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XXX. Investigation of the Alteration produced by Heat in the 

 Velocity of Propagation of Light in Water. By Richard 



RUHLMANN*. 



[With a Plate.] 



I. Critical Review of previous experimental Researches. 



THE fact has been long known that the refraction of light, 

 especially in liquids, is affected by heat. The first nume- 

 rical results in proof of this which attained publicity, were pro- 

 bably those got by Baden Powell f in determining the refractive 

 index of oil of cassia at different temperatures • although Aragof, 

 by means of his interference-refractor, had already instituted an 

 examination on the change in the refractive index of water be- 

 tween 0° and 10°. Arago's results are ambiguous. Making 

 use of the same apparatus, with only slight modifications, 

 Jamin § afterwards undertook an investigation of the index of 

 refraction of water, and found a well-marked decrease in the re- 

 fractive index with the temperature ||. He further observed that 

 the refraction has not, like the density, a maximum at 4° C, 

 but continually diminishes from 0° upwards, a diminution which 

 may be represented by 



K t = K -at-bt% 



where K* denotes the index of refraction at t°, K the same at 

 0°, and a and b are constants. 



Chronologically after this appeared the investigation of Van 

 der Willigen% who, by means of Meyerstein's spectrometer, 

 measured a series of the refractive indices of distilled water at 

 different temperatures. These measurements are, however, so 

 few in number, and are confined within such a narrow thermal 

 range, that we need here do no more than mention them. 



Induced by the experiments of Baden Powell, who was fully 

 alive to the importance of knowing the variations of the refrac- 

 tive index with the temperature, Gladstone and Dale undertook 

 a rather elaborate investigation of this subject, and published 



* Translated from PoggendorfTs Annalen, vol. cxxxii. pp. 1 & 177» by 

 Frederick Guthrie, F.R.S.E., Professor of Physics and Chemistry, Royal 

 College, Mauritius. 



f Poggendorff's Annalen, vol. lxix. 



X Arago' 'sSammtlicheWerke, deutsch, von Hankel, vol. x. pp. 257 &c, and 

 p. 247. He finds that warm water refracts light more powerfully than cold 

 water. 



§ " Description d'un nouvel appareil de recherche fonde sur les interfe- 

 rences," Comptes Rendus, vol. xlii. pp. 482-485. 



|| Jamin, " Sur la vitesse de la lumiere dans l'eau a diverses tempera- 

 tures," Comptes Rendus, vol. xliii. pp. 1191-1194. 



% Poggendorff's A nn alen, vol. cxxii. pp. 190-192. 



