CHEMISTRY OF THE NINETEENTH CENTURY. $99 



It will be observed that if we suppose the existence of atoms of 

 nitrogen and of oxygen of invariable weights, the weight of each nitro- 

 gen atom being to the weight of each oxygen atom in the proportion 

 of 14 to 1 6, the fact of multiple proportions in the compounds of 

 these substances is obviously explained by the union of atoms in the 

 definite groups represented in the last column of the table. 



These are facts, and they were accepted as such by those who re- 

 jected the atomic theory, which Dalton had the merit of introducing, 

 or at least of definitely applying to chemical facts. The idea of atoms, 

 as we have seen, was familiar to the philosophers of ancient Greece 

 Leucippus, Democritus, Epicurus, and others. Modern philosophers, 

 such as Descartes, Spinoza, Leibnitz, Newton had propounded the 

 same conception under different forms. Nearer to Ualton's own 

 time, Boscovich and others had indulged in speculations of the like 

 kind. These speculations have in general no specific reference to the 

 constitution of substances from the chemist's point of view. But an 

 author, whose works Dalton had never perused, anticipated the Man- 

 chester chemist in expressing chemical combinations as the union of 

 ultimate particles. This author was WILLIAM HIGGINS, who published 

 in 1789 a treatise upholding the phlogistic theory. It is, however, 

 certain that the atomic theory of chemistry was not established in this 

 book. The conception is mentioned incidentally as applicable to a few 

 cases, without being followed up or announced as a general law. It is 

 the fact that no student of Higgins' work received from it the concep- 

 tion of the atomic theory. 



The rise of the conception of the atomic theory may be traced to 

 Dalton's study of the physical properties of gases. He gave out as early 

 as 1803 a series of numbers representing the relative weights of the ulti- 

 mate particles of certain chemical elements. Perhaps the best expo- 

 sition of Dalton's theory will be a few paragraphs from the " New 

 System of Chemical Philosophy." " When any body exists in the 

 elastic (gaseous) state, its ultimate particles are separated from each 

 other to a much greater distance than in any other state : each particle 

 occupies the centre of a comparatively large sphere, and supports its 

 dignity by keeping all the rest, which by their gravity or otherwise are 

 disposed to encroach upon it, at a respectful distance. When we 

 attempt to conceive the number of particles in an atmosphere, it is 

 somewhat like attempting to conceive the number of stars in the uni- 

 verse : we are confounded with the thought. But if we limit the sub- 

 ject by taking a given volume of any gas, we seem persuaded that, let 

 the divisions be ever so minute, the number of particles must be finite; 

 just as in a given space in the universe the number of stars and planets 

 cannot be infinite. 



" Chemical anatysis and synthesis go no further than to the separa- 

 tion of particles one from another, and to their re-union. No new 

 creation or destruction of matter is within the reach of chemical agency. 



