242. Temporary activity of inactive matter separated from radio-active substances. We have seen in the last section that the platinum metals and bismuth acquire temporary activity by their admixture with a solution of radio-lead, and that these effects are very satisfactorily explained on the view that some of the products of change of radio-lead are removed with the inactive substances. Very similar effects have been observed by Pegram and von Lerch ([section 186]), when inactive substances were added to solutions of thorium and of the active deposit of thorium. These results, too, are almost certainly due to the removal of one or more of the products of thorium with the inactive matter. Examples of this character may readily be multiplied, and some of the more interesting and important of these will be briefly discussed later.
There have been two general points of view regarding the character of this activity which is temporarily acquired by inactive matter. Some people have supposed that the inactive molecules of the substance, mixed with the solution, acquire by “radio-active induction” temporary activity, the underlying idea being that the close admixture of an inactive and an active substance has communicated the property of radiating to some of the molecules of the former. According to the disintegration theory of radio-activity, on the other hand, the temporary activity of originally inactive matter is not due to any alteration of the inactive substance itself, but to an admixture with it of one or more of the numerous radio-active products. The idea of “radio-active induction” has no definite experimental evidence in support of it, while there is much indirect evidence against it.
We shall now consider how these facts are interpreted according to the disintegration theory. In a specimen of old radium, for example, there are present, besides radium itself, the seven successive products which arise from it. Each of these differs in chemical and physical properties from the others. If now, for example, a bismuth rod is introduced into the solution, one or more of these products are deposited on the bismuth. This action is most probably electrolytic in nature, and will depend upon the electro-chemical behaviour of the bismuth compared with that of the products in solution. An electro-negative substance will tend to remove the product or products which are strongly electro-positive. This point of view serves to explain why different metals are made active to different degrees, depending upon their position in the electro-chemical series.
It seems probable that the activity communicated to inactive matter by precipitation from an active solution occurs only during the precipitation. The correctness of this view could readily be tested by observing whether the time that the inactive substance is present in solution has any effect on the magnitude of the activity imparted to it.
When it is remembered that in pitchblende there are present the radio-elements uranium, thorium, radium and actinium and their numerous family of products, it is not surprising that many of the inactive substances separated from it may show very considerable activity due to the mixture of products which may be removed with them. In carrying out experiments on the separation of radium from pitchblende, M. and Mme Curie observed that the separation of the active substance is fairly complete if the stage of purification is not far advanced. Copper, antimony and arsenic can be separated only slightly active, but other substances like lead and iron always show activity. When the stage of precipitation is more advanced, every substance separated from the active solution shows activity.
One of the earliest observations in this direction was made by Debierne, who found that barium could be made active by solution with actinium. The active barium removed from the actinium still preserved its activity after chemical treatment, and, in this way, barium chloride was obtained whose activity was 6000 times that of uranium. Although the activity of the barium chloride could be concentrated in the same way as the activity of radiferous barium chloride, it did not show any of the spectroscopic lines of radium, and could not have been due to the admixture of that element with the barium. The activity of the barium was not permanent, and Debierne states that the activity fell to about one-third of its value in three months. It seems probable that the precipitated barium carried down with it the product actinium X, and also some of the actinium itself, and that the decay observed was due to the transformation of actinium X. It is interesting to note that barium is capable of removing a large number of products of the different radio-elements. This effect is probably connected with its position in the electro-chemical series, for barium is highly electro-positive.
Giesel showed in 1900 that bismuth could be made active by placing it in a radium solution, and considered that polonium was in reality bismuth made active by the process of induction. In later experiments, he found that the bismuth plate gave out only α rays, and that the activity of the bismuth could not be ascribed to radium, since no β rays were present. We have seen that this activity of the bismuth is due to the product radium F deposited on its surface.
Mme Curie also found that bismuth was made active by solution with a radium compound, and succeeded in fractionating the above bismuth in the same way as polonium. In this way bismuth was obtained 2000 times as active as uranium, but the activity, like that of polonium separated from pitchblende, decreased with the time. In the light of the experiments on the transformation products of radium, it is seen that these early experiments of Mme Curie add additional confirmation to the view that the product (radium F) separated from radium itself is identical with the polonium obtained directly from pitchblende.
CHAPTER XII.
RATE OF EMISSION OF ENERGY.
243. It was early recognised that a considerable amount of energy is emitted by the radio-active bodies in the form of their characteristic radiations. Most of the early estimates of the amount of this energy were based on the number and energy of the expelled particles, and were much too small. It has been pointed out ([section 114]) that the greater part of the energy emitted from the radio-active bodies in the form of ionizing radiations is due to the α rays, and that the β rays in comparison supply only a very small fraction.