4i6 



NATURE 



\Sept I, 1 88 1 



found lo be half of tliose 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 prodacts of the combination of l<nown 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 materi ils, partly of atoms from the other. 

 Thus potassic hydrate is formed by the combination of a mole- 

 cule of potash with one of water. 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 stnicture 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-salts and of cyanides was largely extended 

 in organic chemistry. 



Many families of organic compounds were discovered, in each 

 of which the member^ are connected by close analogy of consti- 

 tution and of properties. Each of these families forms what is 

 called a homologous series, each term of the series being a 

 compound of \^■hich the molecule contains one atom of carbon 

 and two atoms of liydrogen more than the previous term. 



Thus a series of compounds was proved to have reactions 

 siaiilar to those of common alcohol, and molecular weights ran- 

 ging from 32 to 438. The lower terms of the series are distin- 

 guished 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 decomposition at the higher temperatures 

 required for their evaporation, and are distinguished from one 

 another by differences of melting points, that of the alcohol 

 CgfiH^jO being about 85' C. In their constitution these alcohols 

 were found to be analogous to the alkaline hydrates. 



In like manner various other series of alcohols were discovered 

 corresponding respectively in their constitutions to other classes 

 of metallic hydrates. Series were also found of which the mem- 

 bers present analogies of reaction with monobasic, bibasic, 

 tribasic hydrogen salts respectively. 



These and many other such discoveries were made under the 

 guidance of tlie atomic theory, developed to the point of system- 

 atically recognising and studying the mutual reaction of 

 molecules. 



One of the most remarkable and important extensions which 

 our knowledge of molecules has undergone consisted in the dis- 

 covery that various elements in what we are accustomed to con- 

 sider the free state, really consist of molecules containing like 

 atoms combined with one another. 



Thus chemists adopt the formulae Oo, H„, CU, P4, J21 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 formula: CIH, H„0, NH3, 

 cS:c, In all the best-known reactions in which chlorine or hydro- 

 gen are either taken up or evolved we find that those elements 

 behave as chemical compounds of two like atoms ; and, more- 

 over, their molecules, as determined from a study of their re- 

 actions, 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 decomposition of their compound, 

 the direct combination of oxygen and hydrogen, we may 

 expect to be able to resolve into mere proceses 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 con- 



sisted of numbers standing to one another in the sane propor- 

 tions 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 Ijeen tabulated of 

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

 system chemists paid little attention to the essenti.al 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, 



Ij. 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 etiuivalent symbols. 

 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 represented in accordance 

 with the atomic theory, using various multiple proportions of 

 u hat 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 consti- 

 tuent atoms of the reacting molecules ; equivalence could only 

 be said to exist between a like number of atoms when they were 

 known to have similar functions. It became necessary to study 

 the relation of equivalence between elementary atoms, instead of 

 studying them from the point of view of elements divisible in 

 any proportion. 



It is worth while noticing the general process by which this 

 intellectual change was brought about ; for there is a good deal 

 yet to be done in the matter, and our future progress may be 

 guided by experience gained in the past. 



It was es ;entially one-sided. One consideration was brongbt 

 into very prominent relief, and it threw a marvellous light on 

 the matter. It gave us a clear view of the natural order among 

 elements ; but, like every other strong light, it fell on one side 

 only. 



The equality of vapour-volumes had been used with great 

 advantage in conjunction with chemical reactions and other evi- 

 dence as a characteristic of molecules, and the attention of che- 

 mists was greatly arrested by the consideration of four typical 

 compounds, which upon the concurrent evidence of very exten- 

 sive chemical examination and equality of vxpour-vjhimes were 

 known to have respectively a comoosition corresponding to the 

 formulae CIH, 0H„, NH3, CH4. 



It was known that the atom of oxygen in water can be re- 

 placed by chlorine, but that two atoms of chlorine are needed 

 for the purpose. Tlie atom of nitrogen in ammonia requires 

 three atoms of chlorine to replace it, whilst in marsh gas the 

 atom of carbon is replaceable by four atoms of chlorine. Other 

 elements were studied from the point of view of their respective 

 resemblance to these, and arranged in classes, each of which 

 consisted of atoms equiv.alent to one another. Thus chlorine, 

 bromine, iodine, fluorine, hydrogen, potassium, sodium, lithium, 

 silver, &c., constituted a class of atoms of equal value, and were 

 called monads. Oxygen, sulphur, .'elenium, tellurium, calcium, 

 strontium, barium, magnesium, zinc, cadmium, mercury, lead, 

 copper, &c., were classed together as dyads, having equal value 

 amongst themselves, but double the atomic value of the members 

 of the first class. So nitrogen, phosphirns, arsenic, antimony, 

 bismuth, with boron, and some other elements, were considered 

 as forming a class of atoms each of which has three times the 

 value of the monads. The class of tetrads contained carbon, 

 silicon, tin, platinum, &c. 



Many apparent exceptions to these atomic values were satis- 

 factorily explamed as due to the partial combination of like 



