534 THE AIR AND LIFE. 



iiig hollows. These "valleys of death*' have been described by several 

 explorers. No vegetation, no shrubs, not a blade of grass grows there, 

 the sterility is absolute. The ground is bare, stony, and as if under the 

 stroke of death. Here and tliere are scattered the bleacliiug bones of 

 birds, mammals, or even of men, who unaware of the deadly properties 

 of those accursed places, Imve attempted to cross them and have been 

 overcome by the carbonic acid which, heavier than air, has accunuilated 

 in }»laces sheltered from the winds. 



Deadly alike for animals and i^lants, by which it is thrown off as soon 

 as it has been formed in their tissues, the carbonic acid presents indeed 

 the appearance of a death-dealing agent, the most noxious of all gases. 

 The best that can be said is that it m ly act a kiudly part in tlie hour 

 of death of superior beings. It accumulates in the organism by slow 

 degrees on the approach of death, which is almost invariably by 

 asphyxia, and it may be that when man is falling into his last sleep, 

 when his body is about to undergo the final dissolution, tins gas serves 

 to lull Ids intelligence, gently produce insensibility, and assist him 

 through the final act of physical life In any event, this is a likely 

 supposition, and it would thus -appear that this gas, which is said by 

 some physiologists to usher us into this world by promoting child-birth, 

 intervenes again to facilitate our exit. 



This is not however the sum total of tlie action of carbonic acid in 

 life. It has another function, more active, more essential, of deep inter- 

 est, and which we ought not to overlook. 



All animals, whetlier directly or indirectly, feed on plants, and 

 plants take from the .soil the greatest part of their mineral elements. 

 Nitrogen and oxygen they take from the atmospheie, but whence 

 do they draw the carbon that is so abundant in their tissues? Two 

 sources of supi)ly are known. Carbonic acid exists in the ground in 

 the shape of carbonates formed by its combination witli various sub- 

 stances, and in humus or surface soil co iiposed chiefiy of fragments of 

 vegetation dead and decomposed. But inasmuch as humus was not 

 available for the first plants, its carbon can not be taken into account. 

 The carbonates in the ground seem therefore in accordance with the 

 opinion of Mathieu de Dombasle and a number of agriculturists and 

 chemists who followed him, to be the only purveyors of carbon necessary 

 to ])lants. Still the experiments of Sprengel, of Saussure and of others, 

 have shown that carbonates were credited with more importance than 

 they really possess, and at a more recent date it has been proved by 

 Liebig that plants thrive well in a soil destitute of carbonates. But 

 whence then do they drive their carbon! We now know that they 

 take it from the atmosphere. They possess the proi)erty of decompos- 

 ing the carbonic acid of the air and of setting its elements free, releasing 

 the oxygen and retaining the carbon in their tissues. The 41,000,000 

 hectares of cultivated land in Fraiu-e absorb at least sixty millions of 

 tons of carbon each year. There are however two conditions without 



