Publication: Proceedings of the Royal Society of London
On the Colouration of Glass by Natural Solar and other Radiations.
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 neutralize 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 oxidizing the green proto-salt of iron to the pre-salt, and also of imparting a purple shade which neutralizes 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 in land 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 different pieces, which may be attributable to the varied length of the exposure of each to the action of the sun's rays. It seems to me that some of the pieces have lost somewhat of their depth of colour since I picked them up, but this may be an impression only. The two pieces forming together the bottom of a broken tumbler, and which have a deeper tint than the rest, were found about twenty paces apart in an old Oficina that had been uninhabited for 27 years. It is impossible to give any idea of the length of exposure of the remaining pieces to the sun's rays, as I have obtained them from all parts of the Pampa over an extent of nearly 100 miles. The samples I send you were originally white glass, and although an abundance of glass of various colours is to be found, yet I send you none, as it would not be easy to say what the original colours had been previous to exposure."
The pieces of glass referred to above are of all depths of tint, from deep violet, almost black in thick pieces, to pale amethyst. Analysis shows the glass to contain manganese. Heating the glass in a covered crucible to its softening point, discharges the colour, leaving the glass white and transparent.
The colouration is not superficial. On immersing a piece of the coloured glass in a liquid of about the same refractive index as itself, the colour is seen to have penetrated throughout the mass.
At first sight the explanation of this phenomenon would seem to be that it is produced by the action of light, the intense radiation occasioning a re-arrangement of the oxygen molecules in the glass, the ferric salt becoming ferrous, and the manganous salt changing to a manganic compound. * The change of colour might then be expected to be noticed in any part of the world where broken glass is thrown about and the sun's rays are very intense. In the Transvaal, where both of these conditions are well fulfilled, I have neither heard or nor noticed any such colouration, and it would be interesting to hear if travelers in other tropical countries have observed any such change of colour of glass.
* In this connection it may be of interest to recall the fact that in the early days of photographic research the ultra-violet rays of the spectrum were called the "deoxidizing rays."
Probably height above sea level has much to do with the phenomenon. At a height of 4000 metres nearly half the atmosphere is beneath one's feet, and that which remains will allow rays of shorter wave-length to pass through than the atmosphere at sea level will transmit.
For this reason it is not necessary to invoke another mode of explanation that might possibly suggest itself. It now has been well established that many natural bodies, water from great depths, some samples of earth and rock, air from underground sources, together with some minerals, are more or less radio-active. Radium, acting for a few days, even through quartz, will produce as intense a colouration in a piece of glass as exposure to the sun on the Pampa has taken years to effect. It is hardly conceivable that there can be a special radio-activity of the soil in certain parts of Chile and Bolivia sufficiently powerful to produce the effect.
A piece of the coloured glass, bleached by heat, was put close to a quartz tube in which about 15 milligrammes of pure radium bromide was sealed up. In the course of a few hours a faint amethystine tint could be distinguished on the glass, and in a week the tint was equal to the deep colour of the unbleached specimen. A duplicate piece of the same glass which had been bleached by heat, kept away from radium, remained colourless for seven weeks.
A piece of the deepest purple-coloured glass was put on a sensitive photographic film, and kept in the dark in contact with it for 34 days. No trace of action could be detected on developing.
The purple glass which had been bleached by heat and then coloured purple again by radium, was put in close contact with a sensitive film for 24 hours. On developing, no trace of action could be seen.
The darkening effect produced by radium on bodies exposed to its emanations is very general. Quartz, mica, glass of all kinds, and the diamond may be specially mentioned. In a paper recently read before the Royal Society "On the Action of Radium Emanations on Diamond," * I showed that the β-rays (electrons) and λ-rays not only effected a superficial darkening, converting the surface of the diamond into graphite, but the body colour of the st