i8 Clarke, The Atomic 'Hieory. 



It was reserved for the Russian, Mendelceff, four years 

 later, to completely formulate the famous periodic law. 



MendeleefT arranged the elements in tabular form, 

 still following the order of their atomic weights. A 

 periodic variation of their properties, including the 

 property of valency, at once became evident ; and although 

 the scheme was, and still is, open to some criticism, its 

 importance could hardly be denied. In the table, certain 

 gaps appeared, presumably belonging to unknown 

 elements, and for three of these some remarkable pre- 

 dictions were made. The hypothetical elements were 

 described by Mendeleefif, their atomic weights were 

 assigned and their physical properties foretold, and in 

 due time the prophesies were verified. The three metals 

 gallium, scandium and germanium have since been dis- 

 covered, and they correspond very closely with MendeleefPs 

 anticipations. His general conclusion was that all of the 

 physical properties of the chemical elements are periodic 

 functions of their atomic weights, and this conclusion, I 

 think, is no longer seriously doubted. The curves of 

 atomic volumes and melting points which Lothar Meyer 

 afterwards constructed, give strong support to this view. 



The periodic system, then, gives to the numbers 

 discovered by Dalton a much more profound significance 

 than he ever imagined, and is destined to connect a 

 great mass of physical data in one general law. That 

 law we now see, " as in a glass, darkly " ; its complete 

 mathematical expression is yet to be found, but I believe 

 that it will be fully developed within the near future. 

 We may have a spiral curve to deal with, as in the 

 schemes proposed by Stoney or by Crookes, or else a 

 vibratory expression like that suggested by Emerson 

 Reynolds in his presidential address before the Chemical 

 Society last year ; but in some form the periodicity of 



