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I ' 



GG2 



REPORT — 1884. 



it Is also eqiinlly Iinpri.isililt^ to look on tlio opposift; view as tending' towards prnrrrpsv, 

 inasnmcii as In aycviliH to ail olt'inent as many Aalcncios as it pos.'^c.-sc.s comjioimd-; 

 witli some othei* olcment. is only cxprossinrr by civciiitous nictliod.s what tlio old 

 Daltonian law of combination in multiple proportions states in simple terms. Slill 

 we may note certain <reiierally-aocepti d coneliisinns: in tlie iirst place, tliat ol' tln' 

 existence of non-saturati'd conqioinids both iiior^^'anie and orpanie, as carljon-nMii- 

 oxide on tlie one hand, and malio and cilraconie acids on the otlior. Secondly, 

 that th(^ videncyoi'an olenient is not only dependent njxni thenatnre of the element 

 with which it combines, but that this valency is a periodic fnnclion of the aloniie 

 vveifrht of the other comjionent. 'J'hns the elements of tho cidcn'ineL'roup are alwiiy-. 

 monads wlien combined with positive elements or radicals, but triad, pentad, and. 

 heptad with nepalivo ones. Ajrain, the elements of tht> sulphni' j.'rou]) arcj dyads in 

 the tir.st case, but tetrad ami hexad in the second. The periodicity of this projievt', 

 of the atoms, increasinu: and ajrain diminisliinfr, is clearly seen in such a series ds 



AgCl,, CdCl,, InCl,, SnCl., Sbll,, TeTT,,, III, 



as well as in tlie series of oxides. The diillculties which beset this subject may ln' 

 judfied of by the mention of a case or two : \^ vanadium a. <etrail because its lii,Ldi< -i 

 chloride contains four atoms of cidorinc i-' What are wo to say is the vah^ncy of 

 lead when one atom unites with four methyls to form a volatile product, and yi 

 the vapour-density of the chloi'ide shows ihiil the molecide contains one of metal t > 

 two of chloiine!-' Or, how can our method lie said to determine the valency e.f 

 tunjrsten when the hexchloride decomposes in the state of vapour, and the iici.ta- 

 cliloride is the highest volatile stable compound!-' How airaiu are we to deliiie 

 the point at wiucli a body is volatile witliout decomposition!' — thus sulphur tetra- 

 chloride, one of tho most unstable of compounds, can be va])orised without decompe- 

 eition at all temperatures below —'2'1°, wliilst water, one of the most stable of knowr. 

 compounds, is dissociateil into its elements at the tem]ierature of nudtinjj: platimuu, 

 liut, however many doul)ts may liave been raised in special instances against w 

 thorough application ofthe, lawof valency, it caiuiot bo denied that tho general 

 relations of the elements Avbicli this rjuestioii of valency has been the means el' 

 bringing to light are oi the liighest importance, and point to the existence of liiw>' 

 of nature of the widest signilicance ; 1 allude to the periodic law of the i lemer.t- 

 first foreshadowed by Xewlands, but fully developed by ^Mendelejell'and Lotliar 

 3Ieyer. Guided by llio principle that the (diemical projierties of the elements are 

 a periodic function of their atomic weights, or that uuitter becomes endowed with 

 analogous properties when tiie atomic weight of an element is increased Ity the 

 same or nearly the same numlier, wo find ours(dves for tho tirst time in possessioa 

 of a key which, enables us to arrange the hitherto dinjecta memhra of our chemical 

 household in something lik'c order, and tluis gives us means of indicating the 

 family resemblances by winch these elements are characterised. 



And hero we may congratulate ourselves on the fact that, by the recent experi- 

 ments of Brauner, and of Nilsou and Pettersen rt'spectivtdy, tellurium and beryl- 

 lium, two of the hitlierto outstanding memlieis, have been induced to join the 

 ranks, so that at the present time osmium is the only important defaulter amongst 

 the sixty-four elements, and few persons will doubt that a little carefid attention t<> 

 this case will remove Ww stigma which yet attaches to its name. J Jut this periodic 

 law makes it possible for us to do more ; for as the astronomer, by the perturbations 

 of known planets, can predict the existence of hitherto unknown ones, so the 

 chemist, though, of course, with much less satisfacl ory means, has been able to predict 

 with precision the properties, physical and chemical, of certain missing links 

 amongst the elements, such as ekaluminium and elvaboron, then unborn, but which 

 shortly afterwards became Avell known to us in the llesh as gallium and .scandium. 

 AVe must, however, take care that success in a few cases does not blind us to the 

 fact that the law of nature which expresses the relation between the properties of 

 the elementp and the valiu> of the atomic weights is as yet unknown : that many 

 of the groupings are not due to any well-ascertained analogy of properties of tlie 

 elements, and that it is only because the values of their atomic weights exhibit 

 cwrtftin regularities that such a grouping is rendered possible. So, to quote Lothar 



