76 
PROFESSOR W. RAMSAY AND DR, S. YOUNG 
K, stood in the same trough as the gauge G. The heights of the mercury were read 
in both cases by means of the two scales L and M, graduated on plate-glass mirror; 
parallax in reading was thus avoided. By means of this arrangement, the pressure 
in the jacket could be maintained constant for hours together, thus ensuring a 
constant temperature. In order to obtain any desired temperature between that of 
the atmosphere and 360°, the following liquids were used :—alcohol, chlorobenzene, 
bromobenzene, aniline, methyl-salicylate, bromonaphthalene, and mercury. These 
liquids have the advantage of being very stable, and of being easily obtained in large 
quantity in a nearly pure state, and at a low price. 
The vapour-pressures of all these liquids were determined with great care; and as 
this method of obtaining constant known temperatures may prove serviceable to others, 
we have published separate papers on the subject. (Trans. Cliem. Soc., 47, p. 640 and 
49, p. 37.) 
Results of Experiments. 
§ 10 . Chloral Hydrate. —The dissociation of the vapour of chloral hydrate 
q (~)TT 
CCI 3 .CH r Qjq, into chloral, CCl 3 .CHO, and water, H 3 0, has been shown by Naumann 
to be complete even at 78‘5°, under a pressure of 162 millims. The vapour-density 
determined by him was 40'85, the theoretical vapour-density of a mixture of the 
vapours of chloral and water being 41’3, while that of undissociated chloral hydrate is 
82’6 (‘ Berichte,’ 1876, p. 822). Experiments have been made by Moitessier and Engel 
on the distillation of chloral hydrate, and on its vapour-pressure under different con¬ 
ditions (‘Comptes Bendus,’ 86 , p. 971; 88 , pp. 285 and 861; 90, p. 98). Wurtz has shown 
that the vapour of chloral hydrate behaves to an anhydrous salt as though it contained 
water (‘Comptes Bendus,’ 84, p. 977; 86 , p. 1170; see also Friedel, Bull. Soc. Chim., 43, 
p. 56 ; Troost, ‘ Comptes Bendus/ 100 , p. 834); and also that the vapours of anhydrous 
chloral and of water do not evolve heat when mixed (‘Comptes Bendus,’ 89, pp. 190, 
337, 42.9, 1063). Naumann points out that the temperature of decomposition of 
chloral hydrate lies so low that the minuteness of its vapour-pressure probably pre¬ 
cludes a determination of its vapour-density at that temperature (‘ Thermochemie,’ 
p. 135). So long as it retains the solid state, however, it is proved by its physical 
properties to be a compound. It was, therefore, deemed specially important to deter¬ 
mine whether the curve representing its pressures of dissociation was identical with 
that exhibiting its volatilizing points under different pressures. 
