142 ESSAYS BIOGRAPHICAL AND CHEMICAL 



property more rapidly, however, when heated or washed. 

 Even distilled water acquires radio-activity, when placed 

 near radium chloride under a glass bell-jar; the water 

 rapidly loses its power in an open vessel after removing 

 it from the proximity of the radium; and even when 

 sealed into a glass tube it loses power after a few days. 

 On the other hand, a solution of a radium salt (e.g. radio- 

 active barium bromide) loses activity on exposure to air, 

 but regains it on being kept in a sealed tube. 



MM. Curie and Debierne find that this induced radio- 

 activity is greatly increased when the radium compound 

 is placed in a small open vessel under a bell-jar, and sheets 

 of various materials are exposed under the same cover. 

 Even behind leaden screens the activity is induced. If 

 they are in contact with the vessel containing the radium, 

 or with the walls of the enclosed space, only the exposed 

 surfaces are rendered radio-active. The activity of such 

 sheets of material induced by a specimen of barium 

 bromide containing radium, and of which the mean atomic 

 weight of the mixture of metals is 174 instead of 137 (that 

 of barium), is 8000 times that of a piece of uranium of the 

 same dimensions. As long as the sheets are left in the 

 enclosure, the activity persists; if removed, it disappears 

 in a few days. This conveyance of induced radio-activity 

 is equally brought about if the radium compound is 

 placed in one vessel, and the sheets in another, connected 

 with the former by means of a capillary tube ; but if 

 communication between the vessels is cut off, the trans- 

 mission of activity ceases. 



It is very remarkable that this transference of radio- 

 activity is confined to radium and actinium; polonium 

 compounds do not appear to possess the property of 

 giving off emanations. It may be that this difference 

 is connected with the fact discovered by Becquerel that 

 while his rays (those of radium and actinium, probably), 



