THE PROPERTIES OF SEA-WATER 7 i 



The quantity of dissolved salts in sea-water can be given 

 approximately. Assuming the average density of sea-water, 

 with an allowance for increase in the lower layers due to 

 pressure, to be 1*04, the total weight of the seas would be 

 138 x io 16 metric tons. Allowing the weight of salt to be on 

 the average 3^ per cent, of this, the total weight of dissolved 

 salts is 4-84 xio 16 metric tons. The volume of salt which 

 would remain if the waters of all the oceans and seas of the 

 world were evaporated depends on the specific gravity of the 

 salts, which may be taken, on an average, to be 2*22. The 

 volume of the salt would consequently be 2 - i8xio 16 cubic 

 metres, which if spread out on the floor of the ocean would 

 give a depth for the salt layer of 196! feet, nearly 33 fathoms. 

 This mass of salt is considerably more than the whole land 

 volume of Africa (including Madagascar) above sea-level, or 

 three times as great as the volume of Europe, or nearly half 

 that of Asia. 



The density of sea-water is of considerable importance 

 from an oceanographical standpoint, since a great many ocean 

 currents are clearly due to differences in the density of the 

 layers of surface water. 



The determination of the density of sea-water in a physical 

 laboratory is not a particularly difficult operation, and full 

 instructions for determining density are given in the various 

 textbooks on practical physics. At sea, however, an instru- 

 ment known as an aerometer is used. This is a hydrometer 

 of glass, which, except for the long graduated stem, is com- 

 pletely immersed in the sea-water the density of which is 

 desired. Glass, although more brittle than metal, has two 

 advantages : it is less susceptible to increase in volume owing 

 to changes of temperature, and it cannot be indented by rough 

 handling. Since it is only desired to read densities from 1,000 

 to 1,031, it is possible to construct a scale for an aerometer to 

 function between these densities. In order to allow of minute 

 readings the scale would have to be a very long one, so it is 

 found advisable to have a series of aerometers, usually six, 



