524 Sir William Crookes. On the Colouration of [Jan. 17, 



" On the Colouration of Glass by Natural Solar and other Radia- 

 tions." By Sir WILLIAM CROOKES, D.Sc., F.R.S. Received 

 January 17, Read January 26, 1905. 



It is well known that many samples of colourless glass containing 

 manganese slowly assume a violet tint when exposed to sunlight. 

 This effect is frequently seen in plate-glass windows having a southern 

 aspect ; watched from year to year they assume a more and more 

 pronounced amethystine hue. The introduction of manganese into 

 glass is to neutralise the colour caused by the presence of iron. Iron 

 gives the glass a greenish tint, and the addition of manganese binoxide 

 performs the double object of oxidising the green proto-salt of iron to 

 the per-salt, and also of imparting a purple shade which neutralises the 

 green-yellow tint of the silicate of peroxide of iron. 



In 1903, I received from two separate correspondents specimens of 

 glass coloured an intense purple. I quote the following sentences from 

 the covering letters : 



Mr. A. Ernest Williams writes : 



"While residing at Uyuni, in Bolivia, last year, at an altitude of nearly 

 4000 metres, my attention was called to the fact that all transparent 

 white glass when thrown out on the ' Pampa ' in a short time assumes 

 a violet hue, which becomes more marked with time. I was told that 

 all specimens were thus affected, and that when taken to sea level at 

 Antofagasta they lost their colour. This latter statement I hardly 

 believe, as I have had some pieces with me now on low level for nearly 

 a year, and they have not lost the colour. 



" I now notice that all transparent white glass thrown on rubbish 

 heaps, even at low level, assumes this violet colour, though only to a 

 slight degree, and I am curious to know the cause, being more interested 

 since reading that radium so affects glass. 



"I may mention that Uyuni is situated on the great central plain of 

 Bolivia, which plain has evidently formed the bed of an inland sea or 

 lake, for I have found quantities of minute shells there. Not far off to 

 the south and south-west are borax fields, and still further west, nitrate. 

 To the north-east are the mountains of Pulacayo and Cuzco (not the 

 great Cuzco), and electrical disturbances are of almost daily occurrence. 

 I can fully confirm Sir Martin Conway's description of the battles 

 between the mountains, where lateral discharges are plainly visible. I 

 am sending you by post a small specimen of the glass." 



About the same time I received some specimens of purple-coloured 

 glass from Mr. Thomas Wilson, from Iquique, Chile. In a subsequent 

 letter answering some enquiries, he sent a further quantity of the coloured 

 glass, saying : 



" You will notice a great variety in the depth or degree of tint in the 



