178 



PROFESSOR FORBES'S RESEARCHES ON HEAT. 



rays transmitted 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 boil- 

 ing water, but 44 per cent, were polarized, and heat from sources of intermediate 

 intensities gave intermediate results. 



10. 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 

 temperature), the pile was continually receiving a supply of heat by secondary ra- 

 diation from the mica, which, having no relation to the Parallel or Crossed posi- 

 tions 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. 



ScreeTi/ 



Pile. 



Source. 



11. The supposed effect of secondary radiation ft'om 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 intercepting the 

 heat was placed between the source of heat and the polarizing plate K (as shewn in 

 the figure above), so that the mica plates were only absorbing heat during the ex- 

 ceedingly short time (10 seconds) of one swing or dynamical mipulse of the needle, 

 otherwise I do not think he would have urged so infinitesmal an objection.* I 

 endeavoured, however, to meet it directly in this way. I took two mica bundles. 

 G and H (see II. 22), and placed them parallel, as shewn in the figure below. 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 aboi the mica plate, I placed it at^, identically situated with respect to 

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



Screen/ 



V 



* 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. de Chimie, Mai 1837, J9. 13, note. 



