SMELL--THE PHYSICAL SENSE 65 in order to be equal in perception must have equal partial molar free energies in each phase. An alternative approach to the effectiveness of the use of the relation løge P/Ps as a measure of the stimulus is as follows. The properties of the nervous system show that the intensity of perception is directly proportional to the frequency of the electric impulse passed along the nerve, which is a function of the stimulation. The intensity of the impulse in one particular nerve must therefore be proportional to the amount of material adsorbed on the receptor cells (i.e., on their hairs) connected to it. Consider any one gas, if the number of molecules adsorbed per unit surface in unit time is a, the number of molecules impinging on unit surface per second is n, and the average time that each spends on the surface is r. Then tr --=- nr. Now a is the only •neasure of the stimulus which is possible, and, before we can theorise further or understand experimental results, we must be able to express a in known terms. De Boer TM shows that a: Kp where K •- Nro eO/•er 'X/(2•'MRT) for adsorption at constant temperature--this being the case in the nose. It means that the amount of chemical adsorbed is directly proportional to the pressure of the gas, and it involves the assumption that the energy of adsorption is constant for all the molecular species being considered, and that they are not sufficiently closely packed to interfere with each other. Now for a 100 per cent uniinolecular layer (% Mols./unit area) •o = gps and for some other value ,, corresponding to p, * = Kp. ß '. a/ao: PIPs and the fraction covered is the fraction which the vapour pressure is of the saturated vapour pressure. This result was found experi- mentally by Cutting • to be approximately true for the adsorption of volatile insoluble substances on water (which approximates to a surface of uniform activity). Thus, since the stimulus will be proportonal to the amount adsorbed on the receptor hairs, it is proportional to the vapour pressure of the substance, and the ratio of vapour pressure to saturated vapour pressure TABLE III Substance Ethyl alcohol Phenol Pyridine Vanillin Threshold Concentration by Volume Grains/era a Molecules/cm a =n __ 5 X 10 a 6'5 X 10 • 1-2 X 10 -ø 7.7 X 10 • 4 X 10 -• 3.0 X 10 • 2 X 10 -la 7.9 X 10 s by Area __ Molecules/cm 1-6 x 10 t• 1.8 X 10 TM 4-5 • 10" 8.5 x 10 •

66 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS is the measure, as a fraction of the total possible stimulus, of the stimulus applied to the receptor. This is the same conclusion as that reached earlier from consideration of partial free energy. (h) (it) Threshold. Table III summarises available data of the threshold concentration of a few odorants. The enormous difference between substances in the number of molecules striking unit area of the olfactory receptors at threshold concentration is seen. However when, in accordance with the preceding discussion, the threshold concentrations are converted to vapour pressures and the ratio of the threshold vapour pressure to the saturated vapour pressure is cal- culated-using the equation p = nlO-•2x/(MT)/3.52 mm. Hg, we obtain: TABLE IV p calculated from Substance n in above table, P, mm. Hg* mm. Hg Ethyl alcohol 5.37 X 10 -4 88 6.1 X 10 -6 Phenol 8.65 X 10 -•ø 6.7 X 10 -x 1.3 X 10 -9 Pyridine 1.98 X 10 -• 29 6.8 X 10 -x8 Vanillin 5.19 X 10 -•5 I 2 X 10 -a 2.6 X 10 -•' * p, is extrapolated to 32 ø C., using existing vapour pressure data and the relation log p ---- K log T q- C where K and C are constants and T is absolute temperature. Taking into consideration the assumptions which have been made in calculating the values of Ps and p in Cols. 3 and 4 of the above table, we can do no more than say that present evidence suggests that at the threshold concentration the minimum stimulus is of the order of pips = 10 '•2. The extent of the adsorption will apply to those hairs for which the substance has a heat of adsorption Q (which depends on the amino acid structure of the hairs) and will be to different extents on hairs which have varying heats of adsorption, and will vary as the more active spots on the individual hairs are used up. For values of p close to p•, which in turn has a high absolute value, several of the olfactory receptor areas will be covered by a monolayer for the sub- stance which will have a multiple odour quality that will emerge as a single odour perception as p is reduced. It is to be expected that any given hair type will be capable of adsorbing all molecules whose heat of adsorption on it is greater than that of water, and that it will then transmit an impulse message to the brain, the quality of the perception varying with the position of the receptor in the olfactory area. It is also reasonable to expect that hair type will alter over the olfactory area, so that there will be changes in receptor characteristics. This being so, for any one substance Q will vary over the total area on which the molecule will be adsorbed from an optimum to a negligible amount. We have the