Sept. I, 1881] 



NA TURE 



415 



as a compound containing only two atoms of chlorine. It was 

 found that the commoneot and best-known carbonates and 

 sulphates have a fundamentally similar constitution. Thu? pot- 

 as?ic carb:>nate may be represented as a compound in which the 

 two atoms of chlorine in phosgene are replaced by two atoms of 

 the radical O K ; and oil of vitriol, as a compound of two atoms 

 of hydroxyl with the same group, S 0„, which in chlorosulphuric 

 acid is combined with two atoms of chlorine. Chlorochromic 

 acid has not been examined to a^ great an extent a^ the above 

 compounds, but all we know of it points clearly to its having 

 molecular constitution similar to that of chlorosulphuric acid, viz. 

 CI; Cr Oo, for not only do their vapour densities agree, but the 

 chromates in their constitution and cry.-talline forms exhibit a 

 clear analogy lo the sulphates. 



Moreover, the simpler molecular formula?, which a fuller 

 knowledge of their chemical behaviour suggested for these bodies, 

 were found in all cases to agree with the volume belonging to 

 the molecule of every pure substance known in the state of 

 vapour. 



A difficulty of another kind had been foreseen by the great 

 founder of the duali^tic sy-tem, and it was by the investigations in 

 organic chemistry that it assumed serious proportions. 



Carbon compounds were discovered possessing definite and 

 specific properties, and presenting the characteristics of pure 

 substances, h\\\. of which the results of analysis did not agree with 

 any simple proportion between the numbers of their constituent 

 atoms. Their empirical com]>osition could not be decided by 

 the aid of the so-called law of multiple proportions, for two or 

 more atomic formula; required percentages of the constituents 

 differing so little from one another that analysis could not decide 

 which was the true one. 



In order lo select the Ime molecular formula; of such complex 

 substances from among those which approached most nearly to 

 the results of ultimate analysis, and to determine with certainty 

 their empirical composition, it was necessary to find other methods 

 for the determination of molecular weights. It was necessary to 

 study the various properties of coiiipounds of known composition, 

 and of others which could be prepared in a state of purity ; to 

 determine the vapour densities and rates of diffusion of those 

 which could be obtained in the gaseous state without decompo- 

 sition ; to determine boiling points and melting points; to examine 

 crystalline forms of pure compounds and of mixtures ; to deter- 

 mine solubilities and den.sities of solids and of liquids ; but above 

 all it w as necessary to collect fuller and more accurate know- 

 ledge of the chemical changes which take place in the mutual 

 reaction of molecules. 



A vast amount of accurate and careful work of these kinds has 

 been done, and has been subjected to rigid and often hostile 

 scrutiny during the various steps of its progress. We now kmw 

 that compound atoms, or molecules as we call them, which can 

 be identified by their geometrical, mechanical, and other pro- 

 perties, are the same as the compound atoms indicated by the 

 most comprehensive chemical evidences of composition and 

 reactions. The molecular constitution of matter was predicted 

 implicitly by the atmiic theory of the constitution of the 

 elements ; and, wherever the physical properties of the mole- 

 cules are such as afford any basis for the determination of their 

 relative weights, such results agree with those derived from 

 purely chemical considerations guided by the atomic theory. 



Gur know ledge of molecules is as yet in its infancy. Even 

 among the commonest elements and compounds we know the 

 molecular weights of very few, liut what we do know of them 

 proves that the idea of compound atoms invented by chemists to 

 explain the elementary facts of chemical action is, as far as it 

 goes, a true representation of what exists in nature. 



Many of the molecules thus proved to exist were the same as 

 thoje suggested under the dualistic system ; but many were 

 proved, by the more accurate and extensive kno" ledge of their 

 reactions and properties, to have a different weight from that 

 which hail been at first atlributed to them, yet always consistent 

 with the fundamental requirements of the atomic theory. Thus 

 H.,0, CO, CO.J, CHj, SO;, SO3, CaO, FeO, Fe.,03, are the 

 formula; still u-ed to denote the molecules of the respective 

 compounds, though the last three ought probably to be repre- 

 sented by some multiple. On the other hand, the molecule of 

 olefiant gas is ujw represented by the formula C„Hj, instead of 

 CH„. The chloracetate is CjCl^HO.,, instead of CXIg, C.Pj, 

 HjO. The mo'ecule of benzoil chloride is C7H5OCI, instead of 

 one corresponding to (C-H5).,0.j, C.H5CI3, and chlorosulphuric 

 acid is CUSO.,, instead of 2803,' SC'lj. 



In proportion as chemists came to kmw more of the constitu- 

 tion of molecules, and to study chemical reactions from the 

 point of view of the changes which they bring ab^ut in the con- 

 stitution of molecules, did the idea of substitution come to be 

 more and more used in the place of that of mere additive com- 

 bination. A vast number of processes of chemical combination, 

 which had been considered as consisting of direct combination, 

 were found to be processes of double decomposition. 



One of the most important facts which was brought to light 

 by the careful examination of the composition of salts and 

 organic bodies, aided by the molecular method of representing 

 their constitution, was that hydrogen is chemically one of the 

 metals, and that the compounds formed by the combination of 

 water with acids are analogous to other salts of those acids ; 

 while compounds of hydrogen with elements or radicals like 

 chlorine are silts, analogous in their constitution to other 

 chlorides, &c. 



The molecular or unitary mode of viewing the constitution of 

 each substance affords more tru( as well as more simple records 

 of the facts observed in chemical reactions than could be obtained 

 in the dualistic systems. A salt such as hydric sulphate used to 

 be considered as containing sulphuric acid and water, and repre- 

 sented by a formula such as SO3, H„0, implying the presence 

 in it of both the substances from which it was known to be 

 formed. 



When two elements combined, their product was considered 

 and described as containing the elementary atoms which had 

 served to form it, and it was consistent with this habit to rei re- 

 sent a product which had been formed by the combination of 

 two compound molecules as containing those molecules. 



But the main business of chemical investigation is to observe 

 accurately the changes of composition which take place in the 

 reactions of known substances, with a view of discovering the 

 atomic changes to which they are due. 



The compound formed by the com'^ination of sulphuric acid 

 and water differs in many physical and chemical properties from 

 both of those bDdies. Its name and its atomic formula serve to 

 denote the ajgrega'e of properties which are known to belong to 

 it, whereas the dualistic fornuila, SO3, H^O, served to recall the 

 properties of the acid and liase from which it was formed, rather 

 than those of the compound itself. 



Elementary chemical reactions which according to the binary 

 mode of viewing compounds were suppo ed to consist of dual- 

 istic proce-ses involving sometimes the assumption of forces 

 (like predi-posing affinity) of a purely metaphysical character, 

 were now explained as consi-ting of atomic displacements, or 

 interchanges of a kind well known to be of common occurrence. 

 Thus the evolution of hydrogen by the action of zinc or aqueous 

 hydric sulphate was supposed to be the result of a decompo-ition 

 of water by the metal, 'Uch decomposition being induced by the 

 presence of the acid (SO3), which exerted a predisposing affinity 

 for the zinc oxide. Our present explanation is a simple state- 

 ment of the fact, that under the conditions described, zinc dis- 

 places hydrogen from its sulphate. 



The recognition and study of the metallic functions of hydro- 

 gen enabled chemists to obtain far clearer and simpler views of 

 the constitution of salts, and lo observe the differences of pro- 

 perty which are produced in them by the replacement of one 

 element by another. It enabled us to see more and more clearly 

 the characteristic functions of each element, by comparing the 

 constitution and properties of salts containing it w'ith those of 

 the corresponding salts containing other elements. 



Thus in the dualistic system we had for the three common 

 phospihates, P04Na3, POjNa.^H, POjNall;, molecular formula; 

 in which sodium was rciiresented with tnice as great an atomic 

 weight as that which we attribute to it, and which in our atomic 

 weights mav be thus represented, viz. PjOj, 3Na„0 ; P0O5, 

 2Na, O ; PIO3, Na„0. In like manner we had such a formula 

 as PjO;,, 2Na.,0 (for' the phosphite P03Na.,H), and for the hypo- 

 phosphite POoNaHo we had a formula corresponding to P„0, 

 Na^O. 



Determinations of water of crystallisation and of chemically 

 combind 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^ir, POjNaH„, POjNaoH, and PO„NaH„ cannot be 

 deprived of the elements of water without undergoing a funda- 

 mental change of composition and of properties. 



The atomic weights of the alkali metals and of silver were 



