42 



DISCOVERY 



Samarium 

 62 Sa 150-4 



Thulium 

 69 Tm 168-5 



I Gold 

 79 Au 197-2 



Ka.Hmana- 

 tion 

 86 



Europium 

 63 Eu 1520 



Ytterbium 

 70 Yb 1735 



Mercury I 

 80 Hg 2oo'6 



Gadolinium 

 64 Gd 157-3 

 Lutecium 

 71 Lu 1750 



Thallium 

 81 Tl 204-0 



Radium 

 8 Ra 226-0 I 



Terbium 



65 Tb 159-2 



Keltium 



73 Kt ? 



Lead 

 82 Pb 207-2 



Dysprosium 

 66 Ds 162-5 



Holmium 

 67 Ho ? 



Erbium 

 68 Er 167-7 



Tantalum 

 73 Ta 181 -5 



Bismuth 

 83 Hi 208-0 



Thorium 

 90 Th 232-4 



Uranium X2 



Tungsten 



74 W 1840 



Polonium 



84 



Uranium 

 92 U 238-2 



Osmium Iridium Platinum 

 76 Os 190-9 77 Ir 193-1 ; 78 Pt 195-2 



For convenience in printing, the part of the table periodic in 18 has been brought into a reduced number of columns. The Roman figure 

 at the head is the number of the group in which the elements vertically below it are classed. The table is read as follows : — Helium, atomic 

 number 2, chemical symbol He, atomic weight 3-99. Wliere the element is unknown its number is printed large. Among the radio-active 

 elements, only one of several " isotopic " substances has been put in the list. (" Isotopes" are defined later.) Most of the atomic weights 

 of these substances have not been measured. The "rare earths " have no correspondence in properties with the other elements of the columns 

 under which they happen to fall. The three cases where the order of atomic weight must be inverted are A and K, Co and N'i, Te and I. 



hide, which is filled by the elements known as the 

 " rare earths." More than a dozen of these are known 

 to chemists, almost all very rare, and so alike that it is 

 only with the greatest difficulty and patience that 

 they can be separated from one another. After this 

 interlude the periodic system returns and continues 

 to the end of the list. Owing to the complete breach 

 with the periodic law, the chemist cannot say whether 

 there are any elements among the rare earths still to be 

 discovered. The physical experiments which I am 

 going to describe prove conclusively that between 

 hydrogen and uranium there are 5 undiscovered 

 elements — the two of the manganese group, one rare 

 earth, and two more among the radio-active elements 

 which come at the end of the list. These results follow 

 from experiments which go much further into the 

 fundamental nature of atoms than is ever done in 

 chemistry. All trace of periodicity or chemical 

 peculiarity disappears, and is replaced by a perfectly 

 orderly progression of properties from which we can 

 argue with absolute confidence. 



In examining the atomic weights in the table, it will 

 be observed that (especially near the beginning) the 

 majority of them are almost exactly whole numbers. 

 A great deal of speculation has been built up on this 



subject in the face of considerable difi&culty, because 

 a few, especially chlorine at 35'46, resolutely refuse 

 to conform to the rule. Obviously, but for these 

 exceptions, it would have been reasonable to suppose 

 that all atoms were built up out of hydrogen atoms. 

 Some work done only two months ago has entirely 

 revolutionised this question, and in view of its interest 

 and novelty, it will not be out of place to discuss it 

 here, the more so as it has a considerable bearing on the 

 theory of the fundamental nature of matter. 



The electrical theory of matter was developed by 

 J. J. Thomson and his school about twenty years ago. 

 Thomson showed that all atoms contain a common 

 substance — the electron, a particle with a perfectly 

 definite and constant charge of negative electricity, 

 and of mass about one eighteen-hundredth of a hydro- 

 gen atom. The question then arises, How many 

 electrons are there in an atom ? From their mass we 

 can only say less than 1,800 (in the case of an atom of 

 hydrogen), and to get a more definite value we must 

 adopt some other line of attack. Now an atom is an 

 exceedingly impenetrable body, and any ordinary 

 experiment, say of a chemical or of an optical kind, 

 reallj' touches its surface only. In order to find out 



