ON THE PERIODIC ARRANGEMENT OF 

 THE ELEMENTS 



AT the end of the eighteenth century, after the investiga- 

 tions of Black, Scheele, Priestley, Cavendish, and Lavoisier 

 began to crystallise the previous arbitrary collections of 

 chemical facts into more or less of a system, it became 

 evident that the distinguishing feature of a ' compound/ 

 as contrasted with a ' mixture,' was the invariability of its 

 composition. Early in the nineteenth century, Dalton 

 formulated his celebrated hypothesis, by means of which 

 a concrete view was gained regarding the cause of this 

 constancy and invariability of composition. Every one 

 knows that this c explanation ' consisted in the supposition 

 that the combination of two substances, one with another, 

 in definite proportions, involves the union either of one 

 atom of the one with one atom of the other, or of certain 

 small but simple numbers of atoms of the two substances. 

 The atom was regarded, not necessarily as indivisible, but 

 as not having been divided into any smaller particles. 

 The advance made by Dalton consisted chiefly in ascribing 

 to each atom a definite weight ; but as he had no data for 

 determining the absolute weight of any one atom, he was 

 obliged to content himself with relative weights, and chose 

 the smallest known to him, that of hydrogen, as an arbi- 

 trary unit. This choice has proved to be a just one, for 

 as yet no element has been discovered possessing a lower 

 atomic weight than hydrogen, although it is by no means 

 impossible that such an element may exist. 



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