362 Professor Forbes' s Researches on Heat. 



suit in my former paper, art. 45. Under any circumstances the 

 experiment is a troublesome one, but I have succeeded in arrang- 

 ing it in perhaps as satisfactory a way as it admits of being done. 

 The great difficulty arises from the minuteness of the quantity 

 of heat reflected, and consequently the large quantity absorbed 

 by the plates, which complicates and obscures the effect. This is 

 more particularly true with dark heat, which, at the same time, 

 furnishes the most important case to be examined. The effect of 

 the absorbed heat is to produce a powerful secondary radiation. 

 28. My first inquiry on resuming the subject was to ascer- 

 tain the relative order of several different substances as to their 

 power of reflecting heat. This was not proposed to be done with 

 a view to a general inquiry into that important subject, which 

 I reserved to another occasion, but simply to ascertain what re- 

 flecting surfaces might be best employed in polarizing by reflec- 

 tion. Several series of experiments gave the following arrange- 

 ment of substances according to their power of reflecting heat, at 

 an incidence of 45°, beginning with the most perfect reflector. 



Polished speculum metal. 



Mica, split by the hand into thin plates. 



Mica, split by heat (see art. 20). 



Thick plate of mica. 



Rock-salt, with a thin coating of varnish. 

 /Polished rock-salt. 

 ) Glass. 

 ( Alum. 



The three last substances (so different in their diathermancy) 

 were nearly equal in their reflective power for dark heat (from 

 brass about 7000°). The above order did not, however, appear 

 to be changed for heat from incandescent platinum, except that 

 o-lass seemed to stand above alum and even salt. In a general way, 

 we may consider the measure of metallic reflection to be from two 

 to three times as great as that from mica split by heat, which is 



