84 DETERMINATION OF THE MASS [CH. vm. 



Wilson and Metrical Cloud Condensation. 



Mr. C. T. K. Wilson,* in his study at the Cavendish 

 Laboratory of cloud formation under the influence 

 of Rontgen rays and other agents, devised a plan 

 for precipitating a definite and known quantity of 

 aqueous vapour in a visible form. This was done by 

 an arrangement for making a sudden or adiabatic 

 expansion of saturated air, and making it to a 

 carefully measured amount. The apparatus employed 

 is shown in Fig. 12. 



One test-tube moving inside another is employed 

 as a piston, and by a certain arrangement the piston 

 was enabled to drop with great suddenness and thus to 

 produce a measured small exhaustion and consequent 

 cooling in the reservoir containing the gas under 

 experiment ; saturated as it is with vapour, and 

 supplied with electric nuclei. The mist at once 

 formed, and the drops began to fall slowly, as 

 usual. Mr. Wilson tried to get an estimate of 

 their size from the colours, but it was difficult and 

 unsatisfactory. If the size had been known, their 

 number would have been known too, because the 

 measured amount of expansion had produced a 

 known fall of temperature below the dew point, 

 and so had condensed a known amount of aqueous 

 vapour, which would be distributed equally among 

 all the equal globules. 



It occurred to J. J. Thomson that a better estimate 

 of size could be made by observing their rate of 

 falling, which is a thing not difficult to observe since 

 they all fall together, being all of the same size. In 

 any mist formed in a bell-jar it is easy to watch it 

 settling down, by watching its fairly definite upper 



*Phil Trans., A, 1897, vol. 189, p. 265. 



