72 



Mr. W. Makower on the Molecular 



8. Experiments ivith Radium Emanation. 

 The general arrangement of the apparatus is shown in 

 fig. 4. A current of air from a large gasometer was passed 



Fig. 4. 



FttQM GaSOMETE* 



I 



through a solution of radium in water, through a drying- 

 tube, and through a plug of glass-wool to remove dust, 

 into the cylindrical testing-vessel A of 56*3 c. c. capacity, 

 along the axis of which ran an insulated rod reaching nearly 

 the whole length of the cylinder ; thence the current of air 

 passed through the porous vessel V into a second testing- 

 vessel B, and out into the open air. The air-current could 

 be regulated to any desired value by placing weights on the 

 gasometer ; the rate at which the gasometer fell was read on 

 a scale and afforded a measure, in arbitrary units, of the air 

 current passing. The vessel V was attached to the two 

 testing-vessels by the rubber connexions a and h respectively? 

 and could therefore be removed and rejDlaced when necessary. 



The air current was allowed to flow until the concentration 

 of the emanation in the two testing-vessels had become 

 constant, when the ionization currents in A and B were 

 measured by means of an electrometer in exactly the same 

 way as that described above for the experiments on the 

 molecular weight of radium emanation. By means of keys 

 c and d the central rods of A and B could be connected to 

 or disconnected from one pair of quadrants of the electro- 

 meter, so that the saturation ionization current in either 

 could be measured separately. There was no need to allow 

 for the activity excited on the walls of the testing-vessels 

 since the quantity present is proportional to the concentration 

 of the emanation. 



In spite of the fact that the ebonite plugs, used to insulate 

 the central rods, were divided into two parts by metal guard- 

 rings connected to earth, it was found that the rods charged 

 up slightly on disconnecting from earth, although there 

 was no emanation in the testing-vessels. The rate at which 

 they acquired this charge was always measured before 



