926 



SCIENCE 



[N. S. Vol. XXIX. No. 754 



used it takes several minutes to remove all 

 the large ions, on account of their small 

 mobility, whereas the small ions disappear 

 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 Laboratory, has repeated Mr. 

 Wilson's experiments on the supersatura- 

 tion required for condensation,^^ with 

 natural air over mercury. Commencing 

 with a humidity between 60 and 70 per 

 cent., after removing the "dust," no con- 

 densation occurs, not only below satura- 

 tion, but not until the supersaturation 

 becomes four-fold, as in the earlier ex- 

 periments over water. In all our experi- 

 ments the observations have been repeated 

 with air which has remained undisturbed 

 in the apparatus overnight, in order that 

 time might be available for the reproduc- 

 tion of the large ions if they had been 

 initially withdrawn, but the results of the 

 first expansion in the mornings appeared 

 in no case different from those of the later 

 ones. Now Mr. Lusby finds, using two 

 Zeleny 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 tube, they are 

 fully reproduced in number in about 22 

 minutes. Our failure to detect the large 

 ions is not, therefore, because they were 

 removed with the "dust," unless, indeed, 

 large ions are not produced in closed ves- 

 sels, a matter which it would be difficult to 

 determine. 



Considering that in natural air the large 

 ions are fifty times more numerous than 

 the small ones, it is hard to reconcile the 

 fact that the separate existence of the 

 former has never been suspected in conden- 

 sation experiments with the idea of the 

 " Wilson, Phil. Trans., A, 189, p. 2G5, 1897. 



large ion as representing 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 unsatisfac- 

 tory state. 



MM. Langevin and Moulin^^ describe the 

 small and the large ions as playing differ- 

 ent parts in the formation of natural 

 clouds, but the statement is merely one of 

 suggestion. 



As 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 negatives 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 centimeter, or 

 the specific number, as it maj^ 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. Be- 

 tween this estimate and that given by my 

 own experience there is an amazing dis- 

 crepancy. 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 num- 

 ber for the positives being 39 and that for 

 the negatives 38. The European deter- 

 minations are based on observations taken 

 with Dr. Ebert's well-known ion counter, 

 the principle of the apparatus being that 

 of the Zeleny tube. With our present 

 Imowledge of the existence of the inter- 

 mediate ions, it can readily be shown that 

 the inner electrode of the instrument is 

 altogether too long. The apparatus, as 

 ordinarily employed, catches not only the 

 small ions, but a proportion of the others as 



" Langevin and Moulin, loc. cit. 



