CHEMICAL AND PHYSICAL NOTES. 73 
It is also of great importance to ascertain the density of this 
exceptionally cold bottom water. Near the coast of South America 
it was found in the Challenger to be very high, and this was con- 
firmed by an observation of the Gazelle in the same locality. It is 
this density at constant temperature which decides whether a water 
can carry its surface temperature down to great depths, or whether it 
shall remain at the surface, and it is the annual range of temperature 
of such water which gives it its penetrating power. This was clearly 
set forth in a paper sent home during the first year of the voyage of 
the Challenger, and published in the Proceedings of the Royal Society.* 
The highest surface densities are found in the North Atlantic, in the 
Trade-wind regions, and there the surface water has a higher density 
than any layer below it; consequently, when it is cooled in winter to 
the same temperature as the water immediately below it, it sinks 
through it, and in this way a high temperature is disseminated 
through the whole thickness of the water of the North Atlantic. In 
the eastern part of this ocean, the density and temperature of the 
bottom waters are sensibly increased by the “brining down” of the 
Mediterranean. 
The common feature of Antarctic water found by all expeditions 
is the thick warm layer lying between a cold layer at the surface 
and another cold layer at the bottom. It is very important to trace 
these two cold layers Southwards, until they join and the warm 
intermediate layer has disappeared. Every particular connected 
with this will be of interest. 
Freezing Temperature of average Sea Water—Sir James Ross, 
in his description of his voyage, frequently refers to 39° Fahr. as 
the temperature of maximum density of all waters, and draws 
curious conclusions. Now it was well known before the date of 
his voyage that average sea-water continues to contract to a 
much lower temperature than 39°. Indeed, its temperature of 
maximum density is below that of its freezing-point, which may be 
put at 29° Fahr. A similar mistake is often made at the present 
day by geographical writers. Although everybody knows and re- 
cognises that sea-water freezes at a temperature below that of fresh 
water, and that this temperature is the lower the greater the quantity 
of salt contained in the sea-water, it is to some extent not known 
and to a great extent not recognised that pure ice, which when left 
to itself melts at 32° Fahr., begins to melt in salt water at exactly 
the same temperature as that at which the same water begins to 
freeze. A piece of pure lake-ice immersed in average sea-water 
* Proc. RS. (1875), vol. elvii. p. 128. 
