178 •N THE COROLLAS OF FL0WKR8# 



endeavour to ascertain the quantity of mcisture ibrmtcl In 

 tins vapour, wljich plants constantly yield, I have always 

 been foiled, from the quickness of the evaporation. 1» 

 weighing- leaves, I fold them in blottinij paper, first ascer- 

 taining its weight: but though the leaves have often lost 

 near oiie thirtieth of their weight in two hours, yet the paper 

 has scarce gained one third of that thirtieth : which, consi- 

 dering the dilTerent weight of air and water, is nothing, 

 and certainly less infinitely than the moisture in that vapour, 

 ?!ants yield That plants yield vapour only, and that the moisture is 

 7apour on y. ^.^^^^ difficult to be separated from the air, is easily proved, 

 by the method in which it is generally received. Put a plant 

 fresh out of the ground, first wiping of tlie watery crypto- 

 gamiae that may grow on it, and place it under a bell glass, 

 and in an hour or two it will cover the interior with dew : 

 but leave an opening in any part not so big as a sixpence, 

 and no moisture will be found. Why is this ? because in 

 the first pk;ce, the air being pressed by the quantity of 

 oxigen that pours from the plant, is by this pressure forced 

 to give out its moisture ; and the fluid, being detached from 

 the air, is attracted by the glass, to which it has great 

 affinity. But when the oxigen vapour has the power to 

 escape, not being pressed, it retains its moisture, and both 

 leave the'glass in the same condition in which they quitted 

 thej'pores. Sol found it in weighing the leaves, which I 

 have tried in every manner possible: the blotting jjaper was 

 not pressure sufficient to detach the moisture, and the paper 

 absorbed but little, though the leaves had evidently lost so 

 much vapour. 

 Plants do not ^^ former letters I endeavoured to show, that the 

 peripire. watery cryptogamise and the vapour just mentioned were 



taken for the perspiration of plants : every trial, every 

 year's study, convince me still more of this truth : that 

 plants do not perspire. That they yield a quantity of va- 

 pour, I never doubted — but who, that has ever dissected or 

 studied plants, can confound the terms of perspiration 

 and evaporatioji together, in botany at least ? Perspiration 

 is the iiisuing of water from pores formed for the pur-^ 

 pose. It was said by all the advocates of perspiration, to be» 

 accekrated by heat, retarded by cold, and ascertained* 



