350 Dr. L. Bleekrode. On the Determination of the 



The observations led to D= 15*54 millims., with the natrium light 

 d=4*03 millims., n=l*350 at 15, with daylight = 1*357 at 13. 



The index of refraction of the easily liquefied gas was already 

 determined by Ketteler in 1865 with a goniometer, and he found the 

 value of n for the D line= 1'3384 at 24; more recently Bichat* 

 observed n= 1*344 at 18*5. It may be deduced from these quantities 

 that this quantity changes nearly O'OOl for one degree of change in 

 the temperature ; if, therefore, a quantity of 0*003 is subtracted from 

 the value obtained in my experiments, there results 1*347, approach- 

 ing nearly to the amount 1*344, resulting from an exact determination, 

 and demonstrating again what may be expected from the microscopical 

 method in such cases, and how far it is reliable. 



2. Cyanogen (C S N 2 ). 



The gas was prepared from very pure cyanide of mercury strongly 

 dried before, and the liquefaction effected in a bent tube with glass 

 cell provided. 



The observations led to D= 16*03 millims., with the natrium light 

 d=3*93 millims., =1*325 at 18, with daylight n=l*327 at the 

 same temperature. 



In 1826 already D. Brewsterf published the amount of the index 

 as 1*316, yet without indicating the method by which it was found, 

 nor the temperature to which it refers ; no comparison thus is possible. 



3. Hydrocyanic Acid (CNH). 



The gas was prepared by decomposing the cyanide of mercury by 

 well-dried sulphuretted hydrogen (it had been led through anhydrous 

 phosphoric acid), and condensing with a freezing mixture in the glass 

 cell. It was thus obtained extremely pure, as it showed no alteration 

 after a long space of time. 



The observations led to D.=15*26 millims., with the natrium light 

 d=3*09 millims., n= 1*254 at 19, with daylight n= 1*264 at 19. 



Determinations of the index were already made by Bussy and 

 Bnignet J with the prismatic method, giving 1*263 at 17 for the D line. 

 Earlier still, in 1839, Cooper also obtained, in the same way, for day- 

 light, n 1*275. 



4. Nitrous Oxide (N,0). 



I could not succeed in preparing the gas obtained bythe decom- 

 position of ammonia nitrate in liquefied condition in glass tubes, as 



Bichat, " Journal de Physique," t. ii, 1880, p. 279. 



f " Transactions of the Royal Society of Edinburgh," vol. x, p. 407. In this paper 

 there is only said on this subject that Dr. Turner prepared the liquid cyanogen by 

 compression. 



* Bussy and Buignet, " Annales de Chiinie et de Physique " (4), t. iii. 



