Alcohol is hygroscopic, though not remarkably so. It is miscible with water in all proportions. On mixing the two liquids, a rise in temperature occurs, and on cooling the mixture to the original temperature there is found to be a contraction of the total volume, the extent of the decrease depending upon the relative proportions of the constituents. The maximum contraction, when calculated as a percentage on the sum of the initial volumes, is given by a mixture containing one molecule of alcohol and three molecules of water,1 the amount of contraction being 3.64 per cent. at 15.56°. Expressed in another way, the maximum contraction is obtained by mixing 52 volumes of alcohol with 48 volumes of water, and the volume of the resulting mixture (measured at 20° in this case) is 963 instead of 100.

J. Holmes2 has given a table showing the actual contraction, measured in cubic centimetres at 1556°, which occurs when 100 c.c. of alcohol are mixed with increasing quantities of water. From this table, it appears that a maximum amount of contraction, 908 c.c, is reached when the mixture corresponds with one molecule of alcohol to eight molecules of water. This is to be distinguished from the maximum percentage contraction, which, as stated above, occurs with the proportions C2H6O,3H1O. A part of the table is given here by way of illustration: -

 Mixture. Volumes at temperature 15.50°. Actual contraction; initial vol. of alcohol 100 c.c. Percentage contraction, calculated on sum of initial volumes. C2H6O1/6H1O 1.21 c.c. 0.99 C2H6O,H1O 3.79 " 2.89 C2H6O,3H1O 703 " 3.64 C2H6O,6H1O 8.87 " 3.09 C2H6O,8H1O 9.08 " 2.61 C2H6O,12H1O 8.78 " 1.86 C2H6O,20H1O 7.88 " 1.09

Holmes deduces from his experiments that the ' true molecular volume" of ethyl alcohol is 322, that of water at the same temperature being taken as unity. For methyl alcohol, the "true molecular volume" deduced is 223, and for n-propyl alcohol 4 07.