454 Ultra-violet Transparency of Certain Coloured Media* 



at D. The blue colour appeared within a few hours, and with 

 one solution has lasted to the time of writing — nearly three 

 months. 



A piece of black paper was wrapped round the tube and 

 pasted along a narrow strip. In it were made two small 

 holes diametrically opposite. The diameter of the tube was 

 1*5-2 cm., so that the light was incident practically normally 

 at one bole, and it was impossible for light from the arc to 

 reach the spectrograph without having traversed the solution. 



Exposures varying from 10-600 seconds were made with 

 both pale and deep blue solutions, and in all cases total 

 absorption commenced at \2442. The limit of transmission 

 for liquid ammonia is X2393. (In the earlier experiments 

 with calcium absorption seemed to commence at X2618. 



Hence the blue solutions transmit for a considerable distance 

 into the ultra-violet, but they could not be used for screening- 

 purposes in fiat-sided cells on account of the high vapour- 

 pressure of ammonia at ordinary temperatures. Solutions in 

 the primary amines which have higher boiling-points would 

 be more suitable. 



Lithium and caesium dissolve in methyl-amine (boiling- 

 point — 6° C), lithium dissolves in ethyl-amine (19° C). 

 (Moissan, Comptes Rendus, 1899 ; Kengade, ibid. 1905.) 

 Solutions in the higher primary amines have not been 

 obtained. 



Attempts to prepare solutions of lithium and magnesium 

 in ethyl-amine by the above method proved unsuccessful, 

 probably because the ethyl-amine was not properly dry. 



Experiments have also been made on a solution of colloidal 

 gold (after Faraday). When freshly prepared the limit of 

 transmission was X2492. After a day had elapsed it had 

 moved to \2767. 



Discussion of Results. 



One general result is that transparency far into the ultra- 

 violet is much more commonly met with in the case of colour 

 due to colloidal metals than it has been found to be in 

 ordinary coloured salts or aniline dyes. This is clearly 

 illustrated by the natural blue rock-salt and the various salts 

 and minerals coloured by cathode rays. We cannot, of 

 course, expect to find ultra-violet transparency in the coloured 

 substance unless it was present before coloration. Thus the 

 violet Chili saltpetre is opaque in the ultra-violet light, but 

 this opacity is equally met with in white samples : on the 

 other hand, where the uncoloured substance is transparent, 

 coloration in the visible region by bombardment leaves this 

 unaffected. 



The nature of the colouring matter in gems is in many cases 



