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 other, 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, there- fore, 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
