NATURE 



\_June 22, 1882 



easier to classify carbon compounds. The smell of the paraffins 

 is generic ; so is that of the alcohols, the acids, the nitriles, the 

 amines with their irritation like that of ammonia, the bases of 

 the pyridine series, the hydrocarbons of the benzene group, the 

 higher hydrocarbons, such as naphthalene, anthracene, and 

 phenanthrene. Give any one of these to a chemist familiar with 

 the smell of any one of each series, and accustomed to use 

 his sense of smell, and he will at once refer the body to its 

 class. 



The tendency of a rise in the series is to make the smell 

 "heavier," less ethereal, and more characteristic. It also 

 becomes more able to affect the olfactory nerves. 



The rate at which smell travels is doubtless the rate at which 

 the vapour which gives rise to it diffuses. Still it is impossible 

 to test this experimentally. For the ease with which a smell is 

 perceived varies with the molecular weight of the substance. 

 Thus, if a piece of cotton wool be impregnated with ethyl 

 alcohol, and placed in one end of a long tube, which is imme- 

 diately corked, and a similar arrangement be adopted with aniyl- 

 alcohol, the fifth of the series of which the former is the second ; 

 although their specific gravities have the ratio of 23 to 44, and 

 the ethyl-alcohol should diffuse l\ times as rapidly as amyl- 

 alcohol, yet the smell of the latter will be perceived first, because 

 a much smaller quantity produces the sensation. 



It is possible, with practice, to make a fairly accurate analysis 

 by means of the sense of smell. The method is, knowing the 

 constituents of a mixture, to prepare one which has the same 

 smell, measuring the proportions of the ingredients. The only 

 precaution to be observed is that the smell of no member of the 

 mixture be so overpowering as to mask those of the others. 

 Thus I have analysed, or rather synthesised, a mixture of 

 chloroform with ether, alcohol with ether, and these liquids 

 with carbon disulphide, provided the latter be pure, to within 

 2tper cent. ; but I failed with members of the pyridine series. 

 Yet it was possible to detect the proportions of members of that 

 series to each other ; and it is not difficult, however extraordinary 

 it may appear, to guess approximately the boiling-point of a 

 mixture of members of a series, after some practice, purely by 

 its smell. 



So far as I know, no theory has been brought forward to 

 account for the sen^e of smell ; and I therefore venture to supply 

 this want, premising that what follows is merely a tentative 

 explanation, and as such will, I hope, not be too severely 

 criticised. 



There is a probability that our sense of smell is excited by 

 vibrations of a lower period than those which give rise to the 

 sense of light or heat. These vibrations are conveyed by gaseous 

 molecules to the surface network of nerves in the nasal cavity. 

 The difference of smells is caused by the rate and by the nature 

 of such vibrations, just as difference in 'one of musical sounds 

 depends on the rate and on the nature of the vibration, the 

 nature being influenced by the number and pitch of the 

 harmonies. 



Let us see what evidence can be adduced for the theory. 

 Among the lightest substances which have smell are sulphuretted 

 hydrogen and phosphoretted hydrogen, both of which are seven- 

 teen times as heavy a; hydrogen itself. Prussic acid is fifteen 

 times as heavy as hydrogen, and has a smell. But all persons 

 are not able to perceive it. I have remarked an average of one 

 in every five persons who are totally unable to detect its odour. 

 Here we reach the lowest limit of molecular weight. To produce 

 the sensation of smell, then, a substance must have a molecular 

 ■weight at least fifteen times that of hydrogen. If we compare the 

 hydrocarbons of the paraffin series, with each other, and simi- 

 larly the olefine series, we notice that the lower members have 

 no smell. The specific gravity of marsh-gas, CH 4 , is 8; that 

 of ethane, C„H 6 , 15 ; propane, C 3 H 8 , is twenty-two times 

 as heavy as hydrogen, and here we first notice smell, de- 

 fiant gas, C„Hj, has the specific gravity 14, and has no smell; 

 propene, C 3 H 01 has a faint smell with a specific gravity of 21 ; 

 and the higher members of the series increase in intensity of 

 smell with increase in specific gravity. Hydrocyanic acid is 

 smelt by most persons, but not by all. Its specific gravity is 15. 

 The higher members of the series, called the nitriles, have all 

 very characteristic smell. Formic acid vapour has the specific 

 gravity 23, and has a purely pungent odour. Acetic acid, 30 

 times as heavy as hydrogen, has a faint smell when pure; o- 

 pionic, butyric, and valerianic acids have strong smells. Methyl 

 alcohol has no smell ; its specific giavity is 16 ; ethyl alcohol, 

 23 times heavier than hydrogen, has a faint smell; and, as 



usual, the intensity, and if I may so term it, the flavour of the 

 smell, increases as we rise in series. 



These are the most typical instances of the carbon compounds, 

 and they suffice, I think, to show the justice of the assertion 

 that the intensity of smell increases with rise of molecular weight. 

 The hypothesis of vibration satisfactorily explains this. The 

 period of vibration of the lighter molecules is too rapid to affect 

 our sense ; there is a limit to this power; and just as some 

 people have the power of hearing more acute sounds than others, 

 so some senses are limited by a specific gravity of 1 5, and cannot 

 smell prussic acid. Such people also have difficulty in perceiving 

 the odours of bodies of slightly higher molecular weight than 

 prussic acid. 



Let us now inquire what is the probable rate of such vibra- 

 tions. Mr. Johnston Stoney and Prof. Emerson Reynolds have 

 made investigations of the ratio of the bright lines of some 

 spectra, and have calculated their relations to each other. An 

 analogy will make the nature of this relation more evident. 

 When a note, say C, below the treble clef is sounded on a 

 piano, not only the tone C is heard, but its octave C on the 

 third space ; also G above the line, C on the third leger line, 

 E on the fourth, G on the sixth, B flat above the G, and 

 other notes. These are called harmonics, or over-tones. Now 

 if we knew these over-tones, it would be possible to refer 

 them to their fundamental. So with light. The light evolved 

 by incandescent gases consists of certain colours, which have 

 each their own rate of vibration. Knowing these rates it is 

 possible to calculate the rate of vibration of the fundamental. 

 This has been done by Mr. Stoney (Royal Irish Academy, 

 January 9, 1S71 ) with hydrogen, with the following results . — 



Wave-lengths, /(, 4102*37 tenth-seconds 

 F. 4862-11 

 C, 6563-93 

 These are the 32nd, 27th, and 20th harmonics of a fundamental 

 whose wave-length is 0'I3I3 millimeters. The time of vibration 

 is 4-4 fourteenth-seconds. It maybe objected that these coin- 

 cidences are not a proof. But Mr. Stoney and Prof Reynolds 

 have measured the lines of the spectrum of chromyl chloride, 

 and its 31 lines coincide with those calculated. The probability 

 of the correctness of such a calculation approaches to almost 

 absolute certainty. Now we have no means of recognising such 

 fundamental vibrations, unless, indeed, the sense of smell is one 

 means of receiving them. And it is this which appears to me 

 probable ; so probable, indeed, as to form a working theory. 



But it is to radiant heat, I think, that we must look for indi- 

 cations of harmonics of the fundamental vibrations which are, 

 according to this theory, the cause of smell. And a fresh proof 

 may be drawn from the indications already seen. Prof. Tyndal! 

 has shown the power which odours have of absorbing heat-rays, 

 There is no doubt that by refracting such heat-rays by means of 

 a rock-salt prism, after they have passed through an atmosphere 

 of odour, certain portions of the heat-spectrum show colder 

 spaces, each corresponding to the particular rate of vibration 

 which is absorbed by the vapour, through which the heat-rays 

 have passed. By measuring the position of such gaps in the 

 heat-spectrum, calculating the particular rate of vibration of the 

 rays at such gaps, and referring them to their fundamental, we 

 should arrive at the rate of vibration of the molecule which, 

 causes smell. 



We may now inquire what it is which produces quality of 

 smell. This, I think, can also be explained by the vibration 

 theory, and depends on the harmonics of the vibration. Thus, 

 the quality of tone of a violin differs from that of a flute by the 

 different harmonics or overtones, peculiar to each instrument. I 

 would ascribe to harmonics the quality of smell possessed by 

 different substances. And it is to this that compounds of 

 chlorine, phosphorus, &c, owe their peculiarity of odour. The 

 odour of compounds resembles that of these elements to some 

 extent ; this may be accounted for by the similarity of overtones 

 of compounds and their elements. Then we notice a similarity 

 in quality of the odour of a compound of a scries like the 

 alcohols, and yet the quality grows flatter and heavier with 

 increase in molecular weight. 



Smell, then, may resemble sound in having its quality influenced 

 by harmonics. And just as a piccolo has the same quality as 

 a flute, although some of its harmonics are so high as to be 

 beyond the range of the ear, so smells owe their quality to har- 

 monics, which, if occurring alone, would be beyond the sense. 

 It must be remembered that the harmonics are not heard sepa- 



