82 RADIUM PREPARATIONS AND METHODS OF EXPOSURE 



collodion. In the course of a few days this film of collodion becomes 

 so tough that it will strongly resist destruction, thus affording ample 

 protection for the underlying radium. . . ." 



" In the preparation of these coatings both the radium and the 

 collodion solutions are colored with an analine dye. This is done to 

 show the part that has been coated. Besides, if the radium happens 

 to be removed by accident or otherwise, as by scraping, etc., the dis- 

 appearance of the color makes such removal evident. 



"The great difference between radium used in containers, com- 

 posed even of exceedingly thin aluminium, and radium used in the 

 form of the coatings here described, is shown by their relative influ- 

 ences on the electroscope ; a delicate rod coated at its tip with radium 



Fig. 2. Sealed Glass Tubes of Radium Bromide and a Rod Coated with Lieber's 



Radium Coating. 



bromide of 10,000 activity and holding, therefore, very little radium, 

 compared very favorably in its effects with i grm. of radium bromide 

 of 10,000 activity in a glass tube, or with 10 mgrms. of radium 

 bromide of 1,000,000 activity in a very thin aluminium tube." 



That these coatings permit of the escape of the gaseous emana- 

 tion is shown by the fact that the presence of the emanation may be 

 demonstrated in a current of air passed over a coating. That the a 

 rays penetrate through the thin collodion film is proved by the fact 

 that the scintillations produced on a zinc sulphide screen by alpha 

 rays may be caused by the use of a coated rod or disc. The activity 

 of the radium is not affected by this treatment, for Rutherford ^ has 



