204 CONDENSATION NUCLEI. 



has recently studied these visible nuclei and found some of the 

 particles to be positively, 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 simpl}' due to ions which have 

 come in contact wdth 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 — diffuse rapidly to the sides of the 

 vessel, so that a fog is not formed if the radiation be cut oft' even one 

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

 enough to be visible may persist for hours on account of their very 

 sloAV diffusion. 



Other nuclei which, like those produced by ultraviolet light, vary 

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

 studied some time ago b}^ Aitken and recently by Owen. The latter 

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

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

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

 perature of less than 150° C. in air. The nuclei produced by the slow 

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

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

 rated. These clouds have been studied by Barus and others. As in 

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

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

 a considerable amount of contr()^'ersy 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. His measurements 

 of the velocity of the ions showed that they have a ver}^ small mobil- 

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

 little room for doubt that these sloAv-moving ions are identical with 

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

 of the ions formed by X rays. 



Certain experiments of Harms,'' and of Elster and Geitel,'" appear 

 to show tliat by the oxidation of })hospli()rus free ions are i)r()duced 

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

 be rapidly removed by diffusion and reconibinntion, ;nid, after pass- 

 ing thi-ough 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. 



" Bloch. loc. cit. 



!* Harms, Phys. Zeit.. 1st May, 1903. 



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



