president's address SECTION A. 39 



ions, on account of their small mobility, whereas the small ions dis- 

 appear in less than a second, so the nuclei for the drops formed with 

 expansions below 1'27 are small, not large ions. To test whether the 

 large ions become visible at a lower humidity than that at which the 

 small ones appear, Mr. E. P. Norman, at the Sydney University 

 Laboratoiy, has repeated. Mr. Wilson's experiments on the super- 

 saturation required for condensation,* with natural air over mercury. 

 Commencing -with a humidity between 60% and 70%, after removing 

 the " dust," no condensation occux-s, not only below saturation, but 

 not until the supersaturation becomes foiu'-fold, as in the eai"lier 

 experiments over Avater. In all our experiments the observations have 

 been repeated with air which had remained undisturbed in the appa- 

 ratus over night, in order that time might be available for the 

 reproduction of the large ions if they had been initially withdrawn, 

 but the re^sults of the first expansion in the mornings appeared in no 

 case diiferent to those of the later ones. Now Mr. Lusby finds, using 

 two Zelenj tubes in series, joined by earthed piping whose length can 

 be varied, that if all the large ions are removed from a stream of air 

 by the first tul:)e, they are fully reproduced in numl>er in about 

 twenty-tvv'o minutes. Our failure to detect the large ions is not, there- 

 fore, because they were removed with the " dust," unless, indeed, large 

 ions are not produced in closed A-essels, a matter which it would be 

 difficult to determine. 



Considering that in natural air the large ions are fifty times 

 moi-e numerous than the small ones, it is hard to reconcile the fact 

 that the separate existence of the former has never been suspected in 

 condensation experiments with the idea of the large ion as represent- 

 ing a stage in the growth of the small one to a condition of visibility, 

 and the experimental evidence as to the position of the large ion in 

 this connection seems as yet in an unsatisfactoiy state. 



MM. Langevin and Moulinf describe the small and the large ions 

 as playing different parts in the formation of natural clouds, but the 

 statement is merely one of suggestion. 



A^ all the ions have the same charge, the electrical state of the 

 atmosphere is conditioned by the numbers of the ions of each class 

 which exist at the time. Should the numbers of positives and neg-a- 

 tives be equal the air is electrically neutral ; if, however, one kind 

 greatly outnumbers the other the air is thereby highly electrified. 



The number per cubic centimetre, or the specific nuniber as it 

 may be called, of each class of ions in the air is an extremely variable 

 quantity, particularly in the day time. From measurements in other 

 parts of the world it is considered that the specific number of the 

 small ions varies between 500 and several thousands. Between this 

 ' estimate and that given by my own experience there is an amazing 

 discrepancy. In a series of 128 observations, taken at Sydney in the 

 early part of the year 1907, the maximum specific number is 157, the 

 minimum zero, the mean number for the positives being 39, and that 

 for the negatives 38. The European detenninations are based on 

 obsei-vations taken with Dr., Ebert's well-known ion counter, the 

 principle of tlie apparatus being that of the Zeleny tube. With our 



* Wilson. Phil. Trans. A., 189, p. 265, 1897. 

 t Langexin and Moulin loc. cit. 



