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: H—C≡N (hydrocyanic acid) and H C HC C—NO_{2} | || HC CH (nitrobenzol). \ / C H (T. H. Fairbrother, to whom I am indebted for much information on the chemistry of olfaction, would dispose of this criticism of Hcyninx’s by denying that the odours of those two substances are identical. See later, p. 132.) Chemistry, then, having, according to the critics, failed us, we turn to the allied science of physics. Physics deals with matter in its ultimate state, beginning, so to speak, where chemistry, with its work of changes and combinations, ceases, and taking us deep into the heart of matter independent of its chemical properties and behaviour. We have seen that, chemically speaking, elements and their compounds exist as molecules made up of atoms. Now molecules may be minute, and atoms even more minute, but in “electrons,” the name given to the last divisible particle of matter known to the physicist, we are dealing with minuteness inconceivable. Sir Oliver Lodge has said that if an atom could be expanded to fill a space equal to that of the entire solar system, the electrons composing it would each be the size of an orange! There is supposed, indeed, to be an atomic “system” composed of a central nucleus like the sun, with electrons revolving round it, the nucleus having a positive, and the revolving particles a negative, electric charge. Further (whether in virtue of these moving electrons or otherwise is not quite clear), the molecule is supposed to be in a state of constant vibration.