or VITAL MOTION. 45 



condition of the vessels, or else at times when the 

 heart flags there is no evidence of the existence of any 

 power in these vessels which could aid in the circula- 

 tion by rhythmical contractions and dilatations; but 



are bound up as a common whole in mutual subserviency 

 each to the other, each part is nevertheless a complete entity 

 in itself, and not merely a complementary appendage to 

 another. 



It may well be doubted, indeed, that plants are the sole agents 

 endowed with the power of destroying carbonic acid. If it 

 were so we might imagine (what is not the case) that the quan- 

 tities of this gas present in the atmosphere would be greater in 

 proportion as the forests disappear before the progress of civili- 

 zation. In India and China also it may well be doubted 

 whether the vegetable covering of the soil is sufficient to render 

 the air fit for healthy breath. And in the colder realms of the 

 north, where the vegetable carpet is so scantily spread that 

 mighty hordes of animals are compelled to migrate year by year 

 into more southern regions, the insufficiency of the vegetation 

 to the proper purification of the atmosphere is very apparent : 

 and so also in the ocean, for the scanty fields of seaweeds can 

 scarcely be proportionate to the wants of the countless creatures 

 which breathe in the waters. 



And if the vegetable world be insufficient to the task of 

 purifying the air and waters, where are we to turn in order to 

 find the co-operating agencies? It may not be to the realm of 

 animate nature, for here, though under certain circumstances 

 there may be evidence of a partial decomposition of carbonic 

 acid; yet, at all times, a larger amount of this gas will be 

 generated than can by any possibility be decomposed. It is to 

 the inorganic world, therefore, that w^e are obliged to turn, and 

 that here are to be found the complementary agencies for which 

 we seek, appears for a threefold reason. 



In the first place : the stability of the atmosphere is an argu- 

 ment that the cause of this stability rests in an agency which 



