MAY BE CAUSED BY A GAS. 99 



sap did not rise, but fell a little, after I had done 

 pumping." 



In his experiments on the amount of air ab- 

 sorbed by plants, chapter V., he observes, " in the 

 experiments on vines, the very great quantity of 

 air which was continually ascending from the vines, 

 through the sap in the tubes ; which manifestly 

 shews what plenty of it is taken in by vegetables, 

 and is perspired off with the sap through the leaves." 



When we take these facts into consideration, the The rise of 



the sap 



opinion appears not untenable, that the mcompre- maythere- 



i . i f re be 



hensible force, which causes the sap of the vine to caused by 



flow in spring, may be simply referred to a disen- 

 gagement of gas which takes place in the capillary 

 vessels (filled with liquid, and keeping themselves 

 constantly full), in consequence of a kind of germi- 

 nation; and it is possible that the height of the 

 column of mercury, or of water, is only a measure 

 of the elasticity of the disengaged gas. 



Let us suppose a strong glass bottle, in the mouth iiiustra- 

 of which a long tube, open at both ends, and reaching 

 to the bottom, is cemented, to be filled with a 

 liquid in which, from any cause, a gas is disengaged 

 (solution of sugar mixed with yeast, for example), 

 it is evident that the liquid must rise in the tube 

 from the separation of the gas. When it has risen 

 to 32 feet, the gas will occupy only the half, and at 

 64 feet, one-third of its volume under the usual 

 atmospheric pressure. In this case, the height of 

 the liquid in the tube is no measure of a special 



H2 



