Ionization Tracks of Alpha Rays in Hydrogen. 677 



his exceedingly beautiful ionization tracks of alpha, beta 

 and X-ray tracks in air. The apparatus, which was made 

 by the Cambridge Scientific Instrument Co. under his direc 

 tion, is shown in fig. 1 (PL XIII.), and in diagram in fig. 2. 



Fisr. 2. 



j— TO PUMP- 



j— ♦ ro cauce: 



For complete details of the construction and operation of this 

 apparatus the reader is referred to C. T. R. Wilson's original 

 paper *, or to the descriptive catalogue of the Cambridge 

 Scientific Instrument Co. It will suffice here to say that 

 the cylindrical cloud-chamber A (fig. 2), was about 1G : 5 cm. 

 in^ diameter and 3'4 cm. in height. The walls and roof of 

 this cloud-chamber A were coated inside with gelatine con- 

 taining a trace of copper sulphate, and the plate-glass roof, 

 B, of the piston or plunger, C, was coated with gelatine 

 blackened with ink and containing 2 per cent, of copper 

 sulphate. The expansion was effected by opening the valve 

 E, and so connecting up the air space below the plunger with 



* Luc. cit. 



