the Cause and Nature of Radioactivity. 581 



electrically to the highest attainable temperature, and also 

 through the tube cooled by solid carbon dioxide and ether. 

 The tube was then filled with platinum-black, and the 

 emanation passed through it in the cold, and with gradually 

 increasing temperatures, until the limit was reached. The 

 effect of the intense heat was to convert the platinum-black 

 completely into platinum-sponge. In another experiment 

 the emanation was passed through a layer of red hot lead- 

 en romate in a. glass tube. The current of air was replaced 

 by a current of hydrogen, and the emanation sent through 

 red hot magnesium-powder and red hot palladium-black, 

 and, by using a current of carbon dioxide, through red hot 

 zinc-dust. In every case the emanation passed without 

 sensible change in the amount. If anything, a slight in- 

 crease occurred, owing to the time taken for the gas-current 

 to pass through the tubes when hot being slightly less than 

 when cold, the decay en route being consequently less. It 

 will be noticed that the only known gases capable of passing 

 in unchanged amount through all the reagents employed are 

 the recently-discovered members of the argon family. 



But another interpretation may be put upon the results. 

 If the emanation were the manifestation of excited radio- 

 activity on the surrounding atmosphere, then since from the 

 nature of the experiments it was necessary to employ in each 

 case, as the atmosphere, a gas not acted on by the reagent 

 employed, the result obtained might be explained. Red hot 

 magnesium would not retain an emanation consisting of 

 radioactive hydrogen, or red hot zinc-dust an emanation 

 consisting of radioactive carbon dioxide. The correctness of 

 this explanation was tested in the following way. Carbon 

 dioxide was passed over thoria, then through a T-tube, 

 where a current of air met and mixed with it, both passing 

 on to the testing-cylinder. But between this and the T-tube 

 a large soda-lime tube was introduced, and the current of 

 gas thus freed from its admixed carbon dioxide before being- 

 tested in the cylinder for emanation. The amount of emana- 

 tion found was quite unchanged, whether carbon dioxide was 

 sent over thoria in the manner described, or whether an 

 equally rapid current of air was substituted for it, keeping 

 the other arrangements as before. The theory that the 

 emanation is an effect of the excited activity on the sur- 

 rounding medium is thus excluded. It is a 'priori improbable 

 on account of the very different rates of decay of the activity 

 in the two cases. The interpretation of the above experiments 

 must therefore be that the emanation is a chemically inert gas 

 analogous in nature to the members of the argon family. 



