Refraction of Rays of Great Wave-length. 35 



nitrogen is formed. This compound is probably identical 

 with the nitride obtained by Plantamour. 



4. The rate of formation of this nitride, other things being 

 the sarne, is so irregular as to suggest that its formation is due 

 to the interaction of an otherwise inappreciable trace of some 

 other substance. 



5. This compound is dissociable ; but under the circum- 

 stances of the experiment the dissociation process is not 

 entirely reversible, since sufficient heating will permanently 

 destroy the compound. 



6. There are changes in the appearance of the discharge in 

 nitrogen which can be brought about by very slight variations 

 of temperature, and possibly of electromotive force, for which, 

 so far as I know, no explanation is at present forthcoming. 



Sydney, August 5, 1892. 



II. On the Refraction of Rays of Great Wave-length in 

 Rock-salt, Sylvite, and Fluorite. By Heinrich Rubens 

 and Benj. W. Snow*. 



[Plat© II.] 



IN volume xl. of Wiedemann's Annalen one of the present 

 authors recently described a method whereby a know- 

 ledge of the dispersion of rays in the infra-red may be easily 

 obtained. With the aid of this device the dependence of the 

 index of refraction upon the wave-length was determined for 

 sixteen materials ; viz. for nine different samples of glass, for 

 water, carbon disulphide, xylol, benzol, quartz, rock-salt, and 

 fluorite. Inasmuch as in this paper is given a minute de- 

 scription of the methods employed, it will suffice here briefly 

 to refer to the main features of the method of procedure 

 followed in the present determination. 



The rays from the zirconia burner of Linnemann, after 

 being reflected from the front and the rear surfaces of a thin 

 plate of air, enclosed between two parallel glass planes, were 

 then concentrated upon the slit of a spectrometer, by which 

 means two beams of light were produced, capable of mutual 

 interference, so that the otherwise continuous spectrum of the 

 incandescent zirconia plate was crossed by a series of vortical 

 interference-bands. The wave-length X of each such dark 

 band, multiplied by a certain whole number m, always equals 

 the product of twice the thickness d of the layer of air and 

 the cosine of the angle of incidence i of the rays. With tho aid 



* Communicated by the Authors. 



