Bumstead and Wheeler[[403]] have made a very careful examination of the radio-activity of the emanation obtained from the surface water and soil at New Haven, Connecticut. The emanation, obtained from the water by boiling, was passed into a large testing cylinder, and measurements of the current were made by means of a sensitive electrometer. The current gradually rose to a maximum, after the introduction of the emanation, in exactly the same way as the current increases in a vessel after the introduction of the radium emanation. The decay of activity of the emanations obtained from the water and soil was carefully measured, and, within the limits of experimental error, agreed with the rate of decay of activity observed for the radium emanation. The identity of the emanations from the water and soil with the radium emanation was still further established by experiments on the rate of diffusion of the emanation through a porous plate. By comparative tests it was found that the coefficient of diffusion of the emanations from the water and soil was the same as for the radium emanation. Also, by comparison of the rate of diffusion of carbonic acid, it was found that the density of the emanation was about four times that of carbonic acid, a result in good agreement with that found for the radium emanation (sections [161] and [162]).
Bumstead[[404]] has found that a considerable amount of thorium as well as radium emanation exists in the air of New Haven. For a three hour exposure in the open air, 3 to 5 per cent. of the excited activity on the wire is due to thorium. For a twelve hour exposure, the thorium activity was sometimes 15 per cent. of the whole. On account of the comparatively slow decay of the excited activity of thorium, the activity on the wire after removal for three or four hours was due almost entirely to thorium. The rate of decay could then be measured accurately, and was found to be the same as for a wire exposed in the presence of the thorium emanation.
Dadourian[[405]] has made an examination of the underground air in New Haven, and has found that this too contains a large quantity of the thorium emanation. A circular hole about 50 cms. in diameter and 2 metres deep was dug in the ground. A number of wires were wound on an insulated frame and suspended in the hole, the top of the hole then being covered over. The wire was charged negatively by a Wimshurst machine. After a long exposure the excited activity on the wire diminished at a rate that showed it to be a mixture of the excited activities of thorium and radium.
A very large amount of work has been done in examining various hot and mineral springs for the presence of the radium emanation, and it is not possible here to refer more than briefly to a few of the very numerous papers that have been published on this subject both in Europe and America. H. S. Allen and Lord Blythswood[[406]] have observed that the hot springs at Bath and Buxton gave off a radio-active emanation. This was confirmed by Strutt[[407]], who found that the escaping gases contained the radium emanation, and also that the mud deposited from the springs contained a trace of radium salts. These results are of considerable interest, for Lord Rayleigh has observed that helium is contained among the gases evolved by the springs. It appears probable that the helium observed is produced from the radium or radio-active deposits through which the water flows. Many mineral and hot springs which are famous for their curative properties have been found to contain traces of radium and also considerable amounts of radium emanation. It has been suggested that the curative properties may be due to some extent to the presence of these minute quantities of radium.
Himstedt[[408]] found that the thermal springs at Baden Baden contained the radium emanation, while Elster and Geitel[[409]] examined the deposits formed by these springs and found them to contain small quantities of radium salts. Results of a similar character were obtained for a number of waters in Germany by Dorn[[410]], Schenck[[411]], and H. Mache[[412]].
Curie and Laborde[[413]] have tested the waters of a large number of mineral springs and found that the great majority contain the radium emanation. In this connection, it is of interest to note that Curie and Laborde found very little emanation in the waters of Salins-Moutiers, while Blanc[[414]] observed, on the other hand, that the sediment from the spring was very active. A closer examination of this deposit by Blanc revealed the fact that it contained a considerable quantity of thorium. This was proved by finding that it gave out an emanation, which lost half of its activity in one minute, and produced excited activity, which fell to half value in about 11 hours. Boltwood[[415]] has tested a number of samples of spring water from different sources in America and has found that many of them contain the radium emanation.
Most of the results upon the amount of radium emanation from different sources have been expressed in arbitrary units without, in many cases, any comparative standard being given. Boltwood (loc. cit.) has described a satisfactory method for collecting and testing the emanation from different waters, and has suggested that the rate of discharge observed by the electroscope or the electrometer should be expressed in terms of the effect due to the emanation liberated on solution of a definite weight of the mineral uraninite. Since in every mineral so far examined, the amount of radium present is proportional to the amount of uranium, such a standard would be sufficiently definite for practical purposes. The emanation liberated from a few centigrams of the mineral is sufficient to give a convenient rate of discharge of an electroscope. Such a method is preferable to using a known quantity of a radium compound as a standard, since it is difficult to know with certainty the activity of the preparations of radium which may be in the possession of the different experimenters.
277. Radio-activity of constituents of the earth. Elster and Geitel[[416]] observed that, although in many cases the conductivity of the air was abnormally high in underground enclosures, the conductivity varied greatly in different places. In the Baumann Cave, for example, the conductivity of the air was nine times the normal, but in the Iberg Cave only three times the normal. In a cellar at Clausthal the conductivity was only slightly greater than the normal, but the excited radio-activity obtained on a negatively charged wire exposed in it was only ¹⁄₁₁ of the excited radio-activity obtained when the wire was exposed in the free air. They concluded from these experiments that the amount of radio-activity in the different places probably varied with the nature of the soil. Observations were then made on the conductivity of the air sucked up from the earth at different parts of the country. The clayey and limestone soils at Wolfenbüttel were found to be strongly active, the conductivity varying from four to sixteen times the normal amount. A sample of air from the shell limestone of Würzburg and from the basalt of Wilhelmshöhe showed very little activity.
Experiments were made to see whether any radio-active substance could be detected in the soil itself. For this purpose some earth was placed on a dish and introduced under a bell-jar, similar to that shown in [Fig. 103]. The conductivity of the air in the bell-jar increased with the time, rising to three times the normal value after several days. Little difference was observed whether the earth was dry or moist. The activity of the soil seemed to be permanent, for no change in the activity was observed after the earth had been laid aside for eight months.
Attempts were then made to separate the radio-active constituent from the soil by chemical treatment. For this purpose a sample of clay was tested. By extraction with hydrochloric acid all the calcium carbonate was removed. On drying the clay the activity was found to be reduced, but it spontaneously regained its original activity in the course of a few days. It seems probable, therefore, that an active product had been separated from the soil by the acid. Elster and Geitel consider that an active substance was present in the clay, which formed a product more readily soluble in hydrochloric acid than the active material itself. There seemed to be a process of separation analogous to that of Th X from thorium by precipitation with ammonia.