ON THE PERSPIRATION OF PLANTS* 171 



periments, which I shall now detail, prefacing them with 

 an observation which is necessary to begin with, because it 

 is one of the signs given of perspiration, which I cannot False sign of 

 assent to. Hales and Bonnet both observe, that, having I**^ 1 ™ 

 placed a plant under a glass, the water after a time ran 

 down or bedewed the glass. Put a wet sponge under a cy- 

 linder, and it will produce the same effect; and yet we 

 should not say, that the sponge perspired, but that some of 

 the moisture within the sponge had evaporated, and was 

 condensed by the cold of the glass. In short it is merely a 

 sign, that the object thus confined is full of moisture, 



I shall now mention the experimentsjn the order in which Experiments 

 I made them. I wished first to prove, which yielded most JJ^ 16 rose 

 moisture, the earth or plants. I placed a small rose tree 

 under a large glass in a pot of earth, placing at the same 

 time Captain Kater's excellent hygrometer* with it, which 

 then stood at 6'20 from excess of dryness. In 8 hours the 

 moisture ran down the glass, and the hygrometer was at 

 1100, nearly excess of moisture. I then took away the 

 rose tree, and, drying the glass, I put a pot of fresh earth compared witli 

 the same size and weight, and with the same arrangement. fresh <tiiS ** ki 

 In 8 hours the hygrometer, which had been put in at 616, 

 came out 1049, or 433 more moist than it was when placed 

 there. It was the earth therefore, that gave all that excess 

 of moisture, not the plant. 



The next trial was made by fastening down a laurel on a laurel 

 branch, and passing it through a piece of sheet lead, with- ranc 

 out separating it from the tree; making it to fit a very large 

 glass cylinder, then luting it round the lead, and at its en- 

 trance, to keep out the circulation of air, and prevent the 

 wet vapour from passing upwards. After 8 hours the hy- 

 grometer was 130 nearer to dryness than when placed there, 

 and though the glass was steamed, it did not run down with 

 water; nor could I, with the largest magnifiers, discover any 

 dew drops on the leaves. 



I now tried a vast number of plants with the same result, and on Tariow 

 the hygrometer never showed an increase, which it would P lants * 

 certainly have done, had the perspiration been so excessive; ' 



* For a description of this hygrometer, sec our pTesent number. 



and 





