164 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1936 



an electrometer. I then found that, on stopping the current of air, 

 the ionization effect fell off according to a geometrical law with the 

 time, diminisliing to half -value in about 1 minute. It thus seemed 

 clear that thoria emitted some kind of active substance which was 

 carried away with the air stream and decayed in activity with time. 

 I gave the name "emanation" to this unknown substance which 

 readily diffused through paper. This was the first time that the 

 characteristic law of decay of radioactive bodies had been measured. 

 At the same time, I noticed that all substances which came in con- 

 tact with the emanation for some time became radioactive. This 

 "excited" activity, as it was unfortunately termed, decayed with 

 time after the removal of the emanation, according to the same law 

 as the emanation but with a much longer half period, viz., 11 hours 

 instead of 1 minute. Another surprising result was observed in a 

 strong electric field. The activity was to a large extent concentrated 

 on the negative electrode. In this way, a platinum wire could be 

 made strongly active. The activity on the wire could be driven off 

 by heat and removed by solution in acids, but when the acid was 

 evaporated, the activity remained behind. These results were a 

 strong indication that the activity was due to some kind of matter 

 produced either from the emanation or by its action. It seemed 

 likely that the emanation existed in very minute quantity, but it 

 occurred to me that diffusion methods might throw light on whether 

 the emanation was a light or a heavy substance. For this purpose, 

 the relatively long-lived emanation from radium was employed. By 

 measuring the coefficient of diffusion of the emanation into air, Miss 

 H. T. Brooks and I concluded that its molecular weight was large 

 and of the order of 100. 



About this tune, 1901, began that fruitful association with F. 

 Soddy, who was then a teacher in the Chemical Department of 

 McGill University. At this stage, the subject of radioactivity was 

 in a very confused state. A number of substances had been found to 

 show a temporary activity when separated from a radioactive solu- 

 tion or exposed to radioactive bodies, and the idea had arisen that 

 the radiations had in some way the property of "inducing" radio- 

 activity on bodies exposed to the radiation. This was a natural but 

 mistaken idea which had to be cleared away before progress could 

 be made. For this purpose we first made experiments on the 

 thorium emanation to determine its chemical properties and to find 

 whether it originated from thorium itself or from some other sub- 

 stance associated with it. We found that a new radioactive sub- 

 stance, named thorium-X, could be chemically separated from 

 thorium and that this substance and not the thorium itself gave rise 

 to the emanation. It was found that thorium-X was being produced 



