then no less than 280 forms of combination are possible. It is clear that the atomic linking in a given compound can only be determined by calculation in the very simplest

cases.

§ 46. Determination of the Linking by Synthesis and Analysis. The decomposition and building up of compounds afford a valuable means of determining the atomic linking. The conclusions based on these methods depend on the assumption that those atoms which are united together before the union of two compounds remain united together after the act of combination, and that, on the other hand, those atoms which remain joined together after the decomposition of a compound were previously united in the said compound. This deduction was made use of long before the doctrine of atomic linking was known; but, strange to say, the conclusions arrived at in this way were proved to be untenable by the knowledge of atomic linking and were abandoned after a prolonged discussion.

The very ancient observation that a salt is formed from an acid and a base, and can again be decomposed into these constituents, led to the view that the acid and base are present as such in the salt. Calcium carbonate decomposes into lime and carbon dioxide, CaCO ̧=CaO+CO2; from this it was inferred that calcium carbonate contained the proximate constituents CaO and CO2, and that its formula was CaO,CO,.

Analogous formulæ were given to other salts, e.g.

Formulæ of this description are at present used solely for 'molecular aggregations,' such as salts with water of crystallisation. That they are not permissible for real chemical compounds is obvious from the fact that these groups are

already saturated and therefore have no free affinities available for mutual combination.

H--0--H, Ca=0, K--0--K, 0==C==0 0==S==0, 0==P--0--P==0

These formulæ are called dualistic, on account of their separation into two parts. They have been replaced by others in which the groups are united together by means of their oxygen atoms:

H--0--S--0--H

The principle underlying these old formula-viz. the assumption that those atoms which were united in a compound must be regarded as remaining combined-still remains in force, but in its application due care is taken to comply with the laws of atomic linking.

For example, the action of hydrochloric acid on alcohol is represented by the equation

Water

Alcohol Hydrochloric acid Ethyl chloride

In this case two of the carbon atoms and at least four of the hydrogen atoms remain together; we may assume, therefore, that these atoms are contained in alcohol and in ethyl chloride also

The two groups H2O, water, and HCl, hydrochloric acid, are closed groups, in which all the affinities are saturated. We must therefore assume that not more than one hydrogen atom in alcohol is united to oxygen; that is to say, that alcohol contains the monovalent hydroxyl' group, OH, and that this group and the five hydrogen atoms must be directly attached to the carbon thus: C2H ̧

-0

– H.

By the action of hydrochloric acid oxygen is split off; there is no doubt about this taking place, as the oxygen ceases to be united to carbon. The oxygen atom takes away with it the atom of hydrogen to which it was previously united. Where does the second atom of hydrogen come from? Is it taken away from the carbon or the chlorine? Since the monovalent chlorine must detach itself from the hydrogen before it can unite with the carbon, it is more than probable that the hydrogen atom from the hydrochloric acid combines with the hydroxyl. As there is only one interpretation for a group containing two carbon atoms, the formula must be CH‚—CH‚—O--H+H—Cl = CH3-CH2Cl + H—0—H Ethyl chloride

3

Alcohol

2

The groups of atoms which remain united together in these reactions are termed 'radicals,' as they were considered to be the roots from which the peculiarities of the compounds arose; this expression dates back to the time of Lavoisier. Alcohol and ethyl chloride have the same root, the radical ethyl, C,H,.

Further conclusions as to atomic linking may be deduced from the decomposition and the mode of formation of substances. If an atom or a radical replaces another in a compound, we assume that it takes the place previously occupied by the latter. The chlorine from the hydrochloric acid takes the place of the hydroxyl, and the hydroxyl takes the place of the chlorine and unites with the hydrogen atom. This replacement of one atom or radical by another is termed 'substitution,' and is distinguished from addition '-i.e. the simple union or combination of two radicals or atoms; e.g.

6

C2H1Br2

Bromine Ethylene bromide

The formation of additive compounds is frequently used in determining atomic linking, it being assumed that the addition takes place at those points where the available affinities are situated.

§ 47. Determination of Atomic Linking from Physical Properties. Numerous observations show that the atomic linking exercises a marked influence on the properties of substances,

and it often happens that two isomeric compounds containing exactly the same constituents possess totally different properties, solely in consequence of the different linking of their atoms; e.g. ethyl acetate has a pleasant ethereal odour and butyric acid has a rancid, offensive smell. When the constitution of a series of substances has been investigated, the influence of the atomic linking on their properties is soon apparent, and we are now able to determine the atom linking by a study of the properties. The following physical properties chiefly come into consideration: density, fusibility, volatility, colour, solubility, crystalline form, smell, taste, physiological action. The relation of all these properties to atomic linking has not yet been sufficiently investigated. These relations grow clearer day by day, and by this knowledge it is now possible to make new substances possessing certain desired properties. The recent syntheses of dyes and colouring matters offer a brilliant example of our power to accomplish this object.

The application of physical properties to the investigation of atomic linking depends chiefly on the fact that the physical properties change regularly, when a certain definite alteration in the atomic linking is repeatedly made and the rest of the compound is left unchanged. If we examine a series of organic compounds, the members of which differ from each other in their molecular weights by increments of 1C and 2H, it is often observed that the boiling points of these bodies exhibit a certain fixed difference for each CH2 group. This is not always the case, but only when the constitution and linking remain the same, with the exception of the slight difference in composition due to the introduction of the group CH2. When we find that there is a regular increase in the boiling point with the molecular weight, we conclude that the bodies have the same or similar atomic linking. Numerous examples of these relations are found in the text-books on organic chemistry.

§ 48. Determination of Atomic Linking from the Chemical Behaviour. The chemical properties are even more dependent on the atomic linking than the physical properties. The influence which certain groups of atoms exert on chemical

behaviour is first determined in compounds of comparatively simple structure, such as those, for instance, in which only one arrangement of the atoms is possible or such as permit their atomic linking to be easily ascertained. More complicated compounds exhibiting similar properties are assumed to contain the same groups of atoms. If large or small deviations occur, the origin of these is investigated by increasing the number of observations and comparing them with one another. In this way a valuable collection of rules has been obtained, which enable us to deduce the structure of compounds from their chemical properties.

In the case of each of the frequently occurring combinations of atoms, such as hydroxyl, OH, amide, NH2, imide, NH, &c., we have not only determined which properties of the compounds indicate their presence, but we have also ascertained the differences in their behaviour caused by the nature of the atoms or radicals with which they are combined. For example, we are not only acquainted with a whole series of tests for identifying the presence of hydroxyl, but we can also ascertain, from the peculiarities in the behaviour of the substance, whether this hydroxyl is attached to carbon or nitrogen or whether the carbon atom united to the hydroxyl is combined with hydrogen, or oxygen, or only another carbon atom. In other words, we have the means of discriminating between the following formulæ :

2

=N- OH, CH2- OH, =CH-OH, =C—OH, —CO—OH, and many others.

If a compound contains nitrogen we can ascertain from its chemical character whether the nitrogen is directly combined with oxygen, oxygen and carbon, hydrogen and carbon, or with carbon only.

The relations between atomic linking and chemical properties are, at the present time, amongst the chief objects of investigation in the field of organic chemistry, and are generally discussed in connection with this branch of the science. Unfortunately this subject seldom meets with systematic and comprehensive treatment.1

The relationship is systematically treated in E. Lellman's Principien der organischen Synthese, Berlin, 1887.