188 



MR J. Y. BUCHANAN ON THE 



fell from 23'82° C. to 21*99^ C. and the solution remained liquid to the end. The 

 cooling had proceeded for 13 minutes before the temperature fell to 23 16° C, and the 

 loss of heat was taking place quite regularly. The following are the temperatures 

 observed at each half-minute for two minutes before and two minutes after the 

 temperature 23*16° C. was passed : — 



Time in minutes : -2-0 -1-5 - I'O -0-5 O'O +0-5 +1-0 +1-5 +2-0 

 Temperature: 23-23° 23*21° 23*19° 23*17° 23*16° 23*14° 23*12° 23*09° 23*07° 



During the four minutes the temperature fell 0*16° C, whence 0*04° C. per minute 

 represents the mean rate of fall of temperature when the system has the temperature 

 23*16° C. and is cooling in air of constant temperature 19*30° C. 



§ 108. Calculation of Heat liberated during Crystallisation. — The first thermal 

 effect observed was when crystallisation began. The temperature of the system rose 

 in less than a minute from 19*5° to 23*16°. During this phase the glass cylinder, as 

 well as its contents, was warmed 3*66°. The heat liberated in this act depends on 

 the weight of the solution, on its specific heat, and on the rise of temperature. In 

 determining the thermal exchange which has taken place, we have to take account of 

 the capacity for heat, which is generally represented by the " water- value," of the 

 cylinder. The numerical data required in this calculation are the following : — 



Weight of CaCla solution ...... 806*61 grams. 



Its specific heat (Regnault) . . . . . . 0*636 



Whence, water- value of solution . .... 513*00 grams. 



Weight of cylinder 463*58 



Specific heat of glass . . . . . . . 0*2 



Whence, water-value of cylinder .... 92*72 grams. 



Rise of temperature ....... 3*66° C. 



Whence, heat liberated in first act (51300 -I- 92*72)3*66 = 2217 gr.° C. 



After the first minute, when the temperature had become constant at 23*16°, the 

 rate of liberation of heat was exactly equal to its rate of dissipation, which we have 

 found to be represented by a fall of temperature of 0*04° per minute. This state 

 was maintained for 85 minutes, which requires a liberation of heat, in the second act, of 



85 X 605*72 x 0*04 = 2059 gr.° C. 



Adding the 2217 gram-degrees liberated in the first act, we find the total heat evolved 

 during the interval of 85 minutes to be 



2059 -f 2217 = 4276 gr.° C. 



§ 109. In order to verify the state of unrest above described, the experiment was 

 repeated with a 7*196 CaClg solution (§ 113), and after crystallisation was completed, 

 the crystals were removed and freed as far as possible from adherent mother-liquor, 

 and their composition ascertained by estimation of the chlorine contained in a weighed 

 quantity. The results of duplicate determinations gave the composition of the crystals 



