AIR AND LIFE. 177 



have wandered there while in pursuit of food, and in the lower part, 

 where the influence of wind is the least and where the heavy gas nat- 

 urally accumulates, asphyxia rapidly ensues. None who enter come 

 out alive, and the bird of prey soaring- in the heights, whose keen eye 

 perceives the victim in the death struggle, and who pounces down 

 upon this welcome opportunity, is vanquished in turn and rises no 

 more. 



Fatal to animals as well as plants, expelled by both from the organ- 

 ism as soon as it is produced, carbonic acid appears to all under the 

 feature of a death-dealing agent, as a gas whose toxicity is unquestion- 

 able. The only word that cau be said in its behalf is that at the 

 moment of death it may act a kindly part. Death in the majority of 

 cases, as a consequence of disease, is induced by asphyxia. During 

 the death struggle respiration fails gradually, becomes slower and more 

 superficial, with the inevitable result that carbonic acid accumulates 

 in the blood, It is probable that when man is about to fall into his 

 last slumber, when the body is on the point of entering that final stage 

 of dissolution and disintegration which we call death, carbonic acid 

 intervenes and plays its part, slowly drawing the curtain, gently put- 

 ting intelligence to sleep, rendering it unconscious, deaf to sound, 

 insensible to pain, and by beneficial and kind anaesthesia easing the 

 final act of physical life. This may well be so, and this gas which 

 some physiologists consider one of the agents by which each of us is 

 brought into the world by stimulating the contractions of the mater- 

 nal womb, thus also assists us out of it. 



This function, however, is not the only beneficial one which carbonic 

 acid fulfills, and concerning that very unwholesome and toxic constitu- 

 ent of the atmosphere much remains to be said. The unfavorable fea- 

 tures have been put in full light; it is but fair to do the same for the 

 redeeming traits, and this shall proceed to do. 



All animals directly or indirectly feed upon plants, and plants draw 

 from the soil the greater part of their mineral constituents. Nitrogen 

 and oxygen they borrow from the atmosphere. But what about car- 

 bon? The matter is important, as their frame and tissues contain a 

 large quantity of this substance. Two sources are available. Carbonic 

 acid — carbon combined with oxygen — is present in the soil, where it is 

 to be found combined with different substances in the form of carbon- 

 ates, and in humus, the superficial layer of the soil, made up of frag- 

 ments of leaves, of branches, of roots dead and decomposed, of mosses, 

 dead ferns, etc. But we can not take into account the carbon which 

 exists in humus, as the first plants which appeared could not have made 

 use of it. There remain the carbonates of the soil, and it would seem 

 to follow that this must be where plants obtain the larger amount of 

 the carbon they use, as Mathieu de Dombasle and many other agricul- 

 turists after him supposed. A number of experiments by Sprengel, 

 SM 05 12 



