132 PHYSICAL SCIENCE 



The presence of gaseous ions may be inferred 

 from the phenomena of current conduction through 

 the gases, but the existence of charged particles 

 of greater than molecular dimensions has been 

 demonstrated directly by Mr C. T. R. Wilson in 

 a very striking manner. Long ago Aitken showed 

 that the condensation of drops of water from air 

 saturated with aqueous vapourwas much helped by 

 the presence of particles of dust ; in the absence 

 of dust, considerable supersaturation could be 

 attained before condensation set in. Each particle 

 of dust forms a nucleus, round which collect 

 molecules of water ; and, when the drops have 

 grown to a sufficient size, they fall, carrying 

 down the dust particle also. In this way the 

 air is freed from the presence of dust, and to this 

 action, on a large scale, we must attribute partially 

 the clearness of the atmosphere after a downfall 

 of rain. 



Wilson devised an apparatus whereby air 

 could be subjected to a sudden expansion. By 

 this means it was cooled ; and, if previously 

 saturated with water vapour, any desired degree 

 of supersaturation could be obtained by adjusting 

 the amountof expansion. By repeated expansions, 

 the dust particles were removed, and any further 

 expansion then produced only a few drops of water. 

 If, however, when the air had thus been depleted 

 of possible nuclei, Rontgen rays or other ionizing 

 agency were allowed to act on the gas, instead 

 of these few drops, a dense cloud was once more 

 obtained by the same expansion. This cloud was 

 not formed if the ions were removed previously 

 by an electric field, or by some other means. 



Fig. 28 is a photograph of one of Mr Wilson's 



