46 OF VITAL MOTION. 



there is evidence of a general dilatation under the 

 operation of force, by which these vessels are preserved 

 as open channels for the current which flows from the 

 heart. We may imagine circumstances which would 



remains constant and unchanging. This fact seems to point 

 beyond the kingdom of life ; for, if the forms of vegetation were 

 solely concerned in maintaining this equilibrium, it might be 

 supposed that the quantity of carbonic acid present in the air 

 would be greater in winter than in summer. 



In the second place : it may be supposed that inorganic bodies, 

 under certain circumstances, are endowed with a catalytic power, 

 by virtue of which they are enabled, without any manifest 

 change in themselves, to effect changes in other bodies, and 

 hence it may be argued, without any improbable assumption, 

 that the phenomenon of light is expressive of a condition in 

 certain inorganic as well as in certain organic substances, by 

 which the formation of carbonic acid is prevented and its 

 decomposition effected. 



In the third place : the history of sulphuretted hydrogen, 

 (which gas is a joint product with carbonic acid in the process 

 of decay, and not uncommonly also in respiration,) furnishes 

 a strong argument that inorganic bodies are actually concerned 

 in this depuratory office. Both these gases present many points 

 of relationship, and they are closely allied in origin, in properties, 

 in the poisonous effects upon the system, and also in the mode in 

 which they are affected by light, for this agent is unfavourable 

 to their origin and fatal to their existence ; and therefore it is 

 allowable to "Seek some explanation of the action of light in the 

 one case by what is more clearly seen in the other. What, then, 

 it may be asked, is the class of bodies employed in the destruction 

 of the sulphuretted hydrogen which is continually passing off 

 into the atmosphere? As is the case with carbonic acid, it 

 cannot be supposed that this gas is absorbed as food by vegetable 

 structures, neither can it be supposed to be in any way sub- 



