56 Mr. E. J. Evans on the Absorption Spectra 



electric furnace wound with nichrome wire, and the tempera- 

 ture could be adjusted to any value between that of the room 

 and about 1200° C. The temperature was measured by a 

 Pt, Pt — Rh thermocouple, which was connected to a direct 

 reading instrument. When the required temperature had 

 been attained, light from the positive pole of the carbon arc 

 was passed through the vapour inside the quartz tube, and 

 focussed by quartz lenses on the slit of a concave grating, 

 having a radius of 1 metre, and ruled with 15,000 lines to 

 the inch. The spectrum was then examined both visually 

 and photographically. 



Experimental Results. 



Ammonium Chloride. 



It is well known that the vapour of this substance is dis- 

 sociated into NH 3 and HC1 at 350° C, and consequently the 

 pressure of the vapour inside the quartz tube is double the 

 amount calculated from the ordinary gas equation 



m 

 where R = 8*2 x 10 7 , 



= absolute temperature, 

 m = molecular weight of vapour, 

 M = mass in grams of substance in tube, 

 v = volume of tube in c.c. 



From the dimensions of the tube it could readily be 

 deduced that if *017 gram of NH 4 C1 was placed in the tube, 

 the pressure of the vapour at a temperature of 700° O. was 

 approximately 2 atmospheres, and in the experiments under 

 discussion this quantity of NH 4 C1 was used. Photographs 

 of the absorption spectrum were obtained at temperatures 

 varying from 250° 0. to 550° C, and visual observations 

 were also made. As the result of a large number of experi- 

 ments, it was concluded that the vapour of ammonium 

 chloride does not give any well-defined absorption bands in 

 tb- region extending from X 2500-X 6700.. and, furthermore, 

 there is no definite evidence that the vapour shows any 

 general absorption in the same region. It is, however, 

 possible that the vapour absorbs in the extreme red and 

 infra-red regions of the spectrum, which were not examined 

 in this research. 



