AIR AND LIFE. 159 



throughout the atmosphere, and neutral carbonate, which remains in 

 the water. Briefly, so long as the tension of carbonic acid in the waters 

 and that of carbonic acid in the atmosphere is the same, nothing is pro- 

 duced, but as soon as this equilibrium of tension is destroyed the sea 

 restores it by the very simple process just described. This chemical 

 adjustment works automatically at the moment it is needed, and to the 

 extent and in the direction required. It must be added that this equi- 

 librating function is possible mainly through the circumstance that 

 the ocean contains a much larger amount of carbon dioxide than the 

 atmosphere; according to Mr. Schloesing, about ten times as much. 

 However great, then, the production of carbonic acid may be on the 

 surface of the globe by all the agents we have enumerated, it would 

 seem that the proportions of this gas in the atmosphere as a whole 

 can vary but slightly, owing to the power of the sea to absorb it and 

 maintain the equilibrium. 



We have now exhausted the list of the agencies through which the 

 amount of carbon dioxide in the air may be and is reduced when 

 necessary, and they are important and powerful enough, as we have 

 seen, to be equal to probable emergencies. Without them the globe 

 would soon become uninhabitable. Poggendorf, in fact, has found that 

 if all carbon dioxide produced could accumulate in the air the propor- 

 tion would be doubled in eighty-six years. A few centuries would see 

 the last of life as far as superior organisms are concerned. 



Oxygen, nitrogen, carbonic acid, such are the main constituents of 

 air. Those which follow are of less importance, but deserve a passing 

 notice. 



We may begin with ozone. This gas, discovered in 1840 by Schoen- 

 bein, has been made the subject-matter of many investigations by 

 De Marignac, De la Rive, Becquerel, Frcmy, Andrews, Tait, etc. 

 Ozone is oxygen under a peculiar form — condensed oxygen, so to say, 

 oxygen of high potency. It possesses strong oxidizing properties., 

 and the amount which is found in the atmosphere varies considerably 

 according to circumstances and places. This amount is on the average 

 of 1 milligram per 100 cubic meters of air; 3£ milligrams are a maxi- 

 mum. This gas is generally wholly absent from the atmosphere of 

 cities, and in the air which has passed through large centers of popula- 

 tion. Paris offers good opportunities for illustrating this fact. When 

 the wind is northerly, no ozone is found in the air at the Montsouris 

 Observatory, situated in the south of Paris, while, when the wind is 

 southerly and comes over the country without having yet crossed the 

 town, ozone is found in the air. Generally speaking, then, the healthi- 

 est part of all towns is that which lies in the direction from which the 

 prevailing wind comes; the air is purer and fresher and contains more 

 ozone. In western Europe, where the prevailing winds are westerly 

 and northerly, the northwestern and western parts are the most eligible. 



The cause of the difference in the amount of atmospheric ozone is 



