AIR AND LIFE. 153 



uniform, it must not be forgotten that certain local conditions may- 

 tend to increase or decrease its normal ratio. Nor could it be other- 

 wise. However rapid the diffusion of gases, it is reasonable to suppose 

 that when one of the constituents of the atmosphere is being rapidly 

 subtracted or added in great quantities, the normal ratio in that vicin- 

 ity must be more or less altered. In a crowded room where ventilation 

 is inadequate the ratio of oxygen decreases, and the same happens in 

 places where intense combustion is going on — in mine shafts, where 

 slow oxidization of materials is a nearly constant phenomenon. In 

 brief, where the destruction of oxygen is not compensated by rapid 

 ventilation, the proportion of this gas to the remainder of the air must 

 decrease. Under the same conditions, of course, the ratio of carbon 

 dioxide must and does increase, as repeated observations have shown. 

 But such local accidents, such limited alterations of the composition 

 of the air, have no influence on the general atmosphere; they are 

 temporary, very slight, and therefore rapidly obliterated. Even the 

 respiratiou of some two, three, or four million inhabitants, as in a large 

 city, does not affect the composition of the air of the streets ; and 

 Loudon, Chicago, or Paris exert no more influence on the surrounding 

 atmosphere, into which they pour torrents of carbon dioxide, than any 

 forest, for instance, where the case is reversed, and where oxygen is 

 produced in abundance. Diffusion takes place immediately, and no 

 appreciable alteration can be detected, save in very limited spaces and 

 for a short period. And while the one gas is being removed in one 

 place it is being added in another, and thus a compensation is rapidly 

 effected. 



Little need be said concerning nitrogen. This gas, as already stated, 

 was discovered by Priestley, and Lavoisier showed that it is one of the 

 elements of air. Its weight is lighter than that of air as a whole, and 

 in 100 liters of air there are 79 of nitrogen. It neither burns nor main- 

 tains combustion; it plays no part in respiration; it cannot help to 

 maintain life. Not that it has any toxie properties, assuredly ; but it 

 is inert, indifferent, inactive. Little is known concerning its origin. 

 We know that some mineral springs, sulphurous springs particularly, 

 yield a certain amount of nitrogen, and the air ejected from the lungs 

 of animals contains about as much as the same air when inspired. As 

 is the case with oxygen, nitrogen seems to occur in the atmosphere in 

 the same ratio everywhere. 



The two gases, oxygen and nitrogen, are the main constituents of 

 air, and compose the greater part thereof. They are the essentials, the 

 other components, which must now be noticed, occurring only in very 

 limited quantities, some in variable and small proportions. We might 

 almost say that they are accessory components, judging from their quan- 

 tity, had not experience shown that one of them at least plays a very 

 important part in biology, one no less essential, in fact, than that of 

 oxygen, for instance. This latter component is carbonic acid or carbon 

 dioxide. It occurs only in very small quantity, 4 or 5 liters in 10,000 



