Tliird Series. — Variable Polarizahility of Heat. 99 



pared in the method described *, marked I and K, I found 

 that, with heat from an Argand lamp, 72 to 74 per cent, of 

 the incident rays were polarized, that is, — of 100 rays trans- 

 mitted when the plates were parallel, 72 to 74- were stopped 

 when one was crossed or its plane of refraction turned through 

 90°. With heat from boiling water, but 44 per cent, were 

 polarized, and heat from sources of intermediate intensities 

 gave intermediate results. 



M. Melloni ingeniously argued that this appearance might 

 arise from the circumstance that the mica bundles becoming 

 most heated by those kinds of heat which they absorbed most 

 readily, or transmitted least easily (viz. heat of low tempera- 

 ture), the pile was continually receiving a supply of heat by 

 secondary radiation from the mica, which, having no relation 

 to the parallel or crossed positions of the plates I and K, of 

 course tended to diminish the apparent polarization of the 

 heat, or to equalize the effect in the two positions. 



The supposed effect of secondary radiation from plates had 

 been so often urged against my experiments, that, though as 

 often proved to be insignificant or insensible, it gave me no 

 surprise to see it started afresh, and in so plausible a manner. 

 M. Melloni was probably not aware that the screen for inter- 

 cepting the heat was placed between the source of heat and the 

 polarizing plate K, (as shown in PI. II. fig. 2,) so that the mica 

 plates were only absorbing heat during the exceedingly short 

 time (10 seconds) of one swing or dynamical impulse of the 

 needle, otherwise I do not think he would have urged so in- 

 finitesmal an objection f. I endeavoured, however, to meet 

 it directly in this way. I took two mica bundles, G and H, 

 and placed them parallel, as shown in fig. 3. But instead of 

 placing the pile at P, where it receives at once the directly 

 transmitted heat from S (the screen being removed), and the 

 supposed secondary radiation of the surface ab of the mica 

 plate, I placed it at p, identically situated with respect to the 

 surface a b, but wholly removed from the influence of direct 

 radiation from S. When this experiment was performed with 

 dark heat (which, according to Melloni, ought to give the 

 greatest effect) not the slightest movement of the galvanometer- 

 needle was observable on removing the screen, during a far 

 longer space of time than is ever in practice allowed for the 

 absorption of heat. This experiment ought to be considered 

 quite conclusive. 



• Lond. and Edinb. Phil. Mag., vol. xii. p. 650. 



t I might add, too, that, had he been aware of the extreme tenuity of 

 the mica plates employed (of which more hereafter), he must have been 

 led as a necessary consequence of his own reasonings to admit that the 

 effect must be insignificant. — Ann. dc Chimie, Mai 1837, p. 13, note. 



H2 



