SMELL--THE PHYSICAL SENSE 47 (d) Smell in Insects. Many insects' smell-sensilla are all peg-like hair derivatives. Stimulation of the sense of smell of some of them does not require moisture (Dethier2). (e) Smell in Animals. Animals have a "smell response," some having a lower threshold than man. (f) Molecular Structure of Odorants. (i) There is a considerable body of information regarding the change in odour as serial changes in constitution, or changes in atomic arrangement in molecules, are made (Moncrieff-'). Thus stereoiso- mers can have different smells, e.g., the position of groups in the molecule among isomers can alter the smell, and unsaturated substances generally have stronger odours than saturated sub- stances. Moreover, substances of different chemical composition can have similar odours. (ii) When homologous series of organic chemicals are smelled, it is seen that the more evanescent odours are the more volatile members with the lower molecular weights. As the series is ascended, the intensity of the smell increases to a maximum and then decreases as the substance loses volatility. (g) Odour Analysis. This is the ability to dissect a smell into several components whilst being subjected to the stimulus of the whole, the extent to which this can be done being dependent on the past experience of the observer. If two odours of about equal strength are smelled, then either a blend or both individuals can be recognised, but if one is much stronger than the other then the weaker is not recognised. (h) Perception. (i) The intensity of perception. According to the Weber-Fechner law this appears to be a log function of the external chemical concen- tration. (ii) Threshold. There is a limiting concentration below which an odour cannot be smelt: this can be as low as 10 '•-• Molar for some substances. (iii) Differences between odours can be detected much more sensitively when the odours are near to their respective threshold concentra- tions. (iv) Change of quality. The odour of many substances changes in quality as the dilution of the substance is altered. Thus, j asmin absolute has a fatty odour to it which completely disappears when diluted. (v) Effect of vdodty of air stream. The olfactory threshold depends on the air pressure and when determined by air blast methods is different from that determined by other methods (Jones'). (i) Masking. It is sometimes possible to hide one odour by the use of

48 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS another appropriate odour. Thus, methyl ionones--which are very powerful, persistent odours rapidly leading to fatigue will obliterate some odours but not others. This type of masking is really "overpowering." Some odours will neutralise each other so that the characteristic odour of neither can be smelt. This is not necessarily due to chemical interaction or to physical association between them, but is a form of masking known as "neutralisation." Iodoform with balsam of Peru and musk with bitter almonds are often-quoted pairs (this is not well substantiated), and cyclo- hexanol and cineole are another pair which have a camphoraceous smell when blended in the correct proportions. (j) .Fixation. Addition of relatively small amounts of some, usually not very volatile, substances to volatile odorants can reduce their rate of evapora- tion, making them last longer and thereby blend into the less volatile odorants dissolved with them. PART II RELEVANT INFORMATION ABOUT OTHER SENSES Many authors have theorised on the subject of smell and, with the exception of Piesse, have thought it to be related to the visual sense, drawing conclusions which amount to postulating the existence of a small number of primary smells and regarding smell as a "chemical sense"--a chemical sense being one in which a stimulant gives rise to a primary chemical reaction which is converted by a suitable mechanism to a message sent along a nerve to the brain. A physical sense is one which causes an inductive, positional or mechanical change, such as vibration or undulation, of some resonating mechanism which is then converted to a message. Sight is concerned with the response of from one to four chemicals to radiation, the chemicals absorbing quanta of energy, undergoing chemical change, thereby giving rise to a nerve message: it is thus a chemical sense. Hearing is a sense in which vibrations of molecules are transmitted to vibra- tions of hairs, giving rise to the appropriate nerve responses: thus it is a physical sense. Smell could be either a chemical or a physical sense. Molecules of odorants could, on the one hand, react with some sensitive chemical in the olfactory organ, giving rise to a chemical change and associated message to the brain, the chemical change being subsequently reversed by a third reactant to recover the original responding chemical (a favourite mechanism of the nervous system). On the other hand, the odorant molecules could be adsorbed on, or dissolved in, some part of the olfactory organ to change its physical position or state, resulting in a nerve message, the receptor recover-