One great objection to it (to my mind) is that it fails to account for another absorption phenomenon of which I have not yet made any mention. It was first observed by Tyndall nearly fifty years ago.
On submitting odorous vapours to examination Tyndall found, not that they absorbed ultra-violet rays, as this method is of quite recent usage, but that they absorbed heat-rays, or the infra-red rays of the spectrum. So that, if it be correct to say that odours set up ultra-violet rays in the ether, we must be equally ready to credit them with setting up infra-red rays also!
But there is another, and perhaps a stronger, objection to the ultra-violet theory.
In the interesting and highly instructive schema drawn up by Heyninx of the wave-lengths of ultra-violet absorbed by odours, we find one or two discrepancies of a serious character.
For example, iodoform and cinnamic aldehyde show absorption-bands occupying nearly the same position on the spectrum; and presumably, therefore, these substances have the same molecular vibration-rate. Yet their odours are not at all alike!
Again, acetone-methylnonic and butyric acids have precisely the same absorption bands, and yet they also exhale totally different odours.
But the most serious discrepancy remains. The absorption bands of hydrocyanic acid and watery vapour (steam) have precisely the same position in the spectrum, yet one of these has a highly characteristic odour, and the other has none at all!
It is rather difficult, in view of these findings, to believe that this absorption phenomenon can have anything to do with the quality of odour.
My friend Mr. T. H. Fairbrother writes regarding this controversy:—
“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.”