192 Prof. Forbes's Researches ofi Heat. 



very small, which yet differ widely in their polarizability, we 

 infer that that explanation is probably inadequate, and that 

 w^e must look for a mechanical theory of heat differing in 

 some particulars from that of light. 



(2.) This latter conclusion is further confirmed by the re- 

 sults of the second section, in which is deduced, from the 

 singularly accordant results of wholly distinct series of experi- 

 ments with heat from those distinct sources, that the phae- 

 nomena of depolarization differ surprisingly, numerically/ 

 speaking, from those of light, whilst in their general character 

 they are entirely similar. The results at which we have ar- 

 rived oblige us to admit, either that the length of a wave of 

 heat is several times greater than that of a wave of light, or 

 that the velocities of the ordinary and extraordinary ray in 

 doubly refracting crystals are totally different from those of 

 light ; or else a combination of these hypotheses. Now, of the 

 two first alternatives, we are bound at present, I think, to 

 prefer the latter, since we know nothing of the pheenomena of 

 double refraction but from this experiment ; whilst the sub- 

 sequent experiments on the refractive index, would, according 

 to the prevalent theory of dispersion, seem to show that the 

 mean length of a wave of heat cannot differ very materially 

 from one of light. This amounts to admitting that the doubly 

 refractive energy is more feeble for heat than for light; in 

 other words, that a greater thickness of a crystal is required 

 to produce a given effect. The second and third sections 

 also confirm one another in this respect, that the uniformity 

 of the results of depolarization with heat from different sources, 

 and also of the refrangibility, would both be highly impro- 

 bable did the length of a wave materially differ in those in- 

 stances. 



(3.) Of the results of the third section, I have already 

 spoken at sufficient length (p. 189). The mean index of re- 

 fraction for all kinds of heat tried is less than for light; — it 

 ranges within narrow limits; — when the heat from different 

 sources is unmodified by transmission through diathermant 

 bodies, these limits are very narrow indeed; — the measure 

 of dispersion is considerable but unascertained, and opens a 

 fair field for experiment; — dispersion is probably least for 

 sources of low temperature. 



Such are the chief data for speculation afforded by the ex- 

 perimental results contained in this paper: — too imperfect 

 perhaps in themselves to form the basis of a mechanical theory 

 of heat, yet such I hope as may be considered to be fit con- 

 tributions towards its construction at a future period. 



Edinburgh, April 16, 1838. 



