214 THE ROYAL SOCIETY OF CANADA 



NN' was a wet string short circuiting the outsides of the jars and S 

 represents the discharge tubes in which the secondary spark occurred. 

 In taking the photographs the string joined to T, Fig. 3, was cut and 

 the valve B was opened by the falHng weight W. When this weight 

 reached the Hmit of its fall a second weight in the form of a brass ball 

 and supported by a slender cord broke away and fell 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. 



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 sq. mm. This 

 plate of copper was attached to a short stout copper wire which pro- 

 jected into the cloud chamber and was held in position by being fast- 

 ened with wax to the stopper K, Fig. 2. When hydrogen was used 

 the ionisation tracks in most cases extended completely across the 

 cloud chamber. In the present investigation, however, it was the 

 ends of the trails which were to be specially examined and so the 

 tracks in hydrogen were cut down by covering the layer of polonium 

 with sheets of very thin aluminium leaf. This was done in all the 

 experiments in which the photographs of tracks in hydrogen were taken 

 which are described in this paper. The lengths of the reduced tracks 

 were generally about 4 cms. or less. 



III. Experiments. 



Although the investigation was primarily directed to obtaining 

 if possible evidence of the production and of the ionising power 

 of the "H" particles of hydrogen and many photographs were taken 

 with that end in view, not one of the photographs showed any trace 

 of cloud tracks pointing to their production or to ionisation by them. 

 Many of the tracks shewed abrupt bends similar to those obtained by 

 Wilson and occasionally very short spurs were obtained at these 

 bends, but no spurs were obtained such as one should expect to get 

 with "H" particles travelling with velocities such as those Marsden 

 found they possessed. 



This absence of "H" particle cloud tracks in our experiments 

 cannot be taken, however, to mean that "H" particles are not produced 

 in hydrogen or that they do not possess the power to ionise a gas but 

 it goes to confirm, rather, what has been already surmised that when 



