ON EVAPORATION AND DISSOCIATION. 
77 
§11. Pressures of Dissociation of Chloral hydrate in Barometer-tube. 
Temperature. 
Pressure. 
Time required for equilibrium. 
O 
millims. 
127 
6-8 
After standing over night. 
27-9 
19-0 
34' 
39-9 
35-8 
20' 
50-8 
72-0 
24 
567 
96-2 
Constant at once. 
63-3 
146-4 
5 5 55 
78*2 
318-3 
55 55 
The chloral hydrate began to melt and partly liquefied at 50‘8°. These results 
confirm those of Moitessier and Engel. The general form of the curve based on 
these numbers is remarkably similar to that of an ordinary vapour-pressure curve. 
(Plate 1 .) 
§ 12. Temperatures of Volatilization of Chloral hydrate. 
The bulb of a thermometer was covered by dipping it repeatedly in melted chloral 
hydrate, and allowing it to solidify until it had acquired a sufficiently thick deposit. 
A bath of sulphuric acid was used to heat the tube. The temperature rose gradually; 
the pressure remaining nearly constant. At 51‘8° the substance was still perfectly 
solid, although above its ordinary melting-point, 50'6°; the pressure registered 
11 -4 millims. The temperature then rose to 57°, while the pressure rose to 
12’3 millims.; after this the pressure rose only 0'1 millim., while the temperature 
rose to 64°. Even at 64° no sign of melting was to be noticed. The experiment was 
then discontinued, and the melting-point of the sublimed chloral hydrate compared 
with that of the portion left on the thermometer. It was found to be 50*6° for both. 
The substances had, therefore, apparently undergone no change on heating. It is 
right to observe, however, that a specimen of chloral hydrate, which had been re¬ 
peatedly melted, and which was examined after two years, left on treatment with 
water an insoluble residue, the melting-point of which was above 130°. The melting- 
point of the residue on the thermometer-bulb, however, showed that it did not 
consist of this substance, which is possibly a polymeric! e. 
§ 13. In the second experiment, with the temperature at 50‘2°, the pressure was 
7 millims.; the temperature rose to 55°, pressure rising to 7‘4. It was impossible 
to lower the pressure further, for vapours given off by the chloral hydrate attacked 
the mercury of the Sprengel pump, forming mercurous chloride ; this is a remark¬ 
able circumstance, for chloral hydrate had no perceptible action on the mercury in 
the barometer-tube, at a much higher temperature. After this experiment, the 
chloral hydrate adhering to the bulb of the thermometer was carefully examined, 
and it was found that the bulb was completely coated, and that the substance was in 
perfect contact with it. The minimum thickness was 0'8 millim., and the hydrate 
