92 Mr. Herbert Spencer on the Reciprocal Dependence 



To this inference there may, however, be raised objections. 

 It will possibly be said that the carbonic acid that in time past 

 issued by various channels out of the earth arose from the slow 

 combustion of carbonaceous deposits produced in the same 

 way as those now existing ; that the continuance of the like 

 phenomenon in our own day is due to the gradual destruction 

 of the same material, and that the strata of our coal-fields are 

 fated to undergo, by some future volcanic agency, a similar 

 revolution, and have their carbon once more sent into the air 

 in company with oxygen. Or it might perhaps be argued 

 that the oxygen set free by the instrumentality of plants has 

 entered into combination with some other element in place of 

 the carbon with which it was associated, and has thus been 

 again abstracted from the air as fast as it was added to it. 



The first of these objections is plausible, in so far as the 

 possibility of such an arrangement is concerned, though it 

 does not appear to be countenanced by facts. Neither the 

 positions usually occupied by volcanos, nor the phenomena 

 attending their eruptions, seem to indicate that the carbonic 

 acid they evolve proceeds directly from the combustion of 

 carbonaceous matter. They rather imply that it has been 

 driven off from its combinations by heat or chemical affinity. 

 In the cases of calcareous springs it would also appear that 

 the gas liberated by them had been previously in connexion 

 with an earth, it may be for an indeterminate period. More- 

 over it should be borne in mind, that the ultimate tendency of 

 all chemical changes taking place in the interior of the globe 

 must be to oxidize the most combustible elements; and since 

 the greater part of the abundant metallic bases have a stronger 

 affinity for oxygen than carbon has, its continual de- oxidation 

 would result, rather than any action of the opposite character. 

 But even admitting the existence of some play of affinities by 

 which the carbonaceous matter deposited in the course of one 

 sera is transformed into carbonic acid and given back to the 

 atmosphere during another, there is still a link wanting to 

 complete the chain of this circulating system; for it is clear 

 that the oxygen which accompanies the carbon in each of its 

 re-appearances above ground has been derived from some in- 

 ternal source, and when it has once issued into the air and 

 been deprived of its carbon it has no visible means of regain- 

 ing its previous condition, and must consequently remain in 

 the air. On this assumption, therefore, we are still brought 

 in a great degree to the same conclusion. Here indeed the 

 second objection may perhaps be brought in aid of the first, 

 and in such case it would be said that the oxygen after being 

 liberated is again absorbed by other agencies and ultimately 



