Page:Popular Science Monthly Volume 82.djvu/593

This statement is now known to be too broad, but is true in general principle.^[1] Again,

The embryos of the most distinct species belonging to the same class are closely similar, but become, when fully developed, widely dissimilar.^[2]

To cite a few examples: The human embryo, at one stage of its development, possesses the rudiments of gill arches and gill clefts. The larva? of most insects, no matter how diverse, pass through a wormlike stage.

The larvæ of most crustaceans, at corresponding stages of development, closely resemble each other, however different the adults may become, and so it is with very many other animals.^[3]

It is with the latter of the two embryological peculiarities mentioned above, viz., the resemblances between the embryos of different related species, that we are at present concernd.

In the radioactive transformations, we have not the advantage of witnessing a building up of elements from simple to complex forms, as in the process of embryology we observe the formation of complicated organisms from the egg. But, what is almost as good, we observe a devolution of elements, from complex forms to simpler. In the course of their disintegration, the three distinct elements, radium, thorium and actinium give rise to products (i. e., elements) which have very similar properties.

The substances thorium, radium and actinium exhibit many interesting points of similarity in the course of their transformation. Each gives rise to an emanation whose life is short compared with that of the primary element itself. Such experiments as have yet been made, indicate that these emanations have no definite.combining properties, but belong apparently to the helium-argon group of inert gases. In each case, the emanation gives rise to a non-volatile substance which is deposited on the surface of bodies and is concentrated on the negative electrode in an electric field. The changes in these active deposits are also very similar, for each gives rise to a rayless product, followed by a product which emits all three types of rays. In each case, also, the rayless product has a longer period, or, in other words, is a more stable substance than the ray product which results from its transformation.

The disintegration of the corresponding products, thorium B, actinium B and radium C is of a more violent character than is observed in the other products, for not only is an a particle expelled at a greater speed, but a $\beta$ particle is also thrown off at great velocity. After this violent explosion within the atom, the resulting atomic system sinks into a more permanent state of equilibrium, for the succeeding products thorium C and actinium C have not so far been detected by radioactive methods, while radium D is transformed at a very slow rate.

This similarity in the properties of the various families of products is too marked to be considered a mere coincidence, and indicates that there is some underlying law which governs the successive stages of the disintegration of all the radioelements.^[4]

1. Cf. Spencer, loc. cit.
2. "Origin of Species," Vol. II., Ch. XIV.
3. Loc. cit.
4. Rutherford, "Radioactive Transformations," pp. 169-170.