290 MR. C. T. R. WILSON ON THE EFFICIENCY AS CONDENSATION 



that ordinary moist air is always to a very slight extent ionised. The number of 

 these nuclei is so small that the absence of any sensible conductivity in air under 

 ordinary conditions is in no way inconsistent with the view that they are ions. In 

 the latter part of this paper are described some attempts which I have made to 

 decide experimentally whether these nuclei are charged or not. 



It is mainly, however, with the second of the above questions that this paper 

 deals. The experiments to be described prove that there is a great difference between 

 positively and negatively charged ions, with respect to their power of serving as 

 nuclei for the condensation of water vapour ; a much smaller degree of supersatura- 

 tion sufficing to cause water to condense on the negative ions than is required in the 

 case of positively charged ions. They therefore furnish a possible explanation of the 

 preponderance of negatively electrified rain,* which is required by theories which 

 attribute the normal positive potential of the air to the action of precipitation. 



I have shown, in a previous paper, t that the ions produced by various agents 

 (X-rays, uranium-rays, negatively charged zinc exposed to ultra-violet light) are 

 identical with respect to the minimum supersaturation required to make water 

 condense on them. They are also identical in this respect, with the few nuclei 

 apparently always present in moist air. In the present investigation I have therefore 

 felt justified in using exclusively the Rontgen rays as being the most convenient 

 ionising agent, and in assuming that the same results would be obtained with ions 

 from other sources. 



To compare the efficiency as condensation nuclei of the positive and negative ions 

 respectively, expansion experiments were made with moist air containing ions all, or 

 nearly all, charged with electricity of one sign, alternately positive and negative in 

 successive experiments. 



To enable a supply of ions nearly all positive or nearly all negative to be produced 

 at will in the air under observation, this was enclosed between two parallel metal 

 plates, and a narrow beam of Rontgen rays was made to pass between the plates 

 parallel to and almost in contact with the surface of one of them. Under these 

 conditions a supply of positive and negative ions is produced in the thin lamina of air 

 exposed to the rays, and when a difference of potential is maintained between the 

 plates, the two sets of ions move in opposite directions, the positive towards the 

 negative plate and vice versd. If we neglect the slight difference in the velocity of 

 positive and negative ions, shown to exist by the experiments of ZELENY,| the 

 number of ions in unit volume of the positive and negative streams will be the same, 

 assuming (an assumption which later experiments justify) that equal numbers of 

 positive and negative ions are produced, and that the ionisation does not, for example, 



* The earlier observations of ELSTER and GEITEL appeared to show a preponderance of negatively 

 electrified rain (' Sitzungsber. d. k. Akad. in Wien.,' 99, HA, p. 421), but this is not shown in their later 

 observations (' Terrestrial Magnetism,' vol. 4, p. 15). 



t ' Phil. Trans.,' A, vol. 192, pp. 403-453, 1899. 



t 'Phil. Mag.,' vol. 46, p. 120, 1898. 







