182 ORIGIN OF ORGANIC MATTER 



When we consider that the atmospheric air contains only 

 l-1000th of its volume of carbonic acid., it might at first sight 

 appear as if the atmosphere could not supply plants with all their 

 carbon an opinion which was once generally entertained; but 

 certain simple calculations made by Liebig show that, instead of 

 believing the carbonic acid in the atmosphere to be insufficient for 

 the growth of plants, we might rather wonder how it is that, 

 notwithstanding the vegetable kingdom, the quantity of carbonic 

 acid in the atmosphere has not been considerably augmented in the 

 course of ages. The atmosphere exerts a pressure of 22 16' 16 Ibs. 

 on every square foot, and if it were as thick in all parts as it is at 

 the surface of the sea, it would extend 24,555 Paris feet in height, 

 or, after excluding the aqueous vapour, 22,843 feet, or 1 German 

 geographical mile [about 8,100 yards]. If the radius of the earth 

 be assumed at 860 such German miles, the volume of the atmo- 

 sphere (at the pressure of an atmosphere of mean temperature) 

 must be equal to 9,307,500 such cubic miles, in which, in addition 

 to 1,954,578 cubic miles of oxygen, there would be about 3,862 '7 

 cubic miles, or about 28 billions of cwts. of carbonic acid a quantity 

 which must be more than sufficient for the wants of all the vege- 

 tables occurring on the land or in the water of our planet. 



If, on the other hand, we consider that enormous masses of 

 carbonic acid are continually being conveyed to the atmosphere 

 from the earth's surface, we cannot help wondering that it should 

 have experienced no sensible increase in the amount of the 

 carbonic acid which it contains ; at all events, there has been no 

 change in it during the period that has elapsed since the 

 destruction of Pompeii, in the year 79 A.D. (as is proved by the 

 analyses of air which had been contained in funeral urns which 

 had been excavated from that city) ; whilst, on the other hand, 

 geological investigations have rendered it almost certain that at 

 some definite period, ages since, and long before the higher forms 

 of animal life had appeared upon the earth, the atmosphere was 

 far richer in carbonic acid than it is now. A rough estimate of 

 these relations yields the same numbers which Liebig has deduced 

 from a more complicated calculation. Thus, for instance, if a man 

 daily consume 45,000 cubic inches of oxygen, which would give 

 9505-2 cubic feet for the year, 9 billions and 505,200 cubic feet of 

 oxygen will be abstracted from the atmosphere by a thousand 

 millions of men ; and if, further, it be assumed that about double 

 this amount of oxygen is lost by the respiration of animals, and 

 by the processes of decomposition and combustion, it follows that 



