Prof. Forbes on the Refraction and Polarization of Heat, 213 



fave for the proportions of heat transmitted, when the plates 

 \ and F were parallel and crossed, 100 : 93 ; another of eight 

 comparisons, gave 100 : 96 ; a third, of eight, 100 : 92. Among 

 these twenty-two comparisons, only one gave a result slightly 

 negative. 



40. With platinum rendered incandescent by alcohol, the 

 effect appears decidedly greater than with any other source of 

 heat I have tried. Plates E and F; ratios of effect when^a- 

 rallel and crossed, 100 : 59 ; 100 : 62 ; 100 : 66 ; 100 : 54-. 

 The brilliancy of the incandescence affects materially the 

 transmission. 



41. Alcohol fame, Q.^ might be anticipated, is less steady; 

 means from sets of five observations, with plates E and F ; 

 100:66; 100:72; 100:79; 100:42; 100:62. 



42. With the simple oil-lamp of Locatelli ; plates E and F, 

 the ratios are 100 : 76 ; 100 : 73*5 ; 100 : 79. 



43. With Argand lamp, and glass chimney ; plates E and F; 

 ratios, 100 : 70 ; 100 : 72 ; results very steady. 



44. When we combine these results*, and compare them 

 with the quantity of light polarized, which was derived from 

 some rude photometrical experiments, which agreed pretty 

 nearly, we get the following approximations to the degrees of 

 polarization, by a given combination, and depending on the 

 source of heat. 



Source of Heat. I^ys »"' of ^OO, polarized by transmission 



through mica plates £. and F. 



Argand lamp (glass chimney), ... 29 



Locatelli lamp, 24 



Alcohol flame, 36 



Incandescent platinum, . * 40 



Brass, about 700°, 22 



Mercury, about 500° (in crucible), . 17 



Water under 200°, 6 



Proportion of Z?^^^ polarized,* ... 89 



45. So completely and satisfactorily made out does the pola- 

 rization of heat appear by these concurrent experiments, that 

 it was little more than a matter of curiosity to verify it in the 



* It should be remarked, that these experiments contain all the measures 

 I have made with a view to this determination, except two, which were 

 made the very first day I discovered the fact, and which were not accurate 

 enough to be employed. I mention this, because, in such experiments, it 

 is important to be assured of the constancy and marked nature of a result, 

 which can only be appreciated by keeping back no fairly made observation. 



f Though 1 am not aware of any source of error, 1 cannot help thinking, 

 that, in this case, and in that of the tourmaline, art. (21.), the defalcation of 

 light is estimated too high. 



