THE ATOMIC THEOEY. 253 



one aspect of it wa.s capable of experimental investigation. Some of 

 the most exact and elaborate determinations of atomic weight have 

 been made with the direct purpose of testing the truth or falsit}^ of 

 Front's speculation, and science thereby has been notably enriched. 

 The marvelous researches of Stas, for instance, had this specitic object 

 in view. The verdict was finally unfavorable to Prout. At least, the 

 best measurements fail to support his idea; but it still has advocates 

 who believe that the experimental data are vitiated by unknown 

 errors, and that future investigations will reverse the decision. In 

 science there is no court of last appeal. 



Front's hypothesis, then, stimulated the determination of atomic 

 weights, and so helped us to a more accurate knowledge of them. It 

 also led to a search for other relations between these constants, and 

 thus paved the way for important discoveries. Dobereiner, Kremers, 

 Dumas, Pettenkofer, Cooke, and many other chemists published 

 memoirs upon this theme, but not one of them was general or con- 

 clusive." Groups of elements were compared and relations w^ere 

 brought to light, but an exhaustive study of the question was hardly 

 possible until after Cannizzaro had revised the 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 lie showed that the 

 elements, when arranged in the order of their atomic weights, exhibited 

 a certain regular recurrence of properties. Unfortunateh% his views 

 were not given serious attention, and even met with ridicule, })ut they 

 contained the germ of the great truth. It was reserved for the Rus- 

 sian, Mendek'etf, four years later, to- completel}' formulate the famous 

 periodic law. 



Mendeleefi' 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 l)e 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 foretold, and in due time the 

 prophecies were verified. The three metals — gallium, scandium, and 

 germanium — have since been discovered, and they corr(^spond very 

 closely with Mendeleeff's 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 



«A very full account of these attempts is given in Venable's book, The Develop- 

 ment of the IVriodic Law, published at P^aston, Pa., in 1896. 



