50 



SCIENCE PROGRESS 



from a flame can be split up by a prism into a spectrum showing 

 the chemical constitution of that flame, so positive rays emerging 

 from a perforated cathode can be resolved, in like manner, so 

 that the several constituents of the gas in the discharge tube 

 become obvious. 



In order to apply the method to a gas, its particles undergo 

 the following operations : 



(i) they are given a definite charge of electricity ; 



(2) they have a high velocity impressed upon them in a 

 definite direction ; 



(3) they are allowed to pass through an electric and a 

 magnetic field ; 



(4) finally they fall upon a fluorescent screen, a photographic 



Fig. i. 



plate or some other suitable arrangement capable of recording 

 the exact positions of the impacts. 



Fortunately the first two conditions are fulfilled at the same 

 time and automatically by submitting the gas to a high-tension 

 electric discharge at low pressure. The gas is "ionised" by this 

 treatment and the positive ions are projected with prodigious 

 velocity towards the cathode ; if this be pierced with a small 

 hole, so as to allow of their free passage, they will emerge on 

 the other side as a stream of positively charged particles which 

 may then be acted upon by the analysing fields. 



It will be as well now to describe the particular form of 

 apparatus which has been found to give the most satisfactory 

 results. The main features are shown in the accompanying 

 diagram (fig. 1). The discharge tube A, which is very similar 



