310 



Water and its Impurities, with 



to the arrangement of its atoms. There are cavities 

 in which the air is secreted. In these cavities it has 

 the power of secreting a great variety of gases, some 

 more than others. Under a pressure it will take np more 

 of the same gas ; this depends upon the property which 

 gases have of being compressed in larger quantities within 

 the same space. The point which is of most interest to 

 us is, that whenever a surface of water is presented to a 

 gas, the latter is absorbed. Now two gases will not 

 be absorbed to any extent unless they have an affinity 

 and go into chemical union with each other, so that 

 water in which one gas is dissolved will reject another 

 unless the first has been displaced. 



If water containing one gas be exposed to the action of 

 a second, it lets a portion of the first escape and absorbs 

 a correspouding quantity of the second. In this way a 

 very small quantity of a sparingly soluble gas may expel 

 a large quantity of one more soluble. A familiar example 

 of this fact, consists in taking a glass half full of cham- 

 pagne, and having formed the palm of the hand into a 

 hollow cup, to strike the top of the glass, closing the glass 

 and flattening the hand at the same time. The air above 

 the wine is thus forcibly compressed, and a portion is 

 ,then absorbed under pressure by the fluid, from which a 

 quantity of carbonic acid is expelled greater than that of 

 the air absorbed, in the proportion of 1060 to 16 (Kane). 



To apply this to the purification of natural water; — we 

 will suppose that already some gas say sulphuretted 

 hydrogen is dissolved, This exceedingly offensive impu- 

 rity would be likely to escape detection were it not for 

 the agency of the atmosphere ; expose the water to con- 

 tact with air ; the air cannot enter while the other gas is 

 there ; it cannot enter at all unless it will dissolve in water, 

 and examining into this, we find that it possesses the 

 properties necessary to displace other gases. "When 



