Prof. Forbes on the Polarization of Heat. 54S 



the mica plates to the thermal pile affects my results ; and 

 that as more dark heat than bright heat is absorbed by mica, 

 a greater share of the total effect is produced by this cause in 

 the first case than in the second. Since this secondary effect 

 is unchanged when the mica plates are perpendicular and 

 parallel, there appears to be less heat polarized in the first case 

 than in the second. In support of this opinion M. Melloni 

 quotes one of my earliest and rudest experiments, in which the 

 effect of absorbed beat was abundantly obvious, and which 

 could form no certain basis of quantitative deductions. 



M. Melloni does not appear to have observed that in my 

 later quantitative experiments, (which he likewise quotes,) the 

 supposed cause of error was eliminated by observing the 

 dynamical effect, or the extreme arc of impulsion of the nee- 

 dle (a method which I borrowed from M. Melloni), and that 

 (the screen which was removed in order to let the heat act upon 

 the pile being between the source of heat and the mica plates,) 

 the mica plates were only absorbing heat during the few 

 seconds that the arc of impulsion was being observed. I have 

 adopted M. Melloni's mode of ascertaining the inefficiency 

 of the absorbed heat, (which is the same as I suggested at the 

 very commencement of my experiments, for the satisfaction of 

 those who on the very same grounds denied the polarization 

 of heat altogether,) and I find the effect of absorption to be 

 utterly inappreciable, whether for dark or bright heat. 



It is clear that some other mode must be adopted to account 

 for the discrepancy of our results. From M. Melloni's known 

 accuracy and address in the use of the thermo-multiplier I 

 can hardly doubt the correctness of the results he has obtained. 

 I am also tolerably confident as to my own. Perhaps the 

 difference may lie in his having used more plates, and at greater 

 inclinations, to produce polarization ; which of course will 

 always tend to bring to uniformity of state rays of heat, how- 

 ever differently susceptible of polarization by a given plate at 

 a given angle, which is all the distinction I ever meant to draw 

 when I used for brevity the phrase " polarizable nature" of any 

 kind of heat. 



Though I cannot help saying that it seems to me that those 

 who have attended to the memoirs hitherto published on this 

 subject will find almost nothing new either in methods or re- 

 sults in M. Melloni's very long memoir now alluded to, still 

 I feel very grateful to him for having, by delicate and judicious 

 arrangements of the apparatus, confirmed the resultsalready ob- 

 tained, on a scale which may adapt them for class experiments, 

 and which can leave no doubt respecting them, even on the 

 minds of those least accustomed to weigh physical evidence. 



