1903.] Dependence of Refractive Index on Temperature. 441 



The liquid in contact with a growing crystal of sodium nitrate has 

 at 19° C. the index 1-38991, and contains about 48*45 grammes of salt 

 in 100 grammes of solution; a saturated solution at 19° C. has the 

 index P38905, and contains about 48*1 grammes of salt in 100 grammes 

 of solution. 



In each case the liquid in contact with the growing crystal is 

 slightly supersaturated. It was not found to exhibit double refrac- 

 tion even in the case of sodium nitrate. No experiments seem to 

 have previously been made upon the nature of this liquid. 



G. Wulff has suggested that vicinal faces are due to concentration 

 streams in the solution. In order to test this view, crystals of alum 

 were measured after growing for several hours in solution kept con- 

 tinually agitated in order to eliminate the action of the concentration 

 streams. Almost no effect was produced upon the angles of the vicinal 

 faces. 



In sodium chlorate and sodium nitrate the solute is about 45 times 

 more dense in the crystal than in the adjacent liquid. Now planes 

 with high indices in a space4attice contain fewer points in unit area 

 than planes with simple indices. The author suggests that vicinal 

 faces grow upon a crystal in preference to simple forms because the 

 crystallising material descends upon the growing face in a shower 

 which is not very dense. 



" On the Dependence of the Eefractive Index of Gases on Tem- 

 perature." By George W. Walker, M.A., Fellow of Trinity 

 College, Cambridge. Communicated by Professor J. J. 

 Thomson, F.E.S. Eeceived February 26,— Eead March 26, 

 1903. 



(Abstract.) 



The investigations of Professor Mascart on this subject are perhaps 

 the most extensive of any up to the present time. He examined 

 the effect in several gases, and found that in general the tempera- 

 ture coefficient exceeded the theoretical coefficient given by Gladstone 

 and Dale's law. The range of temperature was, however, compara- 

 tively small, and his results for air do not agree with those of 

 Lorenz, von Lange, and Benoit. In fact these four observers dis- 

 agree. Lorenz and Benoit found a coefficient agreeing with the above 

 law, while von Lange obtained a coefficient less than the theoretical 

 value. 



A repetition of the measurements therefore seemed desirable. The 

 gases examined were air, hydrogen, carbon dioxide, ammonia, and 

 sulphur dioxide. The range of temperature was from 10° C. to 



