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. Wilson23. 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 v1 = initial volume of the gas in the vessel, v2 = volume after expansion.
If v2/v1 < 1·25 no condensation is produced in dust-free air. If however v2/v1 > 1·25 and < 1·38, a few drops appear. This number is roughly constant until v2/v1 = 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 v2/v1 < 1·25 no drops are formed, but if v2/v1 = 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 greater the intensity of the rays. The point at which condensation begins is very marked, and a slight variation of the amount of expansion causes either a dense cloud or no cloud at all.
It now remains to be shown that the formation of a cloud by the action of the rays is due to the productions of ions in the gas. If the expansion vessel is provided with two parallel plates between which an electric field can be applied, it is