956 
THE VOYAGE OF H.M.S. CHALLENGER. 
concentration. Although the ocean is the receptacle of the drainage of all the land, it is 
by no means u nif orm in saltness ; there are variations due to the different meteoro- 
logical conditions which obtain in the different regions of the earth. 
The causes which are effective in altering the specific gravity of the sea are those which 
influence the formation of vapour and of ice ; and as these are found at the surface, it is 
there that the greatest variations in saltness are observed. The effect of freezing may be 
taken to apply only to the polar regions. Between these the globe may be divided, or 
that part of it covered by sea, into five zones, namely — two corresponding to the areas 
of prevalence of the northeast and the southeast trade winds, in which evaporation goes 
on actively, and a zone between them corresponding to the equatorial calms, where an 
immense amount of rain falls ; and two to the north and the south of the trade wind 
districts, where on the whole there is a tolerable balance between rain and evaporation. 
At both poles there are areas of concentration due to the formation of ice. 
But the salinity of the sea is affected by the removal of dissolved mineral matter as 
well as of water, whether in form of vapour or ice, and any agency which removes solid 
matter from the water will alter its density. Sea water contains much lime in solution. 
Immense numbers of animals living in it secrete calcareous coverings, drawing on the 
water for the lime and possibly for the carbonic acid necessary for the formation of 
their shells. Let it be assumed that the shell is formed by the direct transference of 
carbonate of lime from the water to the animals. When they die, their shells sink to 
the bottom, or are dissolved before they get to the bottom, thus returning either the 
whole or a part of the carbonate to the water from which it had been taken. Where the 
conditions are such that the shells reach the bottom, a deposit will be formed which will 
constitute a continual drain on the supply of carbonates in the water. In this way the 
composition of the water is altered by precipitation through organic agency. In the same 
way siliceous deposits are formed by animals secreting siliceous skeletons. But this 
cause, though it produces in the course of time very important effects, does not affect 
the composition of the water very sensibly, because the amount of earthy carbonate, or of 
silica, which is held in solution at any one time is, although sufficient for the support of 
this extensive process of transmigration of mineral matter, comparatively small ; moreover, 
these very substances, silica and earthy carbonates, form important solid ingredients in 
solution in river wate£, and the supply is being continuously kept up, consequently 
variations in the amount of them would not be expected to produce a marked effect on 
the density of the water. It is not improbable that in the case of carbonate of lime the 
comparatively high density of bottom water as compared with intermediate water is due 
in part to the fact discovered by Professor Dittmar, that the amount of lime in bottom 
waters is distinctly though slightly greater than in intermediate waters, although this 
surplus in itself would not affect the most exact specific gravity determinations. 
At the surface of the sea in all latitudes there is a constant exchange going on between 
