JlLY 10, 1903.] 



SCIENCE. 



45 



from solution by ammouium hydrate. The 

 filtrate, which is free from thorium, when 

 evaporated to dryness shows an activity 

 measured electrically fully one thousand 

 times as great as that of the original tho- 

 rium. Thoriiun-X, like Ur-X, is present 

 as a trace only. Its radiation consists 

 chiefly of ^ rays. But both thorivuu and 

 Th-X develop rays of both kinds. 



It was early found that both thorium 

 and radium possess the power of exciting 

 temporary activity in bodies placed near 

 them. This excited activity may also be 

 produced by bringing air that has been in 

 contact with i-adium or thorium past the 

 body to be excited. The results are ex- 

 plained by the fact that thorium and 

 radium each give out an 'emanation,' 

 which behaves in all respects like an inert 

 gas. This emanation is itself radioactive, 

 as may be shown by the electrical effects 

 produced by it, and it also has the power 

 of exciting temporary activity in bodies 

 with which it comes in contact. The ema- 

 nations lose their activity rather rapidly; 

 in the case of thorium the activity falls 

 to one half its original value in about one 

 minute, while in the case of radiiun a 

 similar loss occurs in the course of several 

 days. Radium and thorium are the only 

 substances that give emanations ; they are 

 also the only substances which have the 

 power of exciting activity in neighboring 

 bodies. 



That the emanations of radium and tho- 

 rium are gases is confirmed in a great 

 variety of ways. For example, they can 

 be occluded by porous solids, such as the 

 solid salts which develop them. Owing to 

 the fact that the radiiun emanation pre- 

 serves its activity for a long time, the oc- 

 clusion of this emanation is more readilj' 

 studied. A large part of emanation de- 

 veloped by radium is occluded by the ra- 

 dium salt itself; this may be driven ofif 



by heating, after which considerable time 

 is required for the original condition to 

 be restored. When the active salts of ra- 

 dium or thorium are in solution the emana- 

 tions developed are liberated more rapidly, 

 there being in this case no chance for oc- 

 clusion. The rate at which the emanation 

 is developed appears to be constant under 

 all conditions. 



The emanations of both radium and 

 thorium are chemically inert. They may 

 be passed through sulphuric acid, nitric 

 acid and hydrochloric acid without change, 

 and are also unaffected by passing over 

 red-hot lead chromate or magnesium. They 

 may be condensed, however, by passing 

 through a tube immersed in liquid air. 

 The radium emanation condenses at — 150° 

 C, and that of thorium at about — 120° 

 C. The rate of decay of the emanation is 

 unaffected by this low temperature. "When 

 the temperature is raised again the emana- 

 tions are liberated with an activity depend- 

 ing only upon the time that has elapsed 

 since they were developed. 



The excited activity produced by the 

 emanations of radium and thorium is 

 greatest on bodies that are negatively 

 charged. It would seem, therefore, that 

 the substance to which excited activity is 

 due must itself be positively charged. If 

 the substance is sending out more /J rays 

 than a rays such a positive charge would 

 naturally follow. 



Temporary activity may be acquired by 

 exposing a negatively charged body to or- 

 dinary air. Apparently the atmosphere 

 contains a radioactive gas similar to the 

 emanations mentioned above. This view 

 is strengthened by the fact that freshly 

 fallen rain and snow possess temporary 

 activity, probably obtained from the air. 

 Air from cellars and air that has been 

 drawn from a porous soil are especially 

 rich in this active constituent. J. J. 



