240 Of the Propagation of Heat 



same Effects in Water. These Effects are probably pro- 

 duced in both Fluids in the same Manner ', namely >-, by ob- 

 structing the Motions of their Particles in the Operation 

 of transporting the Heat. The conducting Power of 

 Water remarkably impaired by mixing with it such Sub- 

 stances as render it viscous and diminish its Fluidity. 

 These Discoveries respecting the Manner in which Heat is 

 propagated in Water throw much Light on several of the 

 most interesting Operations in the Economy of Nature. 

 They enable us to account in a satisfactory Manner for the 

 Preservation of Trees and other Vegetables, and of Fruits, 

 during the Winter, in cold Climates. 



IT is certain that there is nothing -more dangerous in 

 philosophical investigations, than to take anything 

 for granted, however unquestionable it may appear, till 

 it has been proved by direct and decisive experiment. 



I have very often, in the course of my philosophical 

 researches, had occasion to lament the consequences of 

 my inattention to this most necessary precaution. 



There is not, perhaps, any phenomenon that more 

 frequently falls under our observation than the Propaga- 

 tion of Heat. The changes of the temperature of sen- 

 sible bodies, of solids, liquids, and elastic fluids, are go- 

 ing on perpetually under our eyes, and there is no fact 

 which one would not as soon think of calling in question 

 as to doubt of the free passage of Heat, in all directions, 

 through all kinds of bodies. But, however obviously 

 this conclusion appears to flow from all that we observe 

 and experience in the common course of life, yet it is 

 certainly not true ; and to the erroneous opinion respect- 

 ing this matter, which has been universally entertained 

 by the learned and by the unlearned, and which has, I be- 



