On the ol Particles of Radium. 319 



The results also show that the four groups are alike in all 

 respects save that of initial speed ; and farther, that the 

 a particle spends its energy at a rate which is approximately 

 inversely proportional to the square root of its speed. 



3. Determinations of the loss of range of ol particles in 

 consequence of their passage through various substances, 

 from which it appears that for all the materials examined the 

 loss in traversing any atom is nearly proportional to the 

 square root of the weight of that atom. The loss in the case 

 of a complex molecule is proportional to the sum of the 

 square roots of the weights of the constituent atoms. 



4. Discussion of these results. 



§i. 



In the apparatus which we first used for these experiments 

 we found that our measurements were liable to considerable 

 irregularities. The most troublesome of these was an 

 occasional sudden and violent increase of the ionization 

 current which would cause the electrometer spot to fly off 

 the scale. This would occur in the middle of a period of 

 satisfactory working. It appeared to be due to the liberation 

 of emanation from the radium under observation, and we 

 have therefore enclosed the whole of our apparatus in a metal 

 vessel through which a current of dry air is made to pass 

 continually. 



In other respects the apparatus now used differs but little 

 from that described on p. 728 of the Phil. Mag., Dec. 1904 : 

 except that we have added an arrangement by which an 

 aluminium screen can be drawn over the radium so as to cut 

 off all ol radiation, and we have made it possible to set the 

 ionization-chamber at any desired depth. Both these arrange- 

 ments can be worked from the outside of the enclosing vessel. 

 It is necessary that the air which is drawn through should be 

 fairly dry. In this country a difference of 30° F. between 

 the wet and dry bulb thermometers is not unusual, and at 

 these times artificial drying is hardly necessary ; but we find 

 it more satisfactory to draw the air at all times through a set 

 of large calcium-chloride tubes, the current passing day and 

 night during the progress of any series of experiments. The 

 need for dryness appears to arise from the fact that moist 

 air acts on the thin film of radium which we employ and 

 liberates emanation. 



At one time we endeavoured to secure the complete removal 

 of any liberated emanation by passing the whole stream of 

 air directly over the radium just before it left the vessel ; 



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