42 SCIENCE PROGRESS 



being assumed to unite, under the influence of the rays, with 

 the adjacent crystal molecules with the resultant formation of 

 the colour. The author's opinion that the " Colloidal " ex- 

 planation of such colours is of little value will certainly not be 

 universally accepted. 



The effect of low temperatures on the colour of minerals has 

 been investigated by M. Bamberger and R. Grengg {Cent. Min., 

 65, 1 921). In the case of many of the substances examined, 

 it is found that the colour, compared with that at ordinary 

 temperature, tends to fade at — 190° C. This group includes 

 not only natural minerals, but also substances coloured by the 

 action of radium rays. Many other substances showed no 

 change in colour. Hydrated substances behaved in various 

 ways, some, such as chrome alum, exhibiting a pronounced 

 change in colour, and others, such as pentahydrated copper 

 sulphate, no change, while ferrous sulphate became turbid. 



The production of a rose colour in previously colourless 

 fluorspar by the action of ultra-violet rays is noted by C. Doelter 

 {Cent. Min., 479, 1921). In two other papers, the same author 

 {Koll. Zeit., 26, 23, 1920 ; Sitzber. Akad. Wien, 129, 399, 1920) 

 describes the results of some further experiments on the colora- 

 tion of crystals by means of rays from radium. The results 

 are considered to be favourable to the theory that the colora- 

 tion is due to the presence of material in a colloidal form, the 

 colour varying according to the degree of dispersion of the latter. 



The optical behaviour of certain minerals with regard to 

 long wave infra-red radiations has been further investigated 

 by T. Liebisch and H. Rubens {Sitzber. Akad. Berlin, 876, 191 9, 

 cf. Science Progress, 14, 386, 1920). In another paper 

 {Sitzber. Akad. Berlin, 211, 1921) the phenomena shown by 

 Wurtzite, Zircon, Rutile, Strontianite, and Cryolite are de- 

 scribed. The reflection-curve for wurtzite resembles that of 

 zinc-blende, showing a maximum at 32 /i. The results pre- 

 viously obtained for zircon are shown to have been vitiated 

 by the presence of impurities in the material used, and corrected 

 values derived from the use of a crystal free from inclusions are 

 given. For the ordinary ray DE. is 10-7, and for the extra- 

 ordinary ray, io'4. In the case of rutile, the ordinary ray curve 

 shows a minimum at ii-i, and the extraordinary ray, one at 

 11-7, the values of DE. being 83 and 167 respectively. The 

 curves for strontianite resemble those for witherite and have 

 maxima at 47 and 42, for the rays parallel and perpendicular 

 respectively to the vertical axis, the corresponding values of 

 DE. being 6-6 and 6-5. The curve for the ray parallel to the 

 twin lamellse of a crystal of cryolite showed two maxima, while 

 the curve for the perpendicular ray had at least one, the 

 respective values of DE. being 8*5 and 5*7. The transparency 



