STANISLAO CANNIZZARO 157 



" The atom of every element," Cannizzaro said, " is ex- 

 pressed by that quantity of it which invariably enters as a 

 whole into equal volumes of the simple substance and its 

 compounds ; this quantity may be either the whole quantity 

 contained in a volume of the free element, or a fraction 

 thereof." 



The second part of this sentence opens the way to the exact 

 study of what is now called the atomicity of elements. 

 Cannizzaro said : 



" In order to determine the atomic weight of any element, 

 it is essential to know the molecular weights, and the com- 

 positions, of all or most of its compounds." 



To know the molecular weights of compounds, it was, of course, 

 necessary to determine the relative densities of them in the 

 gaseous state. Even to-day compounds of not more than 

 about two-thirds of the elements have been gasified. Cannizzaro 

 showed chemists how to obtain approximate values for the 

 molecular weights of compounds that have not been gasified, 

 by studying the chemical relations between these compounds 

 and other similar compounds the molecular weights of which 

 have been determined by the direct application of the hypothesis 

 of Avogadro. 



I think one may say that every chemist who worked 

 between the time of the appearance of Dalton's New System 

 and the Congress of Chemists in i860 had drifted away from 

 the definite but too limited conceptions of the founders of the 

 modern atomic theory, Dalton and Avogadro, and had tended, 

 whether he would or not, to divide the theory into parts, held 

 together by no common binding idea. Cannizzaro returned 

 to Dalton and Avogadro. He realised the parts of the theory, 

 showed the meaning of each part, and set before chemists a 

 clear mental picture of the interrelations of all of them — that 

 is, of the complete theory. 



The definitions which Cannizzaro formed of the molecular 

 weight and the atomic weight of a substance are strictly 

 working definitions; they tell what must be done to determine 

 the quantities which they define. 



In 1852 — that is, five years before the appearance of 

 Cannizzaro's Abstract — Frankland applied the notion of equiva- 

 lency to the atoms of the elements. He invited chemists to 



