1885.] 



on the Heat of Dissolution of Salts. 



403 



because I was then unaware of the existence of the allotropic modifica- 

 tions of anhydrous sulphate of sodium since discovered by Mr. Picker- 

 ing (loc. cit.), and partly from errors of experiments, which have sinco 

 been corrected. 



The apparatus I have used consists of two parts, the calorimeter 

 and the constant temperature bath. The calorimeter is a brass drum 

 9 cm. high and 7 cm. diameter, having a paddle which works in a 

 socket in the centre of the vessel, and the spindle of which passes 

 through the lid. At the top of the spindle is a reel, round which 

 passes a silk cord, the ends of which are weighted and drawn over 

 pulleys placed at a little distance on opposite sides. By gently pulling 

 the cord at either end the paddle is made to spin round. Immediately 

 e /he blades of the paddle is a wide brass tube perforated with 

 ~v~i„nd the bottom of which is immersed in the water, which is 

 placed in the calorimeter. In this tube is placed a thin glass bulb 

 containing the weighed quantity of salt and sealed at the blowpipe. 

 Standing in the tube is a thin brass rod having a fork at the end to 

 act as pulveriser, the top which projects through the lid of the calori- 

 meter, being provided with a wooden handle. The thermometer stands 

 in a perforated tube on the opposite side of the calorimeter. The 

 cover of the calorimeter is provided with three holes, one for the stem 

 of the thermometer, one in the centre for the paddle, and the third for 

 the handle of the pulveriser. The vessel is supported on three sharp 

 boxwood points within a covered brass case silvered inside, the space 

 between the calorimeter and the case being about 1 cm. all round. 



Tne bath employed for maintaining a constant temperature is 

 practically identical with that described by Nicol,* but is provided 

 with a well in the middle, into which fits closely the outer case of the 

 calorimeter. The temperature of the water or oil is remarkably con- 

 stant, and when the temperature does not exceed 30° a thermometer 

 in different parts of the bath does not vary by more than "Oo . To assist 

 in maintaining this constancy the bath is covered closely, and is 

 enveloped in thick felt. Three thermometers were used — A, from 8° 

 to 26° C, divided into twentieths; B, from 25° to 50°, and C, from 

 42° to 76°, both divided into tenths Centigrade, and the temperature 

 could easily be read to •005° and "01° respectively, or even half this. 



The course of an experiment is as follows : — The water having been 

 introduced into the calorimeter, and the thermometer inserted into its 

 tube, the whole is placed in an oven until the thermometer indicates a 

 temperature about half a degree above that to which the bath has been 

 previously heated. The calorimeter is then put into the bath, the 

 cover of the case is put on, and the thermometer read from time to 

 time till the temperature is either quite stationary or is falling steadily 

 at a rate which is recorded, and from which a correction can be intro- 

 * " Phil. Mag.," 1883, 339. 



VOL. XXXVIII. ' 2 G 



