536 Royal Institution. 



in the experiments referred to) applies to that portion only of the 

 compound rays, which is also transmissible through glass; the 

 non-transmissible portion is subject wholly to the former condition, 

 as are all the rays from non-luminous sources (as was shown by Leslie 

 and others). 



Hence the distinction of at least two species of heating rays 

 emanating at the same time from the same luminous source. 



From the neglect of this distinction much confusion has been kept 

 up ; and statements involving such confusion have been repeated 

 from one elementary treatise to another. 



Again, notwithstanding that the experiments of Leslie and others 

 on the absorption of heat from non-luminous sources, as well as those 

 of Professor Bache on the radiation from surfaces, demonstrate that 

 the effect has no relation whatever to colour, yet the contrary asser- 

 tion has been often persisted in. 



Again, " dark heat " is often spoken of without recollecting that 

 rays of the very same quality and properties exist in the compound 

 radiation from luminous sources. 



The conclusions drawn from later experiments (performed with 

 all the advantages derived from the beautiful invention of the thermo- 

 electric instrument of Nobili), in many instances, are still vague, 

 from want of attention to the distinction of different species of heat 

 emanating at the same time from the same source. 



Melloni, in a most extensive and valuable series of experiments, 

 taking as the sources of heat successively flame, incandescent metal, 

 boiling mercury, and boiling water, and applying in each instance a 

 long series of substances as screens, estimated the proportion of rays 

 out of 100 stopped, which was very different for each screen and 

 each source : evincing wide differences in " diathermaneity ," while 

 rock-salt alone was almost totally " diathermanous " to rays from all 

 sources alike. 



But we must still ask, what species of rays were those respectively 

 stopped and transmitted? To take the per-centage simply is ambi- 

 guous; the body of rays is not homogeneous; the property of trans- 

 missibility should be viewed in combination with other properties of 

 the specific rays, such as those evinced in their relations to the 

 texture or colour of the absorbing surface. 



Nor is the ambiguity removed, though the difference of source is 

 specially referred to, if the heterogeneity of rays from the same 

 source be overlooked. The mere classification of sources into lumi- 

 nous and non-luminous will not suffice ; still less a reference to their 

 temperatures, it being perfectly well known that the temperature of 

 luminosity is very different for different substances*. 



Again, Melloni has shown that the diathermaneity is not proportional 

 to transparency, by a classified series of transparent screens with the 

 lamp. 



It must however be recollected that the term " diathermaneity " 



* References in detail to all the different researches here mentioned, will 

 be found in the author's two Reports on the state of our knowledge of 

 Radiant Heat in the British Association Reports, 1832 and 1840. 



