574 BEPOKT — 1881. 



were also found of which the members present analogies of reaction with mono- 

 basic, bibasic, and tribasic hydrogen salts respectively. 



These and many other such discoveries were made under the guidance of the 

 atomic theory, developed to the point of systematically recognising and studying 

 the mutual reactions of molecules. 



One of the most remarkable and important extensions which our knowledge of 

 molecules has undergone consisted in the discovery that various elements in what 

 we are accustomed to consider the free state, really consist of molecules containing 

 like atoms combined with one another. 



Thu.s chemists adopt the formulse 0,^, H^, CI2, P,, J.^, As^, to denote molecules 

 of the respective elements, and we have for these molecular formulae evidences of 

 the same kinds as those which serve to establish the molecular formulae OIH, H^O, 

 NH3, &c. In all the best-known reactions in which chlorine or hydrogen are 

 either taken up or evolved we find that those elements behave as chemical com- 

 pounds of two like atoms ; and, moreover, their molecules, as determined from a 

 study of their reactions, have the same volume as that of every compound molecule 

 proved to evaporate without decomposition. 



With this knowledge of the molecular constitution of hydrogen and of chlorine 

 gases, we come to regard the direct formation of hydric chloride as due to a process 

 of double decomposition between two molecules, like the reaction of chlorine on 

 an equal volume of marsh gas. 



Many other reactions, such as the evolution of hydrogen by the action of zinc 

 on a hydrogen salt, the liberation of chlorine and nitrogen on the explosive decom- 

 position of their compound, the direct combination of oxysjen and hydrogen, we 

 mav expect to be able to resolve into mere proce->ses of double decomposition. 



The earliest determinations of combining proportions were made with salts 

 (hydrogen salts and others) which undergo double decomposition by mutual contact, 

 and the term equivalent was subsequently introduced to indicate the proportional 

 weights of analogous substances found to be of equal value in their chemical effects. 

 Tables of equivalent weights of acids consisted of numbers standing to one another 

 in the same proportions as the weights of the respective substances found to be of 

 equal value in neutralising a fixed quantity of a particular base ; and in like 

 manner tables of the equivalent weights of bases recorded the proportions by 

 weight in which certain bases might replace one another in the neutralisation of 

 a particular quantity of a given acid. Similar determinations have been tabulated 

 of the so-called equivalent weights of elements. Under the dualistic system 

 chemists paid little attention to the essential difference between atomic weights 

 and equivalent weights; and some were of opinion that the facts of chemistry 

 might be represented as consistently from the point of view of equivalence as from 

 that of atoms, and that the idea of atoms (which they considered to be hypothetical) 

 might be dispensed with. 



In the system of atomic weights employed under that system, two atoms of 

 hydrogen were generally represented as reacting together, and the symbol of the 

 double atom was marked thus, ff. The alkali metals and silver were represented 

 as having atomic weights twice as great as those which we now adopt, and 

 equivalent to those of the magnesian metals and of oxygen. In a great number 

 of the common reactions of these elements the atomic symbols were consistently- 

 used as equivalent symbol.«. But those who professed to dispense with the atomic 

 theory used atomic symbols, even in cases where they did not represent equivalent 

 weights. Thus nitrogen was always represented by its atomic symbol, and the 

 composition and reactions of nitrogen compounds were always studied and repre- 

 sented in accordance with the atomic theory, using various multiple proportions 

 of what they were still pleased to call equivalent weights, using molecular weights, 

 and various other ideas which formed part of the atomic theory, and which had 

 no known connection with the notion of fixed equivalence. If, however, it be 

 true that all chemical compounds consist of elementary atoms, and that the 

 explanation of chemical reactions consists in stating more and more precisely the 

 changes of combination between the constituent atoms of the reacting molecules ; 

 equivalence could only be said to exist between a like number of atoms when they 



