Page:Outlines of Physical Chemistry - 1899.djvu/79

 the thermometer, 0-0375(760 — b) degrees. The correc- tion is negative or positive, according as the barometric pressure at the time is higher or lower than normal.

In the case of substances which can only be distilled under diminished pressure, data concerning the boiling point should always be accompanied by information of the pressure at which the distillation was carried out.

��Observed 'Regularities

It has already been pointed out that, according to L. Meyer, the volatility of the elements is a periodic function of their atomic weights.

H. Kopp has proposed the following general law for organic substances. For analogous substances a given difference in chemical composition gives rise to a constant difference in the boiling point. The following additions have been made to this law by L. Meyer in his « Modern Theories of Chemistry/ l

1. The boiling points of isomeric substances of analogous constitution do not differ much :

Example. — Butyl acetate boils at 124° ; propyl pro- pionate boils at 122° ; ethyl butyrate boils at 121°.

2. Isomeric substances of dissimilar constitution have different boiling points ; and, for a given difference in the constitution, there is a constant difference in the boiling point.

Example. — Propionic acid, on the one hand, boils at 140°, methyl acetate and ethyl formate, on the other, boil at 56° and 55° respectively.

8. Where several substances have an analogous con- stitution but different molecular weights, there is a constant difference in their boiling points for a given difference in molecular weight.

Example. — In the series of the normal fatty acids the

��1 English translation by Bedson and Williams.

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