47 



express them, a circumstance which, while it be- 

 gets a doubt of their accuracy, tends greatly to 

 puzzle the student, without proportionally advancing 

 the science. It is sufficient lor our purpose to ob- 

 serve, that no direct communication is demonstra- 

 ted to exist between any pores on the surface of the 

 epidermis, and the vessels in which the vegetable 

 fluids move. If, therefore, chemical affinity operate 

 here, it must be between substances, not, as usual, 

 in actual contact, but at a certain assignable distance ; 

 and that distance intercepted by a very compounded 

 organized structure. Let us, however, suppose for 

 a moment the oxygen gas of the air to obtain ad- 

 mission into the juices of the plant, under all these 

 circumstances, by the operation of chemical affinity ; 

 by what power will the carbonic acid, which it 

 is there supposed to form, be afterwards emitted ? 

 Not by chemical affinity, surely, for where is the 

 external agent to effect it ? Not through a living ac- 

 tion, for where is the structure to perform it ? 

 Or how can the reception of air be thus carried on 

 by one law, and its expulsion by another so widely 

 different ? How, also, is this oxygen gas, previous 

 to its supposed attraction into the plant, to be sepa- 

 rated from the nitrogen gas with which it is natural- 

 ly united ? If, as is probable, the separation be 

 brought about only by the superior affinity of carbon 

 furnished by the leaf, then the carbonic acid, in- 

 stead of being emitted from the leaf, is formed exte- 

 rior to it, and out of the very oxygen gas which is sup- 

 posed first to enter into the leaf to form it. Those, 

 also, who favour this opinion of the attraction of 

 oxygen gas, cannot well suppose that plants likewise 



