THE PERIODIC LAW. 165 



tioii 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 ray3 

 which, though they may not be visible to the naked eye, are neverthe- 

 less capable of affecting the photogTaphic plate. The only elements, 

 as far as yet known, which yield such rays are these two of extraor- 

 dinarily 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 prin- 

 cipal uranium mineral, pitchblende, matter which is much more active 

 than uranium. Further investigation seems to show that there are at 

 least tliree such substances present in pitchblende; which have been 

 named radium (from the rays it gives off), polonium (from its dis- 

 coverer'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, con- 

 tinuing to shine indefinitely. There is much dou])t 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 Dobereiner 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 dis- 

 covered 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 



