570 REPORT— 1881. 



respecting their ultimate form or structure will have found a place in the pcience 

 as soon as such speculations have helped to arrange the facts which are known, and 

 to discover new chemical facts. 



At the commencement of our epoch chemists had classified elements according 

 to their electro-chemical properties. Chemical analysis had established the fact 

 that a good many compounds could be represented as consisting of elementary 

 atoms of two kinds combined in small number. Thus carbonic oxide and carbonic 

 acid had been found to possess respectively a composition which could be repre- 

 sented (adopting our present atomic symbols) by the formulae CO and COo, water 

 by the formula 11^ 0, marsh gas CH^, olefiant gas CH.^. The oxides and acids of 

 nitrogen were represented by formulae corresponding empirically to those which 

 we now adopt. So also ammonia and hj'dric chloride had their present formulae. 

 Sulphurous and sulphuric acid had the respective formulae SO.^ and SO3. Phos- 

 phorous and phosphoric acid had the formulae, P.fis ^^^ -Pa^^s- Baryta and the 

 oxides of iron had the formulae BaO, FeO, Fe^Oj. 



Such primary compounds were classified upon the same principle which served 

 for the classification of the elements themselves, into electro-positive or basylous 

 and electro-negative or chlorous compounds, and the smallest quantity of each of 

 them, which consistently with an atomic representation of the results of analysis, 

 was deemed capable of existing, was called an atom of that compound. 



Very simple compounds possessed of prominent characteristics and distinct re- 

 actions had first been isolated and identified. They were found to contain their 

 constituent elements in proportions easily recognisable as multiples of atomic 

 weights. But such simple compounds are rare exceptions among mineral and 

 organic m.aterials, and if the atomic theory could have gone no further than to 

 guide us to an understanding of these few simple compounds, it must soon have 

 given place to some more fundamental conception. It is moreover worthy of notice 

 that in this its most elementary form the atomic theory was not the only con- 

 ceivable interpretation of the proportions of combination between elements. Those 

 proportions could be as consistently represented by fractions as by integral mul- 

 tiples. Thus, instead of representing carbonic acid as containing twice as much 

 oxygen as is contained in carbonic oxide, we might have represented it as containing 

 the same quantity of oxygen combined with half as much carbon, and using for the 

 moment atomic symbols for a non-atomic theory, we might have written carbonic 



acid thus — 0. Or we might represent them both by percentage numbers. 



It was so simple and natural to adopt the atomic hypothesis, and to represent 

 compounds as built up of atoms, that chemists seem to have paid little attention to 

 any other mode of representing the proportions of combination. They assumed 

 that the variable proportions of elements, which were observed in compounds, were 

 due to the various numbers of elementary atoms respectively aggregated together 

 in each compound. They perceived tliat the existence of elementary atoms 

 involved the existence of compound atoms, or molecules, as we now call them, 

 and accordingly they represented each known compound of two elements by a 

 molecular formula as simple as possible, consistently with the view of its atomic 

 constitution. Many of these molecules, such as those of the acids, were found to 

 be capable of combining with others of the other class, forming salts, and those 

 combinations were found to take place in proportions corresponding to the weights 

 of the respective molecules, or to very simple multiples of those weights, and the 

 secondary compounds or salts thus formed combined (if at all) in proportions 

 corresponding to simple multiples of their molecular weights. The dualistic 

 representation of the constitution of salts served to represent the results of their 

 analysis, consistently with the atomic theory, and a vast number of fundamental 

 facts were collected and arranged by the aid of the dualistic theory of combination. 



The actual numbers obtained by analysis of any particular compound exhibited 

 sometimes a very near approximation to those required by an atomic formula of its 

 composition. Sometimes they differed considerably from those required by theory ; 

 but it was always found that the more pure the substance and the more accurate 

 the analytical operations, the more nearly did the result agree with some atomic 

 formula of the substance. 



