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'react as to produce two new bodies, AD and BC, it follows that the electrical polarities are better neutralised in the latter pair of bodies than in the former.'

48. On the basis of this theory Berzelius raised the structure of the dualistic chemistry, which asserted that every compound, whether simple or complex, must be constituted of two parts, of which one is positively, and the other negatively electrified.

The doctrine of dualism is thus introduced by Berzelius': 'If these electro-chemical conceptions are just, it follows that 'every chemical compound is dependent on two opposing 'forces, positive and negative electricity, and on these alone; ‘and that every compound must be composed of two parts 'held together by their mutual electro-chemical reactions. *Therefore it follows that every compound body, whatever be 'the number of its constituents, can be separated into two 'parts, whereof one is positively and the other negatively

electrified. Thus, for example, sodium sulphate is put 'together, not from sulphur, oxygen, and sodium, but from 'sulphuric acid and soda, which again can themselves be 'separated into positive and negative constituents. So also 'alum cannot be regarded as immediately built up from its 'elements, but must rather be looked on as the product of a 'reaction between sulphate of alumina and sulphate of potash, 'the former acting as a negative, the latter as a positive element?

In support of his theory Berzelius appealed to the facts of electrolysis. A solution of sodium sulphate containing a little blue vegetable colouring matter is electrolysed; the colouring matter is reddened around the positive electrode and rendered more distinctly blue around the negative. What can this experiment teach but that the salt is separated by the electric current into alkali and acid ? And can the inference be avoided that the salt is composed of, or contains in itself,

1 Lehrbuch, loc. cit. p. 79.

? See also Berzelius, Theorie des proportions chimiques, et de l'influence chimique de l'électricité dans la nature inorganique; 3rd Ed. Paris, 1835. Also, for a condensed account of the electro-chemical theory of Berzelius, see Ladenburg, loc. cit. pp. 89-93.

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these two compound radicles, soda (NaO) and sulphuric acid (SO)? All salts were to be regarded as dualistic structures. Given the composition of a salt, a dualistic formula-or rather a series of formulæ—was at once devised for it. The following formulæ were employed by various dualistic chemists to express the structure of acetic acid,

(1) C,H,O3. HO (2) CH.04.H, (3) C.1.0.02. H2O
(4) (C2H)C,0g. H2O (5) (C4HC,O,. H (6) (C,H,O)CO,. HÀO
(7) CH.04

(8) C.H..0, (9) C.H.02. H,02 (to) C,H,.0,HGTo choose the proper formula from such a chaos was a task possible only for one whose foible was omniscience. That formula which had the weight of authority on its side was accepted as correct.

49. Lavoisier had regarded oxygen as the 'acidifying principle'. Hydrochloric acid was undoubtedly an acid substance, therefore, in accordance with the dictum of Lavoisier, it contained oxygen. Davy's study of this compound, and of its analogue hydriodic acid, nevertheless established the fact that an acid can exist which contains no oxygen. The further fact, that so many of the oxides—then called acidsexhibited acid properties only in presence of water, led Davy to the belief that very many acids contain hydrogen. Shaking off the trammels of that older philosophy which regarded the introduction of undefined 'principles' as affording explanations of natural phenomena, Davy said that acids are not characterised by the invariable presence of any one element, but that certain compounds of very diverse elements belong to this group'.

Dulonga in 1815 further advanced Davy's conception of acids by recognising no essential difference between those acids which contain oxygen and those which do not. Lavoisier's acid theory was not however generally abandoned until many years later.

1 For an account of the important work of Davy on the non-oxygenised acids, and the arguments of his opponents, see Ladenburg, loc. cit. pp. 81–87.

2 Mem. de l'Acad. 1813–15, p. 198, and Schweigger's Journal, 17. 229.

In 1837-38 Liebig', following up Graham's work on phosphoric acid', distinctly recognised the existence of 'replaceable hydrogen' in acids, whether oxy-acids or acids containing no oxygen, and defined salts, as compounds belonging to the same class as acids, and formed by putting metal in place of an equivalent quantity of hydrogen in acids'.

This view of the structure of salts was altogether opposed to the dualistic theory of Berzelius.

50. Another severe blow was inflicted on the prevailing theory by Faraday's researches on electrolytic decompositions.

Faraday shewed that the quantities of various elements set free from different electrolytes, by the same electric current, were chemically equivalent to one another: thus for each two parts by weight of hydrogen set free from water, there were obtained 16 parts of oxygen, 78'2 parts of potassium, 63.5 parts of copper from persalts and 127 parts of copper from protosalts. But the affinities of the atoms of the various electrolytes were undoubtedly different in each combination. According to Berzelius, the quantity of electricity collected on any group of atoms is greater, the greater the mutual affinity of these atoms; but Faraday's experiments shewed, that in so far as this electricity was measurable by electrolytic decomposition, (and that at least comparative measurements should be thus obtained followed from the terms of the dualistic theory itself), the quantity of it was in no way dependent on the affinities of the combining atoms

51. A bold and partially successful attempt-such an attempt as could be made only by a man of preeminent power—had been made by Berzelius to found chemical classification on the study of composition alone, almost wholly divorced from the study of function, or power of doing. As his authority became greater, Berzelius led chemistry further from the only true path by which she could advance, that namely wherein experiment, and reasoning on experimental data, go hand in hand. And yet no single chemist has enriched the science by the addition of so great a mass of laboriously and accurately determined experimental data as he. The intense concentration of his great intellectual powers upon one view of chemical phenomena led Berzelius to disparage the reasoning of those who sought to view these phenomena from standpoints other than his

1 Compt. rend. 6. 863 (with Dumas): and Annalen, 26. 113, see especially

p. 181.

2 Phil. Trans, for 1833. 253.

3 See, in connection wit acid generally, Laurent, Chemical Method, pp. 39-45.

* See Helmholtz, ‘The Faraday Lecture.' C. S. Journal Trans. for 1881.

pp. 284—6.

own.

Among those who recalled chemistry to the true scientific method, Dumas, Laurent, and Gerhardt stand preeminent.

In 1839' Dumas described trichloracetic acid, obtained by the action of chlorine on acetic acid. The new compound, although containing chlorine in place of hydrogen, was a monobasic acid, and resembled acetic acid in its general reactions. Dumas said there are certain types in organic chemistry which are maintained even when an equal volume of chlorine, bromine, or iodine, is put in the place of hydrogen in the parent substance'.

Berzelius, and the defenders of the dualistic chemistry, violently opposed the idea that the electrically negative chlorine could be substituted for the positive hydrogen, and the identity of type yet be maintained. In Dumas' succeeding papers the conception of types was more fully developed. All bodies containing the same number of equivalents of simple substances, combined in a similar manner, and exhibiting broad analogies of properties, were regarded as belonging to the same type. Such bodies were also, as a rule, simply related to one another by reactions of formation and decomposition :thus acetic and chloracetic acids ; chloroform, bromoform, and iodoform ; ethylene and its chloro

1 Compt. rend. 8. 609, and Annalen, 32. 101. 2 Compt. rend. 8. 621.

3 Annalen, 33. 259: 35. 129 (with Stas), and 289 (with Peligot); or Compt. rend. 9. 813, and 10. 149. M. C.

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derivatives, &c., belonged to the same types, or as Dumas said to the same 'natural families'. Dumas regarded carbonyl chloride as derived from carbonic anhydride by substituting one oxygen by two chlorine atoms-thus COO gives COCI,; this was utterly opposed to the dualistic view, according to which the formula of carbonyl chloride was written co.CCI,, because every compound must be composed of two parts, one of which is electrically positive and the other negative.

52. The new school of chemists naturally opposed the conception of compound radicles, a conception too closely associated with those dualistic theories they were leaving behind, to find favour in their sight. But these chemists found that unless substitution of simple atoms by groups of atoms were regarded as possible, identity of type could not be maintained through groups of compounds undoubtedly belonging to the same natural family.

Inasmuch as the new chemistry based its claims to recognition on an appeal to actual reactions, it was impossible that it should long refuse to recognise the conception of compound, as well as simple, radicles, without proving false to its own method. Liebig and Wöhler, in their researches on oil of bitter almonds, explained the observed reactions of the compounds they obtained by assuming the existence of the compound radicle benzoyl (=C4H1,02) in these bodies (see Annalen, 3. 249).

But what are these compound radicles which the chemists who upheld the unitary system were obliged to recognise, equally with their opponents who supported a dualistic theory? Are they definite groups of atoms always existing as such in compound molecules, or are they only convenient methods of expressing and generalising reactions ?

As chemistry advanced, compound radicles came to be generally recognised as certain groups of atoms, in compound molecules, which remain undecomposed throughout a series of reactions undergone by those compounds'. Thus

See especially Laurent's Chemical Method, pp. 276—300. Also Ladenburg, luc. cit. gth and roth Lectures.

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