396 PHILOSOPHICAL TRANSACTIONS. [aNNO 16QQ. 



received, to bear it on along with it. Water is further fitted to be a vehicle 

 to this matter, by the tenuity and fineness of the corpuscles of which it con- 

 sists : we hardly know any fluid in nature, except fire, whose constituent parts 

 are so exceedingly subtle and small as those of water are : they pass pores and 

 interstices that neither air nor any other fluid can : this enables them to enter 

 the finest tubes and vessels of plants, and to introduce the terrestrial matter, 

 conveying it to all their parts ; whilst each, by means of organs it is endowed 

 with for the purpose, intercepts and assumes to itself such particles as are 

 suitable to its nature ; letting the rest pass on through the common ducts. 

 Nay, we have almost every where mechanical instances of much the same 

 nature. It is obvious to every one how easily and suddenly humidity, or the 

 corpuscles of water sustained in the air, pervade and insinuate themselves into 

 cords, however tightly twisted ; into leather, parchment, vegetable bodies, wood, 

 and the like. And this it is that fits them for hygrometers ; and to measure and 

 determine the different quantities of moisture in the air, in different places 

 and seasons, 



8. Water is not capable of performing this office to plants, unless assisted by a 

 due quantity of heat, which must concur, else vegetation will not succeed. 

 The plants that were set in the glasses q, r, s, &c. in October and in the follow- 

 ing colder months, had not near the quantity of water sent up into them, or 

 near so great an additional increase, as those that were set in June, July, and 

 the hotter months. It is plain that water has no power of moving itself or of 

 rising to the vast height it does in the more tall and lofty plants. So far from 

 it, that it does appear that even its own fluidity consists in the intestine motion 

 of its parts ; there being no need of any thing more, for solving all the phaeno- 

 mena of fluidity, than such a figure and disposition of the parts as water has; 

 for corpuscles of such a make, and that are all absolutely spherical, must stand 

 so very tottering on each other, as to be susceptible of every impression : and 

 though not perpetually in motion, yet must be ever ready and liable to be put 

 into it, by any the slightest force imaginable. It is true, the parts of fire or 

 heat are not capable of moving themselves, any more than those of water; 

 but being more subtle, light, and active, than these are, they are more easily 

 put into motion. In fine, it* is evident, and matter of fact, that heat operates 

 on and moves the water, in order to its carrying on the work of vegetation. 



That the concourse of heat is really necessary in this work, appears not 

 only from the experiments before us, but from all nature, in fields and forests, 

 in gardens and orchards : for we see in autumn, as the sun's power abates, so 

 its eftlct on plants is remitted, and their vegetation slackens by little and 

 little. As the heat returns the succeeding spring, they all recruit again : 



