February 27, 1902 J 



NATURE 



401 



immediately over it, so that nothing but emanations capable of 

 passing through the opaque piper woulJ be subject to experi- 

 ment. Afrer four hours' exposure in total darkness, the film 

 was developed. A good circular patch was obtained over cell 

 A, and a faint diffused darkening showed over the rest of the 

 film, darker at the spot immediately over cell li, fading away at 

 the sides as the distance becaiue greater. That this action was 

 due to the material in the open cell r., and not to general fog 

 over the plate, was seen by the clearness of the film where 

 covered by the lead, and where shadows were thrown by the 

 lead cylinder and pillar. 



^^^m^^mmmmi 



Fig. 1 — Elevation. 



Circles of the same diameter were drawn round the dark im- 

 pression over A, and round the darkest part of the impression 

 over cell E. Measurements were taken of different parts of the 

 spaces enclosed in these circles, and the mean of all these came 

 out — 



Circle over cell A — Opacity log. = -53 ; Opacity = 339. 

 Circle over cell B — Opacity log. = -32 ; Opacity = 2 09. 

 Ratio b/a = 62. 



The experiment was repeated, with the 

 addition of a sheet of aluminium, o'2 mm. 

 thick, under the black paper, the electrons 

 now having to pass through both paper and 

 metal before reaching the film. 



The exposure was for six hours, and the 

 appearance on development was very similar 

 to the last : a dark disc over the protected 

 cell A, and a diffused action over the other 

 part of the film, except in the shadow of the 

 lead supports. Measurements as on the 

 previous occasion gave the following results : — 

 Circle over cell a — Opacity log. = 78 ; 



Opacity = 6 03. 

 Circle over cell B — Opacity log. = -48 ; 

 Opacity = 3 '02. 

 Ratio b/a = -5. 



Finally, I tried polonium subnitrate, which 

 gives off emanations hardly capable of passing 

 through any screen, and greatly obstructed by 

 a few centimetres of air. 



The apparatus was substantially the same as the one just 

 ijescribed, with the modification that the lead cylinder was 12 

 mm. high, and at the other end a rod of glass 12 mm. high was 

 used to support the film. The reduced height was chosen, 

 experience showing that polonium emanations have great 

 difficulty in penetrating many millimetres through air. The 

 exposure was .seven days, at the end of which time the film was 

 developed. Over cell A a dark disc sharply defined the inside 

 of the cylinder, while over cell n was a hazy diftused patch 

 which to the eye looked much the fainter of the two. But 

 measurements of patch .A, and of a disc over cell n of the same 

 size as A, showed that the opacities in each case were practically 

 identical, as shown by the following figures : — 



NO. 1687, VOL. 65] 



Circle over cell A— Opacity log. = 74; Opacity = 5-49. 

 Circle over cell li— Opacity log. = 76 ; Opacity = 575. 



Ratio Y.'\ = I '05. 

 A repetition of the experiment, taking the mean of five 

 concordant results, gave the same opacities as before. 



Without proving that the emanations from polonium are less 

 material than those from actinium and radium, this experiment 

 shows that their behaviour is entirely different as regards 

 difiusibility through air. Whether this is due to the larger 

 mass of the individual particles, or to the less distance they have 

 to travel {12 mm. as against 28 mm. in the case of actinium and 

 radium), or to some other cause, further experi- 

 ments must decide. 



Dr. Rutherford shows that air which has re- 

 mained for some time in the neighbourhood of 

 thotia and then is carried in a current to a dis- 

 tance retains its property of communicating radio- 

 activity to other bodies. He explains these 

 phenomena by supposing that thoria gives off a 

 special kind of emanation capable of being con- 

 veyed by the air, and that this is the cause of 

 the induced radio-aciivity. 



To ascertain if the electrons or corpuscles from 

 radium also possess the property of being carried 

 along in a current of air I fitted up an apparatus 

 shown in Fig. 2. A, B, and c are three brass 

 tubes closed at the lower end and cemented with 

 paraffin to a wooden block. The upper ends 

 were accurately ground to a level surface and 

 then coaled with a thin layer of paraftin wax. 

 Holes were drilled in B and c, to admit glass 

 tubes, cemented air-tight into the cylinders, as 

 shown in the figure. The upper end of the tube 

 in B was closed with a plug of cotton-wool, and the 

 outer end in c was connected to a water puuip, so 

 that when the cylinders were closed at the top a current of air 

 was drawn through B and c. As the radium compound was self- 

 luminous, discs of thin aluminium foil were placed over cylinders 

 A and B to cut off the luminous rays. A sensitive film was laid 

 on the three cylinders over the aluminium, and it was tightly 

 pressed down by a heavy weight, the contact between the film 

 and the tops of the cylinders being sufficient to make the whole air 

 tight. At the bottom of A and B a radium compound was 

 placed, equal weights and equal surface in each. The whole 



pump. 



was put into a light-tight box, and air drawn through. The 

 cylinder .\ was used only as a standard. The air passing into B 

 was expected to carry along with it some of the corpuscles 

 emitted from the active material at the bottom ; and the inlet 

 tube in c was turned up at the end, so that the stream of 

 corpuscles-laden air should impinge on the surface of the centre 

 of the film on c, and if it carried with it any radio-active 

 properties the result should be seen on development, by the 

 production of a dark patch. If, however, the air carried 

 no corpuscles, there would be no image on the sensitive film 

 over c. 



The experiment was continued for eleven hours, 120 litres of 

 air having passed through in the time. 



