484 PRINCIPLES OF CHEMISTRY 



out without a knowledge of the law of periodicity, and I will confine myself 

 to simple substances and to oxides. 



Before the periodic law was formulated the atomic weights of the elements 

 were purely empirical numbers, so that the magnitude of the equivalent, and' 

 the atomicity, or the value in substitution possessed by an atom, could only, 

 be tested by critically examining the methods of determination, but never 

 directly by considering the numerical values themselves ; in short, we wer# 

 compelled to move in the dark, to submit to the facts, instead of being masters 

 of them. I need not recount the methods which permitted the periodic law 

 at last to master the facts relating to atomic weights, and I would merely 

 call to mind that it compelled us to modify the valencies of indium and 

 cerium, and to assign to their compounds a different molecular composition* 

 Determinations of the specific heats of these two metals fully confirmed the 

 change. The trivalency of yttriii/m l which makes us now represent its oxide ^ 

 as Y 2 3 instead of as YO, was also foreseen (in 1870) by the periodic law, and 

 it has now become so probable that Cleve, and all other subsequent investi- 

 gators of the rare metals, have not only adopted it, but have also applied it 

 without any new demonstration to substances so imperfectly known as thosa- 

 of the cerite and gadolinite group, especially since Hillebrand determined the 

 specific heats of lanthanum and didymium and confirmed the expectations 

 suggested by the periodic law. But here, especially hi the case of didymium, 

 we meet with a series of difficulties long since foreseen through the periodic 

 law, but only now becoming evident, and chiefly arising from the relative 

 rarity and insufficient knowledge of the elements which usually accompany 

 didymium. 



Passing to the results obtained in the case of the rare elements leryllium % . 

 scandium, and thorium, it is found that these have many points of contact 

 with the periodic law. Although Avd^eff long since proposed the magnesia 

 formula to represent beryllium oxide, yet there was so much to be said in 

 favour of the alumina formula, on account of the specific heat of the metals 

 and the isomorphism of the two oxides, that it became generally adopted 

 and seemed to be well established. The periodic law, however, as Brauner 

 repeatedly insisted ('Berichte,' 1878,872; 1881, 53), was against the formula 

 Be 2 3 ; it required the magnesia formula BeO that is, an atomic weight! 

 of 9 because there was no place in the system for an element like beryllium 

 having an atomic weight of 18'5. This divergence of opinion lasted for 

 years, and I often heard that the question as to the atomic weight of beryllium 

 threatened to disturb the generality of the periodic law, or, at any rate, to 

 require some important modifications of it. Many forces were operating in 

 the controversy regarding beryllium, evidently because a much more im- 

 portant question was at issue than merely that involved in the discussion of 

 the atomic weight of a relatively rare element: and during the controversy 

 the periodic law became better understood, and the mutual relations of the 

 elements became more apparent than ever before. It is most remarkable that 

 the victory of the periodic law was won by the researches of the very observers 

 who previously had discovered a number of facts in support of the tri- 

 valency of beryllium. Applying the higher law of Avogadro, Nilson and 

 Petterson have finally shown that the density of the vapour of the beryl- 



