164 Biographical Account of [Mabch, 



hypothesis of his own ; but stated experiments which completely 

 overturned all the hypotheses previously contrived to account 

 for the motion of the sap in trees. They often led him to 

 surprising conclusions. Could it have been believed, for 

 example, that the force with which a vine branch draws the sap, 

 at the time when the sap flows, is equal to the pressure of a 

 column of water 36 feet in height. Yet Dr. Hales demonstrated 

 this by cementing glass tubes to the extremities of vine branches 

 cut at that season, and observing how high the sap issuing out 

 from the branch rose. 



Similar experiments, but made in a different season of the 

 year, and upon a great number of plants, informed him of the 

 force and the quantity of transpiration of plants, which he 

 contrived to collect and render sensible. The motion of the sap 

 in trees, and even the existence of vessels of communication 

 which allow it to pass laterally from one side to the other, are 

 rendered obvious to the eyes with an inconceivable ingenuity. 

 He estimates the effect of the heat of the sun upon different parts 

 of trees, and that of the temperature of the earth, which he deter- 

 imnes to as great a depth as the roots usually reach. He 

 explains the use of the leaves, till that time veiy little under- 

 stood, and which, according to him, are the organs by which 

 plants exhale during the day the liquid which they draw from 

 the earth ; and which, during the night, on the contrary, draw 

 back again the moisture which they find in the air. This alter- 

 nate motion in plants is a substitute for the circulation of the 

 blood in animals. It would occupy too much room to specify 

 all the curious experiments related in the first part of this work, 

 and the singular consequences which he deduces from them. 

 But it would be improper to overlook the modest reserve with 

 which he eveiy where restricts himself to a bare statement of 

 facts without hazarding any other conclusions than those which 

 a rigid calculation have converted into certainties. It is unfor- 

 tunate for the progress of physics that this sage conduct has 

 been too seldom imitated. 



The second part of this work is, if possible, still more interest- 

 ing than the first. It constitutes the real foundation of pneu- 

 matic chemistry; for it is doubtful whether Dr. Black, who 

 produced the first great impulse, would have been able to have 

 made out the constitution of limestone had it not been for the 

 previous experiments of Hales. Dr. Black himself informs us 

 that it was the previous experiments of Dr. Hales on limestone, 

 who demonstrated that when this mineral is dissolved in acids, a 

 quantity of air is disengaged from it, that led him first to draw 

 the conclusion, that limestone is a compound of quickhme and 

 air. An examination of the air, which he caWed Jixed air, led 

 him to the knowledge of its nature ; and the properties of fixed 

 air were afterwards fully investigated by Mr. Cavendish and 



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