and the Mode of its Communication. 97 



warmth of any kind of substance used as clothing, or its 

 power of preventing our bodies from being cooled by 

 the influence (frigorific radiations) of surrounding colder 

 bodies, depends very much on the polish of its surface. 



If, with the assistance of a microscope, we examine 

 those substances which supply us with the warmest cov- 

 erings, such, for instance, as furs, feathers, silk, &c., 

 we shall find their surfaces not only smooth, but also very 

 highly polished ; we shall also find that, other circum- 

 stances being equal, those substances are the warmest 

 which are the finest, or which are composed of the great- 

 est number of fine polished detached threads or fibres. 



The fine white shining fur of a Russian hare is much 

 warmer than coarse hair; and fine silk, as spun by the 

 silkworm is warmer than the same silk twisted together 

 into coarse threads ; as I found by actual experiments, 

 an account of which has already been laid before this 

 Society and published in the Philosophical Transactions. 



I formerly considered the warmth of natural and ar- 

 tificial clothing as depending principally on the obstacle 

 it opposes to the motions of the cold air by which the 

 hot body is surrounded ; but, by a patient and careful 

 examination of the subject, I have been convinced that 

 the efficacy of radiation is much greater than I had sup- 

 posed it to be. 



From the result of the experiment No. 31, we might 

 be led to conclude that a very small part only of the 

 heat which a hot body appears to lose when it is cooled 

 in air is in fact communicated to that fluid, a much 

 greater portion of it being communicated to other sur- 

 rounding bodies at a distance ; and, in one of my for- 

 mer experiments, a hot body was cooled, though it was 

 placed in a Torricellian vacuum. 



VOL II. 7 



