ON EVAPORATION AND DISSOCIATION. 
79 
substance melting. As these results are so very unusual, it was deemed advisable to 
check the experiments, showing the possibility of heating the chloral hydrate above 
its melting-point without its fusing; entirely similar results were obtained in two 
separate series of experiments. 
§17. The general conclusion to be drawn from these experiments is, that the rela¬ 
tions between temperature and pressure with chloral hydrate differ from those shown 
in the case of bodies which do not dissociate, and indeed that a fall of pressure allows 
of a rise of temperature, and that a rise of pressure lowers the temperature of this 
dissociating body. It is also to be remarked that this substance undergoes no change 
in composition, so long as it remains solid, while it is known that when the liquid hydrate 
is distilled, separation into its constituents takes place, and that the residuum has a 
different composition from that of the distillate (Naumann, ‘ Berichte,’ 1879, p. 738). 
§ 18. As the results of a determination of the vapour-pressures of chloral by 
Moitessier and Engel are meagre, it was thought worth while to determine these 
constants by the still-method, as chloral attacks mercury, and another cause of error 
is the formation of a small quantity of hydrogen chloride when chloral is heated. 
Vapour-pressure of Chloral. 
Pressure. 
Temperature. 
Pressure. 
Temperature. 
Pressure. 
Temperature. 
millims. 
O 
millims. 
O 
millims. 
O 
5-3 
-13-0 
29-6 
' 18-0 
248-6 
65-2 
7-5 
- 6-9 
37-9 
22-7 
296-6 
69-7 
8-4 
- 5-0 
49-8 
28-2 
358-3 
74-8 
10-3 
- 1-0 
61-9 
32-0 
415-0 
79-7 
12-3 
+ 2-0 
62-9 
32-7 
463-3 
82-7 
13-6 
4-0 
72-6 
35-9 
518-0 
85-3 
15-7 
6-5 
79T 
37-5 
565-0 
88-6 
16-5 
6-9 
83-8 
38-9 
608-0 
90-7 
170 
8-0 
95-5 
41-9 
671-5 
93-8 
18-3 
9-2 
121-3 
47-6 
704-9 
95-3 
20T 
10-8 
160-0 
54-0 
21-6 
12-8 
200-7 
59-7 
§ 19. It will be noticed from § 10 that Naumann found the vapour-density at 7 8 A 0 
under a pressure of 162 millims. to point to complete dissociation. But the vapour- 
pressure of chloral hydrate at 78’5° is 322 millims., hence the vapour in his experi¬ 
ments was very far from being saturated. Since dissociation decreases with increase of 
pressure, it was deemed possible that at pressures nearer saturation, dissociation might 
not be complete. The following experiments were carried out to test this point:— 
A quantity of substance was roughly weighed (about 0’01 6 gram.), so calculated that 
its vapour would exert a pressure nearly that of saturation at 60 ; and its vapour- 
density was then determined by Hofmann’s method; the tube was heated by alcohol 
boiling under reduced pressure. 
