SECT. IV. ELABORATION OF THE SAP. 137 



Imperceptible perspiration is an excretion of sap Impercep- 

 thrown off by the Epidermis of the leaf or other u 

 tender parts of the plant, in consequence of the 

 healthy action of the vegetable organs. It is not 

 discoverable by the external senses, as the name 

 indeed implies, but is legitimately inferred from 

 the following fact: If the branch of a tree is lopped, 

 and the section of the part lopped off covered with 

 mastick, the branch will be found in the course of 

 a few days to have lost in weight. This was ori-^ . 

 ginally an experiment of Mariotte's, and the loss in 

 weight is to be accounted for only on the principle 

 of the perspiration of the sap escaping through the 

 pores of the epidermis. This conclusion may per- 

 haps be regarded as not altogether satisfactory, as 

 being founded on an experiment made only on a 

 lopped-off branch. But the same conclusion fol- 

 lows from experiments on the living plant : Hales 

 reared a Sun-flower in a pot of earth till it grew to 

 the height of three feet and a half; he then covered 

 the mouth of the pot with a plate of lead, which 

 he cemented so as to prevent all evaporation from 

 the earth contained in it. In this plate he fixed 

 two tubes, the one nine inches in length and of but 

 small diameter, left open to serve as a medium of 

 communication with the external air; the other two 

 inches in length and one in diameter, for the pur- 

 pose of introducing a supply of water; but kept 

 always shut except at the time of watering. The 

 holes of the bottom of the pot were also shut, and 



