REVIEWS 703 



(1) On Acquired Radio-Activity. By Sir Wm. Crook.es, O.M., LL.D., D.Sc. 



(From the Phil. Trans., A, vol. 214 [Pp. 433-445], with two plates.) 



(2) On the Spectrum of Elementary Silicon. By Sir Wm. Crookes, O.M., 



etc. (A paper read before the Royal Society, June 25, 1914.) [Pp. 12.] 



(1) In this paper Sir Wm. Crookes gives an account of attempts made for some 

 years past to impart radio-activity to such subjects as diamond, garnet, ruby, 

 quartz, gold, platinum, and various phosphorescent substances by prolonged 

 bombardment in a high vacuum by cathode rays or by exposure to radium 

 emanation. 



After bombardment by cathode rays no activity could be recognised either 

 by photographic or electrical means, even if the cathode stream had acted long 

 enough to discolour the surface in the case of diamonds. 



Exposure to radium emanation conferred temporary activity on all the sub- 

 stances which had been tried, due, apparently, to the condensation of the emanation 

 on the surface ; but this transient activity could be completely removed by washing 

 in dilute acids. 



If, however, instead of exposing the substances to emanation by placing an 

 open bottle containing radium bromide near them in a vacuum, one placed them 

 in the bottle and covered them with the powdered radium bromide and left them 

 so for several months, there were in the case of glass and diamond discolorations 

 of the surface, and the objects acquired a permanent activity. This activity 

 could be removed from glass, quartz, and other substances, except diamond, by 

 boiling in dilute nitric acid, or by immersion in a mixture of fuming nitric acid 

 and potassium chlorate. In the case of diamond the activity could not be so 

 removed ; it has persisted practically unchanged in the case of one diamond for 

 close on ten years. Only by grinding away the discoloured layers of the surface 

 could the acquired radio-activity be removed. 



It is not quite clear that the author attributes this activity really to the 

 diamond. It might obviously be due to radium E and polonium getting so far 

 below the surface of the diamond by process of recoil that the reagents used 

 could not reach them. The writer of this notice is aware of experiments (the 

 results of which will shortly be published) in which platinum foil exposed to 

 thorium emanation in an electric field in the usual manner still retains permanently 

 about 2 per cent, of its original activity even after prolonged and repeated 

 heating in a furnace at 1,200° C. 



(2) This paper gives an account of experiments carried out by Sir Wm. 

 Crookes in the spectrum of silicon. He suggests that some discrepancies 

 between the results of past observers have been due to impurities in the speci- 

 mens of silicon used. After many failures to secure a sufficiently pure specimen 

 he obtained three samples from the Niagara Falls Carborundum Company 

 giving on analysis 99/56, 99'86, 9998 per cent, of silicon, the impurities being 

 titanium, iron, and aluminium. With these samples, and using a spectrograph 

 designed by himself and previously described {Roy. Soc. Proc, vol. 72, p. 295), 

 he has carefully measured the lines of silicon between 3,807 and 2,124 A.U., being 

 enabled in fact by the purity of his specimens to correct the lines given by 

 other less pure samples, and to clear up doubtful points where impurity would 

 interfere with certainty of identification. Where the plates used were insuffi- 

 ciently sensitive, eye observations and measurements could be made by a special 

 piece of apparatus which could be fitted to the spectrograph, being in fact a kind 

 of pantagraph enlarger. 



