46 



SCIENCE. 



[N. S. Vol. XVIU. Xo. 445. 



Thomson lias recently found that the water 

 from certain deep wells in Cambridge also 

 contains a radioactive gas. 



Several general points brought oxA in 

 recent years should not be lost sight of, 

 since their bearing upon the theory of the 

 subject seems to be important. 



Radioactivity seems to be unaffected by 

 temperature. Neither the activity of a 

 substance nor the rate of decay in the case 

 of temporary activity is affected by a 

 change of temperature as great as that 

 from liquid air to a white heat. None of 

 the physical agencies which usually affect 

 physical phenomena seems to influence 

 radioactivity. These facts, together with 

 the fact that the salts of the active ele- 

 ments are active as well as the elements 

 themselves, have suggested the thought that 

 the phenomena of radioactivity are phe- 

 nomena of the atom rather than of the 

 molecule. 



There are many indications that radio- 

 activity is accompanied by some change in 

 the active substances. For example, the 

 Th-X may be removed from thorium com- 

 pletely; but at the end of a month or so 

 the thorium may be again treated in the 

 same way and as much Th-X obtained as 

 before. It seems as though the develop- 

 ment of Th-X was proceeding slowly all 

 the time. The Th-X must itself be under- 

 going some change, since its activity dimin- 

 ishes steadily from the time of its separa- 

 tion. Further evidences of change are 

 furnished by the continual development 

 of the emanations of thorium and radium 

 and the gradual decay of the activity pos- 

 sessed by these emanations. 



Two questions of the greatest importance 

 at once suggest themselves in the considera- 

 tion of radioactive substances. First of 

 all, if the rays consist of particles shot off 

 from the substances it would seem as 

 though a diminution in weight shoiild re- 



sult. It seems hardly probable that matter 

 could be gathered in from the surround- 

 ings to supply this loss. There is in fact 

 some experimental evidence that strong 

 active preparations lose in weight by a 

 measurable amount; but this evidence is 

 not yet to be regarded as conclusive. Ob- 

 servations to test this point are difficult. 

 The change to be expected is extremely 

 minute, even with the most active sub- 

 stances, and might easily be masked by 

 the results of chemical changes produced 

 by the rays in the walls of the containing 

 vessel. But although the question must 

 be regarded as unsettled, I think that most 

 students of the subject are convinced that 

 a loss of weight actually occurs, even 

 though it has not as yet been detected. 



A question of even more fundamental 

 importance, and one that is now attracting 

 especial interest, is that of the source of 

 the energy which the raj's possess. It has 

 recently been shown by the Curies that one 

 gram of an active radium preparation de- 

 velops each hour forty calories of energy. 

 In other words, a gram of this preparation 

 could melt its own weight of ice in two 

 hours. Expressed in a different form, this 

 would mean that this salt of radium de- 

 velops, in the course of a month, as much 

 energy as is liberated by the combustion 

 of an equal mass of hydrogen. When 

 hydrogen is burned its store of energy is 

 exhausted ; but radium can apparently con- 

 tinue to give out this energy month after 

 month, with no diminution in intensity 

 which has yet been detected. The numer- 

 ical results obtained by the Curies may 

 require correction in the light of more ac- 

 curate measurements; but the difficulty 

 will still remain. How is it possible for a 

 radioactive substance to continue radiating 

 energy for an indefinite period without 

 appi'eciable loss? 



Several explanations have been suggested. 



