Experiments with JRontgen's Rays. 455 



influence of the kathode-discharge — gave quite as intense if 

 not more intense radiation than a " focus "-tube made in our 

 laboratory, which appeared to act perfectly, so far as we 

 could judge. 



Experiment to test Hypothesis 1. 



When a vacuum-tube is prepared with electrodes of alu- 

 minium-wire whose ends are about 1 centimetre apart, and 

 exhausted until the discharge will rather jump across three or 

 four centimetres of air between balls of 1*5 cm. diameter 

 than pass in the tube, it is generally noticed that the dis- 

 charge, when forced to pass by the tube, goes rather more easily 

 in one direction than the other. By placing a spark-gap 

 with spherical electrodes in parallel with the exhausted tube 

 and properly adjusting the distance of the balls from one 

 another, it is easy to so arrange matters that the sparks pass 

 mostly by the spark-gap when the current is in one direction 

 and by the tube when it is reversed. An arrangement of 

 this kind is exceedingly sensitive to small changes of pressure 

 in the exhausted tube. In the experiment to be described, the 

 spark-gap was generally so adjusted that when the coil-com- 

 mutator was in one position the whole of the discharge 

 passed over the gap — only the faintest glow being discernible 

 in the tube in a dark room. When the current was reversed, 

 however, the discharge was about equally divided between 

 the gap and the tube. No very delicate adjustment of the gap 

 seems to be necessary, at all events when the discharges follow 

 each other rapidly. 



Having thus obtained a means of testing the vacuum in a 

 discharge-tube more rapidly and probably much more deli- 

 cately than by any kind of gauge, we thought it worth 

 while to try whether Rontgen's rays would project particles 

 into the exhausted tube. If hypothesis (1) be correct, 

 then particles must be carried into the exhausted tube if 

 it is thin enough to be transparent; and if in addition it 

 contains a piece of platinum-foil which stops the radiation, 

 the particles would also be stopped ; also if the particles are 

 not wholly entangled in the platinum, some change in the 

 vacuous state of the tube is to be expected. 



A tube about 12 cm. long and 1'5 cm. in diameter, and 

 having a bulb about 4 cm. in diameter in the middle of its 

 length, was prepared of German glass. It was provided with 

 electrodes fused in from either end, and extending to within 

 1 cm. of each other in the centre of the bulb. A bit of platinum- 

 foil lay in the bulb, and the tube was fused on to a Sprengel 

 pump through about a metre of tubing some millimetres in 



2K2 



