THE AIR AND LIFE. 535 



which this important operation cau not be accomplished; the plant 

 must be supplied with chlorophyll, the green matter which imparts 

 color to the leaves ; there must also be sohir light, and the tempera- 

 ture must not be too low. As a matter of fact, chlorophyll can only 

 decompose carbonic acid in the light and under certain conditions of 

 temperature; it ceases to operate in cold weather or in darkness, and 

 when it is deficient, when there is a lack of leaves, the j)lant droops 

 and dies from starvation. For it is a point worthy of our careful notice 

 that the function of chlorophyll is one of nutrition, decidedly distinct 

 from that of respiration, by the operation of which ])lants, after the man- 

 ner of animals, absorb oxygen and throw oft" carbonic acid. These two 

 functions are not equally active, the former being much more so than 

 the latter, although it only operates by day. If it were not so and 

 if the two functions were exactly balanced, it would be impossible for 

 the plant to grow, as it would lose through the one what it gained 

 through the other. 



It is then chiefly by the medium of the leaves, and in a less degree 

 by the roots, that the carbonic acid of the atmosphere is absorbed, and 

 in any event, that gas must pass through the leaves, the green ijarts 

 fed with chlorophyll, in order to be utilized by the plant. 



We now see that this virulent poison, so decidedly noxious that it 

 destroys aninml life as soon as it accunuilates even to a slight degree 

 in the atmosphere, is one of the essential foundations of life on the 

 globe. If it should disappear from air, vegetation would immediately 

 die, and a few days would suffice to accomplish the deatli of .all that 

 breathes and moves on the surface of our planet. True, carbonic acid 

 by itself is a most noxious substance to life; but when in normal pro- 

 portions in the atmosphere it is as necessary and indispensable to life 

 as it is fatal when the air is surcharged with it. 



The foregoing are the relations of air, studied from the standY)oint of 

 its chemical composition, to life as it manifests itself on earth — of the 

 normal air studied without regard to any artificial cause by which it 

 may be vitiated — of physiological air, so to speak. 



III. — MECHA?nCAL EFFECTS OF THE ATMOSPHERE. 



We shall now discuss another phase of this comi)lex question and 

 turn our atteuti<m to air in connection with its weight. This aspect 

 is quite as worthy of investigation as the foregoing, for there is a very 

 positive relationship between life and atmospheric pressure. 



As already said, the atmosphere has weight and air exerts a pressure 

 on the earth and on all living beings. The pressure varies according to 

 the level, being lighter in elevated regions and greater in low regions. 

 As the barometer is carried from mountain tops toward the plains, 

 then to the level of the sea and finally into the depths of mines, it 

 plainly shows the increase in pressure. Slight variations of pressure 

 have little effect on living beings; but when the difference is great, 



