The rare gases helium and xenon which exist in the atmosphere have been tested and found to be non-radio-active. The radio-activity of the air cannot be ascribed to a slight radio-activity possessed by either of these gases.
281. Amount of the radium emanation in the atmosphere. It is a matter of great interest to form an estimate of the amount of radium emanation present in the atmosphere, for since it comes from the earth, it indirectly serves as a means of estimating the amount of radium which is distributed over a thin crust of the earth.
Some experiments in this direction have been made by Eve in the laboratory of the writer. The experiments are not yet completed but the results so far obtained allow us to calculate the probable amount of emanation per cubic kilometre of the atmosphere near the earth.
Experiments were first made with a large iron tank 154 cms. square and 730 cms. deep, in a building in which no radium or other radio-active material had ever been introduced. The saturation ionization current for the air in the tank was first measured by means of an electroscope, connected with an insulated electrode passing up the centre of the closed tank. Assuming that the ionization in the tank was uniform, the number of ions produced per c.c. of the air in the tank was found to be 10. This is a considerably lower value than has usually been observed in a small closed vessel (see [section 284]). Cooke obtained the value 10 for a well cleaned brass electroscope, surrounded by lead, while Schuster obtained a value about 12 for the air in the laboratory of Owens College, Manchester.
In order to measure the amount of the excited activity from the tank, a central insulated wire was charged negatively to about 10,000 volts by a Wimshurst machine. After two hours, the wire was removed and wound on an insulated frame connected with a gold-leaf electroscope. The rate of decay of the activity on the wire was found to be about the same as for the excited activity produced by the radium emanation. In order to estimate the amount of radium emanation present in the large tank, special experiments were made with a smaller tank in which a known quantity of the radium emanation was introduced by employing a solution of pure radium bromide of known concentration. A central wire was made the negative electrode as before, and, after removal, it was wound on the frame and its activity tested. In this way it was found that the amount of radium emanation present in the large tank, in order to produce the excited activity observed, must have been equal to the equilibrium or maximum amount to be obtained from 9·5 × 10-9 grams of pure radium bromide. The volume of the large tank was 17 cubic metres, so that the amount of emanation present per cubic metre was equivalent to that liberated from 5·6 × 10-10 grams of radium bromide in radio-active equilibrium.
If the amount of the emanation in the tank is taken as the average amount existing in the outside air, the amount of radium emanation present per cubic kilometre of the air is equivalent to that supplied by 0·56 grams of radium bromide.
For the purpose of calculation, suppose the emanation is uniformly distributed over the land portion of the earth (¼ of the total surface), and to extend to an average height of 5 kilometres. The air over the sea is not taken into account as its radio-activity has not been examined. The total amount of emanation present in the atmosphere under these conditions corresponds to that supplied by about 400 tons of radium bromide. In order to maintain this amount of emanation in the atmosphere, it must be supplied at a constant rate from the earth’s surface. Since the greater amount of the emanation probably escapes into the air by transpiration and diffusion through the soil, the emanation cannot reach the surface except from a very thin layer of the earth. The probable thickness of this layer can be estimated if it is assumed that the present loss of heat from the earth is supplied from the radio-active matter contained in it. We have seen (section 271) that, on this hypothesis, there must be an amount of active matter in the earth corresponding to about 300 million tons of radium. If this is supposed to be uniformly distributed, a thickness of layer of about 13 metres will suffice to maintain the calculated amount of emanation in the atmosphere. This thickness of layer is about the order of magnitude to be expected from general considerations.
These results lead indirectly to the conclusion that a large amount of emanation does undoubtedly exist in the surface crust of the earth.
Experiments were also made by Eve with a large zinc cylinder exposed in the open air. Volume for volume, the average amount of excited activity derived from it was only about one-third of that obtained from the large iron tank. This would reduce the amount of emanation, previously deduced, to about one-third.
Before such calculations can be considered at all definite, it will be necessary to make comparative measurements of the amount of emanation in the atmosphere at various parts of the earth. The air at Montreal is not abnormally active, so that the calculations probably give the right order of magnitude of the quantities.