LATEST DEVELOPMENTS WITH TILE X RAYS. 661 



would be the case if it were made of platinum. It is also light, 

 and can be easily fixed to a platinum w^ire. Among the important 

 modifications of the tube are those which enable the operator to 

 control the degree of vacuum in the tube. This is accomplished 

 by sealing to the main tube an appendage containing certain chem- 

 icals which, on being heated, give off a small amount of vapor, and 

 which take it up again on cooling. This modification is made neces- 

 sary by the singular fact that after a Crooke's tube is submitted to 

 an electrical discharge for some time the vacuum becomes more 

 and more complete, and a higher and higher electro-motive force 

 or pressure is needed to produce the discharge in the tube. It 

 prefers in time to jump over the surface. Thus, at the very begin- 

 ning of our use of the X rays we meet with a mystery. Where do 



Fig. 1. — The evolution of the Crooke's tube. 



the remaining particles of air go? It is surmised that they disap- 

 pear in the platinum terminals. 



The manufacture of the X-ray tubes tests technical skill and 

 the patience of the experimenter more highly, perhaps, than the 

 preparation of any apparatus used in science. Glass working is a 

 difiicult art, and requires an absolute devotion to it. There is only 

 one metal known which will enable an electrical discharge to pass 

 into and out of a rarefied space inclosed by glass. This is plati- 

 num. A wire of this metal can be sealed into glass so that no air 

 can leak into an exhausted space around the joints. All electric 

 lamps, so commonly used in electric lighting, have little wires of 

 platinum at their bases, by means of which the electric current 

 enters and leaves the bulb. The Crooke's tube is in principle an 

 Edison lamp with the filament broken. The maker of Crooke's 

 tubes should complete the "making of the tube at one sitting, for 



