THIRD SERIES— REFRANGIBILITY OF HEAT. 207 



tioii of the small effect due to the difference of refrangibility. Now, having proved 

 in the Thhd Section that this difference of mean refrangibility is from most sources 

 very small, which yet differ widely in their polarizability, we infer that that ex- 

 planation is probably inadequate, and that we must look for a mechanical theory 

 of heat differing in some particulars from that of light. 



85. (2.) This latter conclusion is farther confirmed by the results of the Se- 

 cond Section, in which is deduced, from the singularly accordant results of AvhoUy 

 distinct series of experiments with heat from those distinct sources, that the phe- 

 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 arrived 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 phenomena of double refraction but from this experiment ; 

 whilst the subsequent experiments on the refractive index, would, according to 

 the prevalent theory of dispersion, seem to shew that the mean length of a wave 

 of heat cannot differ very materially from one of light. This amounts to admit- 

 ting 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 sour- 

 ces, and also of the refrangibility, would both be highly improbable did the length 

 of a wave materially differ in those instances. 



86. (3.) Of the results of the Third Section, I have already spoken at sufti- 

 cient length (art. 81). The mean index of refraction for all kinds of heat tried 

 is less than for light ; — it ranges within narrow limits ; — when the heat from dif- 

 ferent sources is unmodified by transmission through diathermant bodies, these 

 limits are very narrow indeed ; — the measure of dispersion is considerable but un- 

 ascertained, and opens a fair field for experiment ; — dispersion is probably least 

 for sources of low temperature. 



87. Such are the chief data for speculation afforded by the experimental re- 

 sults 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 

 contributions towards its construction at a future period. 



Edinburgh, \Qth April 18-38. 



