SfK WILLIAM SIEMENS, F.R.S. 435 



The anomalous results which Newton's law and the formula of 

 MM. Dulong and Petit lead to, when applied to the cooling of 

 bodies at a very high temperature, are well illustrated by the 

 attempts at deducing therefrom the temperature of the solar photo- 

 sphere. Waterston, and Pere Secchi (in his work entitled " Le 

 Soleil"), following Newton's hypothesis, obtained 10,000,000*0. 

 as the probable solar temperature, and Captain J. Ericsson, on the 

 same hypothesis but assuming other constants, arrived at a 

 temperature between 2,000,000 and 4,000,000 C. Strangely 

 contrasting with these determinations are those of Pouillet in 1836, 

 and Vicaire in 1872, who, employing Dulong and Petit's empirical 

 formula, deduce the values 1461 and 1398 C. for the solar 

 temperature. Between these extreme estimates we have those of 

 Dr. Spoerer, 27,000 C., of Zoellner, 27,700, Professor James 

 Dewar (1872), 10,000, Rosetti (1878), 9000, and Him (1882), 

 20,000. 



In my own investigations ou this subject, by comparing the 

 spectrum of the sun as regards the proportion of luminous rays 

 with those of the electric arc and gas flames, I have arrived at the 

 conclusion that the temperature of the photosphere does not exceed 

 2800 C., which is in close agreement with the limit assigned by 

 M. Sainte-Claire Deville, deduced from the observations of Frank- 

 land and Lockyer on the hydrogen lines in the solar spectrum. 

 Sir William Thomson, in a paper communicated to the Philo- 

 sophical Society of Glasgow (1882), has compared the power of 

 the sun's radiation per unit of surface with that of a Swan incan- 

 descent carbon filament, and has shown that it is about sixty-seven 

 times greater ; he concludes from these data that the estimate I 

 had formed of the solar temperature, i.e., nearly 3000 C., cannot 

 be very far from the true value. 



These diverse and indirect results have long impressed me with 

 the need of further experimental investigation of the dependence 

 of radiation on temperature ; and it has occurred to me lately, 

 that the difficulties with which Dulong and Petit had to contend 

 in making their measurements by means of a mercurial thermo- 

 meter, where the losses due to conduction and convection are very 

 great, and exceedingly difficult to determine, might be avoided in 

 adopting a method of conducting the experiment which forms the 

 principal subject of my present communication. 



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