154 EVOLUTION OF CARBONIC ACID N VOLCANIC COUNTRIES. 



combustion, or by that of spontaneous decay, tlie entire crop of vegeta- 

 ble produce is apparently, year by year — taking the average of a series 

 of years — resolved into the forms of matter from which it was.originally 

 built up ; — and the substances on which plants feed at length restored to 

 the air in the precise proportion in which they have been taken from it. 



VI. NATURAL EVOLUTION OF CARBONIC ACID IN VOLCANIC COUNTRIES. 



The above apparent conclusion would be absolutely true, were there 

 no causes in operation by which the restoration to the air of a portion of 

 the carbon of animal and vegetable substances is prevented — and no 

 other sources, independent of existing organic matter, from which car- 

 bonic acid may be supplied to the air. 



If the whole of the carbon be not returned to the air, the carbonic acid 

 of the atmosphere may be undergoing diminution ; while — if a large 

 supply be constantly poured into the air from sources independent of 

 vegetable matter, the proportion of carbonic acid may be continually on 

 the increase. 



We have seen that the combustion of fossil coal adds to the air a 

 large quantity of carbonic acid which has never before existed in the at- 

 mosphere of our time. In many volcanic districts also, carbonic acid is 

 observed to issue in large quantity from cracks and fissures in the earth ; 

 — accompanied sometimes by water, forming mineral springs, from 

 which the copious emisson of gas is readily perceived ; more frequently, 

 perhaps, rising up alone, and thus escaping general observation. 



It must obviously be exceedingly difficult to estimate the (luantity of 

 gas which rises into the air in such circumstances over an extensive 

 tract of country, fractured and broken up by volcanic agency — where 

 the outlets are numerous, and the rate at which the gas escapes very 

 variable. That in many localities it must be very great, however, 

 there can be no question. In the ancient volcanic district of the Eifel, 

 comprising an area of many square miles around the Laacher See, on 

 the left bank of the Rhine, the annual evolution of carbonic acid from 

 springs and fissures has been estimated by Bischof at not less than 

 100,000 tons, containing 27,000 tons of carbon. In many other districts, 

 especially where active volcanoes exist, the volume of gas given off 

 may be quite as great, though no attempts have hitherto been made to 

 estimate its real amount. 



Yet though absolutely large, tie quantity of carbonic acid disengaged 

 in this way from the earth, is really small when compared either with 

 the entire quantity supposed to be present in the atmosphere, or with 

 that which is required for the growth of the yearly vegetation of the 

 globe. Suppose that from a thousand spots on the earth's surface a 

 quantity of carbonic acid equal to the above estimate of Bischof escapes 

 constantly into the air, the weight of carbon (27 millions of tons) thus 

 diffused through the atmosphere would be only ecjual to that which is 

 yearly drawn from the air by 54 millions of acres of land under cultiva- 

 tion (p. 147), and only twice as much as that contained in the coal 

 which is annually consumed in Great Britain alone. 



Still if the whole of the carboi: contained in the produce of the general 

 vegetation of the globe be ultimately restored to the air, — either by the 

 respiration of animals, by the natural and slow decay of vegetable mat- 



