ON EVAPORATION AND DISSOCIATION. 
99 
glass thermometers give results closely agreeing with air thermometer readings up 
to 280°. It may he concluded that the temperatures shown by this thermometer, 
corrected in this way, are in sufficiently close agreement with those of Regnault’s 
air thermometer. 
§ 51. The temperature of volatilization of ammonium chloride was then determined 
under ordinary pressure with the apparatus shown in the wood-cut (tig. 4). A block of 
Pig. 4. 
D 
B 
ammonium chloride, B, was drilled with a hole to receive the bulb of a thermometer, A, 
and was placed horizontally at one end of an open combustion-tube, C. The stem of the 
thermometer was protected by a sheet of asbestos, I). The block was heated by 
applying the flame of a Bunsen burner to the combustion-tube, and the temperature 
registered by the thermometer was read, when constant. 
Reading' of thermometer. 
(Stem was heated up to graduation 30°.) 
Mean temperature of cool portion of stem . . 
329'5 C 
Correction for calibration at 329 \5 C 
30-4° 
-3-65 
Zero-point after experiment.Unaltered 
Corrected temperature . 32.9'5-f-14'4 — 3‘65—1 - 9 = 338‘35° 
Barometer. . . .. 762 T millims. at 0° 
By the barometer-tube method, the mean-pressure at 340° was, as before mentioned, 
759'6 millims. The difference, which is less than 2'0°, may well be due to error of 
experiment. 
§ 52. Some Experiments on the Vapour-density of Ammonium Chloride. 
These experiments were carried out by Hofmann’s method at 280° under pressures 
varying from 85 millims. to 135'8 millims. The pressure of the vapour in contact with 
solid, at 280°, is about 145 millims. This temperature is perhaps the only one in which 
Hofmann’s method can be applied ; for at higher temperatures the vapour-pressure 
of mercury becomes too great, and atlower temperatures, the vapour-pressure of 
ammonium chloride is too small. 
No serious error can be introduced by the action of the hydrochloric acid on the 
mercury, because the volume of the gas is so large, the surface of contact with the 
solid so small, and the action so slow at that temperature. 
