74. THE ANTARCTIC MANUAL. 
reduces its own temperature and that of the sea-water in its im- 
mediate neighbourhood to a temperature of roughly 29° Fahr., which 
varies with the concentration of the resultant brine formed by the 
mixture of the sea-water with the pure water formed by the melting 
of the ice. An iceberg consists of pure land-ice, and if it is of 
sufficient thickness to reach the layer of warm intermediate water, 
its lower surface must be always melting at a temperature of about 
29° Fahr., and this temperature must in time be communicated to 
the body of the ice, if it did not have it before. But it must neces- 
sarily be at about this temperature, because it separates from the 
parent land ice after it has been pushed into the sea. If it hada 
temperature below 29° Fahr., it would freeze the sea-water round it 
until it had got rid of its excessive cold; and if it had a temperature 
above 29° Fahr., the sea-water round it would melt its ice until it 
had got rid of its excessive heat. The representative freezing tem- 
perature of sea-water is taken as 29° Fahr., but it varies with the 
salinity. In this respect sea-water was found to agree closely with 
a solution of chloride of sodium containing the same percentage of 
chlorine. The subject was carefully investigated during the winter 
1886-87, and the results were communicated to the Royal Society of 
Edinburgh, in a paper which was read on March 21, 1887.* The 
rule regulating the appearance or disappearance of ice in a solution 
of chloride of sodium is very simple. The number expressing the 
percentage by weight of chlorine in the solution expresses on Celsius’ 
scale the depression of the freezing-point of the solution below that 
of distilled water, and by consequence the temperature at which 
pure ice begins to melt in the same solution. Thus the freezing- 
point of a solution of chloride of sodium, containing 1 per cent. of 
chlorine, is — 1°°0 C.; if it contains 1°75 per cent. chlorine, its 
freezing point is — 1°°75 C.; and so on for concentrations not 
exceeding that of the saltest ocean water. Sea-water, the solid 
contents of which consist chiefly of chlorides, follows this rule 
approximately, but not exactly. The following table, from p. 133 of 
the memoir, is derived from twenty-five determinations made with 
the greatest care in sea-waters of different degrees of concentration 
and freezing at temperatures between — 0°°5 C. and — 2°:22 C. 
Freezing temperature...) .. —2°0C, —1°5C. —1°0C. —0°5C. 
Per cent. by big sini of chlorine 1-940 1°445 0°963 0°475 
Difference .. . es 0:°060 0°055 0-037 0°025 
From this we fae the following approximate rule: The number 
expressing on Celsius’ scale the depression of the freezing-point of a 
* ‘On Ice and Brines, by J. Y. Buchanan (1887). Proc. R.S.E., vol. xiv. p. 129; 
also Nature (1887), vol. xxxv. pp. 516, 608, and vol. xxxvi. p. 9. 
