EXISTENCE AND DISTRIBUTION OF MARINE ORGANISMS. 399 



in the alkalinity of very deep waters, there seems to be no doubt that 

 the quantity of lime increases with the depth. It has long- been known 

 that sea water is distinctly alkaline to test paper; this is due to an 

 excess of base over sulphuric and hydrochloric acids, the surplus base 

 being more or less saturated by carbonic acid, too weak an acid to 

 bring about a neutral reaction. 



The gases in sea water are not only much more variable than the 

 saline matters in their absolute amount, but also vary in the propor- 

 tions among themselves. The gases of general occurrence are oxygen, 

 nitrogen, and carbonic acid, the two former being wholly derived by 

 absorption from the atmosphere, while the last named is partly absorbed 

 from the atmosphere and partly due to excretion from animals and to 

 oxidation of organic matter in situ. 



The gas is absorbed from the atmosphere by the merest surface layers 

 only of the water, and is distributed to the rest of the ocean by descend- 

 ing currents. The quantity absorbed is determined by the temperature 

 of the water and the pressure of the atmosphere, but chiefly by the 

 former. If the water remaining on the surface pass to a warmer region 

 gas is given off, if to a colder region more is absorbed. Once the water 

 is cut off from the surface by overlying layers of water all further 

 absorption of gas ceases. Of the three gases nitrogen alone remains 

 constant in quantity. The oxygen of the water is taken up by animals 

 for the furtherance of their metabolic processes, and in its place they 

 excrete carbonic acid. Thus, if free surface ventilation be denied there 

 is a continual decrease in the proportion of oxygen and a correspond- 

 ing increase in the quantity of carbonic acid ; and in small inclosed seas 

 this process may go so far as to render the water quite unfit for the 

 support of animal life of any order much higher than that of bacteria. 



On account of the coefficient of absorption of oxygen being double 

 that of nitrogen, the proportion of oxygen to the total gas in sea water 

 under full aeration is double that in air, being as a matter of fact 3I£ 

 to 33J per cent. The absolute quantity of gas in solution in surface 

 waters is found to decrease as we go from the poles to the equator, as 

 also does the proportion of oxygen. In high latitudes, indeed, the pro- 

 portion of oxygen is so high as to amount to supersaturation, as much 

 as 36.7 per cent having been found in polar waters. The quantity of 

 oxygen is always less in bottom than in intermediate waters from great 

 depths, but no oceanic water at least is found to be absolutely devoid 

 of oxygen, although in waters from the bottom in great depths the 

 amount is sometimes very small. 



Both the horizontal and vertical circulation of ocean waters is 

 mainly governed by the prevailing Avinds which blow over the surface, 

 and these are again determined by the position of the areas of high 

 and low barometric pressure. Where the winds are dry and constant 

 there we find the saltest water at the surface, as for instance in the 

 trade- wind regions of the North and South Atlantic and South Pacific, 



