AIR AND LIFE, 145 



must exist .some unceasing' agency by means of which new oxygen is 

 added and carbonic acid carried away. Otherwise aquatic life would 

 soon cease. In other terms, there must exist a perpetual exchange 

 between the gases dissolved in the waters and those which make up 

 the atmosphere, just as there goes on a perpetual exchange between 

 the air of any place where the atmosphere is vitiated — a town, a manu- 

 factory, a room — and the air of the streets or surrounding country. 

 And the exchanges which go on between air and water, and between the 

 general atmosphere and those multitudinous centers, great or small, 

 where the normal proportions of the gases of air are being constantly 

 altered, must indeed be most nicely adjusted, since by no method have 

 we yet been able to detect any alteration in the composition of the 

 atmosphere. The equilibrium must be unceasingly maintained. That 

 equilibrium is a very interesting matter. Interesting in two senses — 

 X>ractically, since life depends upon it, and from the scientific point of 

 view, as it is the consequence of a general established law. 



How, then, is that exchange effected between air and water, without 

 which life would soon extinguish all life, without which the living- 

 organisms of water would soon render life impossible to themselves and 

 to their congeners? By means of what may be termed "the breathing 

 of the waters." The waters breathe — that is, expire obnoxious gases 

 and inspire those that are useful; they expel carbonic acid and collect 

 oxygen. Diffusion is the main agency of this grand function of waters, 

 and it is enough that both air and water be in presence and contact to 

 insure the operation. But diffusion is not alone at work; another 

 agency cooperates. It does not at first seem that dust would have 

 any influence, and few would suppose that it plays.any part here. It 

 does, however, and the enormous quantity of it which, imperceptibly in 

 most cases, is carried from the land over the seas, where it falls and 

 slowly sinks to settle at the bottom as a soft red or gray mud — the first 

 stage of new strata of rocks — is a great help toward the respiration of 

 the seas. As J. Thoulet has shown, every particle, however small and 

 minute, carries some air which adheres to it and does not escape when 

 submerged; this air slowly dissolves in the surrounding water. The 

 experimental proof is easy. Bring some water to the boiling point, in 

 order to expel the gases dissolved in it, and then add some potash and 

 pyrogallic acid. This mixture turns black when in presence of oxygen 

 by reason of the action of the latter on the acid. Under ordinary con- 

 ditions, the experiment being thus prepared, what one witnesses is 

 this: The surface of the water blackens and the black color extends 

 slowly toward the bottom, according to the ratio of diffusion of atmos- 

 pheric oxygen in the mixture. The rapidity, or rather slowness, of the 

 change of color is the measure of the slowness of diffusion. Now, throw x 

 some fine dust into the vessel containing the water so prepared. What 

 happens then is that each grain or particle, while falling through the 

 liquid, leaves behind it a black line which marks its path exactly, and 

 sm 95 10 



