1 GO Prof. Forbes's Researches on Heat. 



M. Melloni had hinted that the different dimensions of the 

 sources of heat, and the various angles under which the rays 

 fell on the mica plates must materially affect the results ; and, 

 as I was quite convinced that operating with parallel rays was 

 the most correct method, I proceeded to repeat my experiments 

 on his plan, with a salt lens placed in front of the source of 

 heat so as to render the rays parallel ; I also removed the 

 polarizing and analysing plates to a considerable distance 

 from the pile, and afterwards varied their distance in order 

 to see whether any adequate explanation of the discrepancy 

 could thus be obtained. 



The apparatus was arranged in the following way:— A 

 rock-salt lens was placed between the source and screen 

 (fig. 2 above) so that the heat was refracted into a nearly 

 parallel beam before incidence upon I and K, which were 

 removed to a distance of more than a foot from the pile ; the 

 distance of the source being 2 feet. 



The apparent polarization was somewhat increased, as 

 I had anticipated, from the rays falling more nearly at a con- 

 stant angle when previously rendered parallel ; but the dif- 

 ferent polarizability of the different kinds of heat was even 

 more distinctly marked than ever ; whilst the distance of the 

 mica plates from the pile was now such as to reduce to insig- 

 nificance any effect of secondary radiation, had such before 

 been sensible. 



In prosecuting these experiments, most of which were re- 

 peated many times under various circumstances, I remarked 

 more distinctly than formerly the influence of particular states 

 of combustion of the source of heat upon the index of po- 

 larization, and the accidental variations to which this gives 

 rise on different days, and even during the progress of an 

 experiment. Heat from brass about 700° I have generally 

 found the most uniform on different days, though there occa- 

 sionally occurs in a series of experiments, considerable devia- 

 tions from the mean. The Locatelli lamp seems subject to 

 greater variations, and the Argand still more ; indeed, I have 

 found it so impossible to maintain an Argand lamp in a uni- 

 form state of combustion, even for a quarter of an hour, that 

 I have lately abandoned the use of it. But the quality of 

 the heat from incandescent platinum varies between the widest 

 limits. Nor is this wonderful ; it is composed of heat from 

 two very different sources combined in uncertain proportions, 

 that from the incandescent coil of wire, and that from the 

 alcohol flame which heats it. The intensity of incandescence, 

 too, varies exceedingly. On one occasion, when the incan- 

 descence was unusually bright, and the alcohol flame very 



