1865.] Badiation. 711 



greater heating power is exercised at a considerable distance beyond 

 the red, or thermal end of its spectrum.* When we add that it is 

 even estimated, in round numbers, at what rate each of these rays 

 is projected,f it is not unreasonable to believe that the chief, if not 

 the sole cause of the varied phenomena, known to us as light, heat, 

 &c, are all varieties of the same force; namely, the simple motion 

 or radiation of matter, and it remains for physicists to pursue their 

 inquiries into the behaviour of various forms of matter in an un- 

 combined or combined state. 



In this department of research, also, much has been effected, and 

 Dr. Tyndall's work on " Heat Considered as a Mode of Motion," 

 comprises an admirable resume of almost everything that is known 

 on the subject. It treats of the deportment of the various gases, 

 liquids and vapours, of the metals, woods, simple bodies such as 

 iodine, bromine, &c, and of the effect of radiation upon compound 

 substances, as lamp-black, &c. ; and describes many novel and 

 original experiments bearing on the subject. His second work on 

 " Radiation," however, brief though it be, possesses far greater 

 interest for scientific men, for it deals largely with the question of 

 radiation in " aether," air, vapours, perfumes, &c. ; and reveals many 

 startling novelties concerning the influence of matter in a diffused 

 form upon the passage of heat. Let us extract a single series of 

 experiments : — 



" Absorption of radiant heat by vapours and odours. We commenced 

 the demonstrations brought forward in this lecture by experiments on 

 permanent gases, and we have now to turn our attention to the vapours 

 of volatile liquids. Here, as in the case of the gases, vast differences 

 have been proved to exist between various kinds of molecules, as 

 regards their power of intercepting the calorific waves. While some 

 vapours allow the waves a comparatively free passage, in other cases 

 the minutest bubble of vapours, introduced into the tube already 

 employed for gases, causes a deflection of the magnetic needle. | 

 Assuming the absorption effected by air at a pressure of one atmo- 

 sphere to be unity, the following are the absorptions effected by a 

 series of vapours at a pressure of -gVth of an atmosphere : — 



Name of vapour. 



Absorption 



Bisulphide of carbon 



47 



Iodide of methyl 



. 115 



Benzol .... 



. 136 



Amylene .... 



. 321 



Sulphuric ether . 



. 440 



Formic ether . 



. 548 



Acetic ether 



. 612 



" Bisulphide of carbon is the most transparent vapour in this list ; 

 and acetic ether the most opaque ; tAj of an atmosphere of the former, 



* Tyndall, pp. 23 and 24, and Frontispiece. 



f The extreme red rays making 474,000,000,000,000 vibrations in a second, 

 and the extreme violet 699,000,000,000,000. 



\ The " tube " and " magnetic needle," here mentioned, are instruments which 

 will be found described in " Heat considered as a mode of motion." 



