| Material of vessel | Number of ions produced per c.c. per second | Observer |
|---|---|---|
| Silvered glass | 36 | C. T. R. Wilson |
| Brass | 26 | „ „ |
| Zinc | 27 | Rutherford and Allan |
| Glass | 53 to 63 | Harms |
| Iron | 61 | Patterson |
| Cleaned brass | 10 | Cooke |
It will be shown later that the differences in these results are probably due to differences in the radio-activity of the containing vessel.
285. Effect of pressure and nature of gas. C. T. R. Wilson (loc. cit.) found that the rate of leakage of a charged conductor varied approximately as the pressure of the air between the pressures examined, viz. 43 mms. and 743 mms. of mercury. These results point to the conclusion that, in a good vacuum, a charged body would lose its charge extremely slowly. This is in agreement with an observation of Crookes, who found that a pair of gold-leaves retained their charge for several months in a high vacuum.
Wilson[[444]] at a later date investigated the leakage for different gases. The results are included in the following table, where the ionization produced in air is taken as unity:
| Gas | Relative ionization | (Relative ionization) / (density) |
|---|---|---|
| Air | 1·00 | 1·00 |
| Hydrogen | 0·184 | 2·7 |
| Carbon dioxide | 1·69 | 1·10 |
| Sulphur dioxide | 2·64 | 1·21 |
| Chloroform | 4·7 | 1·09 |
With the exception of hydrogen, the ionization produced in different gases is approximately proportional to their density. The relative ionization is very similar to that observed by Strutt ([section 45]) for gases exposed to the influence of the α and β rays from radio-active substances, and points to the conclusion that the ionization observed may be due either to a radiation from the walls of the vessel or from external sources.
Jaffé[[445]] has made a careful examination of the natural ionization in the very heavy gas nickel-carbonyl, Ni(CO)4, in a small silvered glass vessel. The ionization of this gas was 5·1 times that of air at normal pressure while its density is 5·9 times that of air. The leak of the electroscope was nearly proportional to the pressures except at low pressure, when the leak was somewhat greater than would be expected if the pressure law held. The fact that a gas of such high density and complicated structure behaves like the simpler and lighter gases is a strong indication that the ionization itself is due to a radiation from the walls of the vessel and not to a spontaneous ionization of the gas.
Patterson[[446]] examined the variation of the ionization of air with pressure in a large iron vessel of diameter 30 cms. and length 20 cms. The current between a central electrode and the cylinder was measured by means of a sensitive Dolezalek electrometer. He found that the saturation current was practically independent of the pressure for pressures greater than 300 mms. of mercury. Below a pressure of 80 mms. the current varied directly as the pressure. For air at atmospheric pressure, the current was independent of the temperature up to 450° C. With further increase of temperature, the current began to increase, and the increase was more rapid when the central electrode was charged negatively than when it was charged positively. This difference was ascribed to the production of positive ions at the surface of the iron vessel. The results obtained by Patterson render it very improbable that the ionization observed in air is due to a spontaneous ionization of the enclosed air: for we should expect the amount of this ionization to depend on the temperature of the gas. On the other hand, these results are to be expected if the ionization of the enclosed air is mainly due to an easily absorbed radiation from the walls of the vessel. If this radiation had a penetrating power about equal to that observed for the α rays of the radio-elements, the radiation would be absorbed in a few centimetres of air. With diminution of pressure, the radiations would traverse a greater distance of air before complete absorption, but the total ionization produced by the rays would still remain about the same, until the pressure was reduced sufficiently to allow the radiation to traverse the air space in the vessel without complete absorption. With still further diminution of pressure, the total ionization produced by the radiation, and in consequence the current observed, would vary directly as the pressure.
286. Examination of ordinary matter for radio-activity. Strutt[[447]], McLennan and Burton[[448]], and Cooke[[449]], independently observed about the same time that ordinary matter is radio-active to a slight degree. Strutt, by means of an electroscope, observed that the ionization produced in a closed vessel varied with the material of the vessel. A glass vessel with a removable base was employed and the vessel was lined with the material to be examined. The following table shows the relative results obtained. The amount of leakage observed is expressed in terms of the number of scale divisions of the eye-piece passed over per hour by the gold-leaf:
| Material of lining of vessel | Leakage in scale divisions per hour |
|---|---|
| Tinfoil | 3·3 |
| „ another sample | 2·3 |
| Glass coated with phosphoric acid | 1·3 |
| Silver chemically deposited on glass | 1·6 |
| Zinc | 1·2 |
| Lead | 2·2 |
| Copper (clean) | 2·3 |
| „ (oxidized) | 1·7 |
| Platinum (various samples) | 2·0, 2·9, 3·9 |
| Aluminium | 1·4 |