Tracks of Alpha Rays in Hydrogen. 679 



between the terminals at P, and brought on both the primary 

 spark at P and the secondary discharge at S. The height 

 of the brass ball from the spark-gap, P, was adjusted so as 

 to bring on the discharge at the instant desired after the 

 •expansion took place. 



Fig. 3. 



TO PUMP AND 



In the experiments carried out by us the cloud-chamber 

 above the plunger was filled either with air or hydrogen or 

 with mixtures of air and hydrogen, and the cloud tracks 

 were formed by the condensation of either water vapour or 

 the vapour of absolute alcohol. 



The source of the alpha rays was a layer of polonium on 

 the anterior face of a small sheet of copper of area about 

 1 s(j. mm. The plate of copper was attached to a short stout 

 copper wire which projected into the cloud-chamber, and 

 was held in position by being fastened with wax to the 



