80 proceedings of the Royal Tri-ih Academy. 



at certain points: the mobilities calculated from those points being 2*08, 

 1"69, and 1-50. If we take the point where the rise on the lower curve 

 begins, we have also the ion of mobility 1-35. These are the same four ions 

 that the other methods have revealed. 



One point, however, has to be explained. There is a distinct downward 

 trend in some of the curves, whereas the slope in all cases should be upward, 

 the indication of separate classas of ions being the sudden steepening of the 

 slope. The explanation of this effect is quite clear. The source of radiation 

 available at the time these experiments were carried out did not give pure 

 a-radiation ; a certain amount of y-radiation was also present. The ionization 

 was not therefore confined to the space above the gauze. The existence 

 of a general ionization in the space between the gauze and the electrometer- 

 plate would not matter very much if the positive and negative fields were 

 exactly equal and of equal duration. But the upward field is of slightly 

 longer duration than the downward, and therefore the charge on the plate 

 after one alternation will be diminished by an amount proportional to the 

 difference in the times. This difference tends to increase as the times them- 

 selves increase, and it could easily happen that the resulting effect would 

 increase more rapidly with time than the effect due to the ions which have 

 come through the gauze, and have been captured on the lower plate. The 

 result would be a downward trend of the ciuve. It is clear that this is 

 most likely to happen at the earlier stages of the curve when only a relatively 

 small number of the more mobile ions are concerned ; and this is just what 

 is found experimentally. In practice this effect was not altogether a 

 disadvantage, as it helped to show up more clearly where the sudden rises in 

 the curve occurred. But it was necessary to see that it did not occur in 

 excess. If the rocking de\'ice was adjusted so as to give almost exactly 

 equal times for small values of time, the differences that appeared as the 

 periodic time increased were not generally excessive. 



This matter is entered on here because it is necessary to explain the shape 

 of the curves which are given. It would have been easy to have improved 

 the experiments by using a ptu-e source of a- rays, and by improving the 

 contact-making dcAice — a task which would in any case have been necessary. 

 The experiments in this form were, however, discontinued when it was found 

 that a steady soui-ce of sinusoidal E.M.F was available. The latter makes 

 possible experiments over very wide ranges, and is independent of any 

 commutating device. The results obtained by the method just described, 

 while rather scanty, have this special value, that they furnish a set of proofs 

 from a third source of the composite nature of the ordinai-y small ion 

 in air. 



