1903.] 



The Emanations of Radium. 



407 



and produce phosphorescence when brought near the screen. On 

 t luning the lens to the, apparently, uniformly lighted edge of the 

 screen close to the finger, the scintillations are seen to be closer and 

 more numerous ; what to the naked eye appears like a uniform " milky 

 way," under the lens is a multitude of stellar points, flashing over th? 

 whole surface. A clean finger does not show any effect, but a touch 

 with a soiled finger is sufficient to confer on it the property. Washing 

 the fingers stops their action. 



It was of interest to see if rarefying the air would have any effect 

 on the scintillations. A blende screen was fixed near a flat glass 

 window in a vacuum tube, and a piece of radium salt was attached 

 to an iron rocker, so that the movement of an outside magnet 

 would either bring the radium opposite the screen or draw it away 

 altogether. A microscope gave a good image of the surface of the 

 screen, and in a dark room the scintillations were well seen. No 

 particular difference was observed in a high vacuum ; indeed, if any- 

 thing, the sparks appeared a trifle brighter and sharper in air than 

 in vacuo. A duplicate apparatus in air was put close to the one in 

 the vacuum tube, so that the eye could pass rapidly from one to the 

 other, and it was so adjusted that the scintillations were about equal 

 when each was in air. The vacuum apparatus was now exhausted 

 to a very high point, and the appearance on each screen was 

 noticed. Here again I thought the sparks in the vacuum were not 

 quite so bright as in air, and on breaking the capillary tube of the 

 pump, and observing as the air entered, the same impression was left 

 on my mind ; but the differences, if any, are very minute, and are 

 scarcely greater than might arise from errors of observation. 



It is difficult to form an estimate of the number of flashes of light 

 per second. But with the radium at about 5 cm. off the screen they 

 are barely detectable, not being more than one or two per second. As 

 the distance of the radium diminishes the flashes become more fre- 

 quent, until at 1 or 2 cm. they are too numerous to count. 



[Added March 18. — On bringing alternately a Sidot's blende screen 

 and one of barium platinocyanide, face downwards, near a dish of 

 " polonium " sub-nitrate, each became luminous, the blende screen being 

 very little brighter of the two. On testing the two screens over a 

 crucible containing dry radium nitrate, both glowed ; in this case the 

 blende screen being much the brighter. Examined with a lens, the 

 light of the blende screen was seen to consist of a mass of scin- 

 tillations, while that of the platinocyanide screen was a uniform 

 glow, on which the scintillations were much less apparent. 



The screens were now turned face upwards so that emanations 

 from the active bodies would have to pass through the thickness of 

 card before reaching the sensitive surface. Placed over the " polonium " 



