Page:Radio-activity.djvu/538

 of potter's clay, whitening, and heavy spar, wrapped in linen, were placed in the earth 50 cms. below the surface. After an interval of a month, these were dug up and their activity examined. The clay was the only substance which showed any activity. The activity of the clay diminished with the time, showing that activity had been excited in it by the emanations present in the soil.

Elster and Geitel have found that a large quantity of the radio-active emanation can be obtained by sucking air through clay. In some cases, the conductivity of the air in the testing vessel was increased over 100 times. They have also found that the so-called "fango"—a fine mud obtained from hot springs in Battaglia, Northern Italy—gives off three or four times as much emanation as clay. By treating the fango with acid, the active substance present was dissolved. On adding some barium chloride to the solution, and precipitating the barium as sulphate, the active substance was removed, and in this way a precipitate was obtained over 100 times as active, weight for weight, as the original fango. Comparisons were made of the rate of decay of the excited activity, due to the emanation from fango, with that due to the radium emanation, and within the limits of error, the decay curves obtained were found to be identical. There can thus be no doubt that the activity observed in fango is due to the presence of a small quantity of radium. Elster and Geitel calculate that the amount of radium, contained in it, is only about one-thousandth of the amount to be obtained from an equal weight of pitchblende from Joachimsthal.

Vincenti and Levi Da Zara have found that the waters and sediments of a number of hot springs in Northern Italy contain the radium emanation. Elster and Geitel observed that natural carbonic acid obtained from great depths of old volcanic soil was radio-active, while Burton found that the petroleum from a deep well in Ontario, Canada, contained a large quantity of emanation, probably of radium, since its activity fell to half value in 3·1 days, while the excited activity produced by the emanation fell to half