Chapter I. Method.
Section 1. Determination of the intensity of the Smell-Stimulus for the Normal Organ.
If all the nervous elements concerned in smell are in a normal condition, and if “‘compensation’’ does not come into play, the intensity of an odor depends on the number of odorous particles in gaseous form which are acting on the olfactory nerve-endings at the time. Perhaps it is safe to say that the intensity is ordinarily a function of the number which are acting on the rod-cells of the olfactory mucous membrane.[1] Whether or not individual rod-cells are subject to cumulative stimulation, we do not know, for we do not know even whether the stimulation is chemical, thermal, or electrical,[2] but we do know that the intensity of the smell seems to depend on the extent of membrane and therefore on the number of rod-cells stimulated,—always supposing that the rod-cells are the olfactory cells proper.
Now the number of odorous particles which act at any given time on the olfactory membrane of the normal nose depends, first, on the quantity of vapor which the fragrant body is throwing off ; secondly, on the rate of the diffusion of this vapor ; and thirdly, on the manner and rate of breathing. Let us consider these facts separately.
I. The Quantity of Vapor Thrown off by the Odorous Body. “Whether” says Zwaardemaker, “odorous particles are set free by evaporation or chemical reaction, the mass of odorous molecules which are given off from a solid body or the surface of a liquid is, ceteris paribus, in compound proportion to the time of exposure and extent of surface exposed.[3] Zwaardemaker has invented a “genetic unit” for the measurement of odor in the physical sense. It is the number of seconds of exposure multiplied by the number of square millimeters of surface exposed.[4] It is unnecessary to say that the genetic unit differs from substance to substance. The “other factors” which must remain equal, if the genetic unit of a given substance is to be constant, are the moisture, weight, and temperature of the atmosphere and the amount of ozone present in it.[5]
That heat and dampness affect the intensity of odors is a matter of common observation. Yellow wax smells twice as strong in summer as in winter. Heat promotes evaporation. Dampness also promotes the vaporization of such solids as are
