126 CHEMICAL DISCOVERY AND INVENTION 



The numbers which have been adopted in this table have all 

 been selected so as to comply with certain rules long established 

 and fully explained in all the best textbooks of chemistry of 

 the present day. The first of these is known as the law of 

 Avogadro, who proposed the hypothesis on which it is based in 

 1811. It was not, however, generally recognised till more than 

 fifty years later. 1 The other principle made use of depends on 

 the relation between the specific heat of a solid element and its 

 atomic weight, discovered by the French physicists Dulong and 

 Petit in 1819. In those cases in which both these rules can be 

 applied the result is the same. 



As soon as a table such as the one just given could be 

 drawn up a very important discovery was made.- In 1863 it 

 was observed by Mr. J. A. R. Newlands that in such a table, 

 imperfect as it was at that time, the properties of the elements 

 are related to their position in the series. Every eighth 

 element in the list, starting from any point, exhibits a revival 

 of the chief properties of the element from which counting 

 is begun. Take, for example, the metal lithium as starting- 

 point, the eighth element following it is sodium, and the 

 eighth following is potassium, and these three elements form 

 a natural family, the members of which are very like to one 

 another in chemical properties, and show a gradation in physical 

 properties. This discovery led to further investigation, and in 

 the end the so-called periodic law of the elements was announced 

 by the late Professor MendeleefT in 1869. 2 This principle has 



1 The reader who is interested in such matters as the history of Avogadro's 

 doctrine should read the "Memorial Lecture on Cannizzaro," by Sir Wm. 

 Tilden, in the Transactions of the Chemical Society for 1912, p. 1677. 



8 For a full account of the origin and history of the conception see the 

 "Memorial Lecture on Mendeleeff," in the Transactions of the Chemical Society 

 for 1909, 



