185.8° Equal increments in the molecule are associated with an equal rise in the boiling-point, but this increment varies in different homologous series.
In the aromatic compounds there is no regularity between the increments due to the introduction of methyl groups into the benzene nucleus or side chains; the normal value of 20 0 -21° is exhibited, however, by pyridine and its derivatives.
This is shown in the case of the chloracetic acids: According to van 't Hoff the substitution of chlorine atoms into a methyl group occasions the following increments: The introduction of chlorine, however, may involve a fall in the boiling-point, as is recorded by Henry in the case of the chlorinated acetonitriles: NC CH 3.
Thus bromine and iodine replace chlorine with increments of about 22° and 50° respectively.
If the crystal structure be regarded as composed of 0 three interpenetrating point systems, one consisting of sulphur atoms, the second of four times as many oxygen atoms, and the third of twice as many potassium atoms, the systems being so arranged that the sulphur system is always centrally situated with respect to the other two, and the potassium system so that it would affect the vertical axis, then it is obvious that the replacement of potassium by an element of greater atomic weight would specially increase the length of w (corresponding to the vertical axis), and cause a smaller increase in the horizontal parameters (x and 1/ '); moreover, the increments would advance with the atomic weight of the replacing metal.