The idea has been put forward by Ramsay that the sense of smell is excited by vibrations of a lower period than those which give rise to the sense of light or heat. These vibrations are conveyed by gaseous molecules to the surface network of nerves in the nasal cavity. The difference of smells is caused by the rate and by the nature of such vibrations, just as difference in tone of musical sounds depends upon the rate and on the nature of the vibration - the nature being influenced by the number and pitch of the harmonics. Just as the eye and ear are capable only of appreciating sight or sound vibrations occurring within a limited range, so the nose is unable to appreciate a smell the result of the rapid vibrations produced by substances of low molecular weight. Hydrocyanic acid appears to be at the lowest limit, as one in five are, according to him, unable to detect its odour. It is fifteen times the molecular weight of hydrogen, and he concludes that to produce the sensation of smell a substance must have a molecular weight at least fifteen times that of hydrogen. The intensity of smell in bodies of similar constitution increases with the molecular weight; thus, methyl-alcohol is odourless, but the intensity of smell increases with the molecular weight of each succeeding member of the alcohol group, until the limit of volatility is reached, and they become changed into solids with such a low vapour tension that they give off no appreciable amount of vapour at the ordinary tension.1

1 S. Botkin, junr.: 'Zur Frage uber den Zusammenhang der physiologischen Wirkung mit den chemischen Eigenschaften der Alkalimetalle der ersten Gruppe nach Mendelejeff,' Centralb. fur die med. Wissenschaft. No. 48, 1885.