CONDENSATION NUCLEI. 203 



OTHKH rHorKlM'ir.S <»1- I'lIK IONS. 



There is no room for donht (lint the nuclei produced b}' X rays 

 and similar agents, and reciuiiMug a fourfold or sixfold supersatura- 

 (ioii to make water condense on them, are negatively or j)()sitively 

 charged ions. We know by other methods of studying them a great 

 deal about the pi'operties of ions, their velocity in an electric field, 

 their difi'usion constants and ralt's of recombination under dilfei'enl 

 conditions. Their behavior when studied by condensation has been 

 entirely in agreement with the i-esults obtain<'d by other methods; 

 for example, the rapidity with which their number diminishes after 

 the source of ionization has been cut ofl'. 



NUCLKl Si:\IlLAH IN EFFIC'IENrY TO THE IONS, BT T NOT REIMOVABLE BY AN 



ELECTRICAL FIELD. 



Moist air exposed to weak ultraviolet light is found to contain 

 a plentiful supply of nuclei, which require a degree of supersatura- 

 tion approximately the same as do the ions, in order that a cloud may 

 form upon them. Yet even very strong electric fields appear to be 

 without effect in reducing the number of drops formed on expansion. 

 Certain metals also produce in the air in contact with them similar 

 nuclei, the clouds in this case, however, not generally attaining any 

 considerable density, unless the expansion is great enough to cause 

 condensation on })ositive ions. It is possible that we have in both 

 these cases ions produced as a result of the expansion, there being, 

 therefore, no time for the ions to be removed by the field before the 

 cloud is formed. 



NUCLEI MORE EFFECTIVE IN PROMOTING CONDENSATION THAN THE IONS 



PRODUCED BY X RAYS. 



If we expose moist air to ultraviolet of moderate intensity, the 

 result is not so simple as when the intensity is very small. Nuclei 

 are produced which appear to grow under the action of the light, 

 the expansion required to produce a cloud becoming less than that 

 required by the negative ions, and becoining less and less the stronger 

 the light and the longer the ex])osure. For a given intensity of the 

 light, there appears to be a maximum size beyond which the nuclei 

 cease to grow. A ver}-^ moderate intensity is sufficient to produce 

 nuclei which grow till the slightest expansion will form a cloud, and 

 the growth is very rapid, so that the earlier stages are difficult to fol- 

 low. With very intense ultraviolet light, the growth continues till 

 the nuclei become visible in suital)le illumination, and we get a cloud 

 without expansion, even in unsaturated air. There can be little doubt 

 that the growth of these nuclei into visible drops is to be attributed 

 to the formation of some substance in solution within them. Vincent 



