204 CONDENSATION NUCLEI. 



has recentl}^ studied these visible nuclei and found some of the 

 particles to be j)()sitively, some negatively charged, and others neu- 

 tral; but he finds the evidence to be in favor of the view that the 

 charges are, as it Avere, accidental, being simply due to ions which have 

 come in contact with them. Lenard had previously shown the ioniza- 

 tion of the air by these rays. 



The very small nuclei — i. e., those which require large expansions 

 to make drops form upon them — dilfuse rapidly to the sides of the 

 vessel, so that a fog is not formed if the radiation ])e cut ofi' even one 

 minute before the expansion is made. The nuclei which are large 

 enough to be visil)le may persist for hours on account of their very 

 slow diffusion. 



Other nuclei Avhicli, like those produced b}' ultraviolet, light, vary 

 in size with varying conditions are those produced by heating a wire, 

 studied some time ago by Aitken and recently by Ow^en. The latter 

 has shown that the loAver the temperature at which they have been 

 given otf by the wire the greater is the expansion required to catch 

 them. They can be detected when the wire has been raised to a tem- 

 perature of less than iM)° C. in air. The nuclei ])roduced b}' the slow 

 oxidation of phosphorus, like those formed by the action of strong 

 ultraviolet light, form visible clouds in air which is not supersatu- 

 rat^ed. These clouds have been studied by r)arus and others. As in 

 the cases just considered, the production of the nuclei is associated 

 with the acquisition of conducting power by the gas. There has been 

 a considerable amount of controversy as to the nature of the conduc- 

 tion of electricity in air which has passed over phosphorus. The 

 experiments of Bloch " have, however, proved, from the nature of the 

 curve obtained for the relation between current and potential differ- 

 ence, that we have here a true case of ionization. Ilis measurements 

 of the velocity of the ions showed that they have a very small mobil- 

 ity as compared with the ions due to X rajrs. His experiments leave 

 little room for doubt that these slow-moving ions are identical Avith 

 tlu^ nuclei. The mobility is about a thousand times as small as that 

 of the ions formed by X rays. 



Certain exj)eriments of Ihirms.'' and of Elster and Geitel,'' appear 

 to show that by the oxidation of phosphorus free ions are produced 

 in addition to the visible cloud particles. These we should expect to 

 [)(' ra|ndly removed by diffusion and recombination, and, after pass- 

 ing through any considerable length of tubing, we should expect only 

 the loaded ions to persist. The absence of unloaded ions in Bloch's 

 experiments is perhaps to be explained in this way. 



o Bloch, loc. cit. 



i> nanus, IMiys. Zoit., 1st May, 1903. 



c Elster and Geitel, Phys. Zeit, loth May, 1903. 



