572 REPORT — 1902. 



be represented and sometimes demonstrated to be. Calcium hydroxide and 

 liydrogen sulphide give calcium hydrosulphide and water by two single inter- 

 actions together, which in this case can be eas-ily distinguished, since the calcium 

 hydroxide will also interact with onlj' half as much hydrogen sulphide to form the 

 insoluble crystalline calcium hydroxyhydrosulphide and half as much water as 

 before ; this calcium salt will then interact with as much more hydrogen sulphide 

 as went to form it, and produce the very soluble crystalline calcium hydrosulphide. 

 Or the calcium hydrosulphide and as much calcium hydroxide as yielded it will 

 readily interact to form twice as much as the first-obtained quantity of calcium 

 hydroxyhydrosulphide. Thirdly, the calcium hydrosulphide and half as much 

 water as was formed with it from calcium hydroxide readily interact to produce 

 calcium hydroxyhydrosulphide, and half as much hydrogen sulphide as was 

 needed to form the hydrosulphide. Therefore, and on other grounds, we say and 

 know that one molecule of calcium hydroxide and two molecules of hydrogen 

 sulphide give one molecule of calcium hydrosulphide and two molecules of water. 

 This is, of cour.^e, only the law of multiple proportions introduced into chemical 

 interactions. The expression ' two or more molecules of a substance ' has a 

 meaning only as indicating the number of simultaneous or successive single inter- 

 actions which have led to the conversion of certain substances into others. 



Now a similar but complementary state of things meets us in the case of 

 radicals. Instead of the coefficients of molecules, necessitated by having 1o 

 consider many chemical changes as being cases of two or more single interactions 

 occurring together, there are the valency coefficients of the polyvalent radicals, 

 called out also by such a compound interaction. Thus, in the above case, whilst 

 the single interaction between hydrogen sulphide and calcium hydroxide shows 

 calciumhydroxyl as one of the radicals, the succeeding interaction between the 

 calcium hydroxyhydrosulphide and more hydrogen sulphide shows the radical 

 calciumhydrosulphury], and the common part of these two radicals is the bivalent 

 radical, calcium. It will be evident that to give the atom of tlie calcium radical 

 as bivalent is a statement reciprocal or complementary to that of giving two 

 molecules of hydrogen sulphide as interacting with one of calcium hydroxide. 

 Chemical equality remains still the measure of the atom, but that, in complex 

 changes, whereas the number of molecules of one substance marks the number of 

 single interactions, the valency number of the atom marks the same thing for the 

 radical. It is a matter of valency, and not otherwise a matter of the atom. The 

 radical calcium is never actively bivalent in a single interaction ; in other words, 

 it is never equal to two atoms of hydrogen. As a simple radical it does not take 

 part in such an interaction ; but it does do so as a radical of radicals, such as 

 calciumhydroxyl and calciumhydrosulphuryl, and then has the same measure as — 

 is equal in exchange to — the atom of liydrogen, though carrying with it of necessity 

 other radicals, a thing the hydrogen radical never does or can do. To take 

 another example : when acetamide is formed from acetic acid, the nitrogen of the 

 amidogen and the oxygen of the hydroxyl are equal in exchange, but because of 

 their valencies the one carries with it two atoms and the other one atom of 

 hydrogen. This is no matter of merely academic contention, for upon its recogni- 

 tion rests the doctrine of valency itself. 



The quantity of the radical is the only proper and sufficient definition of the 

 atom, whether the radical be that of a single interaction, or a radical of radicals, 

 that is, a polyvalent radical. The atom is, therefore, the quantified power of a 

 substance, as the compound of the radical, to produce other compounds of the 

 radical, including its compound with itself, where that is possible. As with the 

 molecule of a substance, so with the atom of the radical, it is of no fixed magni- 

 tude, and may weigh a kilogram just as well as only a milligram or something 

 much less. Being a relative quantity and nothing by itself, of its indivisibility 

 there is nothing to be said outside its definition ; whilst, as to its being the smallest 

 relative quantity interchanging in an interaction, it had only thus to be defined 

 when there was uncertainty as to the molecule and the single interaction. 



It has been impossible for me to discuss the nature of the radical and the atom 

 without referring to valencjj but it is itself a subject of such importance as to need 



