The next question that arises is: Do bodies exhaling the same kind of odour resemble each other in the structure of their molecules? In other words, can odour be related to molecular structure?
To the chemist all matter is made up of atoms and molecules. The elements, bodies which cannot be broken up by chemical action into any simpler form, are composed of atoms. On the other hand, when elements combine to form a compound, the unit of the new body, composed as it is of two or more atoms of different elements linked together, is known as a molecule. (Probably the elements also exist in the molecular state, the atoms of which they are composed being linked together in groups.) Both atoms and molecules are, of course, very minute in size.
For reasons we need not enter into here, the molecule is held to have a certain structural form, which form is indicated by what is known as a graphic formula. The graphic formula of water, one of the simplest, may be written as H—O—H, and we may regard it as having a linear form. (Modern views indicate that it is not a simple line, but in two planes.)
Many molecules, however, particularly those of the organic compounds, are highly complex, and their structural form must be very different from that of water.
The question, then, now before us is: Does odour bear any relationship to the molecular structure of bodies? And again it has been maintained that a clue to the problem of the real nature of odour lies here.
There is a well-known series of chemical bodies known as the “aromatics,” by reason of the fact that they possess strong smells more or less similar in quality. With regard to this series, which is made up of groups of what are known as radicles which occupy definite positions on a molecule shaped like a ring—the benzene ring, as it is called—Henning, a German observer, has expressed the opinion that the odour depends, not upon the radicles as such, but upon the position they occupy on the ring.
Transferring his argument to odorous bodies in general, and taking six groups as embracing all (spicy, flowery, fruity, resinous, burnt, and foul), he associates each of these types with some feature in the constitution of the molecule which is common to all the members of each group.
To enter more fully into this branch of the subject would carry us too deeply into chemistry. I shall content myself therefore with saying that Henning’s views have received considerable support from scientific chemists and have led to several interesting and suggestive developments.
Heyninx, however, criticising this theory, points out that hydrocyanic (or prussic) acid and nitrobenzol, two substances with the same smell, have each a molecular structure in no way resembling the other.
The graphic formulæ of these bodies, which I give here, plainly show the difference between them: