quantity of a chemical compound, viz., in a quan-
tity occupying, in tlie state of vapor, the same
vofunie as 2 parts Ijy weight of hydrogen gas im-
der the same conditions of temperature and pres-
sure. This characteristic number is called the
atomic uciyht of the element, and is generally
implied in its chemical symbol ; thus the symbol
CI stands for 3.5.5 parts of chlorine, and the sym-
bol for Ifl parts of oxygen. The formula
ClOj, representing an explosive compound of chlo-
rine and oxygen, shows therefore that that com-
pound contains once 3.5. .5 parts of chlorine and
twice 10 parts of o.xygen; or, briefly, one 'atom'
of chlorine and two "atoms' of o.xygen. The
great advantage of using atomic weights as units
of measurement for the several elements consists
mainly in the fact that it permits of representing
by integral numbers (usually small ones) the
composition of any chemical compound, and that
it therefore greatly simplities inquiry concerning
the similarities and dissimilarities in the chemi-
cal nature of material substances.
In analyzing equal volumes of compounds, it was seen above that the amounts of the elements found would represent either their atomic weights or nuiltijdes of these. Water would yield 2X1 parts of hydrogen and 16 parts of oxygen; hydrochloric acid, 1 jrart of hydrogen and 35.5 parts of chlorine; caustic soda, 23 parts of the metal sodium (Na), 16 parts of oxygen, and 1 part of hydrogen ; common salt, 23 parts of the metal sodium and 35.5 parts of chlorine, etc. If, in studying these compounds, we analyzed, not equal volumes, but equal weights — say, 100 parts by weight of eacli — we would find their composi- tion in the form of percentages as follows: Wa- ter, 11.11 per cent, of hydrogen and 88.89 per cent, of oxygen; hydrochloric acid, 2.74 per cent. of hydrogen and 97.26 per cent, of chlorine ; caus- tic soda, 57.5 per cent, of sodium, 40 per cent, of oxygen, and 2.5 per cent, of hydrogen : common salt, 39.32 per cent, of sodium and 60.08 per cent, of chlorine, etc. Of course, to say that water con- tains 11.11 per cent, of hydrogen and 88.89 per cent, of oxygen is the same as to say that it con- tains 2 parts of hydrogen and 16 parts of oxy- gen. But by comparing the two sets of results just cited it is easy to see that, while the per- centage figures have apparently notliing in com- mon with one another, those derived from com- ])aring equal volumes indicate plainly the simple laws according to which the chemical elements are distributed in their compounds.
The reason for choosing the volume occupied by two parts of hydrogen gas as the standard vol- ume for the investigation of compounds is simply this, that by doing so no compound will be found to contain less than 1 part by weight of hydro- gen; so that the atomic weight of hydrogen will be 1. If, instead of two, one part of hydrogen gas were chosen as the standard, the atomic weight of hydrogen in its compounds would be found to be one-half, the atomic weight of oxygen would be 8, that of chlorine 17.75, etc. The rela- tive magnitude of the atomic weights would, of course, he the same, but the atomic weight of hy- drogen would be one-half. Now, tlie atomic weight of hydrogen being less than that of any other element, chemists prefer to assign to it the value 1, thus making it the unit with which the atomic weights of the other elements are com- pared. That by doing so they do not render their results any less general may he readily seen from the fact that the 'two parts by weight of hydro- gen gas' taken may be 2 grains, 2 grams, 2 pounds, 2 anything.
But why should equal volumes of compounds form their comparable quantities? A plausible answer to this question is furnished by Avoga- dro's hypothesis, according to which equal vol- umes of all gases and vapors contain, under the same conditions of temperature and pressure, equal numbers of molecules. Comparing the com- position of equal volumes is consequently tanta- mount to comparing the composition of single molecules; and that a comparison of molecules should he expected to bring out the laws of chemical composition, is clear. The smallest weights of the elements found in equal volumes of compounds represent, evidently, the relative weights of single atoms of the several elements, and this is why those weights are termed 'atom- ic weights.' It was the application of Avo- gadro's hypothesis that first led chemists to compare equal volumes of compounds. The first to demonstrate clearly the advantage of doing so was the French chemist Gerhardt, from whom the modern system of denoting the composition of substances is called Gerhardt's Notation.
The rule directing us to compare equal volumes of compounds in the gaseous state, and thus leading to a knowledge of atomic weights, has been extended also to the state of bodies when dissolved in some solvent. ( See Molecules — Molecular Weights.) In case, however, com- pounds can he neither vaporized without de- composition nor dissolved in any suitable solvent, Avogadro's rule cannot be applied, and then some other principle has to be employed, if in the absence of better material the refractory compound must be used for determining the atomic weight of one of its elements.
Isomorphism. One such principle is based on the general fact that isomorphous compounds, i. e. substances which have about the same crys- talline form, usually resemble one another also in their chemical composition. An example may serve to explain how this general fact is made use of for the purpose of determining atomic weights. Suppose the oxide known as alumina were the only compound available for determin- ing the atomic weight of its metal, aluminum. Alumina is neither volatile nor soluble, and hence its molecular weight could not be determined by Avogadro's rule. But it is isomorphous with the oxide of iron, which is known to contain 2 atomic weights of iron to 3 atomic weights of oxygen. Alumina is therefore supposed to have a similar composition, i. e. to be made up of 2 atomic weights of aluminum and 3 atomic w'eights of oxygen. A chemical analysis would show that alumina contains 53.03 per cent, of aluminum and 40.97 per cent, of oxygen. Calling the atom- ic weight of aluminum Al, and remembering that the atomic weight of oxygen is 16, we therefore have:
2 Al : 3 X 16 : : 53.03 : 46.97. This atomic weight is consequently found to be 3 X 16 X 53.03 Al = = 27.1—. 2 X 46.97
The Discovery of Puxong and Petit. Dulong and Petit discovered a remarkable fact, which may be stated as follows: When the atomic weight of a solid element is multiplied by its specific heat (i.e. the amount of heat required to
