550 Prof. Forbes's Researches on Heat, Second Series. 



live errors to which the results were liable. I showed at the 

 same time that these errors were of a kind calculated to viask 

 the effects of polarization, but not to produce them. The nu- 

 merical results which I gave (I. Series, art. 44), were intended 

 chiefly to show 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 extensive an inves- 

 tigation in so new a field, can suppose that 1 intended to give 

 these results as to the quantities of heat, of different kinds, 

 polarized by passing through mica bundles, as definitive nu- 

 merical results. I certainly did suppose that the different 

 kinds of heat were polarizable in different degrees under the 

 same circumstances, a conclusion which I am now prepared 

 to establish. 



The extent to which the former paper had swelled, likewise 

 prevented me from inserting the description of a multitude of 

 precautionary measures, taken to show 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 referring to the proofs al- 

 ready stated in this Journal*, in justification of my experiments. 



The polarizing effect takes place only at the surfaces of 

 plates, — the absorptive effect depends upon their thickness. 

 Hence, to polarize heat effectively, a minute subdivision of 

 mica into thin laminae is essential. This I formerly effected 

 by a pen-knife. I have since, however, discovered a method 

 much more effectual. A piece of mica, thrown into a brisk 

 red fire, is split up, by the expansion of the air between its 

 films, into a multitude of pellicles, which reflect light with 

 almost metallic brilliancy, and polarize it intensely by trans- 

 mission. Such mica plates 1 have used, one pair being 

 marked G and H ; the other I and K. 



By experiments with both these pairs of plates I have been 

 led to the conclusion, that some kinds of heat are moke po- 

 larizable AT A given incidence THAN OTHERS. Between 

 heat from an Argand lamp, and that from platinum rendered 

 incandescent by the flame of alcohol, there is little difference ; 

 but heat, unaccompanied by light, is much less polarizable 

 than luminous heat, and that apparently in proportion to the 

 lowness of its temperature. I may take the opportunity of 

 giving one or two examples of such experiments. 



♦ Lond. and Edinb. Phil. Mag., for November 1835, and March 1836. 



