view, and to the surfaces of the human lungs and skin, 

 the power of emitting carbon, it follows,: since this e- 

 mission is carried on through the whole period of living 

 action, and is essential to the continuance of it, that 

 some ulterior source must be provided, from whence 

 its supply may be duly maintained. In seeds, this 

 carbon forms a considerable part of their substance,. 

 and> during the whole period of germination, its pro- 

 portion is constantly diminishing (20.), by uniting: 

 with the oxygen gas of the air. Before this supply is 

 exhausted, roots are sent off from the seed into the 

 earth, by which it draws in nutritious matter,, a large 

 portion of which is considered to be carbon ; and this 

 carbonic matter undergoing the ordinary changes iiL 

 the assimilating organs, is, in part, like other excre- 

 ted substances, thrown off by that expansion of the 

 seed which now forms the plant, and flourishes in: 

 the open atmosphere. So long as water and heat 

 are duly supplied, so long does living action conti- 

 nue, of which the absorption of carbonic matter by 

 the roots of plants, and its expulsion by the leaves, 

 are natural and necessary consequences ; but the va- 

 rious intermediate changes which it is made to un- 

 dergo, we do not pretend to explain : they belong, 

 in fact, to another branch of inquiry, and cannot be 

 understood until the functions of nutrition and se- 

 cretion be more fully ascertained. 



160. In the inferior animals, carbon is likewise a 

 necessary constituent substance; and certain of the 

 laws by which it is expelled from them, we have at- 

 tempted to ascertain. By them too, the means of 

 acquiring it, must be as constant as its expulsion, 

 during living action, has been shewn to be j and 



