ON COLOUKS IN METAL GLASSES, ETC. 265 



(copper vapour) in vacuo respectively. The absorption band in the yellow green 

 shown by the top two curves in fig. 6 was observed by STOKES in the spectrum of a 

 copper ruby glass. 



In fig. 7 the graphs of nV/X for glass (v = 1'5) and of H"K"/\* are reproduced from 

 fig. 6 on such a scale as to have the same ordinate at the D line as that possessed by 

 the continuous curve which has been fitted to the plots of the measured absorption K 

 of the glass Cu (X). The curves in fig. 7 all have a minimum in the red or infra red 

 and a maximum in the yellow-green ; but while that (nV/X) which represents the 

 absorptions of spheres in glass has a secondary maximum near X '480, the dotted 

 curve shows that the absorption of copper molecules in glass continues to increase till 

 X< - 350. Also the maximum in the yellow green for the "sphere" curve occurs for 

 approximately the same value of X as corresponds to the maximum observed absorption ; 

 while the value of X at the maximum of the dotted ("molecule") curve is about lO^i/A 

 less, and the latter maximum is much less marked than are the former two. Finally, 

 the last readings obtained for K in the violet indicate that the continuous (observed) 

 curve rapidly approaches a maximum near X = '480. 



We conclude that copper ruby glass is coloured by metallic copper,! and that the 

 greater part of the copper is present in the form of small spheres, although some 

 probably remains in the form of discrete molecules.| 



The manufacture of copper ruby glass closely resembles that of gold ruby. Like 

 gold ruby, the copper ruby glass becomes turbid if kept too long at a high temperature. 

 This turbidity is also probably due to the formation of crystallites by the coagulation 

 of small spheres, since, when the conditions necessary for the development of turbidity 

 are long maintained, actual crystals, apparently of copper, are formed in the glass. || 



9. Colouring Effects of the Radiation from Radium, Cathode Rays, &c. 



It has long been well known that cathode rays produce a blue-violet coloration in 

 soda glass. Soda glass tubes, after containing the emanation from radium, show the 



* The graph of V/A for v = l can, by increasing all the ordinates in the proper constant proportion, be 

 changed into the graph of w'V'/A for any value of A. Of. 3 above. 



t STOKES (' Math, and Phys. Papers,' vol. IV., p. 242) supposed that the colouring agent was suboxide 

 of copper. The blue colour exhibited by overheated specimens of the glass (lo>;. cit., p. 243) is probably 

 caused by the coagulation of the small spheres into crystallites and crystals which reflect out the red light. 



| Measurements will have to be made with ultra-violet light in order to determine how much copper 

 remains in the molecularly subdivided condition. 



Of. above, 5, p. 251. 



|| Of. 'Phil. Trans.,' A (1904), p. 392. Some of the crystalline glazes made by Mr. BURTON at 

 PILKINGTON'S tile works exhibit the same effect. I have seen a pot with a copper glaze in parts of which 

 the copper was apparently reduced, for in passing from the colourless glaze (where the copper was not 

 reduced) into regions where the reduction had been effected, a deep red (copper ruby) was first reached ; 

 that colour increased in intensity until, in the central portions of the region, crystals, apparently of copper, 

 could be seen. 



VOL. CCV. A. 2 M 



