264 Of the Propagation of Heat 



Among vegetables, those which are the most succulent 

 are annual. Not being furnished with lungs to keep the 

 great mass of liquids warm, which fill their large and 

 slender vessels, they live only while the genial influence 

 of the sun warms them and animates their feeble powers, 

 and they droop and die as soon as they are deprived of 

 his support. 



There are many tender plants to be found in cold 

 countries, which die in the autumn, the roots of which 

 remain alive during the winter, and send off fresh shoots 

 in the ensuing spring. In these we shall constantly 

 find the roots more compact and dense than the stalk, or 

 with smaller vessels and a smaller proportion of Fluids. 



Among the trees of the forest we shall constantly find 

 that those which contain a great proportion o^ thin watery 

 liquids not only shed their leaves every autumn, but 

 are sometimes frozen, and actually killed, in severe frosts. 

 Many thousands of the largest walnut-trees were killed 

 by the frost in the Palatinate during the very cold winter 

 in the year 1788 ; and it is well known that few, if any, 

 of the deciduous plants of our temperate climate would 

 be able to support the excessive cold of the frigid zone. 



The trees which grow in those inhospitable climates, 

 and which brave the colds of the severest winters, con- 

 tain very little watery liquids. The sap which circulates 

 in their vessels is thick and viscous, and can hardly be 

 said to be fluid. Is there not the strongest reason to 

 think that this was so contrived for the express purpose 

 of preventing their being deprived of all their Heat, and 

 killed by the cold during the winter ? 



We have seen by the foregoing experiments how much 

 the propagation of Heat in a liquid is retarded by di- 

 minishing its fluidity ; and who knows but this may 



