Experiments were made on the effect of adding substances to a solution of radio-lead, and then removing them by precipitation. Small quantities of iridium, rhodium, palladium, and platinum, in the form of chlorides, were left in the solution for three weeks, and then precipitated by formalin or hydroxylamine. All of these substances were found to give out both α and β rays, the activity being greatest for rhodium and least for platinum. A large proportion of the β ray activity disappeared in the course of six weeks, and of the α ray activity in one year. It is probable that the two products radium E and F were in part removed with the metals from the radio-lead. We have seen that radium E gives out β rays and loses half of its activity in about six days, while radium F gives out only α rays and its activity falls to half value in 143 days. This conclusion is further confirmed by experiments on the effect of heat on the activity of these substances. By heating to a full red heat, the α ray activity was lost in a few seconds. This is in agreement with the results ([section 232]) where we have seen that radium F is volatilized at about 1000° C. and radium E is left behind.
Salts of gold, silver and mercury added to the radio-lead were found to show only α ray activity on removal. This is in accordance with the view that radium F alone is removed with these substances. Bismuth salts on the other hand showed initially α and β ray activity, but the latter rapidly died away. The presence of β rays in freshly prepared polonium was early observed by Mme Curie. The α and β ray activity of the radio-lead is much reduced by the precipitation of bismuth added to the solution. The α and β ray activity of the radio-lead, however, recovers itself again. This result is exactly what is to be expected if radio-lead contains radium D, E and F. Radium E and F are removed with the bismuth, but the parent substance, radium D, is left behind, and, in consequence, a fresh supply of radium E and F is produced.
While further experiments are required to settle definitely whether the products separated from radio-lead are identical with radium E and F, there can be little doubt that such is the case. This conclusion is strengthened by some experiments which I have made on a specimen of radio-lead, which was kindly forwarded to me by Mr Boltwood of New Haven. This active lead gave out α and β rays, the latter being in unusually large proportion. The active lead was four months old when first tested. The β ray activity in the following six months has remained sensibly constant, but the α ray activity has steadily increased. These results are to be expected if the radio-lead contains radium D. Radium E will reach a practical maximum about 40 days after separation of the product radium D with the lead. The α ray activity due to radium F should increase to a maximum in about 2·6 years (see section 236).
Further experiments are required to settle whether the lead immediately after separation from pitchblende contains only radium D, or whether radium E also appears with it. It seems likely, however, that the bismuth, which is initially present in solution at the time of separation of the lead, will retain both radium E and F, and that the presence of these products in radio-lead is due to their production, after separation, by the parent substance, radium D.
It would be of scientific value to separate radium D from pitchblende and obtain it in the pure state, for, a month after removal, the β ray activity from it would be about 300 times as great as from an equal weight of radium. By placing a bismuth plate in a solution of this substance, radium F (polonium) should be separated, and, provided a sufficient interval is allowed to elapse, a fresh supply of radium F can at any time be obtained.
The rate of transformation of radium D (half transformed in 40 years) is sufficiently slow not to interfere seriously with its utility in most experiments.
The results of the comparison of the products of radium with those contained in polonium, radio-tellurium and radio-lead are summarized below.
Radium D = product in new radio-lead, no rays. Half transformed in 40 years.
Radium E gives out β rays, separated with bismuth, iridium and platinum. Half transformed in 6 days.
Radium F = product in polonium and radio-tellurium. Gives out only α rays. Half transformed in 143 days.