THE TRANSFORMATIONS OF ELEMENTS 537 



In examining a radioactive substance to find what kind of 

 radiations it gives rise to, the method in use is to cover it with 

 varying thicknesses of aluminium. Absorption takes place to 

 different extents ; the ionisation varies accordingly ; this is 

 recorded by electric measurements. From a study of results 

 thus obtained it can usually be determined whether one or 

 more kinds of ray are present ; the results are confirmed by 

 noting the action of a magnetic field on rays which have 

 penetrated the different thicknesses of metal. 



The radiations produce a considerable number of effects — 

 physical, chemical, and physiological. One of the most im- 

 portant is the phosphorescence induced in many substances 

 subjected to bombardment by the rays. Zinc sulphide, a 

 particularly good example, is rendered luminous by both 

 a and /3 rays. Crystals of the different platinocyanides give 

 different colour effects according to their composition. The 

 lithium salt phosphoresces pink, the calcium and barium salts 

 deep green, the sodium salt yellow. Willemite, a zinc silicate, 

 becomes beautifully green in colour and apparently translucent. 

 Kunzite shows a red phosphorescence under the action of /? and 

 7 rays ; a rays are without effect on it. All strongly radioactive 

 compounds phosphoresce strongly themselves. 



The diamond not only phosphoresces when bombarded by 

 radium rays, it also changes colour. Sir William Crookes found 

 that a yellow diamond, after seventy-eight days' exposure to the 

 rays, became bluish green. Prof. Bordas has recently experi- 

 mented with uncoloured rubies ; after some weeks' exposure 

 they became dark brown, and in some cases pink ; sapphires 

 similarly change colour. The effect is in all these cases 

 probably due to the action of /3 rays, and is analogous to 

 their action on different glasses. Soda glass is coloured deep 

 violet, potash glass a dark brown, gold glass ruby colour. 

 Similar effects are produced by cathode rays in vacuum tubes. 

 The colour is discharged quite easily by heating the glass to 

 300-400 C. Maxwell Garnett has made a detailed study of 

 these phenomena, and his work supports the hypothesis that 

 metallic ions are actually liberated in the glass, and that the 

 colour is due to these, violet for sodium, brown for potassium, 

 and so on ; application of heat causes recombination with 

 accompanying loss of colour. It should also be mentioned 

 that glass tubes containing radium emanation show a beautiful 



