194 PRINCIPLES OF CHEMISTRY 



vading water, form salts. And, indeed, salts are found everywhere 

 in nature. In animals and plants they occur, although in but small 



of calcium, 28 parts of iron, 108 parts of silver, 33 parts of zinc, &c. ; and thei'efore, if zinc 

 replaces silver, then 33 parts of zinc will take the place of 108 parts of silver, or 33 parts 

 of zinc will be substituted by 23 parts of sodium, c. 



The doctrine of equivalents would be precise and simple did every metal only give 

 one oxide or one salt. It is rendered complicated from the fact that many metals form 

 several oxides, and consequently offer different equivalents in their different degrees of 

 oxidation. For example, there are oxides containing iron in which its equivalent is 

 28 this is in the salts formed by the suboxide ; and there is another series of salts 

 in which the equivalent of iron equals 18| which contain less iron, and conse- 

 quently more oxygen, and correspond with a higher degree of oxidation ferric 

 oxide. It is true that the former salts are easily formed by the direct action of 

 metallic iron on acids, and the latter only by a further oxidation of the compound 

 formed already ; but this is not always so. In the case of copper, mercury, and 

 tin, under different circumstances, there are formed salts which correspond with 

 different degrees of oxidation of these metals, and many metals have two equivalents 

 in their different salts that is, in salts corresponding with the different degrees of 

 oxidation. Thus it is impossible to endow every metal with one definite equivalent 

 weight. Therefore the conception of equivalents, while playing an important part 

 from an historical point of view, appears, with a fuller study of chemistry, to be but an 

 incidental conception, subordinate to a higher one, with which we shall afterwards 

 become acquainted. 



The fate of the theoretical views of chemistry was for a long time bound up with 

 the history of salts. The clearest representation of this subject dates back to 

 Lavoisier, and was very severely developed by Berzelius. This representation is called 

 the binary theory. All compounds, and especially salts, are represented as consisting 

 of two parts. Salts are represented as a compound of a basic oxide (a base) and an 

 acid (that is, an anhydride of an acid, then termed an acid), whilst hydrates are repre- 

 sented as compounds of anhydrous oxides with water. They employed such an expres- 

 sion not only to denote the most usual method of formation of these substances (which 

 would be quite true), but also to express that internal distribution of the elements by 

 which they proposed to explain all the properties of these substances. They supposed 

 copper sulphate to contain two most intimate component parts copper oxide and 

 sulphuric anhydride. This is an hypothesis. It arose from the so-called electro-chemical 

 hypothesis, which supposed the two component parts to be held in mutual union, 

 because one component (the anhydride of the acid) has electro-negative properties, and 

 the other (the base in salts) electro-positive. Both parts are attracted together, like 

 substances having opposite electrical charges. But as the decomposition of salts in a 

 state of fusion by an electric current always gives a metal, therefore the representation 

 of the constitution and decomposition of salts, called the hydrogen theory of acids, is 

 more probable than that considering salts as made up of a base and an anhydride of 

 an acid. But the hydrogen theory of acids is also a binary hypothesis, and docs not 

 even contradict the electro-chemical hypothesis, but is rather a modification of it. 

 The binary theory dates from Kouelle and Lavoisier, the electro-chemical representation 

 was developed with great power by Berzelius, and the hydrogen theory of acids is due 

 to Davy and Liebig. 



These hypothetical representations simplified and generalised the study of a com- 

 plicated subject, and gave support to arguments, but when salts were in question it 

 was equally convenient to follow one or the other of these hypotheses. But these 

 theories were brought to bear on all other substances, on all compound substances. 

 Those holding the binary and electro-chemical hypotheses searched for two anti-polar 

 component parts, and endeavoured to express the process of chemical reactions by electro- 

 chemical and similar differences. If zinc replaces hydrogen, they concluded that it is 



