Vapour-density and Smell

Abstract

IN a letter to NATURE of May 13 I made a statement to which Dr. Perman very naturally takes exception (May 27, p. 369). He cites ammonia, hydrocyanic acid, and hydrofluoric acid as instances of volatile bodies lighter than air, yet odorous. In considering the physiology of olfaction, however, certain conditions which might lead to misconception must be ruled out. In the first place, a very minute addition of impurity suffices to give odour to an otherwise odourless substance. Formalin was the substance of which I was writing. My judgment, based on sensory experience, absolutely declines to accept the somewhat fatty scent which I recognise with my nose close to a dish of formalin as a property of the vapour which irritates my conjunctiva when far beyond the range of smell. The chief drawback to the ordinary commercial method of preparing formaldehyde is, I am told, the impossibility of preventing polymerisation. In the same way, as Dr. Perman himself points out, hydrofluoric acid at ordinary temperatures “consists mostly of molecules H₂F₂”. Hydrocyanic acid, again, shows a great tendency to polymerisation and to decomposition in the presence of water. The possibility of ionisation in the presence of the film of moisture on the surface of the olfactory membrane and of the moist air in the nasal chambers must also be taken into account. It is also possible that certain gases produce an olfactive effect after the incorporation of water in their molecules.