CONDENSATION NUCLEI. 203 



OTHER PROPEKTIES OF THE IONS. 



There is no room for doubt that the nuclei jti-ochiced by X rays 

 and siniihir aii'ents. and re(|uirin<^' a fourfold oi- sixfohl siipersatura- 

 tion to make water condense on them, aic neiiat i\('ly or positively 

 charged ions. Wv know hy other methods of stiidyiii<>; them a great 

 deal about the properties of ions, their \'elocity in an electric field, 

 their diii'usion constants and rates of recombination umler dift'erent 

 conditions. Their l)ehavior when studied l)y condensation has been 

 entirely in agreement with the results obtained by other methods; 

 for example, the I'apidity with which their luimbci- diminishes after 

 the source of ionization has been cut off. 



NUCLEI SIJMILAR IN EFFICIENCY TO THE TONS, BUT NOT REMOVABLE BY AN 



ELECTRICAL FIELD. 



Moist air exposed to weak ultraviolet light is found to contain 

 a ])lentiful supj)ly of mu-lei, which require a degree of supei'satura- 

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

 form upon them. Yet even very strong electric Helds aj)pear to be 

 without ertect in reducing the number of droj)s formed on expansion. 

 C^M'tain metals also produce in tlie air in contact with them similar 

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

 considerable density, unless the expansion is great enougii to cause 

 condensation on positive ions. It is })ossil)le that we have in both 

 these cases ions pi-oduced as a result of the expansion, there being, 

 therefore, no time for the ions to be removed by the Held befoiv the 

 cloud is formed. 



NUCLEI MORE EFFECTIVE IN PROMOTING CONDENSATION THAN THE IONS 



PRODUCED BY X RAYS. 



If we expose moist aii- to ultraviolet of moderate intensity, the 

 result is not so simi)K' as when the intensity is wvy small. Nuclei 

 are produced which ap]K'ar to grow under the action of the light, 

 the expansion refpiired to produce a cloud becoming less than that 

 required by tlie negative ions, and becoming less and less the stronger 

 the light and the longer the ex])osure. For a gi\-en intensity of the 

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

 cease to grow. A very moderate intensity is sullicient to ])roduce 

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

 the growth is very rapid, so that tlie earlier stages are (Hflicult to fol- 

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

 the nuclei become visible in suitable illumination, and we get a cloud 

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

 that the growth of these nuclei into visil)le drops is to be attributed 

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



