(The Pièce de Résistance)  
The anatomical structure of the olfactory end-organ in the nose is, as we saw in Chapter II., simple.
Contrast it with the eye. Here we have what is obviously an optical instrument, with lens, iris diaphragm, dark walls, and sensitive plate complete—a photographic camera, in a word.
Contrast it also with the ear, which is an acoustic apparatus reminding us in its detail of a recording gramophone leading to a closed box in which are what look like a series of resonators, like the wires of a piano.
In the antechamber of each of those organs the physical vibrations to which they respond undergo considerable modification before they reach the sensory cells.
In the antechamber of the olfactory organ, on the other hand, the amount of modification necessary is evidently but slight, as the olfactory region of the nasal chamber is merely a narrow, open 99passage. As far as we know, all that takes place is that the incoming stimulus, the odorous molecule, is warmed and received by the nasal mucus.
Thus the very complexity of the structure both of the eye and of the ear helps us to comprehend their function.
But what can we deduce from a flat surface in which all we can see is a collection of cells with minute protoplasmic hairs projecting from their distal ends? Obviously, little or nothing. We are, in fact, confounded by simplicity. It may be that we are here dealing with one of the essential properties of all living matter, little, if at all, altered from its primitive condition.
To the physiologist, then, olfaction is the most mysterious of all the senses. It still retains its secrets, and therein lies the fascination of its study.
Of late years, the exploration of this dark region of physiology has been, and is still being, vigorously pushed, and we shall now proceed to give what, however, can only be a brief and superficial account of the progress made and of the opinions held. Even so we shall be compelled to make an incursion into the high and dry realms of modern chemical and physical theory. That may not be good hearing, but what is still worse is that almost every single point we shall be discussing is a matter of controversy.
100Let us commence with a few of the details, mostly unimportant, upon which there is general agreement.
Consider, first of all, the variety, the almost infinite variety, of odours. We have, for example, all the odours of the world of Nature, the emanations of inorganic matter, of the earth itself, its soil and its minerals; to these we must add the multitudinous perfumes of the vegetable kingdom, of barks, roots, leaves, flowers and fruits, including those of growing herbaceous plants, which differ so widely from one another that it is said of Rousseau, whose myopia was compensated for by an unusually acute sense of smell, and who was, moreover, no mean botanist, that he could have classified the plants according to their smell had there been a sufficiency of olfactory terms for the purpose; then we have the thousand effluvia, some pleasant and others not so pleasant, of living animals, including the various races of mankind; next come the—mostly repulsive—odours of decaying vegetable and putrefying animal matter; and finally the products of man’s own proud ingenuity and skill, such as the artificial perfumes and flavours on the one hand and on the other coal-gas, acetylene, carbon disulphide, and the like.
Parker notes it as worthy of remark that man has created, both accidentally and intentionally, 101many new odours—smells, that is to say, which have no fellow in the world of Nature—and he emphasises the fact that the nose is nevertheless capable of appreciating such novel sensations.
In this connection we may mention that the art of modern perfumery can imitate closely many of the natural perfumes, and more particularly the natural flavours, by mixing together essences, or components, which in no way resemble the final product.
Thus the flavour of peaches can be compounded artificially of aldehyde, acetate, formate, butyrate, valerianate, ?nanthylate, and sebate of ethyl, and salicylate of methyl, with glycerine, glycerine being added to the fruit essences, as it is to wines, in order to restrain the evaporation of the volatile bodies. (The fruit essences are used only in the making of flavours. They cannot be employed as perfumes, as they are too irritating to the nose.)
The union of components to form a product different from any one of them is found also in vision. When the colours of the spectrum, for example, are commingled, the resultant white light is devoid of any colour.
Thus the potential responsiveness of the olfactory organ seems to be practically inexhaustible. So far, at all events, it has not yet reached the limits of its capacity.
102The number and variety of recognised smells being so great, then, one can readily understand how difficult it is to construct a classification of odours. Many attempts have, in fact, been made, but, depending as they do more or less upon subjective sensation, no two classifiers give us the same classification. Indeed, a division of all smells into “nice,” “neutral,” and “nasty” would be about as good as many much more ambitious efforts.
Zwaardemaker’s is the classification most usually followed at present, and as it is to him we owe most of our knowledge of scientific olfaction, we shall detail it here:
(1) Ethereal or fruity odours; (2) aromatic, including as sub-classes camphrous, herbaceous, anisic and thymic, citronous, and the bitter almond group; (3) balsamic, with sub-groups floral, liliaceous, and vanillar; (4) ambrosial or muscous; (5) garlicky (including garlic), oniony, fishy, and the bromine type of odour; (6) empyreumatic (guaiacol); (7) caprylic (valerianic acid); (8) disgusting; and (9) nauseating.
The subjective character of these classes is obvious, especially in the last two groups, but, apart from that objection, most people will be inclined to protest when they learn that chloroform and iodoform are put into the first, the ethereal or fruity, group, while it is suggested, though to be 103sure with a query, that coffee, bread, and burnt sugar may belong to the “repulsive” (pyridine) group!
The fact is that Zwaardemaker’s classification is based upon a chemical foundation, that is to say, upon properties which, as we shall see later on, do not necessarily correspond with the odours as we smell them. That, no doubt, explains his inclusion of iodoform among the “fruity” odours.—Iodoform fruity!—Shades of George Saintsbury and his “Cellar Book”!
A shorter classification is that of Heyninx, who, aiming at objectivity, bases his arrangement, to some extent at all events, upon the spectrum analysis of odorous molecules in the atmospheric medium, of which more anon. His list is: acrid, rotten, f?tid, burning, spicy, vanillar or ethereal, and garlicky. But here, also, the coupling of vanillar with ethereal odours seems a little inappropriate.
We stand, perhaps, on rather firmer ground when we turn to the manufacturer’s classification, founded as it is frankly upon subjective sensation, and therefore devoid of any surprises to the logical faculty. Here is Rimmel’s arrangement: rose, jasmine, orange, tuberose, violet, balsam, spice, clove, camphor, sandal-wood, lemon, lavender, mint, anise, almond, musk, ambergris, fruit (pear).
It may be objected, perhaps, that this is a 104catalogue merely, not a scientific classification. That is quite true. But what is also true is that the others we have quoted are little, if any, better. The fact is that we do not yet possess the knowledge necessary to enable us to arrange odours in classes.
The manufacturers, of course, concern themselves with agreeable and attractive odours only. To the great and growing company of the stinks they pay no attention whatever. For that reason their contribution to our knowledge is necessarily but partial and limited.
In their own proper domain, however, they can point to several great successes. They recognise, for practical purposes, about eighty primitive scents. Many natural (to say nothing of many unnatural) perfumes can now be prepared artificially, and some so prepared are said to be even more powerful than the natural productions. Artificial musk, for example, is one thousand times stronger than natural musk, Parker tells us. Deite, on the other hand, says that the smell of artificial musk is not equal to that of the natural! Indeed, according to this authority, although synthetic perfumes play an important part in the concocting of scents, there are only a few of them which can be used instead of the natural product. What happens is that the artificial and the natural are generally used in combination. Thus the 105“mignonette” of the shops is prepared by passing geraniol, an artificial odorivector made from citronella oil, over the natural mignonette flowers, the resulting product being an essence smelling strongly of mignonette, and not at all of geraniol.
One or two, as we said, are purely artificial imitations; coumarin, for example, the “new-mown hay” of sentimental memory, which used to be obtained from the tonka bean, is now entirely made up by the synthetic chemist. But for all the more subtle essences we have still to rely upon Nature’s laboratory. The manufacturer steps in and distils the precious essential oil certainly, but it is from flowers that he obtains it. Attar of roses, for instance, contains, in addition to natural geraniol, a number of other ingredients which have so far escaped analysis, a hundred thousand roses supplying only an ounce of it. In like manner a ton of orange blossom yields but thirty to forty ounces of the odorous essential oil.
Many of the costly plant perfumes come from tropical or semi-tropical countries, such as Ceylon, Mexico, and Peru. But tropical perfumes, though strong, lack the delicacy of those found in temperate climates. Cannes, on the Riviera, gives us roses, acacias, jasmine and neroli; from Nimes come thyme, rosemary, and lavender oil; from Nizza, on the Italian Riviera, we get violets; from Sicily, oranges and lemons; from Italy, iris and 106bergamot. English lavender, until quite recently the most highly esteemed, came from the towns of Hitchin and Mitcham. But I am informed that the growing of lavender in England is no longer pursued with the same success as formerly, and we have to regret the disappearance of this old and truly English industry.
The natural musk, curiously enough, which comes from the musk-deer of Tibet, is not used in making musk perfume. It is, however, widely employed in the perfumer’s art, as it has the curious property of enhancing the strength of other perfumes and of rendering them permanent. Civet, also an animal product, being “the very uncleanly flux” of the civet cat, has similar properties. It is added to other perfumes to strengthen them (“to set them off,” as it were) and to render them more stable.
But the most curious, and also one of the most ancient of perfumes is ambergris, which is a fatty, wax-like substance found floating in the sea or washed ashore. It comes from places as far apart as the west coast of Ireland, China, and South America. The origin of this substance was for long a mystery. But we know now that it consists of the undigested remnants of cephalopods (squids and octopuses) swallowed by the spermaceti whale. Ambergris is used, like musk and civet, to render other scents durable.
107But while the victory of the chemist is by no means so complete as it is in the matter of the dyestuffs, research is steadily going on, and the next few years will almost certainly witness an evergrowing conquest over this department of natural chemistry.
In the meantime chemists are applying themselves to the creation of new varieties of perfume, and, if we may judge from those disseminated by certain ladies in public places, with a success that startles and even irritates us. Compared with them, the love-philtres of olden days must have been but feeble things.
“How d’you know you’re in the right ’bus?” asked the ’bus conductor of the blind man who was confidently boarding his vehicle.
“This is the Maida Vale ’bus,” was the contemptuous reply. “I knows it by the smell o’ musk.”
The inexhaustible capacity of the olfactory organ, to which we alluded above, is by no means its only marvel. It is also of the most wonderful delicacy, equalling, even if it does not surpass, in this respect, the sensitiveness of the eye to light.
This property of the smell-organ has been scientifically estimated. There are many ways of doing so, that by means of Zwaardemaker’s olfactometer being perhaps the most popular:
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“This consists of two tubes that slide one within the other, and so shaped that one end of the inner tube may be applied to the nostril. The odorous material is carried on the inner surface of the outer tube. When the inner tube, which is graduated, is slipped into the outer one so as to cover completely its inner face, and air is drawn into the nostril through the tube, the odorous surface, being covered, gives out no particles, and no odour is perceived. By adjusting the inner tube in relation to the outer one, whereby more or less of the odorous surface is exposed, a point can be found where minimum stimulation occurs. The amount of odorous substance delivered under these circumstances to the air current has been designated by Zwaardemaker as an olfactie, the unit of olfactory stimulation. Having determined for a given substance the area necessary for the delivery of one olfactie, doubling that surface by an appropriate movement of the inner tube will produce a stimulus of two olfacties, and so forth. Thus a graded series of measured olfactory stimuli can easily be obtained. Further, by using outer tubes carrying different odorous substances various comparisons can be instituted as measured in olfacties” (Parker).
Instruments more elaborate and of greater accuracy have, as a matter of fact, been devised and used, but they need not detain us.
The results obtained by these and other methods of determining the minimum stimulus of olfaction are certainly astonishing, and reveal as nothing else can the delicate acuteness of the sense.
Fischer and Penzoldt found that they could plainly smell one milligram of chlorphenol evaporated in a room of 230 cubic metres capacity. This is equivalent to 1/230,000,000 of a milligram to 109each cubic centimetre of air, or, assuming 50 cubic centimetres of air as the minimum needed for olfaction, the amount of chlorphenol capable of exciting sensation is 1/4,600,000 of the thousandth part of a gram—approximately 1/276,000,000 of a grain!
Many other odours have been similarly tested, and although there is much numerical discrepancy in the records made by different observers, all agree as to the extreme delicacy of the sense. (For vanillin and mercaptan, see p. 39.)
Those experiments and estimations explain how it comes about that many odours (musk, for example) may go on giving off their scent until they part with the whole of it without undergoing any appreciable loss of weight.
Thus there is no chemical test known to us so delicate as olfaction.
It has been found, for example, that over-assiduous efforts at filtering and purifying the air used for ventilation so as to remove all noxious chemical and bacterial ingredients defeat their own end. Such air, although to our artificial tests absolutely clean and pure, seems to the sense of smell to lack freshness. And the nose is right. The tests are wrong. For sojourn in such an atmosphere induces lassitude and torpor of mind, as members of the Houses of Parliament, where this method has been tried, know to their cost—and ours.
110But albeit so highly sensitive to minute traces, the sense occasionally fails to perceive a highly concentrated odour.
For example, every one is aware that a bunch of violets which is filling a room with its fragrance seems when held to the nose to have no smell at all, or at the most to have but a vague, indefinable sort of odour.
The effect, as a matter of fact, varies with the perfume employed. Some, like violets, have no smell at all. Others give a different smell when concentrated from what they give when dilute. Muskone, for one, the essential constituent of musk, has an odour of pines when concentrated; and storax, a delightful perfume when dilute, is disagreeable when too powerful, and so on.
It is to be noted that the disagreeable character of these last is not due to the mental “cloying” or “sickening” of excessive sweetness; it is a definite odour. Nor is the anosmia for concentrated violets due to the exhaustion of the sense.
Heyninx, comparing, as we shall see, olfaction with vision, believes the indefinite odour of concentrated violets to be akin to the absence of colour in white light. But this explanation seems to me to be improbable, since the effect is due not to the combination of a number of odours, as white light is the combination of all the colours of the 111spectrum, but to the overpowering influence of a single odour.
Indeed, none of the other senses shows the same phenomenon. If we happen to catch a momentary glimpse of the noonday sun, we plainly see a disc of intense light (it is pale blue in colour to my eye), surrounded by a fiery halo, before it blinds us. In the same way, when a gun is fired close to the ear, we hear the sound before we are deafened by it.
It is for such reasons that perfumers never sniff at a bottle of scent; they take a little, rub it on the back of the hand, and then wait until the spirit has evaporated before they proceed to smell it.
The exquisite delicacy of the sense might lead us to suppose that the olfactory organ must be quick at responding to its proper stimulus. But such is not the case. It is, on the other hand, relatively “slow in the uptake.”
Gleg has estimated that the reaction time for auditory sensation is from 0·12 to 0·15 of a second, whereas the reaction time for smell is as much as 0·5 of a second, only one sensory stimulus being slower, that of pain, namely, which occupies 0·9 of a second.
Odours are conveyed to the olfactory end-organ in the air we breathe. Before they can rise into 112the air from the odorivector (the odorous body) and be transported they must, it is clear, pass into the vaporous or gaseous state. (In the case of fish, of course, the odour must undergo solution, that is pass into the liquid state.) Many of the natural properties manifested by smells have been related to this transformation into vapour.
Everybody knows how rich garden scents become after a shower. It has been claimed that this results from the lightening of the atmosphere by the storm, in consequence of which the diffusion of odorous vapours, following the law that governs the diffusibility of gases, is facilitated. But some of the effect must be due, one would think, partly to the impact of the raindrops breaking up and dispersing the halo of perfumed air that surrounds each flower, and partly also to the evaporation of the rain-water that has absorbed these floral emanations.
We are told also that during the night and in the chill of early morning the air is less charged with odours because cold checks the diffusion of gases. This may be true enough for some odours, but I am inclined to think that the fact is not stated with perfect accuracy, as there are certain perfumes, that of the tobacco-plant for one and that of the night-scented stock for another, which are most prevalent after nightfall. And it has always seemed to me that Mother Earth is never so nicely 113perfumed as on a cool September morning, although I should never be inclined to call any morning “incense-breathing,” like Gray, for anything less like incense could scarcely be imagined.
There is no doubt, however, that frost seals up all odorivectors and renders the air quite odourless.
A physical law appertaining to gases is also invoked to explain the “clinging” of odours. Many, if not all, solids and liquids when exposed to air and other gases adsorb (cause to adhere) to their surfaces a thin, dense layer or film of the gas. If now that gas happens to contain an odour, or is itself odorous, the odour must also be adsorbed, and so in the case of porous materials, such as fabrics, permeated by the odour, it lingers tenaciously in their depths.
Odorous bodies in the solid or powdered form are known to retain their perfume for prolonged periods. Look how long a sandal-wood box remains aromatic. This property is supposed to depend upon the lowered vapour tension of the odorous molecules in the depths of the solid or powder, in virtue of which they rise into the air, or evaporate, but slowly.
It would seem to be natural to suppose that, as vaporisation plays such an important part in the dissemination of odours, the volatile bodies and liquids would be more odorous than the nonvolatile. 114But, as Zwaardemaker has pointed out, this is by no means always the case. Many substances of low volatility are nevertheless highly odorous, and vice versa.
We turn now for a moment to consider the behaviour of the odorous vapour in the nose.
As it passes through the nose the current of inspired air sweeps along the lower and middle regions only; the upper or olfactory region is not directly traversed. But almost certainly some of the air is diverted up into the olfactory region in light eddies, and the act of sniffing, which is a short inspiration abruptly begun and ended, and which we instinctively resort to when trying to detect a faint odour, is obviously of a nature to propel side-streams or eddies up into the olfactory zone. One is reminded of the production of smoke rings from a box.
We smell not only during inspiration, however, but also during expiration, the latter conveying to the olfactory region the flavours of food and drink.
Flavours, that is to say the olfactory elements of so-called “taste,” are not appreciated to the full until after deglutition. To most of us, although experts and connoisseurs can determine it by smelling the wine in the glass, the bouquet of port has really no meaning until after it is drunk, simply because the expiratory current of air as it 115ascends through the throat into the nose receives the concentrated vapours of the warmed volatile higher alcohols which are clinging about the fauces.
We may here remark that although we are usually able to perceive that the odour and the flavour of a sapid food or drink are akin to each other, the sensation of the odour anticipating that of the flavour, yet they are by no means always identical. They may strike us as do a plain and a coloured version of the same print. Sometimes the flavour seems to be the more powerful, sometimes the odour. Nearly all bouillons, for example, possess a flavour more rich and full than the odour they give off with their steam. On the other hand, valerian has a strong, objectionable smell, which, strange to say, becomes subdued and relatively tolerable when that medicine is being swallowed.
It is a curious fact, well known to expert “tasters,” that if the eyes are kept closed during the test, the delicacy of appreciation of flavours, and also of the smell of the wine in the glass, is entirely lost. I cannot suggest any explanation for this curious phenomenon.
Anosmia, absence of smell, which is the next topic for our consideration, is a not uncommon defect. It is generally the result of some form of 116nasal obstruction, such as a bad “cold in the head,” as ?sop’s fox was clever enough to remember. This type is temporary and remediable. But there are other forms that are due to nerve-disease, and for these nothing can be done.
A congenital anosmia is occasionally met with, and a curious partial anosmia, reminding us of colour-blindness or tone-deafness. I myself know people who cannot smell coal-gas unless it is very strong, and I once knew a cook,—a cook who couldn’t smell a bad egg!
Albinos are said to be congenitally anosmic, and there was recorded many years ago by Hutchison the case of a negro who, gradually losing all his pigment, became anosmic in consequence (cited by Ogle). As the sustentacular cells of the olfactory area contain granules of pigment (see Chapter II.), we are forced to conclude that it must exercise a highly important function in the perception of odours. We shall see later on that its presence is supposed by some to support the theory that odour is a specific ethereal vibration similar to light.
We turn now to discuss the real nature of odour, a section of our subject which is still theoretical and highly problematical.
Having accomplished so much in the art of perfumery, the chemist ought, one would think, to be 117able to tell us whether or not there is any relationship or correspondence between odour and chemical constitution.
When investigation of this point was begun, a hopeful fact came to light, as it was pointed out that certain bodies of similar chemical composition had all the same kind of smell. These were the compounds of arsenic, bismuth, and phosphorus, all of which smell of garlic. But it was soon realised that this fact was of little or no significance, as the oxides of many of the metals, although quite different from the former group, also smell of garlic. To these we may add the instance of water and sulphuretted hydrogen, two substances which are related chemically, as their formul? show (H2O and H2S), and yet one of them is odourless, While the other has a strong, unpleasant smell. Finally, according to Deite, natural and artificial musk have nothing in common but their smell. Chemically they are quite different.
The property of odour, then, does not depend upon the Chemical constitution of bodies.
The next question that arises is: Do bodies exhaling the same kind of odour resemble each other in the structure of their molecules? In other words, can odour be related to molecular structure?
To the chemist all matter is made up of atoms and molecules. The elements, bodies which cannot 118be broken up by chemical action into any simpler form, are composed of atoms. On the other hand, when elements combine to form a compound, the unit of the new body, composed as it is of two or more atoms of different elements linked together, is known as a molecule. (Probably the elements also exist in the molecular state, the atoms of which they are composed being linked together in groups.) Both atoms and molecules are, of course, very minute in size.
For reasons we need not enter into here, the molecule is held to have a certain structural form, which form is indicated by what is known as a graphic formula. The graphic formula of water, one of the simplest, may be written as H—O—H, and we may regard it as having a linear form. (Modern views indicate that it is not a simple line, but in two planes.)
Many molecules, however, particularly those of the organic compounds, are highly complex, and their structural form must be very different from that of water.
The question, then, now before us is: Does odour bear any relationship to the molecular structure of bodies? And again it has been maintained that a clue to the problem of the real nature of odour l............