522 THE AIR AND LIFE. 



ological importance, viz, oxygen, nitrogen, and ('arbonic acid. Where do 

 these elements come from ? What is their proportion "? What becomes 

 of them! We naturally ask these questions, and it does not seem out 

 of i)lace to consider them here. In our examination of the relations 

 between the atmosphere and the living being our chief concern will be 

 the influence of organisms on the air, although in the course of our 

 study the influence of air upon organisms will also come under notice. 



I. — CONSTITXTENTS OF THE ATMOSPHERE. 



Oxygen was discovered by Priestley and Scheele in 1774. Not long 

 after, Lavoisier proved by very simple experiments that oxygen is one 

 of the constituent elements of air and that air itself is a compound — a 

 mixture of gases. The reader could not do better than to consult on 

 this point ^Ir. Berthelot's interesting book. La Revolution Chimique. 

 The term oxygen was invented by Lavoisier. Its discovery made a 

 revolution in chemistiy and in physiology and was the beginning of an 

 era which has been fertile in w(Miderful results. 



Oxygen is a gas heavier than air and is specially favorable to com- 

 bustion and respiration. In ],()(H) liters of air there are 208 liters of 

 oxygen and 792 liters of nitrogen. These flgures have been obtained by 

 the numerous and very exact methods now used m chemistry, and the 

 same methods have been employed to determine the constancy of the 

 proportion of oxygen. Such investigations w^ere necessary, because 

 certain chemists, Dalton and Babinet among others, thought that from 

 theoretical considerations the proportion of oxygen in the air ought to 

 decrease as the elevation increases, that at the surfjice of the earth 

 there ought to be a little less oxygen and a little more nitrogen, while 

 in the upper regions of the atmosphere the relative proportions would 

 be reversed, so that at an altitude of 10 kilometers, for example, the 

 volume of oxygen would be LSI to SIO of nitrogen. Direct analysis, 

 however, made by Thcnard of air obtained l>y (Tay-Lussac at an altitude 

 of 7,000 meters, and the experiments of Dumas andBoussingault made 

 by means of the weighing method, have i)roved that the facts do not 

 contirm those theories. The relaiive ])roportions of oxygen and nitro- 

 gen in the air, then, may l)e said to be uniform and constant, except 

 for slight occasional variations. 



Dumas and Boussingault, who studied the air for oxygen in difterent 

 kinds of weather at difterent altitudes, in difterent j^laces and at difter- 

 ent seasons, obtained practically identical results, with the usual allow- 

 ance for errors. Other chemists, Brunner, Regnault, Reiset, ])oyere, 

 and Bunsen, have all by different methods reached the same conclusion, 

 which may, therefore, be considered as flrmly entablished. 



Our tirst question is where does this oxygen in the air come from! 

 What are its sources ? The permanence of the proportion of the gas 

 in the air and the enormous quantity of it consumed by living beings 

 and in combustion naturally lead us t<> desire an answer on this point. 



