April 2, 1903] 



NA TURE 



523 



have contained radium is remarkable. Filters, beakers, and 

 dishes used in the laboratory for operations with radium, 

 after having been washed in the usual way, remain radio- 

 active; a piece of blende screen held inside the beaker or 

 other vessel immediately glowing with the presence of 

 radium. 



The blende screen itself is sensitive to mechanical shocks. 

 A tap with the tip of a penknife will produce a sudden spark 

 of light, and a scratch with the blade will show itself as an 

 evanescent luminous line. 



A diamond crystal brought near the radium nitrate glowed 

 with a pale bluish-green light, as it would in a " Radiant 

 Matter " tube under the influence of kathodic bombardment. 

 On removing the diamond from the radium it ceased to glow, 

 but when laid on the sensitive screen, it produced phosphor- 

 esi ence beneath which lasted some minutes. 



During these manipulations the diamond accidentally 

 touched the radium nitrate in the dish, and thus a few im- 

 perceptible grains of the radium salt got on to the zinc sul- 

 phide screen. The surface was immediately dotted about 

 with brilliant specks of green light, some being a milli- 

 metre or more across, although the inducing particles were 

 too small to be detected on the white screen when examined 

 by daylight. 



In a dark room, under a microscope with a|-inch ob- 

 jective, each luminous spot is seen t<> have a dull centre 

 surrounded by a luminous halo extending for some distance 

 around. The dark centre itself appears to shoot out light at 

 intervals in different directions. Outside the halo, the dark 

 surface of the screen scintillates with sparks of light. No 

 two Hashes succeed one another on the same spot, but are 

 scattered over the surface, coming and going instantaneously, 

 no movement of translation being seen. 



The scintillations are somewhat better seen with a pocket 

 lens magnifying about 20 diameters. They are less visible 

 on the barium platinocyanide than on the zinc sulphide screen. 

 A powerful electromagnet has no apparent effect on the 

 scintillations, which appear quite unaffected when the current 

 is made or broken, the screen being close to the poles and 

 arranged axially or equatorially. 



A solid piece of radium nitrate is slowly brought near the 

 si reen. The general phosphorescence of the screen as visible 

 to the naked eye varies according to the distance of the 

 radium from it. On now examining the surface with the 

 pocket lens, the radium being far off and the screen faintlv 

 luminous, the scintillating spots are sparsely scattered over 

 the surface. On bringing the radium nearer the screen the 

 scintillations become more numerous and brighter, until when 

 close together the flashes follow each other so quickly that 

 the surface looks like a turbulent luminous sea. When the 

 scintillating points are few there is no residual phosphor- 

 escence to be seen, and the sparks succeeding each other ap- 

 pear like stars on a black sky. When, however, the bom- 

 bardment exceeds a certain intensity, the residual phos- 

 phorescent glow spreads over the screen, without, however, 

 interfering with the scintillations. 



If the end of a platinum wire which has been dipped in a 

 solution of radium nitrate and dried is brought near the 

 screen, the scintillations become very numerous and ener- 

 getic, and cease immediately the wire is removed. If, how- 

 ever, the end of the wire touches the screen, a luminous 

 spot is produced which then becomes a centre of activity, and 

 the screen remains alive with scintillations in the neighbour- 

 hood of the spot for many weeks afterwards. 



Polonium " basic nitrate produces a similar effect on the 

 screen, but the scintillations are not so numerous. 



-Microscopic glass, very thin aluminum foil, and thin mica 

 do not stop the general luminosity of the screen from the X- 

 rays, but arrest the scintillations. 



I could detect no variation in the scintillations when a 

 rapid blast of air was blown between the screen and the 

 radium salt. 



A beam of X-rays from an active tube was passed through 

 a hole in a lead plate on to a blende screen. A luminous spot 

 was produced on the screen, but I could detect no scintilla- 

 tions, only a smooth uniform phosphorescence. A piece of 

 radium salt brought near gave the scintillations as usual, 

 superposed on the fainter phosphorescence caused by the 

 X-rays, and they were not interfered with in any degree 

 by the presence of X-rays falling on the same spot. 



During these experiments the fingers soon become soiled 



NO. 1744, VOL. 67] 



with radium, and produce phosphorescent e when brought 

 near the screen. On turning 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 the 

 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 anything 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 radiu n 

 diminishes the flashes become more frequent, until at 1 or 

 2 cm. they are too numerous to count. 



[Added March 18. — On bringing alternatelv a Sidot's 

 blende screen and one of barium platinocyanide, face down- 

 wards, 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 scintillations, 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 eman- 

 ations from the active bodies would have to pass through 

 the thickness of card before reaching the sensitive surface. 

 Placed over the " polonium " neither screen showed any 

 light. Over the radium the platinocyanide screen showed a 

 very luminous disc, corresponding with the opening of the 

 crucible, but the blende disc remained quite dark. 



It therefore appears that practically the whole of the 

 luminosity on the blende screen, whether due to radium or 

 "polonium," is occasioned by emanations which will not 

 penetrate card. These are the emanations which cause the 

 scintillations, and the reason why they are distinct on the 

 blende and feeble on the platinocyanide screen is that with 

 tho latter the sparks are seen on a luminous ground of 

 general phosphorescence which renders the eye less able 

 to see the scintillations. 



Considering how coarse-grained the structure of matter 

 must be to particles forming the emanations from radium, 

 I cannot imagine that their relative penetrative powers de- 

 pend on difference of size. I attribute the arrest of the 

 scintillating particles to their electrical character, and to 

 the ready way in which they are attracted by the coarser 

 atoms or molecules of matter. I have shown that radium 

 emanations cohere to almost evervthing with which they 

 come into contact. Bismuth, 1 lead, platinum, thorium 

 uranium, elements of high atomic weight and density, 

 possess this attraction in a high degree, and only lose the 

 emanations very slowly, giving rise to what is known as 

 " induced radio-activity." The emanations so absorbed 

 from radium by bismuth, platinum, and probably other 



1 I have been quite unab'e to detect any lines but those of bismuth (and 

 of known impurities) in the spectrum of the strongest and most active 

 " polonium " salt I have been able to procure. 



