On the Temperature of 1 /(candescence. 221 



VIII. — Note on the Temperature of Incandescence, and, Us 

 bearing upon Solar Physics. 



BY WALLACE GOOLD LEVISON. 



Read Feb. 2d, 1885. 



It has long been assumed that all solid and liquid substances 

 begin to emit light, or become visibly incandescent, at a common 

 temperature which has been fixed at 



635° by Sir Isaac Newton. 

 812° " Sir Humphrey Davy. 

 947° « jyr r . Wedgewood. 

 980° " Mr. Darnell,— and 

 977° " Dr. J. W. Draper. 



The last figure was obtained by Prof. Draper as the result of 

 a series of experiments which he considered sufficiently conclu- 

 clusive to authorize the suggestion of three laws 1 : — First/ that 

 all substances become visibly incandescent at the same tempera- 

 ture ; — second, that this temperature is 977° ; — third, that the 

 length of spectrum of an incandescent solid or liquid substance 

 is a measure of its temperature. Now, in view of the following- 

 facts, it would seem that these laws must be modified, or at least 

 restricted in application to opaque solids and liquids only. 



When a rod of hard German glass (potash and lime silicate) 

 and a rod of soft glass (alkali or lead silicate) are held side by 

 side in the flame of a Bunsen burner, the soft glass rod soon be- 

 comes red-hot or visibly incandescent, while the hard glass is 

 but faintly luminous ; and even when they have equally acquired 

 the temperature of the flame, about 2.350 C, 2 the hard glass has 



1 Draper. Dr. J. V\ T .. on the radiation cf red-hot bodies and the production of light by heat. 

 Am. Jour. Sci., 2 Series. Vol. IV. 1817. London, Edinburg and Dublin Phil. Mag.. May, 

 1877. Harpers' Monthly. No. 322. Memoirs N. Y.. 1878 8vo. Astor. 446 D. 1st Memoir. 



2 Roscoe, on Spectium Analysis, p 51. 



