AIR AND LIFE. 143 



of gas, of which 3.9 are oxygen, 12 nitrogen, and H>.2 are carbonic acid. 

 Take the Rhone River, on the other hand, and you find 34.8 cubic centi- 

 meters of gas, of which 8.4 are oxygen, 18.4 nitrogen, and 8 carbonic acid. 

 These differences are less surprising after reflection. Each river may 

 and does differ in chemical constitution from other rivers, and even 

 from itself, at different times and places, because of the difference in 

 the nature and quantity of the chemical operations going on in the 

 water. The chemical composition of the banks varies, and the activity 

 of living organisms within the waters also varies. Such differences 

 must exert their influence upon the chemical composition of the latter, 

 and wo have abundant proof that they do so. If the water of the same 

 river, taken at different places at the same moment, is tested chemically, 

 differences are observed which are sometimes considerable. For in- 

 stance, the Thames, above London, contains 7.4 oxygen, at Hammer- 

 smith 4.7, at Somerset House 1.5, at Woolwich 0.2."). Whence arise 

 these considerable variations! They are easily explained by the fact 

 that the river receives a large quantity of organic debris, vegetables, 

 dead animals, and a large number of dead or dying substances or 

 organisms; the debris combines with oxygen, and thus the amount of 

 this gas is greatly diminished. The consequence is that the fish often 

 perish through asphyxia, the amount of oxygen being inadequate. The 

 same occurs in Paris. After its passage through the city, the Seine 

 is generally quite unfit to support the life of most aquatic animals. 

 Many species are not to be seen in the river at Paris, nor below it for 

 some distance, although found above, where the water is sufficiently 

 pure and aerated, and some 10 or 20 miles below, where aeration has 

 been sufficient to make up for the loss, the water having absorbed 

 enough fresh oxygen from the atmosphere. 



So much for one series of differences in aeration. But another series 

 exists which is even of greater interest. The aeration of waters, or 

 the absorption of gases by water, varies according to general external 

 conditions, among which temperature and pressure rank highest — not 

 only general pressure, but, so to speak, individual pressure, or, to put 

 it in other terms, the proportion of any given gas in a mixtivre. Under 

 identical conditions, each gas, moreover, has its own special coefficient 

 of solubility. While nitrogen is feebly soluble, ammonia is highly so. 

 This fact helps us to understand why it is that the gases which 

 spontaneously dissolve in water in contact with the atmosphere do not, 

 when extracted from the water, yield a mixture even distantly compa- 

 rable to air: how it is that the elements of air are not found in water 

 under the proportions they bear to each other in the atmosphere. 

 While water contains the constituents of air, it contains such propor- 

 tions of these constituents as are peculiar to it. However, the latter 

 are provided in sufficient quantity, and normal river water is quite 

 adequate to maintain the life of aquatic animals. This applies to fresh 

 waters generally, for ponds and lakes have the same conditions as 

 rivers. 



