Of the Propagation of Heat 



But there is another circumstance which it is necessary 

 to take into the account, and that is the attraction which 

 subsists between air and the bodies above mentioned, 

 and other like substances, constituting natural and arti- 

 ficial cloathing. For, though the incapacity of air to 

 give a passage to Heat in the manner solid bodies per- 

 mit it to pass through them may enable us to account 

 for its warmth under certain circumstances, yet the bare 

 admission of this principle does not seem to be suffi- 

 cient to account for the very extraordinary degrees of 

 warmth which we find in furs and in feathers, and in vari- 

 ous other kinds of natural and artificial cloathing; nor 

 even that which we find in snow ; for if we suppose the 

 particles of air to be at liberty to carry off the Heat 

 which these bodies are meant to confine, without any 

 other obstruction or hindrance than that arising from 

 their vis inertix^ or the force necessary to put them in 

 motion, it seems probable that the succession of fresh 

 particles of cold air, and the consequent loss of Heat, 

 would be much more rapid than we find it to be in 

 fact. 



That an attraction, and a very strong one, actually 

 subsists between the particles of air and the fine hair or 

 furs of beasts, the feathers of birds, wool, &c., appears 

 by the obstinacy with which these substances retain the 

 air which adheres to them, even when immersed in 

 water, and put under the receiver of an air-pump ; and 

 that this attraction is essential to the warmth of these 

 bodies, I think is very easy to be demonstrated. 



In furs, for instance, the attraction between the par- 

 ticles of air and the fine hairs in which it is concealed 

 being greater than the increased elasticity or repulsion 

 of those particles with regard to each other, arising from 



