably atomic weights.) Inference 2. These results give the following as molecular weights : Hydrochloric acid 36.4 Sodium chloride 58.4 Potassium chloride 74.4 Silver chloride 143.1 Hydrobromic acid 80.8 Sodium bromide 102.8 Potassium bromide 118.8 Silver bromide ^7-S Hydriodic acid 127.6 Sodium iodide 149.6 Potassium iodide 165.6 Silver iodide 234.3 Inference 3. These results suggest the following formulas : H Cl H Br HI Na Cl Na Br Na I K Cl K Br K I Ag Cl Ag Br Ag I CHAPTER XXI. ATOMIC WEIGHT (continued}. FOURTH STEP: THE CHOICE OF A PARTICULAR ATOMIC WEIGHT FROM SEVERAL COMBINING NUMBERS. (b) The Density of Certain Elementary Gases and Vapors. Experimental Fact i. When chlorine gas is weighed it is found to weigh, volume for volume, about 35.4 times as much as hydrogen. Hydrogen is usually taken as a standard of comparison for weight of gases. The number 35.4 is called the density of chlorine gas. Experimental Fact 2. A similar experiment is tried with bromine vapor. Its vapor density is found to be about 79.9. Experimental Fact 3. A similar experiment is tried with iodine vapor. Its vapor density is found to be about 127. Inference I . Certain numbers are obtained that are worthy of atten- tion. Evidently their similarity to the numbers already obtained are not mere coincidences. Inference 2. The density of an elementary gaseous substance at once gives its atomic weight. Inference 3. The gaseous state is a very favorable one for study in this connection : in this state bodies appear to be in a sort of equality of condition that favors their comparison with each other from the point, per- haps, of even other relations than atomic weight merely. (c) The Volume Composition. Experimental Fact. When hydrochloric acid gas is tested, it is found that 209 210 ATOMIC WEIGHT. two volumes of the gas yield by decomposition one vol- ume of hydrogen gas and one volume of chlorine gas. Inference I . This sustains the view assumed in the preceding study, that the compound called hydrochloric acid consists of one atom of hydro- gen and one atom of chlorine. (Of course it is possible that the volume FIG. 123. Regnault's apparatus for filling a globe with gas at the temperature of o C. previous to weighing, for the purpose of determining a vapor density. composition merely teaches that the number of atoms of hydrogen and of chlorine are equal; that the formula is HC1 or H 2 C1 2 or H 3 C1 3 or H n Cl n . Until further information is secured, chemists assume the truth of the sim- plest expression. Further information is not in fact wanting, for subse- quent study strongly sustains the view that the formula is indeed HC1, and no other. See p. 228.) Experimental Facts. Similar results for hydrobromic acid and hydrio- ATOMIC WEIGHT. 21 die acid serve to confirm the numbers already accepted for bromine and iodine. Inference 2. These facts sustain all the inferences previously reached. FIG. 124. Henri Victor Regnault. Distinguished French physicist. Born at Aix-la-Chapelle in 1810; died in 1878. (d) The Vapor Density of Certain Compound Substances. Experimental Facts. The vapor density of hydro- chloric acid gas is found experimentally to be about 18. This means that a given volume of hydrochloric acid gas weighs 18 times as much as the same volume of hydrogen gas, or more formally stated, has 1 8 times the mass of the latter. 212 ATOMIC WEIGHT. Evidently there exists a very simple relation between the number representing the density of hydrochloric acid gas and the number already adopted as its molec- ular weight ; i.e. : 18 : 36 : : i : 2 Density of Molecular weight of hydrochloric acid gas. hydrochloric acid gas. Inference I . Perhaps in case of other compound substances the vapor density and molecular weight are connected by the same simple relation. Perhaps, as a rule, the molecular weight (which is, from its nature, difficult to determine) may be obtained by multiplying by 2 the vapor density (which in many cases it is easy to determine). (By subsequent experiment this principle appears to be sustained, and the inference is accepted as a just one.) Inference 2. Perhaps, in case of elemen- tary substances, the vapor density and molec- ular weight are in the ratio above suggested; that is i : 2. (By subsequent experiment this principle appears to be well grounded, and the inference is accepted as a just one.) A remarkable result follows. The vapor density of chlorine gas is found experimentally to be 35.4; then the molec- ular weight is 35.4 X 2 = 70.8. If, then, the molecular weight is 70.8 (and the atomic weight is 35.4), then the number of atoms in the molecule is 2, and the molecular formula of chlorine is C1 2 . (This generalization is a very important one. By subsequent experi- ment it appears to be sustained with respect to most of the elements capa- ble of existing in the form of gas or vapor. See p. 249.) (e) The Specific Heats of Elements. Definition. The specific heat of a substance is the amount of heat neces- sary to raise one unit of mass of the substance one FIG. 125. Method of intro- ducing into the bulb A a portion of liquid, C, whose vapor density is to be subsequently determined by Dumas' method. ATOMIC WEIGHT. 213 degree of temperature. Water has a high specific heat, and as it is usually taken as the standard, the specific heats of most other substances are expressed by decimal fractions. (See p. 46.) Experimental Facts. Dulong and Petit made a great many experimental determinations of the specific heats of solid substances. (It is more difficult with liquids and gases.) In case of elementary solids, whose atomic FIG. 126. Hot bath, provided with thermometers for determination of vapor densi- ties by Dumas' method. A small but weighed portion of liquid is placed in the globe. It is then vaporized by the heat of the bath. Subsequently the tip of the flask is sealed, whereupon the weight of vapor present at a certain observed temperature may be deter- mined by the balance. weights had been previously determined by other methods, they found that apparently the higher the atomic weight, the lower the specific heat. Inference. They then enunciated the following law, now called the law of Dulong and Petit : The specific heats of solid elements are inversely proportional to their atomic -weights. 214 ATOMIC WEIGHT. The law is likewise expressed in the following proportion : Specific heat of A : Specific heat of B : : Atomic weight of B : Atomic weight of A. This proportion also discloses the following fact : The product of the specific heat of any solid element by its atomic weight is a constant number. This constant is found to be about 6.3. Again, the constant 6.3 divided by the specific heat of a solid element yields as a quotient the atomic weight of the element. FIG. 127. Bunsen's ice calo- rimeter for determining specific heats of substances. FIG. 128. Cooling apparatus for Bunsen's calo- rimeter, already referred to, Fig. 127. Bunsen's method of determining specific heats of substances may be explained by reference to Figs. 127 and 128. S is a glass tube carefully graduated or calibrated. D is an iron reservoir containing mercury; the latter extends from the line through the tube C up into the tube S. The space B is filled with water. A is at first empty. In using the apparatus, intensely cold alcohol is passed in a current through the tube A by use of the apparatus, Fig. 128. In due time the water in B is frozen completely. The cold alcohol is then withdrawn. Next the entire apparatus is placed in melting snow or ice to bring the whole to zero centigrade. Next the posi- tion of the summit of the mercury column in the tube S is carefully observed. Now one gramme of water at 100 C. is placed in the tube A. The water ATOMIC WEIGHT. 21$ melts a certain portion of the ice in B. Thereupon contraction takes place, as is indicated by the rise of mercury in B and the fall of mercury in S. The point to which the summit of the mercury column in S now retracts is observed. The distance this summit has traversed corresponds to the specific heat of water. Subsequently the substance to be tested is raised to the temperature of 100 C. Then one gramme of it is placed in the tube A. Thereupon it melts another portion of ice. Further retrac- tion of mercury takes place. The amount of such retraction being meas- ured and compared with the retraction due to one gramme of water, gives directly the specific heat of the substance in question. Specific Heats of Compounds. The specific heats of many solid compounds have been determined. In a few cases they are in general harmony with the law of Dulong and Petit; in many cases they are not. In some cases the specific heat of a solid compound containing two atoms, and having a given molecular weight, appears to be twice as great as that of a single elementary substance of the same atomic weight; and the specific heat of a solid compound of three atoms appears to be three times as great as that of a mere element whose atomic weight equals the molecular weight of the compound. This seems to show that when a compound body is heated, the rise of temperature is associated with a motion of each different atom in the molecule, and not merely with that of the molecule as a whole. While, then, it requires a certain amount of heat to impart to an elementary substance an amount of motion sufficient to produce a certain change of temperature, in case of a compound body with the same molec- ular weight more heat is required to impart to it the amount of motion that affords the same temperature as that already supposed, twice as much heat is required for compounds of two atoms, and three times as much heat for compounds of three atoms. Atomic Heats of Elements. The constant 6.3 is often called the atomic heat of an element. The significance of this expression may be explained as follows : Upon taking one unit of mass of each of several elementary substances, and then applying equal amounts of heat to each of them, it is observed that the temperature rises to different degrees in the different cases. 2l6 ATOMIC WEIGHT. Suppose, however, different masses of the substances are experimented upon, say 7 parts of lithium, 56 parts of iron, 194 parts of platinum, 204 parts of lead; it is then found that equal amounts of heat added to these several masses produce in all the same advances of temperature. Evidently, then, equal amounts of heat applied to single atoms of these substances will produce the same advances of temperature. This explains the statement that the atomic heats of the elements are the same. Special Cases. Indirect Determination of Specific Heats. In cases of certain elements the specific heat cannot readily be determined directly. This is especially true of the gaseous elements, as hydrogen, fluorine, chlorine, oxygen, nitrogen. But indirect methods have been devised. (In case of certain solid elements, as car- bon, boron, and silicon, the specific heats are abnormal. This is supposed to be due to the tendency of these elements to assume allotropic modifications.) Indirect Method for Chlorine. The specific heat of silver chloride has been learned experimentally. It is .089. Now by independent methods the molecular weight of the compound is found to be 143.1, and it is found to consist of two atoms, one of silver and one of chlorine. Multiplying the molecular weight by the specific heat (143.1 X .089), the molecular heat 12.7 is obtained. Subtracting from this number the atomic heat of silver, 6.1 (as experimentally obtained), there remains 6.6 as the atomic heat of chlorine indirectly determined. Many other similar indirect determinations for chlorine have been made ; generally speaking, they yield the number 6.4. Indirect Method for Carbon. The specific heat of carbon hexachloride (CjjClg) has been learned experimentally. It is .177. By independent methods, its molecular weight is found to be 236.4, and it is found to con- sist of eight atoms as stated. Multiplying the molecular weight by the specific heat (236.4 X .177), the molecular heat, 41.8, is obtained. Sub- tracting from this number six times the atomic heat of chlorine (6 X 6.4 = ATOMIC WEIGHT. 38.4) (41.8 38.4=3.4), there remains 3.4, which is twice the atomic heat of carbon. By this means the atomic heat of carbon is 1.7 indirectly determined. Other similar indirect determinations have given the atomic heat of carbon, in combination, as about 2. Specific Heat of Bromine. Experimental Fact. The specific heat of solid bromine has been found by direct experiment ; it is .08432. Applying the law of Dulong and Petit, i.e. dividing .08432 into the constant 6.3, and there results the quotient 75. Inference. The atomic weight of bromine is nearly 75. But the studies of bromine already referred to (pp. 204 and 209) show that the combining number is about 79.8. Its atomic weight is probably either 79.8, or 2 X 79.8, or 3 x 79.8, or n X 79.8. Now, as has been said before, while the specific heat does not give the exact atomic weight, it enables us to decide that 79.8, and not some multiple (or fraction) of it, should be accepted. Specific Heat of Iodine. Experimental Fact. The specific heat of solid iodine has been found by direct experiment to be .0541. Applying the law of Dulong 63 and Petit, - about 116. .0541 Inference. The atomic weight of iodine is about 116. But the results previously and otherwise obtained point to 126.6. Evi- dently the specific heat confirms this selection. Specific Heat of Sodium. Experimental Fact. The specific heat of sodium is found directly to be .293. 6.3 Now - = about 22. Inference. The atomic weight of sodium is about 22. But results previously and otherwise obtained have given the number 23. Evidently the specific heat confirms this selection. 2l8 ATOMIC WEIGHT. Specific Heat of Potassium. Experimental Fact. The specific heat of potassium is found directly to be .166. Now A = about 38. . I DO Inference. The atomic weight of potassium is about 38. But results previously and otherwise obtained have given the number 39. Evidently the specific heat confirms this selection. Specific Heat of Silver. Experimental Fact. The specific heat of silver is found directly to be .057. 6.3 Now - = about in. Inference. The atomic weight of silver is about in. But results previously and otherwise obtained have given the number 107.7. Evidently the specific heat confirms this selection. General Inference. - If now reference is made to the provisional table presented on p. 208, it is seen that the numbers there given secure marked confirmation from the experiments subsequently detailed. They are repeated here as SEVENTH TABLE. WELL-ESTABLISHED ATOMIC WEIGHTS. Atomic weight of hydrogen adopted as .... i. " " " chlorine found to be . . . . 35.4 " " bromine " .... 79.8 " " " iodine " .... 126.6 " " " sodium " . . . / 23.0 " " " potassium " .... 39.0 " " " silver " .... 107.7 ATOMIC WEIGHT. 2IQ < II. A STUDY OF OXYGEN AND SOME OF ITS COMPOUNDS. The study of chlorine and its affiliated elements has afforded a considerable number of suggestions. These may well be applied to other elements. Oxygen may well be studied first, because of its great importance in this as well as other relations. (a) Experimental Fact. The density of oxygen gas is about 1 6. Inference. The atomic weight of oxygen is about 1 6. (b) Experimental Fact. The volume composition of water vapor is as follows : two volumes of hydrogen and one volume of oxygen. Inference I. The formula of water is H 2 O (see pp. 226 and 229). Inference 2. The molecular weight of water is about 2 + 16 = 18. (c) Experimental Fact. The vapor density of water vapor is about 9. Inference. The molecular weight of water is about 9X2= about 18. This inference is based on results obtained under chlorine. It sustains the views already adopted in this section. (d) Experimental Fact. Gravimetric analysis shows that water is made up as follows : Hydrogen .... 1 1 . 1 1 parts by weight. Oxygen ; . . . 88.88 99-99 The ratios are as i : 8 or 2 : 16. 22O ATOMIC WEIGHT. Inference. These facts add support to the views previously accepted, and contribute greatly to create confidence in the general principles as well as the numerical results adopted. (e) Experimental Facts. Oxygen forms compounds with sodium, potassium, and silver, having the composi- tion given in the following table. (As a matter of fact it forms many others, but this set is selected as afford- ing a strict continuity to the argument.) EIGHTH TABLE. PERCENTAGE BASIS. Sodium and Oxygen. Potassium and Oxygen. Silver and Oxygen. Per cents. Per cents. Per cents. Sodium, 74.19 Potassium, 82.98 Silver, 93-9 Oxygen, 25.81 Oxygen, 17.02 Oxygen, 6.91 100.00 loo.oo 100.00 If now the results of the eighth table are computed on another basis, i.e., using the atomic weights accepted for sodium 23, for potassium 39, and for silver 107.7, there is afforded a new table. Its results are surprising, but they present a strict statement of facts in a. special form merely. NINTH TABLE. ATOMIC WEIGHT BASIS. Sodium and Oxygen. Potassium and Oxygen. Silver and Oxygen. Sodium, 23 Potassium, 39 Silver, 107.7 Oxygen, 8 Oxygen, 8 Oxygen, 8. 3i 47 "5-7 Inference. Either the atomic weight of oxygen is 8 instead of 16; or, if it is indeed 16, then the atomic weights accepted for sodium, potassium, ATOMIC WEIGHT. 221 and silver are only one-half what they should be; or else the compounds have the formulas NajO, KjO, Ag 2 O, respectively. This latter supposition satisfies all the foregoing facts (and many others) so well, that it has been universally adopted, and with it the atomic weight 16 (or thereabouts) for oxygen. On this view the following table may be arranged. It is, as to numerical relations, a strict statement of experimental facts. i TENTH TABLE. NEW FORMULAS. Na,, 23x2 = 46 K,, 39x2=78 Ag 2 , 107.7X2=215.4 O, 16 O, 16 O, 16. 62 94 231.4 Inferences from the Tenth Table, From various facts already pre- sented, the following two groups of formulas have been accepted : Hydrochloric acid, HC1 Hydrogen oxide (water), H 2 O Hydrobromic acid, HBr Hydriodic acid, HI Sodium chloride, NaCl Sodium oxide, Sodium bromide, NaBr Sodium iodide, Nal Potassium chloride, KC1 Potassium oxide, Potassium bromide, KBr Potassium iodide, KI Silver chloride, AgCl Silver oxide, Ag 2 O Silver bromide, AgBr Silver iodide, Agl An inspection of these formulas recalls the striking and very important suggestion that oxygen possesses a different numerical nature from chlo- rine, bromine, and iodine, not only in that it has a different atomic weight from theirs, but, further, in this, that while chlorine, bromine, and iodine are satisfied to combine with hydrogen, sodium, potassium, and silver, atom for atom, the atom of oxygen is only satisfied when it combines with two atoms of the elements in question. 222 ATOMIC WEIGHT. This fact is emphasized by a great many other compounds, so much so that oxygen is called a dyad, and the other elements mentioned are called monads. Further, an atom of oxygen is said to have two points of attraction, while an atom of hydrogen (and one atom of each of the other substances mentioned) is said to have one point of attraction. Yet further, the property of the elementary atoms by virtue of which they attract different numbers of atoms is called equivalence or valence. And an atom of hydrogen is accepted as the unit of valence. (See p. 223.) III. A STUDY OF SULPHUR AND SOME OF ITS COMPOUNDS. Sulphur may be discussed very much as oxygen has been. (a) Experimental Fact. The density of sulphur vapor is found to be about 32.2. Inference. The atomic weight of sulphur is about 32. (b) Experimental Fact. Sulphuretted hydrogen gas is found to be composed of hydrogen and sulphur, and to have the density about 17.2. Inference I . Its molecular weight is 34. Inference 2. It is probably made up of one atom of sulphur weighing 32, and two atoms of hydrogen weighing 2. Inference 3. Its formula is probably H 2 S. (c) Experimental Fact. Sulphuretted hydrogen gas has been found to have the percentage composition : Hydrogen . . . 5.88 parts by weight. Sulphur. ... 94.12 " Total .... 100. The ratios of these numbers are evidently as i : 16, or as 2 : 32. ATOMIC WEIGHT. 223 Inference. These facts sustain the views already presented as to the composition of sulphuretted hydrogen, and that the atomic weight of sulphur is 32. (d) Experimental Fact. The specific heat of sulphur has been found to be .188. But ~ = about 33. . I oo Inference. This affirms the selection of the number 32 as the atomic weight of sulphur. Experimental Fact. The most exact determinations of the atomic weight of sulphur have been based upon its combination with silver. The composition of sul- phide of silver has been learned by experiment to be Silver . ... . . . . 87.07 per cent. Sulphur ... . . . . 12.93 " 100. Inference i. If the atomic weight previously found for silver is 107.7, then the atomic weight of sulphur is 16, or some multiple of it. But the results already stated suggest the number 32 as the proper atomic weight. If this view is accepted, the following inference may be obtained. Inference 2. The formula of sulphide of silver is Ag. 2 S. Then The molecular weight of Ag 2 = 215.4 (87.07 per cent) The atomic weight of S = 32. (12.93 P er cent ) 247.4 (100. per cent) Inference 3. From these results sulphur is placed in the class of dyads, as oxygen was. The notion of valence already reached is therefore sus- tained by the studies of sulphur above described. Inference 4. As previously intimated, it has been learned that certain elements hydrogen, chlorine, bromine, iodine, sodium, potassium, silver have the equivalence one, and certain elements oxygen, sulphur have the equivalence two. The suggestion naturally arises that perhaps other elements have the equivalence three or four, or indeed higher num- 224 ATOMIC WEIGHT. bers yet. It may be added that this suggestion is amply sustained by facts, some of which will be presented soon. The general notion of valence is adopted as a fundamental fact of chemistry. NOTE. By gravimetric analysis of binary compounds containing on the one hand elements whose atomic weights have been provisionally adopted as just de- scribed, and on the other hand other elements, atomic weights of these latter elements may be secured sub- ject, of course, to revision in the light of additional facts such as have been already presented. CHAPTER XXII. ATOMIC WEIGHT (continued}. FIFTH STEP: CONFIRMATION OF THE ATOMIC WEIGHTS CHOSEN. THE preceding chapters on atomic weights have suf- ficed to show how a few of these important constants can be secured. The discussion has called attention to certain methods pursued and certain precepts accepted. Of course, when the atomic weight of a substance is determined by a sufficient number of independent methods, such weight may be used in fixing the molecu- lar formula of compounds of the element. On the other hand, it is a fact of deeper significance that when the molecular formula of a compound can be determined by independent methods, this formula may be of great service in securing atomic weights of ele- ments, or in confirming those already secured. Every effort is made, therefore, to determine molecular formulas of elements and compounds. I. MOLECULAR FORMULA SECURED BY VOLUME COM- POSITION. In illustration, the four type-compounds of modern chemistry may be referred to 225 226 ATOMIC WEIGHT. Hydrochloric Acid. First Fact. Hydrochloric acid consists of hydrogen and chlorine, and nothing else. Second Fact. Hydrochloric acid is a gas. Third Fact. Two volumes of hydrochloric acid gas yield by decomposition one volume of hydrogen and one volume of chlorine. Inference. The molecule of hydrochloric acid has some one of the following formulas : HC1, H 2 C1 2 , H 3 C1 3 , HCl n '. On a previous page certain reasons have been assigned for adopting the formula HC1. Yet other reasons will be assigned hereafter. (See p. 228.) Fourth Fact. Hydrogen and chlorine do not form any compound but this one, called hydrochloric acid. Inference. Hydrochloric acid is the simplest possible compound of the elements hydrogen and chlorine. Therefore it is a compound containing one atom of each. NOTE. This inference is not a particularly valid one, but it naturally arises in cases of a single compound of two elementary substances. In some cases it is distinctly misleading. Thus, previous to the recog- nition of hydrogen dioxide, but one compound of hydrogen and oxygen was known; viz. water. On this general ground the formula HO was adopted. Subsequently the volume relations and other considerations (some of them stated in this chapter) have led to the adoption of the formula H 2 O. Water. First Fact. Water consists of hydrogen and oxygen, and nothing else. Second Fact. Water may be changed into a vapor and experimented upon in that form. Third Fact. Two volumes of water vapor yield, by ATOMIC WEIGHT. 22/ decomposition, two volumes of hydrogen gas and one volume of oxygen gas. Inference. The formula of water is probably one of the following : HA HA,' H 2n O M '. But reasons have been heretofore stated, favoring the view that the Ammonia Gas. First Fact. Ammonia gas consists of hydrogen and nitrogen, and nothing else. Second Fact. The substance is a gas. Third Fact. When two volumes of ammonia gas are decomposed, they afford one volume of nitrogen gas and three volumes of hydrogen gas. Inference. The formula for ammonia gas is one of the following : H 3 N, H.N t , H 9 N 3 , The simplest formula, H 3 N, may be accepted for the present, with the intention of changing it if facts hereafter discovered demand such change. Marsh Gas. First Fact. Marsh gas consists of car- bon and hydrogen, and nothing else. Second Fact. The substance is a gas. Third Fact. Two volumes of marsh gas yield, by decomposition, four volumes of hydrogen gas. (The volume relations of the carbon cannot be stated, since carbon cannot be obtained in a state of gas.) 228 ATOMIC WEIGHT. Inference. The formula of marsh gas is one of the following : H 4 C, H 8 C 2 , The simplest formula H 4 C may be adopted at present, with the inten tion of changing it if facts subsequently discovered demand such change. (Evidently the fact that carbon is not obtainable in a gaseous form diminishes, to some extent, confidence in the formula adopted.) II. MOLECULAR FORMULA BASED UPON CHEMICAL SUBSTI- TUTION. Hydrochloric Acid. First Fact. Many chlorides may be formed by substituting certain metals for the hydro- gen in hydrochloric acid : such are the well-known chlo- rides, sodium chloride, potassium chloride, silver chloride. Second Fact. When such substitutions as those just referred to are made, it is found that the whole of the hydrogen may be replaced by a metal, but no fractional part can be. Thus it is not possible to form a chloride in which part of the hydrogen has been replaced by potassium and part of the hydrogen remains unreplaced. Replacement must be of the whole of the hydrogen or of none at all. Inference I. The amount of hydrogen in hydrochloric acid is chem- ically indivisible. In other words, it is an atom. NOTE. It is true that the chemist cannot so carry out his experiment as to work upon a single molecule of hydrochloric acid. The smallest quantity upon which he can experiment must necessarily contain millions of molecules. But this does not in any way invalidate the conclusions just reached. If in a portion of hydrochloric acid containing (say) four molecules of ATOMIC WEIGHT. 22Q hydrochloric acid, one-half of the hydrogen were replaced by potassium, there might be a change somewhat as indicated below : First Stage. Second Stage. HC1, HC1, HC1, HC1, HC1, KC1, HC1. KC1. In a sense, however, all the hydrogen in the hydrochloric acid experi- mented upon has been replaced by potassium. But the formulas given represent simply an incomplete operation. They do not represent a new and single compound containing part hydrogen and part potassium. Instead, they represent a mixture of compounds, the one having all its hydrogen replaced by potassium, as already intimated, the other having none of its hydrogen yet replaced. By a process of experiment and reasoning entirely similar to the fore- going, it may be shown that the chlorine in hydrochloric acid may be replaced by another element; for example, bromine or iodine. It is found, however, that bromine and iodine respectively replace the whole of the chlorine in the hydrochloric acid, or none at all. They cannot replace the one-half or one-fourth, nor any other fractional part of the chlorine. Inference 2. The chlorine in the molecule of hydrochloric acid is practically indivisible. That is, it is a single atom. Thus it seems to be proved that the formula for hydrochloric acid is HC1. Water. First Fact. Many compounds may be formed by substituting proper metals for the hydrogen in water. Thus the well-known compounds, potassium oxide, so- dium oxide, may be easily formed. Second Fact. When such substitutions as those just referred to are made, it is found that they may be accom- plished in two different ways ; i.e. either the whole of the hydrogen may be replaced by the potassium and sodium, respectively, or the half of the hydrogen may be so re- placed. In these cases two entirely different substances are produced. When the whole of the hydrogen is re- 230 ATOMIC WEIGHT. placed, the substance called potassium oxide is produced. It contains 78 parts by weight of potassium, and 16 parts by weight of oxygen. When the half of hydrogen is replaced, a distinct and well-known compound is pro- duced, called potassium hydroxide. It contains 39 parts by weight of potassium, I part by weight of hydrogen, 1 6 parts by weight of oxygen. Here, then, it is seen that potassium may replace the half of the hydrogen in water or the whole of it. But the displacement of hydrogen in water is not possible in any other fractional way. By a course of experiment and reasoning similar to that pursued with hydrochloric acid, it may be shown that the oxygen of water is not drv isible ; in other words, is a single atom. Inference I. It appears that the one-half of the hydrogen that is in a molecule of water is the indivisible portion of hydrogen; i.e. is one atom. In other words, it appears that the molecule of water has two atoms of hydrogen. Inference 2. It appears that the proper formula for water is H 2 O. Ammonia Gas. By the use of undoubted facts of observation and a method of reasoning similar to that pursued in the two preceding illustrations, it may be shown that the hydrogen of ammonia gas is divisible into three parts, and that its nitrogen is not divisible. Thus the inference is secured that the formula for ammonia gas is H 3 N. Marsh Gas. In similar fashion, marsh gas may be shown to have an amount of hydrogen that is divisible into four parts and not into any other fractional amounts, and that its carbon is indivisible. (See p. 168.) ATOMIC WEIGHT. 23! Thus it is apparent that the formula H 4 C should be adopted for marsh gas. The foregoing discussion has sustained an opinion previously expressed with reference to the valence of chlorine and oxygen; viz. chlorine has been made out to be a monad, oxygen a dyad, and so also nitrogen and carbon respectively a triad and a tetrad. Many other facts of chemical substitution could be presented to sustain the views just enunciated, and thence, of course, to sustain the formulas already adopted. III. RELATION OF MOLECULAR WEIGHT TO MELTING-POINTS OF SOLIDS AND BOILING-POINTS OF LIQUIDS. Chemistry as a system cannot be complete until ele- ments and compounds can be arranged in orderly lists, showing regular progression of the various physical and chemical properties of the substances. Many small groups are recognized where such a prin- ciple as this is successfully carried out. There are certain marked cases of the simultaneous advance of molecular weights and boiling-points in the case of elementary substances. For example note the following : Name. Atomic Weight. Molecular Weight. Boiling-point. Chlorine, 35.4 70.8 - 33.60 C. Bromine, 79.8 !59-6 59.27 C. Iodine, 126.6 253.2 250 C. Such advance, however, is by no means uniform. If the list of elemen- tary substances be arranged in an order commencing with that of lowest atomic weight, and ending with that of highest atomic weight, it will be found that while there is a general tendency toward increase of melting- point (for most of the elements are solid at ordinary temperature), this increase is by no means uniform or even regular. 232 ATOMIC WEIGHT. The facts with respect to compound substances are so marked, however, that the chances are that the order of arrangement of elements by atomic weights is not quite correct; that some of the elements in the solid state may have a larger number of atoms than others; and so the proper molec- ular weight of elements (at present in most cases unknown) may be at present recognized only in those few that can be given in regular order of melting-points. Here is a group showing such progress : Name. Formula. Molecular Weight. Boiling-point. Sulphur dioxide, SO 2 64 - 10 C. Sulphur trioxide, SO 3 80 46 C. In case, however, of the compounds of carbon and hydrogen, several series can be constructed which ad- vance with very striking regularity at once in molecular weight and in boiling-point. Name. Formula. M ^^ r B ^' Weight. point. Methane (marsh gas), CH 4 16 (g a s) Ethane, C 2 H 6 30 (gas) Propane, C 3 H 8 44 (gas) Butane, C 4 H 10 58 i C. Pentane, C 5 H I2 72 38 C. Hexane, C 6 H 14 86 71.5 C. Heptane, C 7 H 16 100 98.4 C. Octane, C 8 H 18 114 125.5 C. A Study of Certain Nitrogen Compounds. One use that can be made of boiling-point may be illustrated by a short study of the compounds of nitrogen and oxygen. These compounds are as follows : Nitrogen monoxide, N 2 O; vapor density, 21.99; melting-point . 99 C. Nitrogen dioxide, N 2 O 2 or NO, not liquefied at 1 10 C. Nitrogen trioxide, N 2 O 3 ; vapor density, 37.95; liquefies . . . 10 C. ATOMIC WEIGHT. 233 Nitrogen tetroxide, N 2 O 4 or NO. 2 ; v. d. anomalous \ solidifies at - 9 C. ( liquid boils at 22 C. Nitrogen pentoxide, N 2 O 5 ; solid melts at 30 C. The vapor density for nitrogen monoxide (about 22) points to a molec- ular weight 44, and this corresponds to the requirements of the formula N 2 O. The vapor density for nitrogen trioxide (about 38) points to the molecular \veight 76. This corresponds to the requirements of the formula N 2 O 3 . There are reasons that need not be specified here for adopting for nitrogen pentoxide the formula N 2 O 5 . The following question then arises with respect to the two compounds remaining : Is the formula N 2 O 2 or the formula NO to be pre- ferred for nitrogen dioxide ? Comparing the boiling- points of this compound with the compound N 2 O, called nitrogen protoxide, it is seen that the boiling-point of N 2 O is very much higher. In accordance with a general rule, substances having lower boiling-points should be of simpler constitution. A substance having the formula N 2 O 2 is of more complex constitution than a substance having the formula N 2 O. If, however, we assign to the substance designated as N 2 O 2 the formula NO, its con- stitution becomes simpler than that of N 2 O ; it then accords with the general rule. There are certain facts with respect to the substance designated as N 2 O 4 that lead to the conclusion that this is the correct formula at low temperatures. It is thought at high temperatures it dissociates, forming the simpler molecule NO 2 . (See p. 144.) NOTE. It appears probable that the following general law may be safely accepted : General Law. The more complex compounds of a series condense more easily to liquids and solids and decompose more readily by heat than the less complex compounds. 234 ATOMIC WEIGHT. IV. RELATION OF MOLECULAR FORMULA TO CRYSTALLINE FORM. Mitscherlich's law of isomorphism may be stated as follows : FIG. 129. Mass of alum crystals. In general, when two solid compounds are isomor- phous that is, have the same crystalline form, they have the same number of atoms and the same molecular arrangement. It hardly needs mention that in applying the law it must be remembered that compound radicles, like am- monium, methylamine, ethylamine, etc., must be counted as elements. ATOMIC WEIGHT. 235 It must be admitted that there are well-recognized cases of substances of similar crystalline form that are evidently not chemically analogous; again, substances possessing most marked chemical resemblances are known that solidify in differing crystalline forms. But while there are many exceptions to the law, and it is not an authoritative guide, yet it is of occasional value in confirming results obtained by other methods. I. The law is well illustrated by the alums. A certain substance called alum has been recognized with more or less distinctness for at least two thousand years. During the last century its chemical composition has been distinctly made out. It is usually expressed by the formula K 2 SO 4 AL 2 (SO 4 ) 3 24 H 2 O. FIG. 130. Diagrams showing different forms assumed by crystals of the first or regular system. It crystallizes easily and distinctly in cubes or regular octohedrons, or some simple modification of these belonging to the first or regular system. Within a few years, at least a dozen substances have been recognized which bear such marked structural resemblance to ordinary alum that they have all been called alums. Examples : Potassio-aluminic alum (ordinary alum), K.jSO 4 Al^SO^g 24 H 2 O. Sodio-aluminic alum, Ns^SC^ AL^SOJ,, 24 H 2 O. Ammonio-aluminic alum, (NH 4 ) 2 SO 4 A1 2 (SO 4 ) 3 24 H 2 O. Potassio-chromic alum, K^SC^ Cr 2 (SO 4 ) 3 24 H 2 O. Ammonio-ferric alum. (NH 4 ) 2 SO 4 Fe 2 (SO 4 ) 3 24 H 2 O. The fact that these all crystallize in forms similar to ordinary alum leads chemists to confidently adopt for them the same general molecular formula as that assigned to ordinary alum. Yet, further, two other inferences are drawn ; viz. that potassium, sodium, and ammonium are radicles of analo- 236 ATOMIC WEIGHT. gous character, and that aluminium, chromium, and iron are also of anal- ogous chemical character. 2. The substances calcium carbonate, potassium nitrate, and sodium nitrate have some marked crystalline resemblances. Thus a certain form of calcium carbonate, found crystallized in nature and called aragonife, is rec- ognized as having the same crystalline form as potassium nitrate. Again, a slightly different form of calcium carbonate, though of the same chemical composition, but found crystallized in nature in the form called calcspar, has a crystalline form similar to that of common sodium nitrate. Here, then, are three substances closely related as to crystalline forms. Analysis has shown that possible formulas for these substances are CaC0 3 , KNO 3 , NaNO 3 . The crystalline resemblances then favor the adoption of these formulas. It may be added that the formula CaCO 3 leads to the approval of the num- ber 40 as the atomic weight of calcium, and the other formulas favor the employment of the numbers 23 and 39, already well substantiated, for sodium and potassium respectively. V. RELATION OF MOLECULAR FORMULA TO MOLECULAR STABILITY. The question has arisen whether the substance known as nitrogen dioxide should have the formula N 2 O 2 , or the formula NO. Facts already given, under the head of boiling-points, indicate pretty clearly that the formula should be NO. Certain facts with respect to the decomposition of this substance as compared with the decomposition of the substance nitrogen monoxide (N 2 O) sustain this view. Thus the general rule is that substances of more complex composition possess less chemical stability than substances of less complex composition. Now when a piece of glowing phosphorus is plunged into the gas nitrogen monoxide (N 2 O), it readily decomposes the molecule, withdrawing oxygen; ATOMIC WEIGHT. 23 / and the phosphorus continues to burn by combining with this oxygen. But when burning phosphorus is introduced into the gas called nitrogen dioxide (N 2 O 2 or NO), it is extinguished; that is, it does not withdraw the oxygen. The conclusion is that the molecule of nitrogen dioxide is of greater chemical stability than the molecule N 2 O, and therefore is of simpler constitution than the molecule N 2 O. This favors the assumption that the formula of nitrogen dioxide should be taken as NO, and not as N 2 O 2 . For if the formula were accepted as N 2 O 2 , we should have the more complex molecule, having the greater chemical stability instead of the less, as the general law demands. (See p. 232.) VI. MOLECULAR FORMULAS SUGGESTED BY RELATIONSHIP. The substances marsh gas, methyl alcohol, and formic acid are naturally related. The accepted formulas are as follows : Marsh gas . . . CH 4 , Methyl alcohol . . CH 3 OH, Formic acid . . . HOCHO, or HCOOH. So the substances ethane, ethyl alcohol, and acetic acid are naturally related. The accepted formulas for these are as follows : Ethane .... C 2 H 6 , Ethyl alcohol . . C 2 H 5 OH, Acetic acid . . . HO(C 2 H 3 O), or CH 3 COOH. Now the formulas chiefly in question are those of formic acid and acetic acid. But the fact that the marsh gas and methyl alcohol, whose formulas are established, have each one atom of carbon, favors the assump- tion that formic acid contains one atom of carbon, and therefore that it has the formula assigned. Again, the fact that ethane and ethyl alcohol, whose formulas are well established, have each two atoms of carbon, favors the assumption that acetic acid has two atoms of carbon, and therefore that it has the formula assigned. 238 ATOMIC WEIGHT. VII. MOLECULAR FORMULA SUGGESTED BY PRODUCTS OF DECOMPOSITION. When the two organic compounds, formic acid and acetic acid, are made to combine with alkaline sub- stances to produce respectively formates and acetates, it is observed that 46 parts of formic acid do the same work as 60 parts of acetic acid. These numbers may then be taken temporarily as the molecular weights of the two substances. Taken in conjunction with other facts of analysis, the following statement may be pre- pared : Formic Acid. The formula HCOOH corresponds to molecular weight 46. Acetic Acid.^he. formula CH 3 COOH corresponds to molecular weight 60. But analysis has shown that 46 parts of formic acid contain 12 parts of carbon, and that 60 parts of acetic acid contain 24 parts of carbon. Next consider the products of decomposition. Experiment has shown that when these acids are subjected to the cur- rent of the galvanic battery they are decomposed. Now it is observed that from formic acid but one carbon compound is produced, i.e. carbon dioxide. But from acetic acid two compounds of carbon are produced, carbon monoxide and ethane. These facts suggest that the carbon in formic acid acts somehow as a unit, while in acetic acid there is such a difference in the condition of the carbon that it is easily susceptible of division into at least two parts, the one part doing one thing, the other part doing another thing. These facts while not absolutely conclusive favor the opinion that formic acid contains one atom of carbon, while acetic acid contains two atoms of carbon. ATOMIC WEIGHT. 239 VIII. THE ADOPTED MOLECULAR FORMULA SUPPORTED BY CERTAIN EXCEPTIONAL COMPOUNDS. Certain double salts, produced naturally or artificially, may point out the existence of definite molecular groups. Thus in the Solvay soda works, at Syracuse, N.Y., a salt was artificially formed in considerable quantity (al- though decomposable by water) which had practically the following formula : MgCO 3 Na 2 CO 3 NaCl. This combination was evidently of those single groups which are accepted as molecules. Thus it seems to establish the three formulas adopted for the three substances taking part in it. 1 IX. OTHER ILLUSTRATIONS OF THE CLOSE CONNECTION BE- TWEEN THE PROPERTIES OF SUBSTANCES AND THEIR MOLECULAR WEIGHTS. (a) Density of Liquids as related to their Molecular Structure. The following law, called Groshans's law, has been enunciated : At the temperature of ebullition, the density of compound bodies in the liquid state is in proportion to the number of the atoms in their molecules. (&) The Relation of the Physiological Action of Inorganic Compounds with their Molecular Weights. In a Study of the influence of different salts in solution when intro- duced into the blood of living animals, it has been observed that the acid radicle of the salt has but little 1 Chemical News, Ivii. 3. 240 ATOMIC WEIGHT. influence. Any physiological action produced depends almost entirely on the electro-positive component of the salt, i.e. upon the metal. Again, it has been noted that practically all the elements found in organized bodies have atomic weights of less than 40. Thus it appears that all the positive elements among them are monads and dyads. Now, it has been noted that the physio- logical action of substances increases from the monads onward. Dr. J. Blake, who has studied this subject, points out that the monads tend to affect but one set of tissues or organs, the pulmonary arteries. The dyads affect two or more, i.e. the centres of vomiting, the voluntary and cardiac muscles, while with elements of higher equivalence the influ- ence is more widespread and therefore more considerable : it extends to the ganglia and even the brain itself. Again, experiments seem to show that the physiological efficiency of substances belonging to one and the same isomorphous group is directly proportional to the atomic weights; i.e. the higher the atomic weight, the greater the action. The law has been studied with respect to the following substances : first, lithium, sodium, rubidium, thallium, silver; second, magnesium, iron, manganese, cobalt, nickel, copper, zinc, cadmium; third, calcium, strontium, barium, lead; fourth, palladium, platinum, osmium, gold. In case of chlorine, bromine, and iodine, however, the action seems to vary inversely as the atomic weights. In case of potassium and ammonium the influence is also partially exceptional. The whole study is an important one and may be looked upon as likely to offer more valuable information in the future. Thus it may be that the biological relations of chemical substances may assist in determining the position of atoms and molecules in the chemical scale. 1 (c) The Magnetic Rotary Polarization of Compounds as related to their Chemical Constitution. Chemists have long felt assured that such rotation was dependent 1 Blake, J., Chemical News, xliii. 191 ; xlv. in ; Ivii. 194. ATOMIC WEIGHT. 24! upon the kind of molecules involved as well as their number, yet difficulties in the way of demonstration have prevented the attainment of any satisfactory con- clusions. Dr. Perkin has studied this subject very carefully. He has adopted a new system of unit lengths for those portions of the substances experi- mented upon; that is, he has employed such portions of liquid compounds as would produce unit lengths of columns of vapor when in the latter con- dition. As a result, it appears that certain definite relations do exist between this magnetic rotary power and the molecular constitution of bodies. It is not practicable to express these results in a few words. Reference, therefore, must be made to Perkin's original paper. 1 ( ' bd = 1 P fe to to u> O 1 1 "O So 3 s w "o"^ P ft S 1 3 Sfi i |'& H ! pg K Q I I ? I ^ c^ 246 ATOMIC WEIGHT. SECOND. It throws most of the elements into groups and series which accord with many of their undoubted geological, physical, and chemical properties. It cannot be denied that in this system some elements are brought together that do not appear to be closely related. This is merely equiva- lent to admitting that the system is not yet perfect. Probably, also, some so-called elements are wrongly placed because of their peculiar compound nature. THIRD. It helps in the decision as to which of several combining num- bers of an element shall be accepted as its atomic weight. Thus indium might have the number 75.6 or the number 113.4 (one and a-half times the former). The latter number is now chosen under guidance of the periodic law. FOURTH. The periodic table has shown some gaps in the series of numbers representing atomic weights. On this basis as long ago as 1871 Mendeleeff predicted the existence of two new elements, and more, he stated their general range of properties. To one he gave the provisional name eka-aluminium. Now in 1876 the element gallium was discovered, and it proved to be the predicted eka-aluminium. So scandium, discovered in 1879, proved to be Mendeleeff 's predicted eka-boron. The recently discovered element samarium fell into a place not previ- ously occupied, thus contributing to support the system. Algebraic Expression of the Periodic Law. Professor Carnelley has recently studied the periodic law with a view to expressing its numerical relations in the form of an algebraic formula. For reasons which are given in detail in the memoir, an expression of the form is adopted, where A represents the atomic weight of the element ; c, a constant ; m, a member of a series in arithmetical progression, depending upon the horizontal series in the periodic table to which the element be- longs ; and v t the maximum valence, or the number of the vertical group of which the element is a member. ATOMIC WEIGHT. 247 From a number of approximations Professor Carnelley finds that m is best represented by the value o in the lithium-beryllium-boron, etc., hori- zontal series; by 2i, in the sodium series; 5, in the potassium series; and 81, 12, 15 1, 19, 22.1, etc., in the subsequent series. Thus m is a member of an arithmetical series of which the common difference is 2\ for the first three members and 3] for all the rest. On calculating the values of the constant c from the equation A m + Vz> for 55 of the elements, the numbers are all found to lie between 6.0 and 7.2, with a mean value of 6.6. In by far the majority of cases the value is much closer to the mean 6.6 than is represented by the two extreme limits; thus in 35 cases the values lie between 6.45 and 6.75. If the number 6.6, therefore, is adopted as the value of c, and the atomic weights of the ele- ments are then calculated from the formula the calculated atomic weights thus obtained approximate much more closely to the experimental atomic weights than do the numbers derived from an application of the atomic heat approximation of Dulong and Petit. The number 6.6 at once strikes one as being remarkably near to the celebrated 6.4 of Dulong and Petit, and Professor Carnelley draws the conclusion that there must be a connection between the two. This assumption appears to be supported by several interesting facts. 1 Prout's Hypothesis. As early as 1816 the theory was suggested that the atomic weights may be represented by numbers that are exact multiples of that of hydro- gen,, This led to the further suggestion that possibly hydrogen is a sort of " primordial matter which forms the other elements by successive condensations of it- self." The most critical determinations of the atomic weights seem at present to afford numbers that are not integral 1 Nature, xli. 304. 248 ATOMIC WEIGHT. multiples of that for hydrogen. But in most cases the variations are but slight, and it cannot be declared with certainty that the atomic weights at present held may not be subject to corrections such as will in future afford numbers sustaining Prout's proposition. Thus recent and careful recalculations of the atomic weights show that " thirty-nine out of sixty-five elements have weights varying each by less than the tenth of a unit from even multiples of the atomic weight of hydrogen." Of the remaining elements, twenty-six have atomic weights that are known to be defectively determined. Thus Prout's hypothesis acquires new interest. CHAPTER XXIV. ATOMIC WEIGHT (continued}. ELEMENTARY SUBSTANCES AS MOLECULAR. IT is believed that in most cases elementary sub- stances are arranged in groups which may be properly called molecules. FIRST. This view is sustained by the volume condi- tions of certain chemical unions. Example a. Two volumes of hydrogen and two volumes of chlorine combine to form four volumes of hydrochloric acid gas. In accordance with Avogadro's law, two volumes of hydrogen may be considered as having n molecules of hydrogen, and two volumes of chlorine ;/ molecules of chlorine. Then four volumes of hydrochloric acid gas must have 2 n molecules of the substance represented by HC1. But each of these 2n molecules contains at least one atom of hydrogen, a total of 211 atoms of hydrogen. But these 2n atoms of hydrogen are derived from n molecules of hydrogen. Therefore each molecule of hydrogen has two atoms of hydrogen. By a similar process of reasoning it is appar- ent that each molecule of chlorine contains two atoms of chlorine. In certain other cases, elements may be shown to contain four atoms in the molecule. In some cases, it is true, the number of atoms in a molecule cannot be readily determined. 249 250 ATOMIC WEIGHT. Example b. When sulphur burns in oxygen gas to form the molecule SO 2 , there is neither increase nor decrease of volume; i.e. apparently one molecule of oxygen gas has furnished one molecule of sulphur dioxide, SO 2 . This favors the theory that the amount of oxygen gas in the molecule SO 2 is the same as the amount of oxygen gas in one molecule of oxygen. Whence one molecule of oxygen gas appears to consist of two atoms. When carbon burns in oxygen gas to form the molecule CO 2 , there is neither increase nor decrease in volume. By a course of reasoning similar to that just employed with respect to sulphur dioxide, the view then brought forward is sustained by the burning of carbon. SECOND. This view harmonizes with certain facts of tkermo-chemistry. Example a. Phosphorus burns in the gas nitrogen monoxide, N 2 O. It also burns in oxygen gas. Carbon, also, will burn in either of these gases. Now it is ob- served that more heat is evolved when the combustible burns in nitrogen monoxide than when it burns in oxygen. According to the molecular theory, oxygen has mole- cules as well as nitrogen monoxide. The heat afforded by the combustion represents a difference between the true amount of heat evolved by the chemical action and the amount of heat absorbed in decomposing the mole- cule containing oxygen. The less amount of heat given off by the combustion in pure oxygen suggests that when the phosphorus withdraws an atom of oxygen from its companion atom, more energy is expended than when it draws the oxygen from the companion nitrogen. Example b. When the very explosive compound called chloride of nitrogen decomposes, the reaction is as follows: 2 NC1 3 exploded N 2 + 3 C1 2 . Now when this operation goes on, 38,100 units of heat are liberated. This extraordinary result is believed to be explainable, on the molecular theory, as due to the fact that the affinity of an atom of nitrogen for another atom ATOMIC WEIGHT. 2$ I of nitrogen, and the affinity of an atom of chlorine for another atom of chlorine, are far greater than is the affinity of the atom of nitrogen for the atoms of chlorine with which it was combined in the explosive substance. It is true that nitrogen as an elementary gas, and under certain other circumstances, is very inert. But nitrogen is found as a constituent of a very large number of compounds. Some of these are very stable, and the nitrogen holds to the other constituents with great affinity. Ammonia gas (NH 3 ) is an example. This shows no fundamental lack of affinity in the nitrogen atom. Perhaps the very inertness of the free nitrogen molecule may be due to the affinity binding the two atoms of this molecule together. THIRD. The facts of allotropism accord with this theory. Some elementary substances are capable of undergoing a great change in their properties without any change of weight, and without any addition or withdrawal of other kinds of matter. Thus a given portion of ordi- nary oxygen may be changed at least partly into ozone and back again. So ordinary phosphorus may be changed into red phosphorus and back again. One of the forms of such substances is called the allotropic form of it, and this general property of bodies is termed allotropism. The view now held is that such modifica- tions represent some change in the number of atoms in the molecule of the element, or some change in their arrangement within the molecule. Occasionally the term allotropism is applied to compounds to describe a kind of physical isomerism. Indeed, the three states of aggregation in which a compound may exist may be considered as representing a sort of allotropism. Many natural minerals display a kind of allotropism in their differences from the same substances artificially produced. The properties of ozone support the theory in ques- tion. When oxygen changes to ozone, condensation occurs without loss of matter ; and when ozone changes 252 ATOMIC WEIGHT. to oxygen, corresponding expansion occurs. Ozone is i times as heavy as oxygen ; in other words, ozone has the density 24, and oxygen has the density 16. According to Avogadro's law, and the molecular theory of elements, ozone should have the molecular weight 48, and oxygen the molecular weight 32. This means, then, that the molecule of ozone should have three atoms of oxygen, and a molecule of ordinary oxygen should have two atoms of oxygen. Now ozone is characterized by two striking properties at first incon- sistent; i.e. it readily oxidizes certain substances, like metallic silver, add- ing oxygen to the silver. Again, it readily reduces certain substances like oxide of silver; i.e. withdrawing oxygen from the silver. But the incon- sistency is immediately explained by the molecular theory. Ozone (O 3 ), by imparting oxygen to silver, is itself changed to the more stable form O 2 ; i.e. ordinary oxygen. Again, ozone (O 3 ), in withdrawing oxygen from oxide of silver, at once gives rise to 2 O 2 ; i.e. to two molecules of the stable form of oxygen called ordinary oxygen. FOURTH. This theory affords the best explanation of the properties of certain compounds like hydrogen dioxide (H 2 O 2 ). Like ozone, it readily withdraws oxygen from certain substances and it readily adds oxygen to certain substances. The apparent inconsistency is at once explained by the molecular theory. By adding oxygen to certain substances the unstable molecule H 2 O 2 is changed to the stable molecule H 2 O. On the other hand, in withdrawing oxygen from certain sub- stances, one atom of oxygen withdrawn unites with one atom of oxygen from the H 2 O 2 , thus producing the sta- ble molecule O 2 . But the existence of such a molecule as O 2 is the very point that this discussion is now intended to support. FIFTH. The superior activity of substances in the ATOMIC WEIGHT. 253 nascent state is best explained by this theory. It is supposed that atoms of a gas v/hen freshly liberated, in other words, when in the nascent state, have not yet combined to form molecules of that particular element. They are then more ready to combine with other sub- stances than when, as in their ordinary condition, they have to leave companion atoms to do so. SIXTH. The theory accords with the facts enunciated in the law of Charles. If the expansion of compound gases by increase of heat is due to a forcing apart of a certain number of mole- cules, the fact that the elementary gases obey the same law shows that they must have molecules of the same size as those of compound gases. If elementary gaseous substances were composed of single atoms of smaller size, they should be expected to expand at least twice as much as compound gases by accession of heat. It must not be supposed that all the substances known to chemists afford satisfactory information as to their molecular structure. This is true of elementary sub- stances, and of compound substances as well. Chemists are still in the dark with respect to the proper molec- ular formulas of certain elements and compounds. For the present, molecular formulas as well as atomic weights are adopted that are recognized as only approximate, and that are likely to demand^ revision hereafter. NDEX. [THE NUMBERS REFER TO PAGES.] Acid, acetic, 237, 238. formic, 237, 238. hydrochloric, 77, 226, 228. Adhesion between gases and gases, 136. between liquids and gases, 134. between liquids and liquids, 131. between solids and gases, 130. between solids and liquids, 114. between solids and solids, 114. Affinity, chemical, 3, 141. Air, atmospheric, 139. Allotropism, 251. Alloys, 128. fusible,' 49. Alum, 126. crystalline form of, 235. Amalgams, 129. Ammonia fountain, 134. gas, 227, 230. gas, liquefaction of, 59. gas, thermolysis of, 146. Ammonium, 27. Ampere, A. M., 78, 80, 188, 190. Analysis, 26. Andrews, Thos., 20, 35, 58. Antiseptics, 165. - Arragonite, 236. Atmosphere, terrestrial, 137. Atoms, 3, 14, 15, 29, 30, 31, 32. Attraction, chemical, 168, 170, 187, 188, 189. Avogadro, 12, 78. Bacteria, 156, 158, 165, 166. Balance, 7, 12, 138. Barium, 3. Barometer, 8, 9, 10, 69. Hecquerel, 188. Benzene, 32. Berthelot, M., 176, 177. Berthollet, 188. Berzelius, J. J., 12, 188, 194, 195, 200. Bismuth crystals, 108. Botany, i. Boyle, 63, 64, 65, 67. Brodie, 29. Bromine, 217, 231. Bunsen, ice calorimeter, 214. Cadmium, 3. Cailletet's apparatus, 61. 254 Calcspar, 236. Calorimetry, 178, 181. Calory, 177. Cane sugar, 124. Capillarity, 115, 116. Carbon, 216. dioxide, 139. Carnelley, Thos., 29, 244, 246. Carre, ice-machine, 51. Cathetometer, 69. Charles, J. A. C., 65, 66, 68. Chemistry, 2. Chlorine, 216, 231. Cleavage, 103. Cohesion, 101, 102. Colloids, 134. Compound radicles, 27, 34. Compounds, specific heat of, 215. Cooke, J. P., 194. Corn starch, 153. Corrosive sublimate, 165. Critical point, 58, 63, 64. Crookes, Wm., 30, 38, 39, 41, 194, 244. Cryohydrates, 128. Crystals, 109, no. Crystallization, 103, 104, 108. Dalton, John, 6, 29, 196, 200. Davy, Sir H., 188, 189. Deleuil, apparatus, 58. Deliquescence, 122. Deville, H. St. C., 145. Dialysis, 132, 133. Diathermancy, 19. Diffusipmeter, 68. Dissociation, 49, 50. Dobereiner's lamp, 131. Dulong, 213. Dumas, J. B. A., 12, 188. Efflorescence, 128. Eka-aluminium, 246. Eka-boron, 246. Electricity, 93, 94, 96, 97. related to chemical change, 147. Electric arc, 96, 147. spark, 148. Electrolysis, 24, 147. Electroplating, 97, 147. Elements and compounds, 5. INDEX. 255 Elements considered molecular, 249. atomic heats of, 215. specific heats of, 212. Energy, 82, 172. Ether, the, 83. Ethylene, 237. Eudiometer, 12. Eutexia, 49, 123. Evaporation, 54. Expansion, 85. Faraday, M., 29. Formula, molecular, 225, 234, 236, 237, 238. Freezing mixtures, 123. Gas, 23, 36, 57, 67, 209, 212. Gay-Lussac, 75, 76. Geissler tubes, 24, 25, 91. Geology, i. Gerhardt, 12. Germicides, 165. Gladstone, J. H., 29. Gmelin, L., 188. Goniometer, 105. Graham, Thos., 29, 67, 130. Granite, 114. Gravitation, 99. Gunpowder, 171. Guthrie, F., 127. Hannay, 131. Heat, 42, 82, 83, 144. latent, 53, 54, 57. specific, 45, 46. from chemical combination, 184, 185. Heterogeneity, 17, 18, 21, 23, 25. History, natural, i. Hoff, J. H. van 't, 118. Hofmann, A. W., 27, 28. Hogarth, 131. Holtz machine, 148. Hydrargyrum, 3. Hydrogen, 70. dioxide, 252. Ice calory, 177. Iceland spar, 21. Ice-machine, 51, 52, 53, 56. Infusoria, 157. Iodine, 183, 217, 231. Kekule", 32. Koch, 155, 156. Latent heat, 53, 54, 57. Laurent, 12. Lavoisier, A. L., 6, 8. Laws, of definite proportions, 12, 72. of Charles, 65. of Dulong and Petit, 213. of Gay-Lussac, 75. of Henry and Dalton, 71. of Groshans, 239. of insolubility, 171. of isomorphism, 104, 234. of Mariotte, 63. Laws, of Mitscherlich, 234. of multiple proportions, 12, 73. of maximum work, 177. of volatility, 172. Lead, 128. boro-silicate of, 23. Leucomaines, 166. Light, 20, 22, 86, 87, 88, 146. Liquid, 36, 52, 59, 60, 132, 134, 239. Liquefaction, 47, 48, 121. Lockyer, J. N., 29, 92, 149. Mallet, J. W., 194. Mariotte, law, 63. Marsh gas, 227, 230, 232. Masses, 3, 6, 99. Matter, 3, 7, 15, 35, 38, 63, 172. Matthieson, 130. Maxwell, Clerk, 176. Mechanics, 2. Mercury, 3. as a solvent, 119. Melting, 43, 46, 47, 48. Mendeleeff, D. I., 29, 197, 243. Methyl alcohol, 237. Meyer, L., 29, 244. Meyer, V., 146. Microbes, 150, 155, 157, 159. Microcrith, 3, 24, 75, 199. Mills, 29. Mitscherlich, 104, 234. Moistening, 115. Molecule, 3, 4, 15, 28, 78, 101. Muir, M. M. P., 176, 177. Mycoderma, aceti, 158, 159. vini, 158. Nascent state, 253. Naumann, A., 176. Newlands, J. A. R., 29, 197, 243. Newton, Sir Isaac, 188. Nitrogen, oxides of, 73, 74, 232, 233, 236. Occlusion, 130. Organic compounds, 151, 153. Organized bodies, 105, 154. Osmose, 132. Oxygen, study of, 219. unit of atomic weight, 200, 202. Ozone, 5, 23, 251, 252. Palladium, 130. Pasteur, L., 34, 155, 160, 161, 163. Pattinson, 128. Peiiodic law, 243, 246. Perkin, W. H., 241. Petit, 213. Philosophy, natural, 2. Physics, 2. Platinum, 130. Polariscope, 23. Polarity, 102. Polarization, 22, 240. Potassium, 205, 218. Potassic nitrate, no. Potato starch, 152. 2 5 6 INDEX. Pressure, in liquefaction of gases, 58. Protagon, 5. Protyle, 30. Front's hypothesis, 247. Ptomaines, 166. Quartz, 112. Radicles, compound, 27, 234. Radiometer, 40. Raoult, 38, 118, 241. Rayleigh, 194. Refraction, 21, 22. Regnault, H. V., 211. Ruhmkorff, 148, 150. Saccharimeter, 23. Sodium, 205, 217. chloride, 26. sulphate, 126. Salt, crystals of, in. Saturation, 122. Science, i. Silver, 127, 136, 218, 223. Slit, for spectroscope, 89. Solid, 36, 124. Solubility, of gases, 71, 134, 135. of solids, 118. Specific heat, 214, 216, 247. Spectroscope, 86, 87, 89, 90, 91. Spencer, Herbert, 29. Spheroidal state, 116. Starch, 5, 152, 153. Stas, 194. Steam, latent heat of, 53. Sterilizing apparatus, 160, 162. Stokes, 29. Sugar, no, 125. Sulphur, a study of, 222. crystals, 109. dioxide, 56, 60, 232. density of vapor, 146, 222. specific heat of, 223. Sulphur trioxide, 232. use as disinfectant, 164. Sulphuretted hydrogen, 222. Synthesis, 26. Syphon, 71. Systems, of crystals, 105. Temperature, 58, 84. Theory, atomic, 6. Thermal units, 177. Thermo-chemistry, 183. Thermo-dynamics, laws of, 176. Thermometer, 65, 67, 84, 85. Thermolysis, 144. Thomson, Sir Win., 31. Thomsen, Julius, 38, 176, 177. Type compounds, 202, 225. Uranium, 200. Vapor, 37. density, 211, 213. rressure, 52. processes, 150. Water, 226, 229. as a solvent, 119. calory, 177. electrolysis of, 148. latent heat of, 47. spheroidal state of, 117. vapor, 77, 219. Weight, atomic, 193, 196, 199, 204, 209, 245. molecular, 231, 239, 241. Wheat starch, 152. Wislicenus, 34. Work, 191. Yeast plant, 152. Zero, absolute, 85. Zoology, i. Zinc, 3, 120, 168. , Apple ton. T.H. A7 1897 Lessons'ii T) Vi ilon <*>r>Vi a chemical p Bioiaib. ft V7 743121 UNIVERSITY OF