AIR AND LIFE. 147 



noticed, diffusion is not the sole agency, and it is quite clear that the 

 white crested waves, all foam and sparkling with air bubbles, that the 

 winds, the currents, the tides, and lastly the dust particles have done 

 and are doing much to hasten the process, and accelerate the execu- 

 tion of the great respiratory function of the deep. No method, unfortu- 

 nately, has yet been devised for measuring the rapidity of this process; 

 and before it can be done, some manner by which the approximate 

 number of dust particles falling into the seas can be ascertained should 

 of course be discovered. The problem is a difficult one, truly. 



II. — Air from the Chemical Point of View. 



Considered by the ancients, and even by modern philosophers till 

 a very recent period, as one of the four initial elements (earth, air, 

 water, and fire), air was unable to keep this position after the birth of 

 modern chemistry. Like most other substances it has had to reduce 

 considerably its pretensions. They were of no avail in presence of 

 the methods of chemistry. Instead of being, as formerly supposed, an 

 element, a homogeneous matter out of which no known method of 

 reduction can obtain two or more differing substances, air has shown 

 itself to be nothing more than a mixture of different elements. A mix- 

 ture, a mechanical mixture; not a compound. Air is not like water, in 

 which two gases, oxygen and hydrogen, are combined and make up a 

 third body exceedingly different in properties from those out of which 

 it is made, nor like the enormous number of compounds known to 

 chemistry in which two or more elementary substances are combined 

 in definite proportions and form new substances more or less pecul- 

 iar, but invariable, and possessing properties Avhich neither of the 

 elements possesses; it is a mixture only. This may be demonstrated 

 in various ways. When nitrogen and oxygen , the fundamental elements 

 of the air, are mixed together, no heat is evolved, no heat is absorbed, 

 as is the case in the preparation of most compounds. Again, the 

 refringency of air is equal to the mean of the refringency of oxygen 

 and nitrogen when experimentally mixed in the proportions in which 

 they occur in the atmosphere; and the ratio of oxygen to nitrogen is 

 not a simple one; lastly, when in presence of air, water dissolves differ- 

 ent proportions of the different constituents of the former; it dissolves 

 each gas according to its own proper coefficient of solubility. 



These four proofs are considered as more than sufficient to show that 

 air is a mixture, not a compound. It may be added, moreover, that 

 while the composition of the atmosphere is fairly uniform as a whole, 

 it is not absolutely so; the one or the other constituent is more or less 

 abundant according to circumstances. No chemical compound offers 

 such variability in composition; its constituents are constant, always 

 the same, and in the same ratios, while in a mixture every variation is 

 possible, and may be expected. 



