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SCIENCE. 



[N.S. Vol. XVIII. No. 4C0. 



atomic weights and indicated their proper 

 values. 



In 1865, Newlands presented before the 

 London Chemical Society a communication 

 upon the law of octaves, in which he 

 showed that the elements, when arranged 

 in the order of their atomic weights, exhib- 

 ited a certain regular recurrence of prop- 

 erties. Unfortunately, his views were not 

 given serious attention, and even met with 

 ridicule, but they contained the germ of 

 the great truth. It was reserved for the 

 Russian, Mendeleeff, four years later, to 

 completely formulate the famous periodic 

 law. 



Mendeleeff 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 predictions were made. 

 The hypothetical elements were described 

 by Mendeleeff, their atomic weights were 

 assigned and their physical properties fore- 

 told, and in due time the prophecies were 

 verified. The three metals gallium, scan- 

 dium and germanium have since been dis- 

 covered, and they correspond very closely 

 with Mendeleeff 's anticipations. His gen- 

 eral 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 expres- 

 sion 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 the elements must 

 be recognized, and one set of relations will 

 connect them all. In the arrangement 

 proposed by Reynolds the inert gases, the 

 elements of zero valency, appear at the 

 nodes of a vibrating curve, a circumstance 

 which gives this method of presentation a 

 peculiar force. Biit for the consideration 

 of physical properties the curves drawn by 

 Lothar Meyer seem likely to be the most 

 useful. In one respect, however, the peri- 

 odic system is still defective; it fails to 

 take adequately into account the numerical 

 relations between the atomic weights, a 

 phase of the problem which should not be 

 ignored. Such relations exist ; some of 

 them have been indicated by your distin- 

 guished fellow member. Dr. Wilde; and, 

 elusive as they may seem to be, they are 

 surely not meaningless. The final law must 

 cover the entire ground, and then atomic 

 weights, physical properties and valency 

 will be completely correlated. Prout's 

 hypothesis is discredited, and yet it may 

 prove to be a crude first approximation to 

 some deeper truth, as the probability calcu- 

 lations of Mallet* and of Struttf would 

 seem to indicate. The approaches of the 

 atomic weights to whole numbers are too 

 close and too frequent to be regarded as 

 purely accidental. But this is aside from 



* Phil. Trans., Vol. 171, 1881, p. 1003. 

 "^ Phil. Mag. (6), 1, p. 311. 



