Cathode Rays and certain Chemical Effects. 219 



temperature of liquid air during the experiments ; this 

 prevented the diffusion of phosphorus vapour into other parts 

 of the apparatus. (To prevent diffusion of the vapour while 

 no experiments were being made the taps T x and T 2 could be 

 kept closed.) 



The orifice o was a small slit parallel to the axis of the 

 solenoid, *7 mm. broad and 2 mm. long. Such a slit ensured 

 the admittance into I of a maximum density of the cathode- 

 ray stream without sacrificing homogeneity of velocity, o 

 could be closed by a shutter S, shown in detail in fig. 1 o. 

 Several attempts were made to adapt an ordinary tap to this 

 purpose, the tap being placed at t. As this tap must 

 necessarily be earthed it was essential that a metal one be 

 used. With a view to preventing any heating of the tap 

 grease by the cathode rays in their passage through the bore, 

 the taps tried were grooved immediately above and immedi- 

 ately below the bore and left ungreased between these 

 grooves. The experiment proved fairly satisfactory ; but no 

 tap was entirely reliable, and unless repeatedly greased soon 

 failed to hold a vacuum. The idea was abandoned and a 

 shutter system substituted. S is a heavy brass slab shaped 

 as in fig. lb, as perfectly plain as possible and polished to a 

 mirror surface with emery and rouge. It was completely in 

 contact with the wall of I at the centre of which is o. Its 

 normal position was such as to cover o completely. W is a 

 tap deeply grooved at the centre so as to allow of its taking 

 a few turns of thread. It operated as a windlass and allowed 

 of S being manipulated from outside the apparatus. S 

 rotated about the peg p and, by shortening or lengthening 

 the thread, o could be exposed or covered at will. Although 

 this system did not, of course, prevent diffusion of gas through 

 o when dealing with ordinary pressures, it was entirely 

 efficient at the low pressures required for cathode-ray work 

 (*050 mm. of mercury). Knudsen* has shown that for such 

 an orifice and at such pressures as we are here concerned 

 with, the ordinary laws of diffusion do not apply. The mean 

 free path of the gas molecule becomes the important factor 

 in the problem, and any diffusion through the orifice is 

 determined only by the chance of its lying directly in the 

 path of any gas molecules. Diffusion, then, becomes very 

 much reduced ; being, as a matter of fact, only proportional 

 to the first power of the pressure difference between the 

 gases on the two sides of the orifice. The experimental 

 application of this principle proved perfectly satisfactory. 



* Knudsen, Ann. der Phys. vol. xxviii. p. 75 et seq. 



Q2 



