“Whilst I do not for one moment suggest that the whole phenomena of smell can be explained entirely in terms of chemical constitution, I do maintain that it has much to do with it, and I certainly think that more valuable information about the cause of various odours has been obtained from considerations of chemical constitution than from the many extravagant physical theories which do not lead us very far. In my view the physicists are begging the question, because they usually postulate something which we cannot prove, and whilst it is possible that the vibration of electrons causes smell, how much wiser does that statement make us? One might easily say that it was possible that the bombardment of electrons caused smell, etc. On the chemical side, however, we are bound down to experimental facts, and we do know that esterification of carboxylic acids does bring about a fruity odour invariably, etc. Chemical constitution cannot explain fully all these phenomena, because chemical formulæ themselves are only approximations, but the effect of groups in a nucleus has done much to help synthetic production of odorous bodies. When the physicist can control the vibrations of his electrons and make them rotate in accordance with his will, then he may be able to synthesise new odours—till then we have no means of testing his theories.” The older view of olfaction—and many modern scientists, as we see, still adhere to it—is that the odorous molecule acts as a chemical reagent upon the olfactory hairs. And there is something to be said for this opinion. To begin with, no one doubts nowadays that odours are material. They pass through the air as vapours, and they are known to travel miles on the wind. That is to say, apart from those hypothetical varieties of odour (if we can call them odour at all) discussed by Fabre earlier in this book, odours do not emanate from a point and disperse in all directions as light and sound do. Why then drag in the ether? Is it not more probable that the odorous molecule acts on the olfactory hairs by direct material contact, and that it sets up chemical changes in them? We are asked to believe that the ultra-violet rays of odour stimulate the olfactory hairs as visible light-rays stimulate the retina. But it must not be forgotten that in the eye those rays may induce first of all chemical changes in the retina, just as they would act on the silver salt of a photographic plate, and that it may be by these changes that the retina is stimulated. In the phenomenon of olfactory exhaustion, as we said in our first chapter, we have a circumstance which suggests the presence of some chemical reagent in the olfactory area. It may be, of course, that in the nose as well as in the eye the process is a combination of chemical and physical changes. And in any case we are here dealing with that obscure region where chemistry and physics meet and mingle.