CHEMICAL AND PHYSICAL NOTES. 79 
s,, were found to be identical with those observed at the same tem- 
perature during cooling. 
Another solution of the same salt was now made, and of strength 
represented by salinity ss. It was cooled down to close on ¢,, and then 
snow was mixed with it. The vessel having been removed from 
the cooling bath was exposed to the heat of the laboratory, and the 
thermometer carefully observed as the temperature rose. When the 
temperature was exactly ¢4, ¢3, t2, 4, samples of brine were taken and 
analysed, and the resulting salinities sy, 83, s2, s;, were found to be 
identical with those observed in the two previous experiments. 
It was thus shown that when a saline solution of concentration 
comparable with sea-water is gradually frozen, certain crystals 
which we call icc-crystals separate out, aud during the process the 
temperature of the mixture gradually falls while its concentration 
increases. When the mixture of brine and ice-crystals is warmed 
the ice-crystals gradually melt, the temperature rises and the con- 
centration diminishes; but when in the process of cooling and 
freezing the temperature has fallen to a certain point, say ¢, and the 
salinity is s, it was found that when the process was reversed and 
the same temperature ¢ was reached during the process of warming 
and melting, the solution was found to have the salinity s, There- 
fore the substance, which forms the ice-crystals which separate 
out at a temperature ¢ during cooling, melts again at the same 
temperature and concentration during warming. When the same 
solution, having the highest concentration which was used in the 
previous experiments, was cooled down and mixed with snow and 
then gradually warmed, it was found that the snow melted exactly 
as the ice-crystals formed in the solution itself had done, and at 
exactly the same temperature for the same salinity. But it is only ' 
a question of whether the salt is in the ice or in the brine. There is 
no salt in snow, and it behaves in a saline solution in exactly the 
same way as the crystals formed by freezing that solution ; therefore 
the crystals formed by freezing the saline solution must be equally 
free from salt, and 2 has thus been proved that the crystals formed 
in freezing saline solutions of moderate concentration are pure ice, 
and that the salt from which they cannot be freed docs belong to the 
adhering brine. 
Analogy between Snow and Sea Ice.—Snow is the result of the 
crystallisation of water-vapour dissolved in a gaseous mixture of 
nitrogen and oxygen. The freezing-point of this gaseous mixture is 
commonly called the “dew-point” of the air. The freezing of a 
saline solution is analogous. It is a homogeneous liquid, and the 
