﻿792 On the Theory of Freezing Mixtures. 



It is most convenient to write 



6 =K. 



I + S 



A few curves (rig. 2) have been drawn for different values of 



Fig. 2. 



Z5 



Tx 



20 



15 



IO 

























^s 



n 















o 



\\ 





r- 







H 



\ 







1 



1 



I'l 



1 1 



■% 



\ 





v 





f~ 



\ 



s- I 





;i i 



j 





*>. 





<£ 



\ 



l \ 



o T 



Q j 



r I 



j if 







k e >> 



. 





\ 



\ 

 \ 



1 



V'l 



1 1 









\ \ 







\ 



\ 



\ \ 



i 



hi 



i 













s. 





\ 



'4 



















\\ 



in/ 







































^ 



o io zo SO 40 So 6o yo So o,o soo. 

 ferce vtaQe of Salt. 



K to show the effect produced. Possibly the most interesting- 

 case is to find the value of K for which the cryohydric is 

 just reached. Assuming that all the salt and ice is used and 

 that mjM = \ t we have 



21-6r| + |+^]=216m 



6>=8-3S-2-l(I + S), 



or 



and hence 



K=2-l; 



therefore to cool a body whose thermal capacity is 6 down to 

 — 21-6° C, the quantities of materials to be used are 0/8 '4 gm. 

 of salt and 0/2*8 gm. of ice. In practice it is always 

 necessary to take somewhat larger quantities to allow for the 

 formation of dew on the exposed surfaces. This isa fairly 

 serious factor, as 1 gm. of dew is equivalent in its heat 

 change to roughly 8 gm. of ice. It will be seen, therefore, 

 that if K<2-1, there exists a central horizontal portion of 

 the curve, whilst if K > 21, the two sloping lines intersect at 

 a vertex lying on the 25 per cent, ordinate. 



