We may therefore conclude that this number expresses the volume of one atom of carbon and two of hydrogen -that is to say, the volume of CH2.

2. Two organic compounds which differ from each other by the addition of nC and the loss of nH2 possess the same molecular volume. We may, therefore, conclude that C occupies the same volume in these compounds as H2, and as the molecular volume of CH2 is 22, it follows that the atomic volume of carbon is 11 and that of H is

=5·5.

3. The molecular volume of water at boiling point is 18.8 (instead of 18). If we subtract 11, the volume of H2, we have 7.8 for the atomic volume of oxygen. According to Hermann Kopp, oxygen only occupies this volume in organic compounds when it is contained in a typical residue, to use the expressions of that time—that is to say, when it is connected with two different atoms which it unites, as, for instance, the two atoms of hydrogen in water. It occupies a different volume when it is contained in a radical-that is to say, combined by its two points of saturation to the same atom of carbon as in aldehyde and acetone.1

Aldehyde containing C,H,O-that is to say, 2CH, + O -the volume which is here occupied by oxygen may be found by subtracting from the molecular volume of aldehyde (56 to 56.9) that of 2CH2=44. We thus obtain 12 to 12.9 as the atomic volume of oxygen when contained in an organic radical.

Hermann Kopp adopts the mean 12.2.

1 The two forms of oxygen compounds are given in the follow. ing table, which will explain the distinction in question :—

Having thus calculated, by means of the above considerations, the volumes occupied in organic compounds by the atoms of carbon, hydrogen, and oxygen, the eminent chemist was able to calculate à priori the molecular volumes of a number of ternary organic compounds, by adding together the sum of the atomic volumes of the elements, in accordance with one of the propositions given above. The molecular volume of a compound containing a atoms of carbon, b atoms of hydrogen, c atoms of oxygen in the radical, and d atoms of typical oxygen, may therefore be given by the formula

MV = all+b5·5 + c12·2 + d7·8.

The molecular volumes calculated in this manner have been compared with those deduced from experiment, when the molecular weights are divided by the densities taken at boiling point. The agreement between calculated and experimental results in a great number of cases is sufficient to justify a serious consideration of Hermann Kopp's conclusions.

Omitting the consideration of the facts relative to the atomic volumes of other elements, such as sulphur, nitrogen, chlorine, bromine, and iodine, indirectly deduced from the molecular volumes of liquid compounds containing these elements, by means of processes similar to those just discussed, we must add a few words upon the molecular volumes of solid bodies. We must here confine ourselves to a few results regarding certain bodies endowed with a similar constitution and obtained under the same physical conditions.

4

2

4

It is found that a great number of isomorphous bodies have the same molecular volume. This is the case with the sulphates of the magnesian series SOM”+7H2O, with the double sulphates of the magnesian series SO1M”+SO,R2+6H,O, and with the alums. It seems, however, impossible to calculate the molecular volumes of solid compounds by means of the atomic volumes from the principles laid down for liquid bodies. Here the data of the problem are different. In proof of this we may, in conclusion, refer our readers to the relations pointed out by Playfair and Joule between the molecular volumes of certain crystallised

We

salts and that of the water which they contain. should suppose that the molecular volume of the crystallised salt would be equal to the sum of the volumes of the anhydrous salt and the water. But it is not so. In certain salts rich in water of crystallisation, such as the arsenates and phosphates which contain 12 molecules, and in the crystals of carbonate of soda which contain 10, the volume of this water (taken as solid) is equal to the volume of the molecule of the crystallised salt, the molecules of the anhydrous salts being as it were interposed between the molecules of water, without augmenting the volume of the latter.

BOOK II.

ATOMICITY:

OR VALENCY OF ATOMS IN COMBINATION.

CHAPTER I.

DEFINITION AND HISTORIC DEVELOPMENT OF THE IDEA OF

ATOMICITY.

In the preceding pages we have traced the origin and foundation of the atomic theory. We have seen this simple and correct idea which was brought forward by Dalton-namely, that the invariable proportions in which bodies combine represent the relative weights of their ultimate particles-gradually gain ground in science. We have explained the principles upon which the determination of these weights rests, as well as the physical laws by means of which these determinations are guided and controlled, thus rendering to the hypothesis of atoms, which belongs to