300 



SCIENCE 



[N. S. Vol. XXX. No. 706 



miliar to science. So far no direct evidence 

 has been obtained of the possibility of 

 building up an atom of higher atomic 

 weight from one of lower atomic weight, 

 but in the ease of the radioactive sub- 

 stances we have decisive and definite evi- 

 dence that certain elements show the 

 convei'se process of disintegration. It may 

 be significant that this process has only 

 been observed in the atoms of highest 

 atomic weights, like those of uranium, 

 thorium and radium. With the exception 

 possibly of potassium, there is no reliable 

 evidence that a similar process takes place 

 in other elements. The transformation of 

 the atom of a radioactive substance appears 

 to result from an atomic explosion of great 

 intensity in which a part of the atom is ex- 

 pelled with great speed. In the majority 

 of cases, an a particle or atom of helium is 

 ejected, in some cases a high-speed electron, 

 while a few substances are transformed 

 without the appearance of a detectable 

 radiation. The fact that the a particles 

 from a simple substance are all ejected 

 with an identical and very high velocity 

 suggests the probability that the charged 

 helium atom before its expulsion is in rapid 

 orbital movement in the atom. There is at 

 present no definite evidence of the causes 

 operative in these atomic transformations. 



Since in a large number of cases the 

 transformations of the atoms are accom- 

 panied by the expulsion of one or more 

 charged atoms of helium, it is difficult to 

 avoid the conclusion that the atoms of the 

 radioactive elements are built up, in part 

 at least, of helium atoms. It is certainly 

 very remarkable and may prove of great 

 significance, that helium, which is regarded 

 from the ordinary chemical standpoint as 

 an inert element, plays such an important 

 pai't in the constitution of the atoms of 

 uranium, thorium and radium. 



The study of radioactivity has not only 

 thrown great light on the character of 



atomic transformations, but it has also led 

 to the development of methods for detect- 

 ing the presence of almost infinitesimal 

 quantities of radioactive matter. It has 

 already been pointed out that two methods 

 — one electrical, the other optical— have 

 been devised for the detection of a single 

 a particle. By the use of the optical or 

 scintillation method, it is possible to count 

 with accuracy the number of a particles 

 when only one is expelled per minute. It 

 is not a difficult matter, consequently, to 

 follow the transformation of any radioac- 

 tive substance in which only one atom 

 breaks up per minute, provided that an a 

 particle accompanies the transformation. 

 In the case of a rapidly changing substance 

 like the actinium emanation, which has a 

 half period of 3.7 seconds, it is possible to 

 detect with certainty the presence, if not 

 of a single atom, at any rate of a few 

 atoms, while the presence of a hundred 

 atoms would in some cases give an incon- 

 veniently large effect. The counting of the 

 scintillations affords an exceedingly power- 

 ful and direct quantitative method of 

 studying the properties of radioactive sub- 

 stances which expel a particles. Not only 

 is it a simple matter to count the number 

 of a particles which are expelled in any 

 given interval, but it is possible, for ex- 

 ample, by suitably arranged experiments 

 to decide whether one, two or more a par- 

 ticles are expelled at the disintegration of 

 a single atom. 



The possibility of detection of a single 

 atom of matter has opened up a new field 

 of investigation in the study of discontinu- 

 ous phenomena. For example, the experi- 

 mental law of transformation of radioac- 

 tive matter expresses only the average rate 

 of transformation, but by the aid of the 

 scintillation or electric method it is possible 

 to determine directly by experiment the 

 actual interval between the disintegration 

 of successive atoms and the probability 



