356 Professor Forbes's Researches on Heat. 



larization, but not to produce them. The numerical results which 

 I gave (I. Series, 44), were intended chiefly to shew that, even 

 under all these disadvantages, the effects which I observed were 

 of an extremely important and obvious character, such as no slight 

 or capricious anomaly could possibly have produced. No one 

 who candidly reads the paper, or is aware of the labour of so ex- 

 tensive an investigation in so new a field, can suppose that I in- 

 tended to give these results as to the quantities of heat, of dif- 

 ferent kinds, polarized by passing through mica bundles, as defi- 

 nitive numerical results. I certainly did suppose that the differ- 

 ent kinds of heat were polarizable in different degrees under the 

 same circumstances, a conclusion which I am now prepared to 

 establish. 



18. The extent to which the former paper had swelled, like- 

 wise prevented me from inserting the description of a multitude 

 of precautionary measures, taken to shew that errors, of whose 

 existence I was perfectly aware had no influence in producing 

 the effects which I stated ; nor am I now going to enter into 

 details of manipulation, which the experimentalist must learn for 

 himself, and which would be highly tedious to any other reader. 

 I will content myself with recalling two proofs (which I have else- 

 where* stated), in justification of my experiments. Could an 

 effect similar to polarization have been produced by the con- 

 duction of heat, it must have been in the following way : My 

 earliest experiments were made with bundles of thin mica, A 

 and B, Fig. 2, cemented to soles or bases of wood, C and D, 

 forming, with the mica, an angle of about 34°. Two of these 

 being placed between the source of heat S, and the thermo-elec- 

 tric pile P, the fact observed was, that when either of these bun- 

 dles was set on edge, as in Fig. 3, so that the planes of refraction 

 in the two bundles were perpendicular, less heat reached the 

 pile than in the first position. If conduction produced this ef- 

 fect, it must have been owing solely to the different quantities 



* Philosophical Magazine and Annals, for November 1835 and March 1836. 



