ATOMIC VOLUMES OF SALTS IN SOLUTION.

967

by the number of grains of water of crystallization which they contain: thus 143 grains, or one equivalent in grains of crystallized sodic carbonate, or 161 grains of the crystallized sodic sulphate, each of which contains 90 grains or 10 equivalents of water of crystallization, causes an increase of 90 grain measures in the bulk of the water in which it is dissolved; 123 grains, or an equivalent of magnesic sulphate, containing 7 equivalents or 63 grains of water, occasions an increase of 63 grain measures of water when brought into solution.

Volumes in grains of water of an equivalent in grains of several anhydrous salts, both when dry and when in solution (H=1).

(1736) 4. Atomic Volumes of Liquids of Analogous Composition. -In the year 1842 an important paper was published by Kopp (Liebig's Annal. xli. 79, 169), in which he showed that when liquids belonging to one homologous series are compared with corresponding terms of other collateral homologous series (vide table, p. 40), like differences are observed in their atomic volumes. When, for instance, the atomic volume of a normal acid (HA) (A representing an atom of the salt radicle of any monobasic acid) is compared with its corresponding vinic ether (¤‚H ̧Ã), the atomic volume of the acid, at ordinary temperatures, is about 42.7 less than that of the corresponding compound ether. The atomic volume of the acid (HA) is about 24 less than that of its corresponding methylic compound (ЄH,A); and as a necessary consequence, the atomic volume of an ethylic compound is about 187 greater than that of the corresponding methylic compound. A more careful examination of the subject

968

ATOMIC VOLUMES OF LIQUIDS OF ANALOGOUS COMPOSITION.

has shown that the somewhat considerable divergences observed between the atomic volumes calculated according to these laws, and those actually found by observation, are materially reduced if the liquids be compared, not at equal temperatures, but at corresponding temperatures. Corresponding temperatures are those at which the cohesion of the liquids compared is equal; or temperatures at which the liquids emit vapours of equal tension. The tension of the vapours of different liquids through the required range of the thermometric scale is, however, only known in a few instances; but it may be assumed without any very serious error, that corresponding temperatures in liquids are those situated at equal distances below the boiling points of the liquids under comparison. For instance, if the boiling point of alcohol be 173°, and if that of ether be 94°, the temperature of 60° F. would not be a corresponding temperature for these liquids; but if the alcohol be at 60°, which is 113° below its boiling point, the corresponding temperature for ether would be 113° below its boiling point, or -19°.

The atomic volume v of a compound increases as the temperature rises. This, indeed, must be evident from the consideration that the specific gravity d diminishes as the temperature rises, whilst the atomic weight q remains constant; and sincev, the atomic volume must necessarily vary inversely as the density of the body, and consequently must increase as the temperature rises. In making comparisons of atomic volumes of compounds. it is found advantageous in practice always to calculate them for the boiling point of the respective liquids under a pressure equal to that of 29'92 inches, or 760 m.m., of mercury.

At the time when Kopp first called attention to the atomic volume of liquids, few really exact data existed, by means of which his conclusions could be rigidly tested. Since that period both Pierre (Ann. de Chimie, III. xv. 325; xix. 193, and xx. 5), and Kopp himself (Poggendorff's Annal. lxxii. 1 and 223, Liebig's Annal. xciv. 257, xcv. 307), have published a series of important and elaborate researches upon the specific gravity, the expansion, and the boiling points of a considerable number of liquids, by means of which the atomic volume of these liquids at the boiling point may be calculated. The following table contains the principal results of Kopp's inquiries arranged for the convenience of comparison :

Atomic Volumes of Liquids of Analogous Composition.

Diff. in at. vol. for CH2.

3× 20'6

3×236

148 09832 4568
78 08991 1760
134 08778 3470 185 2
128 097748 424 4 149'2
114 07135 2264 186-8

Atomic Volumes of Liquid Sulphur Compounds.

970

ATOMIC VOLUMES OF LIQUIDS OF ANALOGOUS COMPOSITION.

(1737) Discussion of Kopp's Conclusions.-The atomic volumes contained in the foregoing table are those calculated by Kopp for the boiling point of their respective liquids given in the fifth column of the table. These boiling points are not in all cases the actual numbers obtained by experiment, but are in some instances modified according to considerations explained in (1738), since the boiling point of many substances is only approximatively known, the estimates of different observers sometimes varying several degrees. The atomic volumes contained in this table, therefore, differ from the earlier calculations already referred to, which were all made at ordinary temperatures. Now if we compare the hydrated acids contained in Division II. of the table, with the ethers in Division III., it appears that the atomic volume of the hydrated acid is about 43 less than that of the corresponding ether; for instance:

The difference between the atomic volume of the normal acid and that of the corresponding methylic compound is about 208Methyl butyr. = 127'3 Butyric acid = 107.8

= 418

Methyl acetate = 858
Acetic acid = 63.8

Methyl formiate = 63'4
Formic acid

19'5

22'0

216

and, as may be seen by inspection of the table, the mean difference between the atomic volume of the corresponding compounds in the ethyl and the methyl series is about 22:2:

Ethyl acetate = 107.8
Methyl acetate = 85.8

22'I

22'0

Now, since the corresponding compounds of the ethylic and the methylic series differ by (EH2), this mean difference in the atomic volume of such corresponding compounds has been assumed by Kopp to represent the volume occupied by each group of (EH) when in combination. This difference, it will be seen from the foregoing table, ranges in a large number of compounds between 20 and 24. Kopp fixes 22 as its average value at the boiling point of each compound.

Isomeric liquids of analogous composition, such, for instance, as the different metameric compound ethers, possess atomic volumes which are sensibly equal to each other, and they consequently at their boiling points have the same specific gravities; such, for example, as :

Numerous other cases may also be seen by comparing the volumes of the ethers which are bracketed together in the table, p. 969. But if the isomeric compounds belong to different and dissimilar series having a dissimilar typical constitution, the same correspondence is not observed; for example:

By comparing together liquids which contain the same number of elementary equivalents, but in which a certain number of atoms of oxygen have been substituted for a corresponding number of atoms of hydrogen, it is found that the atomic volume of oxygen is somewhat greater than double that of hydrogen; as for example:

From a similar comparison of compounds in which a certain number of atoms of carbon have taken the place of a corresponding number of atoms of hydrogen, it has been concluded that the volume of 1 atom of carbon and 2 of hydrogen are equal. This is exemplified, according to Kopp, in the corresponding compounds of the valeric (¤¡H12), and the benzoic (E,H ̧ ̧) acids; thus :

Methyl benzoate= 150'3 Methyl valerate = 149'6 0'7

10

The difference is so slight, that in computing atomic volumes from the composition of a body, each atom of carbon may be considered to occupy a volume equal to two of hydrogen. Kopp concludes that the atom of each element, in entering into combination, does so with its own specific volume; and he further assumes that the same element may have different specific volumes according to the position which it occupies in the compound. Oxygen, for example, has a value when it forms a part of the radicle, different from that which it possesses when external to the radicle:-Thus, if →=16 and H=1 at the boiling point of the different compounds,