Chemistry and Physics. 325 



paths of gaseous ions which are constrained to pass through an 

 uncharged gas, and by interpreting the data in terms of the mean 

 free path of the molecules of the electrically neutral gas through 

 which the ions are sent. In order to make the interpretation 

 legitimate two conditions must be fulfilled. In the first place 

 the experimental conditions must be so arranged as to prevent 

 the ions from becoming complex by the process of "loading" with 

 neutral molecules. The second condition requires the influence 

 of the electric charge on the number of collisions, (that is, upon 

 the free path), to be negligible. By measuring the^rs^ free path 

 which an ion describes after its genesis the first condition is satis- 

 fied, for loading can only occur when an effective collision with 

 a molecule takes place. Although the apparatus partially described 

 below is so designed as to make the determination of the first 

 uninterrupted ionic path possible, the investigators have made 

 their method doubly sure by using pressures lying between 0*03 mm 

 and 0"14 mm of mercury, since B. Todd has shown experimentally 

 that complex ions are not formed at these pressures. Again, by 

 employing ions of sufficiently high speed the influence of the 

 charge on the mean free path of the neutral molecules can be 

 made smaller than the remaining possible experimental errors. 



The novelty of the apparatus consists essentially in using two 

 parallel-plate condensers which are maintained accurately parallel 

 to each other and which can be set at different distances apart. 

 A small hole was made in the center of the top plate of the upper 

 condenser and in this hole a small platinum spiral coated with 

 aluminium phosphate was so adjusted as to be flush with the lower 

 surface of the upper plate of the condenser. When the spiral was 

 caused to glow by the passage through it of a suitable electric 

 current, the coating became a sufficiently steady source of ions. 

 The lower and movable condenser had a guard ring but no hole; 

 otherwise it was exactly like the upper condenser, but inverted. 

 The ions were first accelerated by a drop of 40 volts in descend- 

 ing through the upper condenser, then they continued uniformly 

 through the space between the two condensers, and finally they 

 were disposed of by the reverse field of 38 volts in the lower con- 

 denser. Denoting the distance between the condensers by x, it 



was shown that the familiar formula JV=JV e *> applied under 

 the experimental conditions. The probable nature of the ions 

 emitted by the spiral led the investigators to use hydrogen be- 

 tween the condensers. The mean free path, A, was found to be 

 0'157 mm , corresponding to a pressure of l mm of mercury. Assum- 

 ing n = 2*80 X-10 19 , and also that the spiral emits "charged 

 hydrogen molecules," the investigators deduce <r = 2-41 x 10 ~ scm , 

 where o- symbolizes the diameter of a molecule of hydrogen. This 

 result agrees fairly well with the value 2-17 X I0~ 8cm as calcu- 

 lated by Sutherland from the kinetic theory of uncharged gases 

 and on the basis of quite different experimental data. — Verh. d. 

 Deutsch. Phys. Gesellsch., No. 11, 1912. h. s. u. 



