116 On the Distribution of Heat in the Spectrum, 



heliostat-mirror^ which was of speculum-metal, might exert 

 some disturbing influence on account of its faint reddish tinge, 

 I replaced it with one of glass silvered on the front face, but 

 could not detect any substantial difference in the results. 



The important fact clearly brought into view by these experi- 

 ments is, that, if the visible spectrum be divided into two equal 

 portions, the ray having a wave-length of 5768 being considered 

 the optical centre of such a spectrum, those portions will pre- 

 sent heating-powers so nearly equal that we may impute the 

 differences to errors of experimentation. Assuming this as true, 

 it necessarily follows that any two series of undulations in the 

 spectrum will have the same heating-powers, no matter what 

 their wave-lengths may be. 



But this conclusion leads imavoidably to a most important 

 modification of the views now universally held as regards the 

 constitution of the spectrum. AVhen a ray falls on an extin- 

 guishing surface, heat is produced ; but that heat did not pre- 

 exist in the ray. It arose from the stoppage of the aether waves, 

 and is a pure instance of the conversion of motion into heat — an 

 illustration of the modern doctrines of the conservation and 

 transmutation of force. 



From this point of view the conception that there exist in an 

 incident ray various principles disappears altogether. We have 

 to consider an incident ray as consisting solely of sethereal vi- 

 brations, which, when they are checked by an extinguishing 

 substance, lose their vis viva. The effect that ensues depends 

 altogether on the quality of this substance. The vibrations im- 

 parted to it may be manifested by the production of heat, as in 

 the case of lampblack; or by chemical changes, as in the case 

 of many of the salts of silver. In the parallel instance of 

 acoustics, clear views have long ago been attained and are firmly 

 held. No one supposes that sound is one of the ingredients of 

 the atmosphere, and it would not be more incorrect to assert that 

 it is something emitted by the sounding body than it is to affirm 

 that light or heat or actinism are emitted by the sun. 



The progress of actino-chemistry would be greatly accelerated 

 if there could be steadfastly maintained a clear conception of the 

 distinction between the mechanism of a ray and the effects to 

 which that ray may give rise. The evolution of heat, the sen- 

 sation of light, the production of chemical changes, are merely 

 effects — manifestations of the motions imparted to ponderable 

 atoms. And these in their turn can give rise to converse re- 

 sults — as, when we gradually raise the temperature of a sub- 

 stance, the oscillating movements of its molecules are imparted 

 to the cether, and waves of less and less length are successively 

 engendered. 



