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XIX. Intelligence and Miscellaneous Articles. 



ON THE TEMPERATURE OF THE SUN. BY J.-B. SORET*. 



Geneva, 14th Oct. 1874. 

 T HAVE read with very great interest the Notes, on the tempera- 



ture of the sun, which you have communicated to the Academy in 

 the name of M. Violle, a distinguished philosopher, who, besides 

 his original works, is entitled to the gratitude of all physicists for 

 the important part he has taken in the publication of Verdet's 

 (Euvres. Ton, perhaps, remember that I have for several years 

 been occupied in researches which, as regards the means of obser- 

 vation, are very similar to those of M. Violle. I have already made 

 known some of the results f; and I hope to be able shortly to make 

 these investigations the subject of a more complete publication, 

 which I have been obliged to defer because a correction rather 

 difficult to obtain remained to be determined. 



These measurements of the calorific intensity of solar radiation 

 are, I believe, very interesting in divers respects ; but I doubt 

 whether, in the present state of science, they can lead to the deter- 

 mination of the temperature of the sun. 



The principle of the apparatus, or actinometer, successively em- 

 ployed for these observations, first by Pouillet (who soon adopted 

 a different method), then by Mr. Waterston, Father Secchi, Mr. 

 Ericson, M. Violle, and myself, consists in placing a thermo- 

 meter with a blackened bulb in an enclosure of which the tempera- 

 ture 6 is known. An aperture admits into the enclosed space a 

 pencil of solar rays, which falls upon the bulb of the thermometer, 

 which takes a temperature t. 



In order to be able to deduce from this the temperature of the 

 sun, it would be necessary first of all to know the law of the ra- 

 diation of heat at very elevated temperatures. Sometimes Newton's 

 law has been assumed, sometimes that of Dulong and Petit. Now 

 neither the one nor the other is accurate for high temperatures : 

 this appears to me to follow very distinctly from a series of ex- 

 periments which I made known two years since J. Permit me to 

 recapitulate the results, referring for the details to the Notes which 

 I have published. 



With the actinometer which I employed, the solar radiation at 

 Geneva produces an excess of temperature t—d which exceeds 14°-5. 

 If, instead of exposing the actinometer to the sun, I make use of a 

 disk of zirconium or magnesium heated by the oxyhydric lamp (illu- 

 minating-gas and oxygen), placing it at a distance such that its ap- 

 parent diameter referred to the bulb of the thermometer is the 



* Extract from a letter to M.H. Sainte-Claire Deville, published in the 

 Annates Scientifiques de VEcole Normale, 18/4, No. 12. 



t See Comptes Rendus de VAcad. des Sciences, 1867, vol. Ixv. p. 526, 

 and 1868, vol. lxvi. p. 810; Comptes Rendus de V Association francaise 

 pour Vavancement des Sciences, premiere session, 1872, p. 282. 



% Archives des Sciences Physiques et Naturelles, 1872, vol. xliv. p. 220, 

 and vol. xlv. p. 252. 



