Amount of Emanation from Radium and Thorium.
171. It has been shown in section 93 from experimental data that 1 gram of radium bromide at its minimum activity emits about 3·6 × 1010 α particles per second. Since the activity due to the emanation stored up in radium, when in a state of radio-active equilibrium, is about one quarter of the whole and about equal to the minimum activity, the number of α particles projected per second by the emanation from 1 gram of radium bromide is about 3·6 × 1010. It has been shown in [section 152] that 463,000 times the amount of emanation produced per second is stored up in the radium. But, in a state of radio-active equilibrium, the number of emanation particles breaking up per second is equal to the number produced per second. Assuming that each emanation particle in breaking up expels one α particle, it follows that the number of emanation particles present in 1 gram of radium bromide in radio-active equilibrium is 463,000 × 3·6 × 1010, i.e. 1·7 × 1016. Taking the number of hydrogen molecules in 1 c.c. of gas at atmospheric pressure and temperature as 3·6 × 1019 ([section 39]), the volume of the emanation from 1 gram of radium bromide is 4·6 × 10-4 cubic centimetres at atmospheric pressure and temperature. Assuming the composition of radium bromide as RaBr2, the amount from 1 gram of radium in radio-active equilibrium is 0·82 cubic millimetres. Quite independently of any method of calculation it was early evident that the volume of the emanation was very small, for all the earlier attempts made to detect its presence by its volume were unsuccessful. It will be seen, however, that, when larger quantities of radium were available for experiment, the emanation has been collected in volume sufficiently large to measure.
In the case of thorium, the maximum quantity of emanation to be obtained from 1 gram of the solid is very minute, both on account of the small activity of thorium and of the rapid break up of the emanation after its production. Since the amount of emanation, stored in a non-emanating thorium compound, is only 87 times the rate of production, while in radium it is 463,000 times, and the rate of production of the emanation by radium is about 1 million times faster than by thorium, it follows that the amount of emanation to be obtained from 1 gram of thorium is not greater than 10-10 of the amount from an equal weight of radium, i.e. its volume is not greater than 10-13 c.c. at the ordinary pressure and temperature. Even with large quantities of thorium, the amount of emanation is too small ever to be detected by its volume.
172. Volume of the emanation from radium. The evidence already considered points very strongly to the conclusion that the emanation possesses all the properties of a chemically inert gas of high molecular weight.
Since the emanation continuously breaks up, and is transformed into a solid type of matter, which is deposited on the surface of bodies, the volume of the emanation, when separated from radium, should contract at the same rate as it loses its activity, i.e. it should decrease to half value in about four days. The amount of emanation to be obtained from a given quantity of radium is a maximum when the rate of production of new emanation balances its rate of change. This condition is practically attained when the emanation has been allowed to collect for an interval of one month. The probable volume of the emanation to be obtained from 1 gram of radium was early calculated on certain assumptions, and from data then available the writer[[265]] deduced that the volume of the emanation from 1 gram of radium lay between ·06 and ·6 cubic millimetre at atmospheric pressure and temperature, and was probably nearer the latter value. The volume to be expected on the latest data has been discussed in the preceding section and shown to be about ·82 cubic mm. The volume of the emanation is thus very small, but not too small to be detected if several centigrams of radium are available. This has been proved to be the case by Ramsay and Soddy[[266]] who, by very careful experiment, finally succeeded in isolating a small quantity of the emanation and in determining its volume. The experimental method employed by them will now be briefly described.
Fig. 61.
The emanation from 60 milligrams of radium bromide in solution was allowed to collect for 8 days and then drawn off through the inverted siphon E ([Fig. 61]) into the explosion burette F. This gas consisted for the most part of hydrogen and oxygen, produced by the action of the radiations on the water of the solution. After explosion, the excess of hydrogen mixed with emanation was left some time in contact with caustic soda, placed in the upper part of the burette, in order to remove all trace of carbon dioxide. In the meantime the upper part of the apparatus had been completely evacuated. The connection C to the pump was closed, and the hydrogen and emanation were allowed to enter the apparatus, passing over a phosphorous pentoxide tube D. The emanation was condensed in the lower part of the capillary tube A, by surrounding it with the tube B filled with liquid air. The process of condensation was rendered manifest by the brilliant luminosity of the lower part of the tube. The mercury from the burette was then allowed to run to G, and the apparatus again completely evacuated. The connection of the pump was again closed, the liquid air was removed and the volatilized emanation forced into the fine capillary tube A. Observations were then made, from day to day, of the volume of the emanation. The results are given in the table below.
| Time | Volume | Time | Volume |
|---|---|---|---|
| Start | 0·124 cub. mm. | 7 days | 0·0050 cub. mm. |
| 1 day | 0·027 „ | 9 „ | 0·0041 „ |
| 3 „ | 0·011 „ | 11 „ | 0·0020 „ |
| 4 „ | 0·0095 „ | 12 „ | 0·0011 „ |
| 6 „ | 0·0063 „ | 28 „ | 0·0004 „ |
The volume contracted with the time, and was very small after a month’s interval, but the minute bubble of the emanation still retained its luminosity to the last. The tube became deep purple in colour, which rendered readings difficult except with a strong light. There was a sudden decrease in the first day, which may have been due to the mercury sticking in the capillary tube.