PROCESS OF NUTRITION. CHAP. III. 



has been found also to increase as the heat of 

 summer advances ; being more abundant in July 

 than in June, and still more in August than in 

 either of the preceding months, from which last 

 period it begins again to decrease. 



But the most remarkable instance of rapid tran- 

 spiration yet observed is that which is related by 

 Guettard, who found that a small sprig of the 

 Corneil-tree or Cornelian Cherry, Cornus mascula, 

 transpired in the course of a day 1 oz. 3 drams, 

 a quantity almost double its own weight. He found 

 also in general that branches deprived of their 

 leaves afford but little transpired matter, and that 

 branches furnished with their leaves afford a great 

 deal ; it follows, therefore, of necessity that the 

 leaves, as Hales suspected, are the principal organs 

 of transpiration. 



The substance thus transpired by the plant may 

 be obtained by enclosing a bough in a glass vessel 

 of proper dimensions luted to the branch. Its pro- 

 perties have not yet been very minutely investi- 

 gated ; Hales and Guettard could discover in it 

 nothing different from common water except that in 

 some cases it had the odour of the plant ; but Du 

 Hamel found that it became sooner putrid than water. 

 Or per- Such then are the facts that have been ascertained 

 with regard to the imperceptible perspiration of 

 plants, from which it unavoidably follows that the 

 sap undergoes a very considerable modification in 

 its passage through the leaf. But it often under- 



