of Atmospheric Nitrogen and Oxygen by Water, 93 



the process of solution in this case is the same as before. 

 These values of /are given in the last column of Table III., 

 and on comparing them with the values already deduced for 

 tap-water and sea-water some interesting relations become 

 evident. 



It will be seen that the value of / for the tube through 

 which undried air was drawn is approximately the same as 

 that derived for tap-water from previous experiments, but 

 the value for the tube with the dried air is very much 

 greater, and that for the tube with the air saturated with 

 aqueous vapour is very much less than the mean value. 

 This shows that when the air above the water is very dry 

 the absorption of atmospheric gases takes place very much 

 more rapidly than usual. 



On the other hand, when the air is saturated, or nearly 

 saturated, with aqueous vapour, the rate of solution is very 

 greatly retarded. This would seem to indicate that the 

 process by which the dissolved gas is carried down into the 

 body of the liquid is affected by evaporation from the surface 

 of the liquid, since the evaporation will be at a maximum 

 when the air is dry, and a minimum when it is moist. 



In considering, therefore, this question of the mechanism 

 of solution, there are two factors which, whatever others 

 may affect it, are obviously of primary importance, namely : 

 (1) concentration of dissolved salts in the surface-layers due 

 to evaporation ; (2) cooling of the surface-layers produced 

 by evaporation. 



Both these factors are of such a nature as to result in an 

 increase of density of the surface-layers, which would tend 

 to set up vertical currents in the body of the water. In the 

 case of waters containing large quantities of dissolved salts, 

 the concentration of these salts by evaporation must play a 

 very important part in the process of solution of air. In 

 this connexion it is important to notice that the rate of 

 solution of air by quiescent bodies of sea-water may be much 

 more rapid than that by similar bodies of fresh water, 

 as shown by the relative magnitudes of the values of f in 

 each case. 



In the case of water containing only small quantities of 

 salt in solution the effect of concentration would not be so 

 great, but would probably be of somewhat the same magni- 

 tude as the effect produced by the cooling of the surface- 

 layers. This cooling will, of course, result in an increased 

 density of the surface-layers, provided the temperature is 

 not below 4°C, and consequently it will in most oases 

 operate in such a way as to hasten the process of solution. 



