158 SCIENCE PROGRESS 



consider facts which led him to assign to each elementary 

 atom a certain " combining power . . . which is always satisfied 

 by the same number of [other] atoms." Frankland acknow- 

 ledged, in 1877, that his hypothesis could not have been satis- 

 factorily developed had not Cannizzaro " placed the atomic 

 weights of the metallic elements on their present consistent 

 basis." But Cannizzaro did much more to advance the subject 

 of atomic equivalency, and to prepare the way for the over- 

 whelming multiplicity of its applications in recent chemical 

 work, than is indicated in the words quoted from Frankland's 

 Researches. 



In the Abstract of his Lectures Cannizzaro tabulates the 

 molecular compositions of series of chlorides, and shows that 

 one atom of each of certain elements combines, sometimes with 

 a single atom of chlorine, sometimes with two atoms of chlorine : 

 as one atom of chlorine combines with one atom of hydrogen, he 

 concludes that a single atom of each of the elements in question 

 is equivalent to one atom of hydrogen in some compounds, and 

 to two atoms of hydrogen in some other compounds. He then 

 shows that the weight of chlorine which combines with one 

 atom of certain elements — call these, elements of Class I. — com- 

 bines with two atoms of certain other elements — call these, 

 elements of Class II.; and then, expanding his data, he finds 

 that the weight of any element which combines with a single 

 atom of an element of Class I. combines with two atoms of an 

 element of Class II. He thus arrives at the conception that each 

 atom has a definite capacity of saturation. Having defined unit 

 capacity of saturation of an atom as the power of combining 

 with a single atom of hydrogen, chlorine, bromine, or iodine, to 

 form a molecule, he classifies many atoms in accordance with 

 their saturation-capacities. 



The consideration of all the facts which he had marshalled 

 convinced Cannizzaro of 



** the unchangeability of the proportions between the atomic 

 weights of the bodies which mutually replace one another, what- 

 ever be the nature and number of the other constituents of the 

 compounds." " This is a law," he said, " which limits the number 

 of possible compounds and more especially applies to all cases 

 of double exchange." 



Having thus got a firm hold of the law of atoms, following 

 from the study of the compositions of molecules, and the law of 



