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



days, even through quartz, will produce as intense a colouration in a 

 piece of this 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 Koyal Society " On the Action of Radium Emanations on 

 Diamond,"* I showed that the /3-rays (electrons) and y-rays not only 

 effected a superficial darkening, converting the surface of the diamond 

 into graphite, but the body colour of the stone was changed from pale 

 yellowish-brown to bluish-green ; and I suggested the explanation that 

 the action might be chemical, the ferric state of the iron being reduced 

 to the ferrous state, and the colour thereby changing from yellow to 

 blue-green. 



In the year 1855, I tried a series of experiments with a spectrum 

 camera furnished with two quartz prisms and a quartz lens, with the 

 object of ascertaining if the atmosphere exerted any absorptive action 

 on the more refrangible rays of light. Photographs of the solar 

 spectrum were found to reveal lines of higher and higher refrangibility 

 the nearer they were taken to mid-day, and arguing from this I 

 concluded that the "noon-day spectrum at midsummer ought to contain 

 more and higher rays than are possessed by the corresponding spectra 

 at any other time of the year." The examination of the photographed 

 spectra was continued through the summer, photographs being taken at 

 noon whenever the sun was clear, and I found that " as the light came 

 less obliquely through the atmosphere, new rays began to be apparent, 

 until at midsummer, when the sun was on the meridian, I succeeded in 

 obtaining evidence of the existence of rays which the most prolonged 

 exposure failed to detect at any other time." 



* ' Roy. Soc. Proc.,' June, 1004, vol. 74, p. 47. 



