TEANSACTIONS OF SECTION B. 573 



the result of a decomposition of water by the metal, such decomposition being 

 indui-ed by the presence of the acid (SO,), which exerted a predisposing affinity 

 for the zinc oxide. Our present explanation is a simple stateuient of the fact, that 

 under the conditions described, zinc displaces hydrogen from its sulphate. 



The recognition and study of the metallic functions of hydrogen enabled 

 •chemists to obtain far clearer and simpler views of the constitution of salts, and 

 to observe the differences of property which are produced in them by the replace- 

 ment of one element bj' another. It enabled us to see more and more clearly the 

 characttristic functions of each element, by comparing the constitution and pro- 

 perties of the salts containing it with those of the corresponding salts containing 

 '•"ther elements. 



Thus in the dualistic system we had for the three common phosphates, 

 PO.Nag. PO^Na.^II, P04NaH2, molecular formulte in which sodium was represented 

 with twice as great an atomic weight as that which we attribute to it, and which 

 in our atomic weights may be thus represented, viz. P^Oj, 3Na,0 ; PoOj, 2Na, ; 

 PjO^, NajO. In like manner we had such a formula as P0O3, 2Na^0 (tor the phos- 

 phite PO^NajH), and for the hypophosphite PO^NaHj we had a formula cor- 

 responding to PjO, Na20. 



Determinations of water of crystallisation and of chemically combined water 

 proved that many of the compounds assumed on the dualistic system to exist 

 are either not obtainable or have different properties and a different constitution 

 from those which have been described. Thus we now know that the salts 

 PO^Na^H, POjNaH.j, POgNa.^H, and PO^NaH^ cannot be deprived of the ele- 

 ments of water without undergoing a fundamental change of composition and of 

 properties. 



The atomic weights of the alkali metals and of silver were found to be half of 

 those of the dualistic system, and an atom of one of these metals, in common 

 double decompositions between their salts and hydrogen-salts, changes place with 

 one atom of hydrogen. 



Many products of the combination of known molecules were found to be 

 formed by processes of double decomposition, so that each molecule of such 

 products is built up partly of atoms derived from one of the materials, partly of 

 atoms from the other. Thus potassic hydrate is formed by the combination of a 

 molecule of potash with one of wai-er. Yet each molecule of the hydrate is built 

 up of half a molecule of potash and half a molecule of water. 



The study of organic compounds played an important part in the improvement 

 -of our processes of reasoning. Many of their molecules having a very complex 

 structure were found to undergo in most of their reactions very simple changes, of 

 the same kind as those which mineral compounds undergo. Most of the elements 

 of each organic molecule remained combined together with functions analogous to 

 those of hydrogen or chlorine. 



The theory of radicals which had been suggested by the reactions of ammonia- 

 aalts and of cyanides was largely extended in organic chemistry. 



Many families of organic compounds were discovered in each of which the 

 members are connected by close analogy of constitution and of properties. Each 

 of these families forms what is called a homologous series, each term of the series 

 being a compound of which the molecule contains one atom of carbon and two 

 atoms of hydrogen more than the previous term. 



Thus a series of compounds was proved to have reactions similar to those of 

 common alcohol, and molecular weights ranging from 32 to 438. The lower terms 

 of the series are distinguished from one another by differences of boiling points 

 approximately proportional to the number of atoms of carbon and hydrogen by 

 "which they differ from one another ; whilst the higher terms undergo decompo- 

 sition at the high temperatures required for their evaporation, and are distinguished 

 from one another by differences of melting points, that of the alcohol CjQHgjO 

 being about 86° C. In their constitution these alcohols were found to be analogous 

 to the alkaliue hydrates. 



In like manner various other series of alcohols were discovered corresponding 

 respectively in their constitutions to other classes of metallic hydrates. Series 



