The following table shows the comparative values obtained for air and carbon dioxide.
| Air K1 | Air K2 | Air K2/K1 | CO2 K1 | CO2 K2 | CO2 K2/K1 | |
|---|---|---|---|---|---|---|
| Direct method (Langevin) | 1·40 | 1·70 | 1·22 | 0·86 | 0·90 | 1·05 |
| Current of gas (Zeleny) | 1·36 | 1·87 | 1·375 | 0·76 | 0·81 | 1·07 |
These results show that for all gases except CO2, there is a marked increase in the velocity of the negative ion with the dryness of the gas, and that, even in moist gases, the velocity of the negative ions is always greater than that of the positive ions. The velocity of the positive ion is not much affected by the presence of moisture in the gas.
The velocity of the ions varies inversely as the pressure of the gas. This has been shown by Rutherford[[63]] for the negative ions produced by ultra-violet light falling on a negatively charged surface, and later by Langevin[[64]] for both the positive and negative ions produced by Röntgen rays. Langevin has shown that the velocity of the positive ion increases more slowly with the diminution of pressure than that of the negative ion. It appears as if the negative ion, especially at pressures of about 10 mm. of mercury, begins to diminish in size.
34. Condensation experiments. Some experiments will now be described which have verified in a direct way the theory that the conductivity produced in gases by the various types of radiation is due to the production of charged ions throughout the volume of the gas. Under certain conditions, the ions form nuclei for the condensation of water, and this property allows us to show the presence of the individual ions in the gas, and also to count the number present.
It has long been known that, if air saturated with water-vapour be suddenly expanded, a cloud of small globules of water is formed. These drops are formed round the dust particles present in the gas, which act as nuclei for the condensation of water around them. The experiments of R. von Helmholtz and Richarz[[65]] 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[[66]]. 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.