which act as nuclei for the condensation of water around them. The experiments of R. von Helmholtz and Richarz[1] 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[2]. 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_{1} = initial volume of the gas in the vessel, v_{2} = volume after expansion.
If v_{2}/v_{1} < 1·25 no condensation is produced in dust-free air. If however v_{2}/v_{1} > 1·25 and < 1·38, a few drops appear. This number is roughly constant until v_{2}/v_{1} = 1·38, when the number suddenly increases 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 v_{2}/v_{1} < 1·25 no drops are formed, but if v_{2}/v_{1} = 1·25 there is a sudden production of a cloud. The water drops of which this cloud is formed are finer and more numerous the
