44 OF VITAL MOTION. 



The phenomena of capillary movement in perfect 

 organisms appear to point to a common law in the 

 operation of force, and this equally, whatever may be 

 the nature of the force. Except in a very rudimentary 



therefore, to pause a moment and investigate the antagonizing 

 influences which prevent and correct these several evils. 



The history of respiration appears to imply that this function 

 is most vigorous in the absence of light. In plants, for example, 

 the formation of carbonic acid is nearly suspended during the 

 day, or at any rate is much less marked than at night. In 

 animals these results are obscured by many circumstances, but 

 here also, all other things being equal, the same facts are 

 observable. In that form of force, therefore, which is accom- 

 panied with the sensation of light, we find, as it would seem, a 

 condition unfavourable to the formation of carbonic acid, and 

 light, on this account, may be supposed to be unpropitious to 

 the act of respiration. Light, moreover, is not merely unfa- 

 vourable to the formation of carbonic acid, but it is entirely 

 inimical to this process, for the history of vegetation affords 

 sufficient and ample proof that this gas is actually decomposed 

 under the influence of this agent. And thus it would appear, 

 that the conditions unfavourable to the act of respiration are 

 those which restore the equilibrium of the atmosphere by 

 destroying the poisonous gas which is the product of the act. 



The actual mode in which light prevents the formation, or 

 effects the decomposition of carbonic acid, is but very imper- 

 fectly understood. The belief is, that the gas in question, 

 though poisonous to animals, is a necessary element of food 

 to plants : it is supposed that the latter are enabled to assimilate 

 this gaseous food under the influence of light, and in this way 

 to preserve the air in the requisite degree of purity. It is 

 an apparent fallacy, however, to imagine that plants are the 

 agents solely concerned in the process of atmospheric depu- 

 ration. The difference between animal and vegetable is one 

 of degree and not of kind, and though all parts of the creation 



