Radio- Active Matter in ordinary substances. 393 



as before. The water containing the powder thus differs from tap 

 water, for if the emanation is once expelled from the latter, it 

 does not again accumulate to anything like the original extent, 

 though even in this case there is, as Mr Adams has shown, a very 

 slight recovery of the emanation. 



Properties of the Emanation. The emanation given out by 

 all these substances seems to be the same as that given out by 

 radium ; thus its radio-activity falls to one-half its value in about 

 four days, while the induced radio-activity due to it, falls to 

 about one-half its value in 40 minutes. In fact the phenomena 

 are just those which would occur if radium were a very widely 

 diffused substance, minute traces of it occurring in almost all 

 specimens of clay, sand or shale. The emanation obtained from 

 these substances is due to an impurity mixed with them, and 

 does not come from the main bulk of the substance ; this is shown 

 by the great differences in the amount of emanation given off by 

 specimens of the same substance obtained from different sources ; 

 thus for example in the case of precipitated silica, one specimen 

 bought in Cambridge was richer in the emanation than any sub- 

 stance (other than radium or thorium) I have ever examined, 

 while two other specimens, one of which had been for a long 

 time in the Cavendish Laboratory, and the other obtained from 

 the Chemical Laboratory, hardly gave a trace of emanation. The 

 same variation was observed to a smaller extent in all the sub- 

 stances examined. 



The question as to whether all substances give off emanations 

 to a slight extent is one to which I have given a good deal of 

 attention, but so far I have not obtained any emanations other 

 than those whose capriciousness indicated that they were due to 

 minute traces of a radio-active impurity. Two methods were 

 used; in the first of these, salts of the metal to be tested (for the 

 most part lead, tin, zinc, bismuth, or copper) were dissolved in 

 distilled water, and the air expelled from the solution by boiling ; 

 tests were made soon after the solution was formed and again 

 after it had stood for some weeks, but in neither case was any 

 emanation found in the air expelled from the solution. In the 

 second method in order to get a large surface of the metal exposed 

 to the water I deposited the metal in a colloidal state by making 

 an electric arc pass beneath distilled water between terminals of 

 the metal, the metals tested in this way were platinum, bismuth, 

 and lead ; enough metal was deposited to make a litre flask of the 

 solution opaque, the solution was boiled and the air from it tested, 

 but I could not obtain any evidence of an emanation. 



I also tried whether a metal could be stimulated to give off 

 emanation by exposure to Rontgen rays ; for this purpose I used 



vol. xn. pt. v. 26 



