and Surface- Col OK )' of N'lt roso-dimethyl-aniline. 107 



have described in a previous paper on screens transparent 

 only to ultra-violet light *. 



Th absorption-spectrum was photographed by pressing out 

 a drop o£ the fluid substance between a quartz lens and plate, 

 the whole being kept warm by a current of hot air. In this 

 way a lilm of variable thickness was obtained which, when 

 brought close to the slit of the quartz spectrograph, enabled 

 a record to be made of the relative intensities, positions, and 

 general form of the absorption-bands. One of these photo- 

 graphs is shown in fig. 7, PI. VIII. 



The nitroso begins to vaporize at a temperature only a 

 little above its melting-point, and the absorption of the 

 vapour is interesting, for unlike most absorbing vapours and 

 gases this substance shows a broad absorption-band, similar 

 to the liquid only shifted well down into the ultra-violet. 

 The nitroso was vaporized in a glass tube, the ends of 

 which were closed with quartz plates. The tube was previously 

 exhausted, and was heated by a water-bath through which it 

 passed. The absorption-spectrum was photographed at dif- 

 ferent temperatures ranging from 85°^ the melting-point, to 

 100°. The absorption does not begin as a narrow line^, as is 

 usually the case with gases, but with a broad band, which 

 increases in intensity as the density of the vapour increases. 

 The centre of this band is at wave-length 34_, while the centre 

 of the band in the case of the liquid is at 43. 



I made numerous experiments to see whether the density 

 of the vapour could be sufficiently increased to cause a shift 

 in the position of the band towards the red, all of which 

 failed owing to the fact that the nitroso decomposes at 

 temperatures above 150°. The substance was heated in 

 strong sealed bulbs in an air-bath_, but decomposition always 

 resulted before a density sufficient to shift the absorption-band 

 in a measurable degree had been obtained. 



On the other hand, the position of the absorption-band can 

 be shift(3d by increasing the density of the medium in which 

 the nitroso- vapour is present. If a solution of nitroso in 

 ether is heated above its critical temperature in sealed glass 

 tubes_, the centre of the absorption-band can be given almost 

 any position between that of the vapour-band and the solution- 

 band, by varying the amount of ether in the tube, or, in other 

 words, by varying the density of the vapour. 



I made a number of attempts to prove that the double 

 overlapping band shown by the solid was connected with the 

 double refraction of the nitroso crystals, by placing a nicol 

 before the slit of the spectrograph and photographing the 



* Phil. Ma^. Feb. 1903. 



