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matter is removed, the product immediately begins to lose its activity at its characteristic rate, viz. the activity falls to half value in about six days.

233. Separation of the constituents by means of a bismuth plate. The active matter of slow decay was obtained in solution by introducing dilute sulphuric acid into a glass tube in which the emanation from 30 milligrams of radium bromide had been stored for a month. The solution showed strong activity and gave out both α and β rays, the latter, as in other cases, being present in an unusually large proportion.

When a polished bismuth disk was kept for some hours in the solution, it became strongly active. The active matter deposited on the bismuth gave out α rays, but no trace of β rays. After several bismuth disks had been successively left in the solution, the active matter, which emits α rays, was almost completely removed. This was shown by evaporating down the solution after treatment. The β ray activity remained unchanged, but that of the α rays had been reduced to about 10 per cent. of its original value. Three bismuth disks, made active in this way, were set aside and their activity measured at regular intervals. The activity fell off according to an exponential law with the time during the 200 days since their removal, while that of each fell to half value on an average in about 143 days.

At the same time it was observed that the solution, from which the α ray activity was removed, gradually regained its activity, showing that the active substance which gave out α rays was continuously produced from the matter left behind in the solution.

234. Explanation of the results. We have seen that a close examination of the active deposit of slow change has disclosed,

(1) the presence of a β ray product which loses half of its activity in about six days;

(2) the presence of an α ray product, which is deposited on bismuth and is volatilized at 1000° C. This product loses half of its activity in 143 days;

(3) the presence of a parent substance, which produces the β ray product at a constant rate.