and its Products of Transformation. 835 



emitted from each part was counted by the scintillation 

 method. In the tube of shape shown in fig. 4 the mica 

 plate was removed. The open tube was fixed centrally in a 

 glass tube (as in fig. 5), at one end of which was placed a 



Fi*r. 5. 



B. Zinc-sulphide screen. A. Microscope. 



piece of zinc sulphide screen. The tube was so arranged 

 that the a particles from each part of it could fall on the 

 screen. The apparatus was exhausted to a low pressure. 

 The number of scintillations which fell per minute on a 

 definite small area of the screen was counted by a low-power 

 microscope in the usual way. The diameter of the circular 

 area of the screen in the field of the microscope was 1*05 mm. 

 and its area -0086b* sq. cm. The part of the screen used for 

 the counting experiment had been previously calibrated by 

 Dr. H. Geiger, who found that the number of scintillations 

 observed was on the average 10 per cent, less than the 

 number of a particles falling upon it. To correct for this, 

 the number of scintillations observed is in all cases increased 

 by 10 per cent. As the emanation tubes were somewhat 

 irregular in shape the following method was adopted in 

 deducing the total number of a particles emitted per second 

 by the polonium in the tube. Suppose the tube to be divided 

 into a number of rings of equal length. As a first approxi- 

 mation, it may be supposed that the amount of polonium on 

 the walls of the ring is proportional to the volume included 

 in the ring. This" should evidently be the case, for the 

 deposit from the emanation should be proportional to the 

 amount of emanation initially contained in that part. Since 

 the diameters of the rings are in all cases small compared 

 with the distance from the screen, the polonium may be 

 supposed concentrated at the centre of each ring. Let 

 v u r. 2 , ... be the volumes of the rings, </ 1? d 2 ... the distances 

 of the middle parts from the screen, then 



cf-dS + d.^ 



where v is the total volume of the tube, d the mean distance, 

 to be determined on the assumption that all the polonium is 

 concentrated at a point. If 0=1, then t^, t* 2 , ... are the 



