214 SCIENCE PROGRESS 



fractional diffusion through pipeclay. If molecules of different 

 masses exist in a gas, they must be separable, at least partially, 

 by this method ; but unless the masses are very different, it is 

 extremely slow. The two neons only differ by lo per cent., 

 and it was only after many months of tedious work that a 

 positive result was obtained, and a difference of o-y per cent, 

 measured between the two extreme fractions. Further diffu- 

 sion experiments were in progress when the war interrupted 

 the work, for although evidence as to the existence of the 

 isotopes of neon was then available from several different lines 

 of reasoning, and a positive result had been obtained of their 

 partial separation, the latter was considered too small to settle 

 such an important point in a satisfactory manner. 



By the time work was recommenced in 191 9, the " isotope " 

 theory was generally accepted as regards the radio-active ele- 

 ments, and a good deal of interesting theoretical speculation 

 had been made as to the possibility of it applying to the elements 

 in general ; it was therefore very desirable that the case of 

 neon should be put beyond dispute. As diffusion experiments 

 were at the best extremely slow and laborious, attention was 

 once more turned to the possibilities of positive rays. Thom- 

 son's method of analysis, as previously used in the work, was 

 not capable of very great resolution ; it could separate masses 

 such as the two isotopes of neon, but, as has been already 

 stated, was incapable of distinguishing between 20*0 and 20*2 

 with certainty. If a method could be found of sufficient pre- 

 cision, and it could be demonstrated that the accepted atomic 

 weight of neon lay between the two values given by positive 

 rays, the question would be settled ; this it was found possible 

 to do as follows : 



The " Mass-Spedrograph." — The principle of this instru- 

 ment is indicated in Fig. i. The positive rays, which are 

 charged atoms and molecules of matter moving with high 

 velocities, are generated in the usual manner in a large discharge 

 tube similar to an X-ray bulb, and are allowed to pass through 

 an aperture in the cathode. In Thomson's apparatus the 

 aperture consisted of a long fine tube giving a narrow circular 

 beam or string of rays ; in the new arrangement, two very fine 

 slits Si, S2 are employed, giving a thin ribbon of rays. These 

 slits are made of aluminium by a special method. In the 

 present apparatus they are i mm. long by -05 mm. wide, and 

 mounted about 100 mm. apart, so that the beam produced is 

 exceedingly narrow and would, if undeflected, make a fine 

 sharp line on the recording plate. The space between the slits 

 is exhausted as highly as possible, as also is the " camera " in 

 which the analysis takes place. 



The beam of rays produced then passes between the two 



