78 THE ANTARCTIC MANUAL. 
at +16°°8 C. These specific gravities correspond to salinities of 
2°5,1°3 and 1:2 per cent. respectively. The average specific gravity 
of the sea-water is 1-025,” 
Weyprecht assumed that the ice formed by freezing sea-water 
was pure ice, and that its saltness was derived from brine mechanic- 
ally adhering to it, from which it was impossible to free it. There 
was no experimental evidence to prove this. Others thought that 
as the ice formed by freezing salt water has a different melting-point 
from fresh-water ice, and it is impossible by melting it to produce 
water free from salt, that the salt is an essential constituent of 
the ice. Experiments made by the writer* in the Antarctic area 
during the cruise of the Challenger, supported the latter view, and he 
held it until subsequent experiments, f which he made some twelve 
years later, enabled him to supply direct experimental evidence that 
the contrary is the case, that the ice-crystals are indeed pure ice, and 
that their saltness is due only to adhering brine from which it is 
impossible to free them. 
Demonstration that the Ice produced by Freezing Sea-water and 
similar Solutions is pure Ice—The principle guiding the research 
was, that if the ice, which forms when sea-water or saline solutions 
of similar concentration are partially frozen is pure ice, then pure ice 
of independent origin such as snow, must, when mixed with the sea- 
water or saline solution, melt at the same temperature as the ice which 
is formed by freezing the solution, when the concentration is the same. 
Sea-water and various saline solutions were experimented with on 
these lines, and the following was the scheme of experiment :— 
The solution of a salt, for instance, chloride of sodium, was 
gradually cooled in a freezing mixture, and the temperature watched 
as more and more crystals separated out; at suitable intervals the 
temperature was accurately noted, and simultaneously a sample of 
the brine was taken; in this way a series of freezing temperatures 
4, ta, ts, etc., was obtained, and after analysis of the samples a corres- 
ponding series of salinities s,, s9, 83, etc., was obtained. When the 
lowest temperature, say ¢,, was obtained, the salinity was s,; then 
the vessel containing the solution was removed from the freezing 
mixture, and it was exposed to the heat of the air of the laboratory. 
The temperature of the mixture was observed to rise slowly, and 
when it arrived again at /4, ts, tg and ¢, respectively, samples of the 
brine were again taken and analysed. The resulting salinities s,, 83, 89, 
* On Sea-water Ice.” Proc. R.S. (1874), vol. xxii. p. 431. 
+ ‘On Ice and Brines, by J. Y. Buchanan (1887). Proc. R.S.E., vol. xiv. p. 129; 
also Nature, vol. xxxv. pp. 516, 608, and vol. xxxvi. p. 9 
