H—C≡N (hydrocyanic acid) and
(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.
The physical theory of odour, then, refers that quality to the vibration of the molecule. It suggests that the molecules of an odorous body passing in the gaseous or, in fishes, the liquid state into the olfactory region of the nose, are there received by the film of mucus in which the olfactory hairs lie, and stimulate these hairs by their molecular vibration. No chemical change is supposed to take place, only, as it were, a mechanical stimulation, comparable to the mechanical stimulation of the retina by the waves of light.
A recent development of the theory which we owe to Heyninx, a Belgian scientist, brings the process very closely into harmony with what occurs in the eye. According to this authority, olfaction is in reality a perception of ethereal undulations of the same character as the undulations of light, these undulations being provoked by the intra-molecular vibrations of the odorous vapour in the nasal mucus and transmitted to the olfactory hairs not by immediate contact, but through the medium of the ether.
We owe this last suggestion to the curious fact, but recently discovered, that many odorous substances (in their gaseous form in the air) absorb the rays of ultra-violet light.
In order to make clear what this means, we must say a preliminary word regarding the spectrum and spectrum analysis.
The passage of a beam of white light through a glass prism breaks it up into its component parts, beginning with red, then orange, yellow, green, blue, and ending with violet. Beyond the violet end of the spectrum we know there are rays invisible to us, but capable of acting on a photographic plate. These are called the ultra-violet rays.