136 Proceedings of Royal Society of Edinburgh. [mar. 21 , 
namely 09 6 5 for III., and 0*9825 for IV. The mean freezing 
temperature was - 2° '05 for III. and - 1°*05 for IV. The latent heat 
of water freezing at 0° is 79*25. The specific heat of ice being 0*5, 
the latent heat of water freezing at -2 o, 05 is 78*22, and that of 
water freezing at - 1°*05 is 78*73. In Experiment III. the total 
heat extracted during the freezing was 4230x0*965 = 4082 heat 
units (gramme-degrees), and dividing this by 78*22 we find 52*2 
grammes as the weight of pure ice formed at — 2° *05 C., equivalent 
to this abstraction of heat. In Experiment IV. the heat abstracted 
was found to he 5193 x0*9825 = 5102. Dividing this by 78*73 
we find 64*8 grammes as the equivalent weight of ice formed. 
We have calculated the weight of ice which would he found, first 
on the basis of the salinity of the solution ; and second, 011 the 
basis of the observed thermal exchange, assuming in both cases 
that, in the act of freezing pure ice is formed. Thus — 
Table III. 
No. of Experiment, . 
III. 
IY. 
Calculated from thermal exchange, 
,, ,, salinity, 
Difference, ....... 
grammes. 
52*2 
51*0 
1*2 
grammes. 
64-8 
607 
4*1 
The agreement between the two quantities of ice formed as 
calculated by the different methods is as close as could be expected, 
and renders probable the truth of the common assumption that the 
solid body formed is pure ice. 
It has, moreover, been proved by Guthrie, Riidorff, and others 
that, in solutions of the salts occurring in sea-water, ice does 
separate out at first, and continues to separate out until the con- 
centration has become many times greater than that of sea-water. 
Assuming that in sea-water all the chlorine is united to sodium, 85 
per cent, of the water would have to be removed as ice before a 
cryohydrate would form, and if it contained nothing but sulphate 
of soda in the proportion corresponding to the sulphuric acid found 
in it, over 90 per cent, of the water would have to go as ice, before 
the cryohydrate would be formed. 
In my experiments about 15 per cent, of the weight of the water 
was frozen out as ice, causing a lowering of freezing-point by0°*3 C. 
