418 



Sir W. Crookes. 



was just as radio-active as before the partial removal of the iron. 

 Therefore the presence of iron does not interfere with the activity of 

 the substance. 



26. Having thus definitely proved that the supposed radio-activity 

 of uranium and its salts is not an inherent property of the element, 

 but is due to the presence of a foreign body,"^ it is necessary patiently 

 to determine the nature of the foreign body. Several radio-active 

 Tjodies claimed to be new have already been extracted from pitch- 

 blende, and experiments have been instituted to see if the newly found 

 body UrX had similar chemical properties to those of the older active 

 substances. 



27. Polonium was first tried. A photographic plate had a thin 

 sheet of celluloid laid on it, and over this a sheet of aluminium 

 foil, 0'05 mm. thick. On this double layer were put two cells, one 

 containing basic polonium nitrate, the other active UrX. Action 

 was allowed to proceed for twenty-four hours, and the plate was then 

 •developed. A disc of blackening was seen under where the UrX 

 stood, the action having passed through the glass, celluloid, and 

 aluminium. Under the polonium nitrate no trace of action could be 

 detected. 



The experiment was repeated, minus the aluminium foil, and the 

 action continued only two and a quarter hours. On development, the 

 UrX was found to have acted well, while the polonium showed no 

 trace of action. 



28. This behaviour of polonium being excentric or contrary to 

 published accounts,! I put some polonium nitrate in a very thin 

 gelatine capsule, and laid it for eight hours on a sensitive plate. No 

 trace of an image could be seen on development. 



The same polonium nitrate was put in a watch-glass, and the sensi- 

 tive plate put over it face downwards, so that it might be expiosed 



* For tlie sate of lucidity tlie new body must hare a name. Until it is more 

 tractable I will call it provisionally UrX — the unknown siibstance in uranium. 



f " The rays emitted by compounds of polonium render barium platinocyanide 

 fluorescent ... To make tbe experiment, place on the active substance a very 

 thin sheet of aluminium, and on this a thin layer of barium platinocyanide ; in 

 the dark the barium platinocyanide appears feebly luminous over the active 

 substance " (M. and Mdme. Curie and M. Bemont, * Comptes Rendus,' vol. 127, 

 p. 1215; ' Chem. News,' vol. 79, p. 1). "Polonic rays act on sensitive plates. 

 The substance we call sulphide of polonium gives a good impression after only 

 three minutes, and there is a decided action noticed after even half a minute " 

 {Mdme. Curie, 'Revue Grenerale des Sci.,' January 30, 1899; ' Chem. News,' 

 vol. 79, p. 77). In the same paper the authoress, after describing the power pos- 

 sessed by a polonium compound to excite phosphorescence, says : " the rays 

 emitted by this latter body have traversed the aluminium and excited the fluore- 

 scence of the platinocyanide above it," It is evident from the above extracts 

 that I was justified in thinking that polonium rays were not entirely stopped by 

 thin aluminium, glass, or celluloid. 



