tion to the table is as yet hardly more than speculative, and they have been likened to the asteroids of the solar system.
From tantalum to bismuth the table is very regular, except that the element of the manganese series, which would have an atomic weight of about 188, is unknown, as we have already seen is the case with eka-manganese. Above bismuth, with its weight of 208, we know but two elements, thorium, 831, and uranium, 240. Since the discovery of the Rontgen rays, great interest has been excited by different kinds of rays which, though they may not be visible to the naked eye, are nevertheless capable of affecting the photographic plate. The only elements, as far as yet known, which yield such rays are these two of extraordinarily high atomic weight, thorium and uranium. Closely connected with this phenomenon is that of giving off luminous rays when not exposed to light. It has recently been discovered that while uranium can give off comparatively feeble rays, there is contained in the principal uranium mineral, pitchblende, matter which is much more active than uranium. Further investigation seems to show that there are at least three such substances present in pitchblende; which have been named radium (from the rays it gives off), polonium (from its discoverer's native land), and actinium (from its ratio-activity). Of these radium alone has been studied at all extensively, and even its claim to be called a chemical element is by no means established. It strongly resembles barium, but it gives off rays easily visible in the dark, continuing to shine indefinitely. There is much doubt as to whether it be not really a peculiar form of barium, but recent determinations of its atomic weight, in a condition only partially purified, indicate that it has a higher atomic weight than barium, and that it may in this respect resemble thorium and uranium.
It may seem rather remarkable that, inasmuch as Döbereiner had brought out the resemblances between elements of the same group in his triads, nearly half a century should have elapsed before the essential features of the Periodic Law were discovered. This is due, chiefly at least, to three causes. First, there would have been many more gaps in the table then than now, so many new elements having been discovered since that day; second, the atomic weights of the elements were then so imperfectly known that, using the weights then accepted, it is impossible to construct a periodic table; the third great difficulty lay in the fact that nine very important elements refused to be reduced to order, and finally were excluded and relegated to an outlying group of unique properties. These nine elements are iron, cobalt, nickel and the so-called platinum metals, platinum, palladium, iridium, rhodium, osmium and ruthenium. As a matter of fact, these nine metals cannot be brought into any of the seven regular groups, but must be placed by themselves in a single group of three series, or in three groups. Thia
