
44 IONIZATION THEORY OF GASES [CH. 
which act as nuclei for the condensation of water around them. 
The experiments of R. von Helmholtz and Richarz had shown that 
chemical reactions, for example the combustion of flames, taking 
place in the neighbourhood, affected the condensation of a steam- 
jet. Lenard showed that a similar action was produced when ultra- 
violet light fell on a negatively charged zinc surface placed near 
the steam-jet. These results suggested that the presence of electric 
charges in the gas facilitated condensation. 
A very complete study of the conditions of condensation of 
water on nuclei has been made by C. T. R. Wilson?.- An apparatus 
was constructed which allowed a very sudden expansion of the air 
over a wide range of pressure. The amount of condensation was 
observed in a small glass vessel. A beam of light was passed 
into the apparatus which allowed the drops formed to be readily 
observed by the eye. 
Preliminary small expansions caused a condensation of the 
water round the dust nuclei present in the air. These dust nuclei 
were removed by allowing the drops to settle. After a number of 
successive small expansions, the air was completely freed from 
dust, so that no condensation was produced. 
Let v, = initial volume of the gas in the vessel, 
v. = volume after expansion. 
Vp nied : 
If —<1-25 no condensation is produced in dust-free air. If 
1 
Vo s , ’ sl? 
however — > 1:25 and < 1°38, a few drops appear. This number is 
2, 
roughly constant until 2 = 1°38, when the number suddenly in- 
creases and a very dense cloud of fine drops is produced. 
If the radiation from an X ray tube or a radio-active substance 
is now passed into the condensation vessel, a new series of phenomena 
is observed. As before, if < 1:25 no drops are formed, but if 
1 
“ _ 1-25 there is a sudden production of a cloud. The water drops 
1 
of which this cloud is formed are finer and more numerous the 
1 Wied. Annal. 40, p. 161, 1890, 
* Phil. Trans. p. 265, 1897; p. 403, 1899; p. 289, 1900. 
