THOMSON'S METHOD OF CHEMICAL ANALYSIS 51 



to an ordinary X-ray bulb, is a large spherical flask about 

 i£ litres in capacity. Pushed into the neck of the flask and 

 closely fitting it is the cathode B : this is made of aluminium and 

 is so shaped that it presents to the bulb a hemispherical front 

 provided in the centre with a funnel-shaped depression. The 

 long, fine " canal-ray " tube extends from the bottom of this 

 depression. If carefully centred and fixed so that its hemi- 

 spherical head just projects into the bulb, this type of cathode 

 gives a very intense beam of positive rays down its axis, i.e. 

 into the <( canal-ray" tube. The latter has been made in several 

 different ways : as the accuracy of the method depends on the 

 fineness of the emergent beam, it is essential that the tube should 

 be perfectly straight and extremely fine. The best results have 

 been obtained with brass or copper tubes drawn down until their 

 internal diameter was of the order of o'l mm. The fine tubes are 

 most carefully straightened — tested by sighting a bright light 

 through them — and mounted in a thick soft-iron tube (shown 

 black in the diagram), which not only protects them from injury 

 but also effectually shields the rays passing through them from 

 external magnetic fields ; the latter is a very important point, 

 as in so narrow and long a barrel — 80 mm. is a convenient 

 length — the smallest magnetic deflection would be sufficient to 

 drive the particles against the walls of the tube and so pre- 

 vent them from emerging. The cathode is kept cool during the 

 discharge by means of a small water-jacket C. 



The anode of the discharge bulb is an aluminium rod D, 

 which is generally placed for convenience in a side tube. In 

 order to ensure the gas under examination being as nearly pure 

 as possible and also to keep its pressure constant, a steady 

 stream of the gas is allowed to leak through an exceedingly fine 

 glass capillary tube E and after circulating through the 

 apparatus is pumped out at F by a Gaede rotating mercury 

 pump. By varying the speed of the pump and the pressure in 

 the gas-holder communicating with E } the pressure in the 

 discharge tube may be varied at will and maintained at any 

 desired value during considerable lengths of time. The pressure 

 is usually adjusted so that the discharge potential corresponds 

 to a spark-gap between brass balls 1-2 cm. apart in air, i.e. 

 30,000-50,000 volts. Positive ions, i.e. particles of gas carrying 

 a positive charge of electricity, are formed in A by the discharge 

 which is maintained by a large X-ray coil made by Cox. Under 



