[JOHNSTONE] EMANATING POWER OF xAIIXERALS 103 



taining thorium and radium compounds show a marked variation in 

 the proportions of emanation which escape from them, and that these 

 variations are influenced by changes in temperature and the amount 

 of moisture present. It will be shown in the course of this paper 

 that the emanating power of powdered uranium minerals is subject 

 to similar variations from the same causes. 



For calibration purposes the use of some solid material containing 

 radium having an emanating power, which would not be appreciably 

 affected by the changes in temperature and atmospheric humidity 

 under the usual conditions of experiment, would obviously possess a 

 great advantage over other methods now in use. This material could 

 be sealed up in a glass tube and the radium emanation allowed to 

 collect for a definite, known period and would furnish a convenient 

 source from which, as desired, a definite quantity of radium emanation 

 could be obtained. After having once been calibrated by comparison 

 with a standard radium solution or standard uranium mineral it 

 would thereafter be available for the convenient calibration of an 

 electroscope. On account of the variation in the emanating power 

 of a uranium mineral, such minerals in their natural state are not 

 suitable for the purpose which has just been considered. However, 

 it is known that by heating a uranium mineral to a comparatively 

 high temperature, its emanating is greatly reduced. It was thought 

 possible that simultaneously with the reduction of the emanating 

 power other cha.n^es might take place in the heated mineral which 

 would cause the rate of escape of emanation to be less susceptible 

 to variations in temperature and humidity of the air coming in contact 

 with the material. In the course of the work described in this paper 

 evidence has been obtained that the anticipated changes in the 

 properties of the mineral actually do occur so far as the temperature 

 effect is concerned, and if the humidity of the air coming in contact 

 with the mineral be kept constant it will be shown that the heated 

 uranium mineral furnishes a sufificiently constant source of radium 

 emanation to satisfy the requirements which have been already 

 outlined. 



Apparatus and Method 



The measurement of the ionization currents were made in an air- 

 tight, emanation electroscope,^ which had a capacity of about 3 liters. 

 The ionization chamber was fitted with two side tubes which could 

 be opened or closed by means of stopcocks. The charged electrode 



^Previously designed and used by Professor Boltwood. 



