Compounds capable of union in this way are termed unsaturated. When the capacity of saturation of a carbon nucleus is less than

CnH2n+29 on account of its being closed in a ring form, the mole

cule acts as though saturated-i.e. cannot combine directly with halogens, nascent hydrogen, &c. In open chains the maximum capacity of saturation reachable is 2 n + 2, in single closed rings = 2 n, in molecules with double rings 2 n-2, &c.

Whilst, for example, the molecule

the nucleus valency increasing from 2 n, to 2 n + 2; by analogy, the body represented by the symbol

would not be easily affected, the nucleus valency remaining 2n, as there is in it only monovalent carbon union, difficult to disjoin.

ISOMERISM.

43. In the foregoing paragraphs frequent mention has been made of compounds which, though possessing the same general molecular formulæ, exhibit different properties in consequence of the varying arrangements of their elementary atoms, being, in fact, different chemical bodies. Such compounds are termed isomeric (from iooμephs, composed of equal parts.) The term isomerism has in general a still further meaning, being applied to all bodies which on ultimate analysis show the same percentage composition—that is, contain the same elements in the same ratio. In consequence isomerism is divisible into several varieties.

44. Polymerism.-Bodies of the same percentage composition but of different molecular weight are termed polymeric compounds.. Their molecular formulæ are either whole multiples of one another or at least of the same simplest atomic-ratio formula.

Such a difference has already been mentioned in § 22.

Another group of polymeric bodies is formed by those hydrocarbons whose formulæ are multiples of CH2.

These latter show considerable resemblance in their properties.

From this variety of polymerism, in which the difference is in the molecular weight only, there is distinguishable the further case of genetic polymerism. There is, namely, a pretty large number of organic bodies, which under certain circumstances are transformed directly into polymeric compounds of similiar chemical properties, in which several similar molecules have united to a single new one. quently this latter can be reconverted into the less complex original molecules by very simple means for instance, by action of a high temperature.

Fre

Aldehyde, a liquid boiling at 21°, and which is miscible with water in every proportion, by contact with acids is converted into paraldehyde, boiling at 124°-and scarcely soluble in water; this latter, on superheating its vapour, is reconverted into aldehyde.

45. Metamerism. In many cases of equal molecular weight and equal percentage composition, the respective compounds are decomposed into several organic bodies by reagents that are incapable of breaking the connection between the carbon atoms in the nuclei. Such compounds, therefore, contain several carbon nuclei united to one another by atoms of polyvalent elements, such as oxygen, sulphur, nitrogen, &c. In these cases the bodies are said to be metameric. There are, e.g.; three compounds of the molecular formula C3H6O2,

which yield, on treatment with caustic potash, salts of organic acids, propionic acid, methylic acetate, and ethylic formate. are expressed by the following equations:

The reactions

46. Structural Isomerism.-When the difference between several organic bodies of the same molecular formula is not due to metamerism, they are isomers in the strict sense of the word, true isomers or structural isomers. The organic radicals contained in them are of the same weight; the carbon nuclei contain the same number of atoms; the difference in properties is caused by difference of position of points of attachment of other atoms to the nucleus (§ 39), or the difference of structure in nuclei of like number of carbon atoms.

As an instance of isomerism of the first kind-place isomerism— there may be mentioned, in addition to those given in § 39—

Of the many instances of isomerism of the second kind due to the method of mutual union of the carbon atoms, nucleus isomerism, there may be mentioned

When in truly isomeric bodies the carbon is united to atoms of one monad element only, the isomerism must be due to nucleus isomerism. From the position isomeric bodies

there results, on replacement of the chlorine by hydrogen, identically the same hydrocarbon :

and on complete removal of all hydrogen atoms from the position

isomers

by chlorine there results the same chloride of carbon:

CI

Cl

CI

Cl

-CI

Cl

As our knowledge of the inner constitution of organic bodies is still

a great many cases very imperfect, and frequently entirely wanting, it results that for numerous undoubted cases of structural isomerism we are unable to settle to which kind it is due.

47. Physical Isomerism. In a very few cases organic compounds of the same molecular formulæ, and possessing the same chemical properties, and in which any difference in structure is not only nonrecognisable but highly improbable, exhibit differences in certain of their physical properties, more especially in their behaviour with polarised light. Such bodies are termed physical isomers.

One of the most striking instances of this kind of isomerism is given by ethylidene lactic acid:

CH3

CH.OH

соон

CO.OH

which, prepared from the juice of flesh, rotates the plane of polarisation of a beam of polarised light, whilst that modification of the acid obtained by the fermentation of sugar is without any action on polarised light.

Very probably the reason of the difference between bodies only physically isomeric lies in an easily imaginable difference in the arrangement in space of atoms which are still united chemically in exactly the same way (geometric isomerism).

HOMOLOGY AND HOMOLOGOUS SERIES.

48. In opposition to the remarkable differences between organic bodies of like molecular formulæ, is the fact that bodies of different molecular composition frequently exhibit great similarity in all their chemical and physical properties.

The compounds in which these analogies are most marked are those whose formulæ differ by CH2, or a whole multiple thereof, nCH2, and whose molecular weights differ therefore by 14n. It is of course obvious, from what has been said before, that a difference in composition of CH, does not make similar properties a necessary consequence; this only occurs when the respective substances agree with one another in their chemical constitution, their structure, and all essential points. This analogy of structure is especially shown in the fact that like reagents produce like changes, and further that the resulting products of such changes agree in properties, but differ in composition by CH2. Such bodies are termed homologous compounds. They form members of a natural family of bodies which can be arranged according to their increasing contents of carbon. The similarity of physical properties between the members of a homologous series is greater the nearer they stand to one another on such a list— that is, the less they differ in chemical composition.

Such a homologous series is formed, for example, by