64 OUTLINES OF PHYSICAL CHEMISTRY
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��The volume of liquid displaced by the sinker is the same as in the ease of water, since the operation is carried out at the same temperature (15°C).
The water displaced weighed 1*0 and the liquid dis- placed weighs 1*8687; this number, therefore, expresses the specific gravity of the liquid. would not require to be suspended on the link, but no other change in the method would be necessary.
The Moleculab Volume. — The volume of a gram- molecule of water (at 4°C.) is 18 cubic centimetres. The volume of a gram-molecule of any other substance of molecular weight m would be m cubic centimetres if the substance had the same density as water. But the density
being <£, the molecular volume becomes 7 .
a
The molecular volume of a substance is proportional directly to its molecular weight and inversely to its density.
In order to discover any regularities which the molecular volumes of liquids might present it is essential, according to H. Kopp, to take the different substances in comparable states. Kopp selected as the temperature of comparison the boiling points of the liquids, i.e. the temperature at which their vapour tension is equal to one atmosphere.
The determination of the density of a liquid at its boiling point can be made by means of the direct method of Ramsay and Lothar Meyer. Eopp proceeded by indirect means; he took the density of the liquid at several different temperatures and thus found the cubical coefficient of expansion, and from it he calculated the density at the boiling point.
Comparison of the results obtained led to the establish-
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