82 EXPERIMENTS OF HALES ON THE 



evidence. HALES says, in his experiment XXII., 

 " This height of the mercury did in some measure 

 shew the force with which the sap was imbibed, 

 though not near the whole force ; for while the 

 water was imbibing, the transverse cut of the 

 branch was covered with innumerable little hemi- 

 spheres of air, and many air-bubbles issued out of 

 the sap-vessels, which air did in part fill the tube 

 e r, as the water was drawn out of it ; so that the 

 height of the mercury could only be proportionable 

 to the excess of the quantity of water drawn off, 

 above the quantity of air which issued out of the 

 wood. And if the quantity of air which issued from 

 the wood into the tube, had been equal to the quan- 

 tity of water imbibed, then the mercury would not 

 rise at all ; because there would be no room for it in 

 the tube. But if 9 parts in 12 of the water be 

 imbibed by the branch, and in the mean time, but 

 three such parts of air issue into the tube, then the 

 mercury must needs rise near 6 inches, and so pro- 

 portionably in different cases." 

 The ab- When, in his experiments, the root, the stem, or 



sorbent 



power di- a twig had been injured at any part, by the cutting 

 by injury to off of buds, root-fibres, or small twigs, the absorbent 

 power of the remainder was diminished in a very 

 obvious degree (because, from these places, by the 

 entrance of air the difference of pressure was more 

 easily equalized) ; the absorbent power was greatest 

 on freshly-cut surfaces, on which, however, it gra- 

 dually decreased, till, after several days, it was not 



