98 THE RISE OF THE SAP 



" If in the morning while the sap is in a rising 

 state, there was a cold wind with a mixture of sun- 

 shine and cloud ; when the sun was clouded the sap 

 would immediately visibly subside, at the rate of an 

 inch in a minute for several inches, if the sun con- 

 tinued so long clouded ; but as soon as the sun- 

 beams broke out 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 plen- 

 tiful rise of the sap in the vine in the bleeding 

 season, is effected in the same manner." 

 HOW is this If we consider, that the sap in spring, even with 



effect to be 



accounted a clouded sky, does not cease to rise and flow, for 



for ? 



this even goes on during the night, we cannot ex- 

 plain 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. 



Gas is HALES says, in his experiment XXXVIII. " In 



with the 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. 



" 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 



