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we find Kekulé in 1857 citing the case of sulphuric acid, H,SO, which when acted on by zinc gives ZnSO, and may therefore be said to contain the radicle so, but when acted on by phosphorus pentachloride, the compound SO,Cl, is produced, hence the acid may be said to contain the radicle S0,

53. The conception of types was destined to bear much fruit. Let us briefly trace its development.

Liebig and Dumas had regarded salts as substituted metallic derivatives of acids, they had spoken of a quantity of metal as taking the place of an equivalent quantity of hydrogen; Dumas had even ventured to regard the negative chlorine as capable of replacing an equivalent amount of the positive hydrogen. In doing this, these chemists had returned to the old conception of equivalents—too much forgotten by the Berzelian school—as quantities to be determined by the study of reactions, but they had given this conception fresh life by engrafting on to it the notion of natural families or types.

In writing the formulæ of sulphates, selenates and chromates, as MO.S03; MO. Se03; and MO. CrOz, Berzelius had undoubtedly recognised the principle of types; but so long as this principle was dominated by the necessities of the dualistic system it was unfruitful. The idea of the chemically reacting unit as one whole, one structure with parts capable of replacement by other parts without the necessary destruction of the building, gave meaning to what was before but a form of words.

From its earliest beginnings to its present form the theory of types has been interwoven with the atomic theory; without the latter, the former had never had being. If the value of a scientific idea is to be measured by its fruitfulness, then is Dalton's New System of Chemical Philosophy the most important work yet produced by any chemist.

. Now if the reacting unit of any substance is possessed of a definite atomic structure, only those bodies can be said to belong to the same type, or natural family, whose reacting units are built on a similar atomic plan: but our only method of discovering similarity of structure is by studying reactions; hence only those bodies which are characterised by similarity of chemical function ought to be classified under the same type'. And as modification of structure has been recognised as not necessarily implying destruction of type, it follows that those quantities of radicles, simple or compound, are equivalent, which can perform similar functions in similarly constituted compounds.

1 The modern development of the conception of compound radicle will be better understood by considering pars. 70 to 74 in Section 4 of this chapter.

At last a method of chemical classification has been found by Dumas, Liebig, Gerhardt, and Laurent which, when more fully developed, will reconcile those who regard composition as all important, with those for whom function is supreme; which will preserve the fundamental conception of equivalent, but interpret it in terms of the wider theory of atoms; and which will recognise the connection, while yet emphasising the importance of the difference between the atom of Dalton and the molecule of Avogadro.

But in its development the theory of types must necessarily be largely modified. Classification by types cannot be final in a science which has advanced so far towards becoming an abstract science as chemistry.

"By the classification of any series of objects is meant 'the actual, or ideal, arrangement together of those which ' are like and the separation of those which are unlike; the 'purpose of this arrangement being, primarily, to disclose the 'correlations or laws of union of properties or circumstances, ‘and, secondarily, to facilitate the operations of the mind ‘in clearly conceiving and retaining in the memory the “cha‘racter of the objects in questiona”.

Those 'properties or circumstances' which are correlated must be such as are really characteristic of the objects classified, they must be essential properties of these objects, not mere surface appearances; they must be capable of accurate

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1 See especially Laurent's Chemical Method, pp. 298-300.

2 W. Stanley Jevons (modifying the words of Huxley), Principles of Science, 3. p. 348.

definition, and at the same time of fairly easy recognition; and that property-or properties-chosen as the mark of a class must belong to all the members of that class.

But the properties of a type are necessarily somewhat vague: properties regarded by one observer as essentially belonging to the type may by another be regarded as accidental; a given substance may possess so many of the properties of the type as at one time suffices to ensure its admission into the class, but at a future time new properties may be discovered which necessitate the removal of the substance to a class whose type shews considerable divergence from that under which the substance was originally placed.

The very elasticity, and even vagueness, of the theory of types ensured it an important place in the development of chemical science.

SECTION III. Equivalency of atoms. 54. Dualism had reigned supreme, but only because it was despotic; when the rebellion, headed by Dumas, once got a footing the fate of the older theory was sealed. The new system succeeded because it was not too systematic.

In attempting to preserve unity of type through large series of compounds, the builders of modern chemistry were obliged to make free use of the conception of compound radicles as substituting simple radicles; they thus became familiarised with the general notion of each radicle possessing a definite substituting power.

In 1852 Frankland' extended this conception to the atoms of the elementary bodies; in 1855 Odlingintroduced the use of dashes placed over the atomic symbols to express what he called the replaceable, or representative, or substitu

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1 Phil. Trans. 142. 417, see especially p. 440.

? C. S. Journal, 7. 1. (The recognition of two 'replaceable values' for the iron atom, and other atoms, shews the close connection between the theory then coming into existence and the older theory of equivalents.)

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'tion value of these atoms, he also recognised that an elementary atom may have more than one 'replaceable value'. Odling applied this fruitful conception to the formulæ of many salts, especially the phosphates, and succeeded in shewing analogies until then overlooked.

The inherent fascination of the idea of compound radicle may be realised, by considering that in less than twenty years after Dumas' discovery of the chloracetic acids—which marks the beginning of the revolt against the compound radicles of dualism-Kekulé, and independently of him, Couper? (in papers of the greatest importance) found it necessary to recall chemists to the consideration of elementary atoms as being the true units by the combinations of which all compound molecules are built up, and by whose properties those of the compounds are determined. Couper criticised Gerhardt's development of types, objecting to the vagueness of the idea as a basis for classification; and especially opposing Gerhardt's opinion that the molecular constitution of bodies can never be ascertained by chemists. Would 'it not be rational,' says Couper, ‘in accepting this veto to ‘renounce chemical research altogether?' This dictum of Gerhardt is to be traced, in Couper's opinion, to the overdue employment of compound radicles, to forgetting that these can have no properties which are not a direct consequence of the properties of the individual elements of which they are 'made up,' and hence to endowing these radicles with some unknown and ultimate power which it is impossible to 'explain.' Returning then to a study of the elements, Couper finds chemical affinity as a property inherent in, and common to them all; he distinguishes 'affinity of kind' and 'affinity ‘of degree;' applying the latter to carbon, he cites the oxides CO and CO, (in his notation C,0, and C,02), the former expressing the first, the latter the second and last degree: CO, is the ultimate affinity, or combining unit for carbon.'

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1 Annalen (1837), 104. 129. ? Phil. Mag. (1858) [4], 16. 104.

Kekulé in 1857, and more especially in a paper published in March 1858'—a paper the importance of which can hardly be overrated—distinguishes more clearly than Couper “affinity of kind' from 'affinity of degree;' or rather he distinguishes chemical affinity from what he calls the basicity of atoms, both conceptions being needed, he says, for the explanation of chemical combinations. Kekulé clearly distinguishes—and this distinction has been too much forgotten in recent developments of chemical theory-between equivalent weights of elements, and equivalency (or basicity) of elementary atoms; he shews that the new theory deals with definite entities, called atoms, having defined properties, and not with 'unit weights,' and that it is these atoms which he proposes to compare as to their substituting power for the hydrogen atom. Having shewn that one atom of carbon, so far as our knowledge goes, is never combined with more than four atoms of hydrogen in a compound molecule, Kekulé also shews that two atoms of carbon do not bind to themselves more than six atoms of hydrogen, three atoms of carbon not more than eight atoms of hydrogen, and so on.

The tetravalency of the carbon atom, and the power which two, or more, atoms of carbon possess of binding themselves together in a molecule, are enunciated by Kekulé in this paper, which forms the foundation stone of the modern theory of 'atom-linking?'

Kekulé and Couper insisted, that if a definite theory of the connections between properties and structure of compounds is to be obtained, it must be based on the study of the combining powers of the elementary atoms : The whole is simply a derivative of its parts,' said Couper.

55. A theory which shall attempt to explain the atomic structure of compound molecules, must, in the present state

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1 Annalen, 106. 129.

? In comparing Couper's paper with Kekulé's it may be well to notice how Couper attempts to trace a close connection between the basicity of atoms and chemical affinity; his statements are here much vaguer than Kekulé's, yet this dynamical method of regarding 'valency' at the very outset of the theory is important.

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