42 POPULAR SCIENCE MONTHLY. 



gaseous emanation from radium. The conclusion therefore followed 

 that in all probability these bodies are gases of the argon group, the 

 atomic weight of which, and consequently the density, is very high; 

 indeed, several observers, by means of experiments on the rate of 

 diffusion of the gas from radium, believe it to have a density of approxi- 

 mately 100, referred to the hydrogen standard. This conclusion has 

 been confirmed by the mapping of the spectrum of the radium emana- 

 tion, which is similar in general character to the spectra of the inactive 

 gases, consisting of a number of well-defined, clearly cut brilliant lines, 

 standing out from a black background. The volume of the gas pro- 

 duced spontaneously from a given weight of radium bromide in a given 

 time has been measured; and it was incidentally shown that this gas 

 obeys Boyle's law of pressures. The amount of gas thus collected and 

 measured, however, was very minute; the total quantity was about the 

 forty-thousandth of a cubic centimeter. 



Having noticed that those minerals which consist of compounds 

 of uranium and thorium contain helium, Rutherford and Soddy made 

 the suggestion that it might not be impossible that helium is the product 

 of the spontaneous change of the emanation; and Soddy and I were 

 able to show that this is actually the case. For, first, when a quantity 

 of a radium salt which has been prepared for some time is dissolved 

 in water, the occluded helium is expelled, and can be recognized by 

 means of its spectrum; further, the fresh emanation shows no helium 

 spectrum, but after a few days the spectrum of helium begins to 

 appear, proving that a spontaneous change is in progress ; and last, as 

 the emanation disappears its volume decreases to zero; and on heating 

 the capillary glass tube which contained it, helium is driven out from 

 the glass walls, into which its molecules had been imbedded in velume 

 equal to three and a half times that of the emanation. The a-rays, as 

 foreshadowed by Rutherford and Soddy, consist of helium particles. 



All these facts substantiate the theory, devised by Rutherford and 

 Soddy, that the radium atom is capable of disintegration, one of the 

 products being a gas, which itself undergoes further disintegration, 

 forming helium as one of its products. Up till now, the sheet anchor 

 of the chemists has been the atom. But the atom itself appears to 

 be complex, and to be capable of decomposition. It is true that only 

 in the case of a very few elements, and these of high atomic weight, 

 has this been proved. But even radium, the element which has by far 

 the most rapid rate of disintegration, has a comparatively long life; 

 the period of half-change of any given mass of radium is approximately 

 1,100 years. The rate of change of the other elements is incomparably 

 slower. This change, too, at least in the case of radium, and its em- 

 anation, and presumably also in the case of other elements, is attended 

 with an enormous loss of energy. It is easy to calculate from heat 

 measurements (and independent and concordant measurements have 

 been made) that one pound of emanation is capable of parting with 



