84 PHYSIOGRAPHY. [chap. 



naturally be assumed that this gas would unduly accumulate, 

 and at length vitiate the entire bulk of the atmosphere. 

 Such accumulation is, however, prevented by the action of 

 living plants. To show that so small a proportion of car- 

 bonic acid in the atmosphere as &"o35 per cent, is sufficient 

 to supply the vegetable world with its carbon, it is simply 

 necessary to calculate the weight of this gas in the atmo- 

 sphere resting on a square mile of land. The weight of air 

 on this area is about 590,129,971,200 lbs., and the carbonic 

 acid which it contains weighs not less than 3,081,870,106 lbs. 

 or about 1,375,834 tons. The weight of carbon in this car- 

 bonic acid is 371,475 tons. The carbonic acid, so in- 

 jurious to the animal, is the source whence ordinary plants 

 derive the whole of the carbon in their structure. Wood, 

 for example, contains about half its weight of carbon ; yet 

 every particle of carbon in a forest of trees has been de- 

 rived from the gaseous carbonic acid invisibly distributed 

 through the surrounding atmosphere.^ 



Before leaving the subject of carbonic acid, it should be 

 remarked that this gas is one of great density, being in fact 

 about half as heavy again as an equal bulk of atmospheric 

 air. It might, therefore, not unfairly be assumed that the 

 carbonic acid in the atmosphere would tend to settle down 

 in a stratum near the ground. If we shake up a mixture of 

 liquids of different densities — say mercury, water, and oil — 

 the liquids soon settle down, after agitation, in the order of 

 their relative weights ; the heavy quicksilver sinking to the 

 bottom, and the light oil floating on the top of the water. 

 Such a separation does not however take place when gases 

 of different densities are mixed. The following table shows 

 the densities, or specific gravities, of the three gases which 

 compose the atmosphere : — - 



1 This subject will be further discussed in Chapter XIV. 



