266 Of the Propagation of Heat 



vert to the additional embarrassments to the passage of the 

 Heat which arise from the increased viscosity of the sap 

 in winter, and to the almost impenetrable covering for 

 confining Heat which is formed by the bark, we shall 

 no longer be at a loss to account for the preservation of 

 trees during the winter, notwithstanding the long contin- 

 uation of the hard frosts to which they are annually ex- 

 posed. 



On the same principles we may, I think, account in a 

 satisfactory manner for the preservation of several kinds 

 of fruit — such, as apples and pears, for instance — which 

 are known to support, without freezing, a degree of cold 

 which would soon reduce an equal volume of pure water 

 to a solid mass of ice. 



At the same time that the compact skin of the fruit 

 effectually prevents the evaporation of its fluid parts, 

 which, as is well known, could not take place without 

 occasioning a very great loss of Heat, the internal mo- 

 tions of those fluids are so much obstructed by the thin 

 partitions of the innumerable small cells in which they 

 are confined, that the communication of their Heat to 

 the air ought, according to our hypothesis, to be ex- 

 tremely slow and difficult. These fruits do, however, 

 freeze at last, when the cold is very intense ; but it must 

 be remembered that they are composed almost entirely 

 of liquids, and of such liquids as do not grow viscous 

 with cold, and, moreover, that they were evidently not 

 designed to support for a long time very severe frosts. 



Parsnips and carrots, and several other kinds of roots, 

 support cold without freezing still longer than apples and 

 pears, but these are less watery, and I believe the vessels 

 in which their fluids are contained are smaller; and both 

 these circumstances ought, according to our assumed 



