712 Properties of Radium in Minute Quantities. 



changed by altering the degree o£ concentration of the 

 product. For example, the constant of decay of the radium 

 emanation has been tested over a range of at least one million 

 times, but no alteration of its value has been detected. 



The experiments of Eve undoubtedly show that the activity 

 of the radium bromide, kept in a closed vessel, after reaching 

 its maximum, does not exhibit the decrease observed by 

 Voller for a deposit of radium exposed to the open air. This 

 shows that the decrease of activity, noticed by Voller, has 

 nothing to do with the actual change in the rate of disinte- 

 gration of radium, but is a result of the exposure to the open 

 air. Not the slightest evidence has so far been obtained to 

 indicate that the rate of disintegration of radium is in any 

 way altered by the amount of radium present. In this respect 

 its behaviour is analogous to the other numerous radioactive 

 products. 



On the other hand, there is some evidence that small 

 quantities of radium, deposited on a plate by evaporation of a 

 radium solution, apparently lose their activity fairly rapidly 

 when left in the open air. I have observed that the activity 

 of a plate 50 sq. cms. in area over which half a milligram of 

 radium was deposited, lost more than half of its activity in 

 the course of a year. 



This disappearance of the activity has no connexion with 

 the actual life of the radium itself, but is probably due to 

 the escape of the radium from the plate into the surrounding 

 gas. A quantity of 10~ 9 grams of radium bromide, spread 

 over an area of 10 sq. cms., is far too small to form a layer of 

 even molecular thickness. During the process of evaporation, 

 the radium bromide would tend to collect together and form 

 small crystals which are deposited irregularly over the surface 

 of the plate. These particles would not be held firmly to the 

 plate, and would gradually escape from it. 



Such an effect would probably be also shown by inactive 

 matter, if deposited in such minute amount. This action, 

 however, can only be experimentally tested with a substance 

 like radium, where the activity can be used as a means of 

 detecting the change in an excessively minute quantity of 

 deposited matter. 



McGill University, Montreal. 

 March 28, "l 905. 



