DISTRIBUTION OF HEAT IN A PURE SPECTRUM. 57 



oration, absorb certain of those rays which are beyond the extreme red ; and if these 

 obscure rays, by superposition or overlapping, extended from their proper places into 

 the coloured regions above, the absorptive action of the diaphanous media would then 

 be exercised, and apparent movements of the point of maximum take place, such as have 

 been actually observed the point shifting successively from the obscure spaces into the 

 red, from the red to the orange, from the orange into the yellow. 



201. Now, in those spectra which were formerly employed, both by Sir W. HERSCHEL 

 and M. MELLONI, this very difficulty must have taken place, and the results obtained 

 were therefore not simple, but compound. It is obvious, if we would avoid these in- 

 terferences, that we must employ a spectrum capable of showing the fixed lines, the 

 parts of which are properly separated from one another. 



202. On making use of such a spectrum, M. MELLONI found the maximum temper- 

 ature uniformly at the extremity of the red ray, it being immaterial what diapha- 

 nous substance had been used as a prism or as an absorbent medium. When glasses 

 of a brownish colour were interposed, a colour which results from their exerting an ab- 

 sorbent action on all the colours of the purified spectrum, it appeared that so intimately 

 were " these colours allied to their temperatures, that during the transmission they lost 

 as much heat as light, so that the ratio of these two agents remained always unaltera- 

 ble." From these things it is deduced, "That the luminous radiations, disengaged from 

 every other heterogeneous radiation, have a heat of their own, which follows exactly 

 the same vicissitudes, so that the different phases of a given ray of simple light may be 

 measured indifferently by its luminous or calorific relations." 



203. Of all the departments of physical science, there is none about which so many 

 experimental difficulties gather as that which is connected with investigations on the 

 solar spectrum, which is the point of reunion of the most energetic and powerful im- 

 ponderable agents, visible and invisible ; and probably we shall not obtain true views 

 of its constitution and properties until our doctrines have changed over and over again, 

 and years have elapsed. In the views just cited, M. MELLONI returns to the opinions 

 held at the beginning of this century ; those views which led to the invention of Les- 

 lie's photometer. 



204. That some splendid generalization will hereafter unite all these imponderable 

 principles, we have repeatedly said ; but there are very many facts now known which 

 none of the views hitherto brought forward can embrace. Under these circumstances, 

 it would seem that the proper course to pursue is to regard each one of these agents as 

 physically distinct. "Notwithstanding the clearness with which M. MELLONI has put 

 forth his recent doctrines, those who have read with attention the beautiful memoirs 

 which he has formerly written, will pause before they assent unconditionally. It is not 

 by a difficult and delicate experiment, such as that now under consideration, nor even 

 by a series of them, that that assent will be readily obtained. There are former exper- 

 iments to be explained away, former measures to be accounted for, before this desirable 

 simplicity can be effected. It is very true that, admitting the doctrine of the identity of 

 light and heat, or their common dependance on one higher agent, the experiment we have 

 just described meets with a ready explanation ; but we become involved in difficulties 



H 



