352 Prof. Forbes on the Polarization of Heat. 



perishably connected with the science of heat, the one in its 

 experimental, the other in its most recondite mathematical, de- 

 partment. In these experiments no less than Jifty per cent. 

 of the whole heat incident upon the pile in the circumstances 

 of figg. 1. and 3. was polarized, that is, stopped, in the posi- 

 tion of the plates in fig. 2. : the effect on the galvanometer 

 was instantaneous, and the relative positions and distances 

 of the plates were varied in every possible way without sensi- 

 bly affecting even the quantitative measure of the results. 



It would be easy to devise a vast variety of modes of opera- 

 ting which would equally annihilate the mathematical ob- 

 jection ; at present, being anxious to confine myself to the 

 narrowest possible limits, I will only add that in the beautiful 

 and admirably marked phaenomena of depolarization by a 

 uniform mica plate introduced between the plates A and B, 

 and perpendicularly to the incident ray, there subsists the 

 most perfect symmetry which the mathematician can desire. 



I have had the pleasure of learning within a few days, from 

 M. Melloni himself, that he is now satisfied that heat is 

 polarized by tourmaline, which was that of all my experi- 

 ments in which conduction produced the most troublesome 

 effects. 



London, October 5, 1835. 



Postscript. Having communicated the substance of the 

 above note to Professor Powell, I had the satisfaction of 

 learning from him that the statement in the October number 

 of the Phjlosophical Magazine had led me to an erroneous 

 apprehension of what he stated, at the Dublin meeting, rela- 

 tive to the polarization experiment. In relation to the ques- 

 tion discussed above, he says : 



" Allow me to assure you that if any of the reports repre- 

 sented it as urged for an objection to your results, they must 

 have entirely mistaken my meaning. I proposed it merely as 

 an abstract problem which would affect the small correction 

 which (if the heat were intense enough) should be applied for 

 the secondary radiation ; but I have no doubt it is quite in- 

 sensible." 



I suffer the statement which I have made above, relative to 

 this experiment, to remain, because it may serve as a general 

 answer to a natural enough prima facie objection; whilst it 

 gives me the highest pleasure to be able to quote Professor 

 Powell's opinion in support of my views instead of the re- 

 verse. 



Edinburgh, October 21, 1835. 



