68 WAVE-MOVEMENT OF HEAT AND LIGHT. 



has been obtained, we occasion a manifestation of light* 

 It has been concluded, somewhat hastily, that heat and 

 light differ from each other only in the rapidity of the 

 undulations of an hypothetical ether. 



It must be admitted that the mathematical demon- 

 strations of many of the phenomena of calorific and 

 luminous power are sufficienlty striking to convince us 

 that a wave-movement is common to both heat and 

 light. The undulatory theory, however, requires the 



* Dr. Draper, On the production of light by heat, in the Phil. 

 Mag. for 1847. 



Sir Isaac Newton fixed the temperature at which bodies be- 

 come self-luminous at 635; Sir Humphry Davy at 812; Mr. 

 Wedgewood at 947 C ; and Mr. Daniell at 980; whilst Dr. 

 Draper from his experiments gives 977; and Dr. Robinson 865. 



In a review of the above paper by Melloni, entitled Researches 

 on the Radiations of Incandescent Bodies, and on the Elementary 

 Colours of the Solar Spectrum, translated for Silliman's Journal 

 for August, 1847, he remarks : 



tf l say that they conduct, as do others heretofore known on 

 light and radiant heat, to a perfect analogy between the general 

 laws which govern these two great agents of nature. I will add 

 that I regard the theory of their identity as tho only one admis- 

 sible by the rules of philosophy ; and that I consider myself 

 obliged to adopt it, until it shall have been proved to me that 

 there is a necessity of having recourse to two different princi- 

 ples, for the explanation of a series of phenomena which at pre- 

 sent appear to belong to a solitary agent.' 1 



Reference should also be made to a paper by Dr. Robinson, 

 On the effects of Heat in lessening the Affinities of the Elements 

 of Water, in the Transactions of the Royal Irish Academy, 

 1848, where he says that " when a platinum wire is traversed by 

 a current gradually increased till it produces ignition, the first 

 gleam that appears is not red, but of a colour which, when I 

 first saw it, I compared to the ' lavender ray ' discovered by Sir 

 John Herschel beyond the violet, though I was surprised at 

 seeing the tint of that most refrangible ray preceding the ray 

 which is least so. It is quite conspicuous at about 865; and 

 as the mode in which it makes its appearance presents nothing 

 abrupt or discontinuous, it seems likely that it is merely a 

 transition from invisible rays excited at a lower temperature 

 to ordinary light." p. 310. 



