566 EBPORT— 1886. 



between the first and the last members of each series, we can scarcely help admitting 

 the existence of a point of mean variation within each system. In general, the 

 fourth element of each series possesses the properties we might expect a transition- 

 element to exhibit. If we examine a particular period — for instance, that one 

 whose meso-element is silicon, we note : — First, that the three elements of lower 

 atomic weight than silicon, viz. sodium, magnesium, and aluminium, are distinctly 

 electro-positive in character, while those of higher atomic weight, viz., phosphorus, 

 sulphur, and chlorine, are as distinctly electro-negative. Throughout the best known, 

 periods this remarkaljle subdivision is observable, although, as might be anticipated, 

 the differences become less strongly marked as the atomic weights increase. 

 Secondly, that the members above and below the meso-element fall into pairs of 

 elements, which, while exhibiting certain analogies, are generally in more or less 

 direct chemical contrast. Thus, in the silicon period we have — 



Si'" 



+ A1'" P'"- 



+ Mg" S"- 



+ Na' Cl'- 



This division also happens, in many cases, to coincide with some characteristic 

 valence of the contrasted elements. It is noteworthy, however, that the members 

 on the electro-negative side exhibit the most marked tendency to variation in atom- 

 fixing power, so that valence alone is an untrustworthy guide to the probable posi- 

 tion of an element in a period. 



Thus for the purpose of graphic translation Professor Reynolds considers that 

 the fourth member of a period — silicon, for example — may be placed at the apex of 

 a symmetrical curve, which shall represent, for that particular period, the direction 

 in which the properties of the series of elements vary with rising atomic weights. 



In the drawing before you (fig. 1) I have modified Professor Reynolds's 

 diagram m one or two points. I have turned it the reverse way, as it is more 

 convenient to start from the top and proceed downwards. I have represented 

 the pendulous swing as gradually declining in amplitude according to a mathe- 

 matical law, and I have introduced another half-swing of the pendulimi between 

 cerium and lead, which not only renders the oscillations more symmetrical, but 

 brings gold, mercury, thallium, lead, and bismuth on the side where they are in 

 complete harmony with members of foregoing groups, instead of being out of har- 

 mony with them. This modification has another advantage, inasmuch as it leaves 

 many gaps to be hereafter filled in with new elements just when the development 

 of research is beginning to demand room for such expansion. 



I do not, however, wish to infer that the gaps in Mendeleefi's table, and in this 

 graphic representation of it, necessarily mean that there are elements actually 

 existing to fill up the gaps ; these gaps may only mean that at the birth of the 

 elements there was an easy potentiality of the formation of an element which 

 would fit into the place. 



Following the curve from hydrogen downwards we find that the elements form- 

 ing Mendeleefi's eighth group are to be found near three of the ten nodal points. 

 These bodies are ' interperiodic,' both because then- atomic weights exclude them 

 from the small periods into which the other elements fall, and because their 

 chemical relations with certain members of the adjacent periods show that they are 

 probably interperiodic in the sense of being transitional. 



This eighth group is divided into the three triplets — iron, nickel, and cobalt ; 

 rhodium, ruthenium, and palladium ; iridium, osmiiun, and platinum. The members, 

 of each triplet have often been regarded as modifications of one single form of matter. 



Notice how accurately the series of like bodies fits into this scheme. Beginning 

 at the top, run the eye down analogous positions in each oscillation, taking either 

 the electro-positive or electro-negative swings : — 



Ta Au Ha- Tl Pb Bi 



