SMELL--THE PHYSICAL SENSE 71 The types of receptors can be identified from the study of their physio- logy. This is being attempted by Lord Adrian '•a, and here the work of Derbier -ø" ,•nd others on the physiology of the receptors of insects will be helpful. An adaptation of the "flicker technique" should also be useful, for if odours at concentrations near to their thresholds could be presented in pairs to the receptors in alternating impulses, those which stimulated different types of receptor would be perceived as separate odours at higher frequencies of presentation than odours which overlapped several types of receptor. This can be done by using "Maxwell's colour mixing device", by painting solutions of odorants on to a disc whose speed of rotation can be varied in a known manner the disc can be made of wire mesh and a low pressure jet of air can be smelled after passing through the disc. Such an apparatus lends itself to the very necessary simple and rapid alternating change of odour. Work needs to be done on the evaluation of threshold concentrations in terms of threshold vapour pressure as a fraction of saturated vapour pressure. This involves the measurement at 30-:38 ø C. of the saturated vapour pressures of a wide range of substances whose vapour pressures are very small. Once the threshold is known, the relation between the intensity of perception and the activity of the stimulus should be investigated. It will be interesting to discover the minimum number of quanta needed to perceive a smell. It is striking that nearly everyone who investigates olfaction omits to give details of the purity of the chemicals which they have used as odorants mostly they seem to take no steps whatever to make sure that they are dealing with a single substance. Until steps are taken by workers to determine the purity of their materials, it cannot be expected that a great deal of progress will be made, since synthetics sometimes owe their characteristic odour to impurities present in small quantity--e.g., see Krajkeman •. To know the threshold of an impure chemical is useless if the impurity is an odoriferous body. Detection of impurities is not difficult with the modern methods of infra-red spectra analysis, and the even newer--and possibly more useful--gas chromatograms. When it is recalled that individuals vary in their perception of odour and that therefore the effect of each odorant will have to be determined on a large number of them, it is seen that there is an enormous amount of work to be done. The author suggests that all workers in this field should set up a co-ordinating body to ensure that their efforts do not overlap and to advise those wishing to commence work on oilaction of the areas of research which most need attention and of the most successful methods of investigation. In this way the various research projects could be kept in balance and arranged to complement each other. One very important function of such a central body could be that of issuing samples of odorants of known purity.

72 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS The author wishes to thank Messrs. R. S. Lacey and J. Pickthall for reading the manuscript and for their comments thereon: he also thanks his wife for her help in typing the manuscript. REFERENCES • Basic Odour Research Correlation. Ann. N.Y. Acad. Sci., 1954, õ8. 13-260 e Dethier, V. G., ibid., 139. a Moncrieff, R. W., The Chemical Senses. Leonard Hill, Ltd., 1954. 4 Jones, F. N., Ann. J. Psychist, 1953, 66, 81. s Hainer, R. M., et al., Ann. N.Y. Acad. Sci., 1954, õ8, 159. • Mozell, M. M., and Pfaff mann, C., ibid., 97. 7 Allison, A. C. and Warwick, R. T. T., Brain, ?g, 186. s Warwick, R. q_. T., and Le Gros Glark, W. E., J. Neurol. Neurosurg. Psychiat., 9, 101 9 Aitken, J., Proc. R. S. Edin., 1905, gõ, 894. 20 Boer, J. H. de, The Dynamical Character of Adsorption. Clarendon Press, 1953.34. n Boer, J. H. de, ibid., 42. • Adrian, E. D., Advancement of Science, 1952, 9, 417. •a Middleton, A. W., J.S.C.C., 1953, IV, 230. •'• Hainer, R. M., etal., Ann. N.Y. •lcad. Sci., 1954, õ8, 170, Fig 5. • Dethier, V. G., ibid., 146, Fig. 7. • Ferguson, ]., Proc. R S., 1939, B, lg?, 887. •* Boer, ]. H. de, The Dynamical Character of Adsorption, 45. •s Cutting, Thesis Ph. D., London, 1937. •9 Boer, J. H. de, The Dynamical Character of Adsorption, 86. .,0 Poucher, W. A., J.S.C.C., 1955, ¾I, 80. • Boer, J. H. de, The Dynamical Character of ,•dsorption, 52. •2 Pickthall, J., J.S.C.C., 1954, V, 194. •a Adrian, E. D , Electroenceph and Clin. Neurophysiol..1., 1950, g, 377 ,4 dvancement Science, 1947, 4, 287 1952, 9, 417. 34 Krajkeman, A. J., J.s.c.c., 1956, VII, 38 ERRATA to Vol. VI, No. 4. Page 264, line 26, "Ca•H• ", should read "C•He• ". Page 273, line 21, "higher ", should read, "lower". Page 286, line 13, "F.P.S." should read "F. R. S." Page 292, line 3, "--3" should read "--0'3 ". In the advertisement on page viii we regret that the name of the advertiser was misspelt it should, of course. have been Honeywill-Atlas Ltd.. and the sole selling agents Honeywill & $•ein Ltd. Pubhsl•ccI on bel•aH ot tBe Society of Cosmetic C•em•sts of Great Bream trom 3s Devercux Dr•ve. Wattoral, HerIs. January, 1955, Printed and Bound by Fisher. Knigh• & Co,. Ltd., Gainsborough Pre.• gr. Alban.q. Hertfordshire. England