44 MOTION OF THE JUICES OF THE ANIMAL BODY. 



again, the sap would immediately return to its then rising state, just as any liquor 

 in a thermometer rises and falls with the alternacies of heat and cold ; whence it 

 is probable, that the plentiful rise of the sap in the vine in the bleeding season, is 

 effected in the same manner." 



If we consider, that the sap in spring, even with a clouded sky, does not cease 

 to rise and flow, for this even goes on during the night, we cannot explain the fall 

 of the sap from the moment that the sun was covered by a cloud by a mere change 

 of temperature in the juice, because the time was too short for the cooling and 

 contraction by cooling (one inch in a minute.)* Heat determined the more rapid 

 rise, and cold the fall, but they acted on a cause which lay higher than the root, 

 and which was more sensitive to heat than the liquid itself. 



HALES says, in his experiment XXXVIII. In very hot weather many air 

 bubbles would rise, so as to make a froth an inch deep, on the top of the sap in 

 the tube.t 



" I fixed a small air pump to the top of a long tube, which had twelve feet height 

 of sap in it ; when I pumped, great plenty of bubbles arose, though the sap did 

 not rise, but fell a little, after I had done pumping." 



In his experiments on the amount of air absorbed 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 mani- 

 festly shows 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 opinion appears not untenable, 

 that the incomprehensible force, which causes the sap of the vine to flow in spring, 

 may be simply referred to a disengagement of gas which takes place in the 

 capillary vessels (filled with liquid, and keeping themselves constantly full,) in 

 consequence of a kind of germination ; and it is possible that the height of the 

 column of mercury, or of water, is only a measure of the elasticity of the dis- 

 engaged gas.J 



Let us suppose a strong glass bottle, in the mouth 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 power residing 

 in the walls of the vessel ; it only shows the tension of the gas. 



If the walls of the vessel were permeable to the gas under a certain pressure, no 

 further rise, beyond that point, could occur. 



If, in the apparatus, Fig. 4, we push the tube a through the cork down to the 

 little lead drop ; if we then fill the tube c with water to which some yeast has 

 been added, and a with solution of sugar, and expose the whole to a temperature 

 of from 68 to 75, the liquid rises in 6, from the gas disengaged in c, very 

 rapidly, so as to overflow. If c be filled with solution of sugar, and a with yeast, 

 the same rise occurs, and lasts till the disengaged gas puts an end to the contact 

 between the membrane and the liquid 



It is hardly necessary to point out, that the idea above expressed as to the 

 cause of the flow and pressure of the spring sap, is nothing more than an indication 

 of the direction in which experiments must be made. When we know with 

 accuracy the volume of the liquid which flows out of a vine at the time of flowering, 

 and the quantity of gas which is developed at the same time, we shall, I trust, 

 find ourselves a step nearer to the explanation of this phenomenon. According 

 to the experiments of GEIGER and PROUST, the sap of the vine is rich in carbonic 

 acid ; and it is possible that the gas which is disengaged, may be no other than 

 carbonic acid gas. 



* How is this effect to be accounted for ? 



t Gas is given off with the sap. 



t The rise of the sap may, therefore, be caused by th evolution of gas. 



