THE ATOMIC VIEW OF NATURE. 



415 



German, and the " type " theory to the French, school of 

 chemists. But the idea of the " atomicity " and " valency " 

 or saturating capacity of the element of any substance was 

 not possible without the clear notion of the " molecule " 

 as distinct from the " atom." This idea had lain dormant 

 in the now celebrated but long forgotten law of Avogadro, 

 which was established in the year 1811, almost immedi- 

 ately after the appearance of Dalton's atomic theory. 



The atomic theory may be regarded in two distinct 

 ways, and it is instructive from the point of view of the 

 history of thought to see how -these two different aspects 

 of the theory have gradually presented themselves. The 

 older and vague atomic theory professed to be a theory of 

 the constitution of bodies, and to afford the basis for an 

 explanation of physical phenomena ; in order to do this, 

 forces of attraction and repulsion between the particles of 



22. 



Two aspects 

 of the 

 atomic 

 theory. 



It appears that this theory was 

 largely based upon a compound 

 prepared by Bunsen, and called 

 ' ' cacodyl." This compound was one 

 of the few organic radicles which 

 contained a metal arsenic. Frank- 

 land, partly alone, partly in union 

 with Kolbe, entered upon a series 

 of researches which had two distinct 

 objects. Both these objects were 

 foreign to that school which had 

 given up the radicle theory, and 

 which, by looking upon organic 

 compounds as essentially different 

 from inorganic compounds, had lost 

 that important clue the connec- 

 tion of the two branches of chemis- 

 try. These objects were the isola- 

 tion of the so-called radicles or 

 compound elements and the pre- 

 paration of other " orgauo-metal- 

 lic " bodies. The latter research 

 led to new insight into the nature 

 of chemical combinations. " I had 



not proceeded far," says Franklaud, 

 " in the investigation of the organo- 

 metallic compounds before the facts 

 brought to light began to impress 

 upon me the existence of a fixity in 

 the maximum combining value or 

 capacity of saturation in the metal- 

 lic elements which had not before 

 been suspected. ... It was evi- 

 dent that the atoms of zinc, tin, 

 arsenic, antimony, &c. , had only 

 room, so to speak, for the attach- 

 ment of a fixed and definite number 

 of the atoms of other elements, or, 

 as I should now express it, of the 

 bonds of other elements. This 

 hypothesis, which was communi- 

 cated to the Royal Society on May 

 10, 1852, constitutes the basis of 

 what has since been called the doc- 

 trine of atomicity or equivalence of 

 elements ; and it was, so far as I 

 am aware, the first announcement 

 of that doctrine" (ibid., p. 145). 



