166 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1936 



the form of electromagnetic radiation of high frequency (y-rays). 

 Since there was a large emission of energy in the change of one atom 

 into another, it was natural to infer that a large store of energy was 

 contained in a heavy atom. It was clear, too, that the atom must be 

 the seat of intense internal forces in order to be able to hurl out a 

 fragment of itself with such high speed. 



Since experiments are usually made with small quantities of active 

 matter or with elements like radium of slow rate of transformation, 

 it is not easy to realize except in imagination the extraordinary effects 

 that would be observed if we could, for example, experiment with a 

 reasonable quantity of a short-lived element like the gas radon. Sup- 

 pose we were able to obtain a kilogram of this gas and introduce it 

 into a bomb made of heat-resisting material. At the end of about 2 

 hours heat would be evolved corresponding to about 20,000 kilowatts, 

 and the bomb would be melted unless it were very efficiently cooled. 

 Penetrating y-rays would be emitted with energy corresponding to 

 about 1,000 kilowatts. The heating effect would die away with the 

 decay of radon (half period 3.8 days). At the end of about 2 months 

 the radon would mostly have disappeared, but in its place would 

 remain about 54 grams of the gas helium, and deposited on the walls 

 946 grams of a lead isotope (radium-Z>, atomic weight 210), mixed 

 with a small quantity of radium-5' and polonium. After allowing 

 about 200 years for most of the radium-Z? to disappear, we should 

 then find remaining about 72 grams of helium and 928 grams of an 

 inactive lead isotope of atomic weight 206. I cannot imagine a more 

 convincing experiment to illustrate the striking nature of these radio- 

 active transformations, but unfortunately, or rather fortunately hav- 

 ing regard to the safety of the investigator from the radiations, there 

 is little chance of trying such a large-scale experiment. 



The property of radioactivity, apart from uranium and thorium 

 and their products, is shown only by a few other elements, potassium 

 and rubidium — to which may now be added samarium — and then 

 only to a very feeble degree. All the rest of the chemical elements 

 appear to be permanently stable when tested by the criterion of 

 radioactivity. 



When once the nature of the a- and )8-particles had been established 

 and the long series of radioactive changes in uranium, thorium, and 

 actinium had been mostly made clear, it appeared that the main con- 

 tribution of radioactivity to our knowledge was nearing an end. But 

 it is characteristic of this subject that no sooner does it appear to be 

 visibly moribund than it flashes out again into vigorous life, leading 

 to new and unexpected additions to our knowledge. Tliis is well illus- 

 trated by the work of the next few years, 1911-13, which saw three 

 new advances of great significance for the future — I refer to the idea 



