SMELLS AND PERFUMES

The old saying, "De gustibus non disputandum," is based upon the fact that both liking and the repulsion evinced by human beings for different odours (including those odours which we call flavours) are not matters of general agreement. Thus the smells of garlic and of onions, and even of assafoetida, are to many men among the most attractive and appetising in existence—to very many they are, on the other hand, repulsive. High game, a certain kind of putrid fish ("Bombay ducks"), and again rotten cheese are attractive to many men and offensive to as many more. Many animals revel in the smell and flavour of carrion, and even of manure, which they devour. There are well-known flowers which attract insects, not by the possession of the sweet perfumes appreciated and extracted by mankind, but by a smell like that of putrid meat, which so far misleads blue-bottle flies as to cause them to lay their eggs on the reeking blossom. So diverse are the tastes of men and animals in these matters that it is remarkable when we find agreement among them, as, for instance, in the attraction for butterflies of those delicate scents which also are agreeable to ourselves in such flowers as the rose, the jasmine, the heliotrope and the honeysuckle.

There seems to be no rule or principle at work by which smells can be definitely classed as either pleasant or unpleasant. Even perfumes carried by some of the inhabitants of Western Europe with the intention of making themselves attractive to their fellow-citizens are often repulsive to a certain proportion of those who come near them, as, for instance, is the case with the extract of the East Indian herb "patchouli." In regard to our other senses there is a general agreement amongst mankind, which extends also to all animals, as to what is agreeable and what is disagreeable. There are definite mathematical laws as to harmony and melody in sound and colour which affect animals and ourselves to a large extent similarly. Sweets are agreeable and bitters are disagreeable, though it is the fact that the snail, which loves sugar, recoils from saccharine, and there are "mites" (Acari) which feed with avidity on bitter strychnine! Excess of heat and of cold is disliked by animals and all men, whilst the sense of touch is pleasurably or painfully affected in much the same way in most men and animals, more than is the case with regard to any other of the senses. The sense of smell depends upon immediate and personal experience of "association" for the determination of pleasure or pain, attraction or repulsion, as the result of its being called into operation. It is a very general experience that odours are more efficient in arousing memory than are mere colour effects or sounds. Not only in animals with acutely developed olfactory powers, but also in man, an odour—a peculiar perfume—will start a whole chain of reminiscence when sight and sound have failed to do so. It is due to this close association with memory (conscious or unconscious) that an odour is agreeable or disagreeable.

In itself an odour is neither attractive nor repulsive. The acrid fumes of sulphur, chlorine, ammonia, and such bodies are not simply "odours" but corrosive chemical vapours, which act painfully upon the nerves of common sensation within the air-passages of the nose and throat and not exclusively, if at all, on the terminations of the olfactory nerves. An odour—that which acts on the special nerves of smell distributed in chambers of the nose—acquires its attractive or its repulsive quality only as the result of mental association with what is beneficial (suitable food, mates, friends, safety, home, the nest), or with what is injurious (unsuitable food, poison, enemies, danger, strange surroundings, solitude). Hence it is intelligible that the man accustomed to garlic or onions in his food is strongly attracted by their smell. So too the man whose tribe or companions have learnt by necessity to eat slightly putrid meat, fish, and cheese is attracted by their odour, though for others these odours are associated rather with what is poisonous and injurious. The dislike of the smell of sewer-gas and foul accumulations of refuse was not known to former generations of men (even in European cities a couple of hundred years ago) any more than it is to-day to the more unfortunate poorer classes, to many modern savages, to hyenas, and several other animals and birds which inhabit lairs and caves which they make foul. The odour of putrescence has become actually painful and almost intolerable to the more cleanly classes of mankind, owing to the association with it, as the result of education, of fear of disease and poisoning. Either conscious or unconscious association of an odour with what is held, either as the result of tradition or through personal experience, to be beneficial and of pleasant memory, or, on the contrary, injurious and of painful connection, determines man's liking for and choice or rejection of, odours and flavours. One can account with fair success on this basis for one's own preferences and dislikes in the matter.

On the other hand, odours exist in vast variety amongst plants and animals which have not acquired any special association or significance. We find that some organisms produce as a result of their chemical life material which oxidises and gives out light and so these organisms are "phosphorescent" without any consequence, good or bad, to themselves. And then we come upon others (as, for instance, the glow-worms and fire-flies) which have made use of this "accidental" quality, and produce phosphorescent light in special organs so as to attract the opposite sex. Again, we find that the red-coloured oxygen-seizing crystalline substance hÆmoglobin exists in the blood of a vast number of animals, and might as well be green or colourless for all the good its colour does them. Yet here and there the splendid red colour which this chemical gives to the blood becomes of great importance as a "decoration," or "sex-ornament." The comb of the domestic fowl, the wattles of the turkey, but above all the supreme beauty of the human race—the cherry-red lips and the crimson-blushing cheek of healthy youth—owe their wonderful colour to the red blood which flows through them. So at last the redness, of the oxygen-carrier is turned to account. So it must be also with odorous substances. Many have been called into existence, but few have been chosen in the long course of animal evolution and selected as the important means of repulsion or attraction.

There are odorous substances attached to many of the lower animals which seem to have no significance, but just happen to be the result of necessary chemical changes, not aimed (so to speak) at their production. Of course, it is very difficult to form a certain and definite conclusion as to their uselessness as odours. For instance, nearly all the sponges when fresh and filled with living protoplasm have a curious smell which reminds one of that given off by a stick of phosphorus. Marine sponges have it and so has the beautiful green or flesh-coloured liver sponge (common on the wood of rafts and weirs in the Thames). A rather uncommon marine worm, called Balanoglossus or the acorn worm, has a very strong and unpleasant smell like that of iodoform. In neither case is the nature of the odorous body known, nor its use to the animal suggested. Smelts smell like cucumbers: the green-bone fish and the mackerel smell alike. One of the common earth-worms has a strong aromatic smell, and the common snail, as well as the sea-hare and one of the cuttle-fishes (Eledone), smells like musk. Musk itself is produced, as a scent attracting the opposite sex, by several animals—musk-deer, musk-sheep, musk-rats. I am not now attempting to enumerate the well-recognised odours of animals such as are extracted from them by man in order to "opsonize" himself, but am pointing to the more obscure cases. There is not a very great or marked variety in the odours of fishes; but reptiles with their dry, oily skins give off various aromatic smells, none of which are valued by man. Toads have distinct odours, and one kind (Pelobates fuscus, or the heel-clawed toad), common in Europe, but not British, is known locally as the garlic toad on account of its smell. There are amongst carnivorous mammals various smells allied to that of civet which are not so agreeable to man as that substance; for instance, the odour of the fox and of the badger, and yet more celebrated, the terrible, awe-inspiring smell of the fluid emitted in self-defence by the skunk from a sac in the hinder part of the body. Horses, cows, goats, sheep, and the giraffe have their distinctive odours. Many of the herbivorous animals secrete a colourless fluid from large glands opening on or near the feet, and also from a gland in front of the eye (similar glands occur in other strange positions), which has not a smell familiar to man—that is to say, not one which has been recognised and described—yet seems to be readily "smelt" by the animals of its own kind. The bats—especially the large frugivorous bats—have a very unpleasant, frowsy smell.

An important fact about animal smells is that many which we might be inclined to attribute to the animal which diffuses them, are really due to the fermentative or putrefactive action of bacteria which swarm on the skin and in the intestines of animals. It is often difficult to decide how far a peculiar animal odour is due directly to a substance secreted by the animal, and how far the odour of that substance is modified or even entirely produced by the chemical changes set up in secretions of the body-surface by bacteria. Several distinct repulsive smells liable to occur on the human body are due to want of cleanliness in destroying bacteria by proper antiseptics. The fatty and waxy secretions of the skin are often decomposed by bacteria, even before complete extrusion from the glands in which they are formed, whilst the decomposition of food in the mouth and intestines by bacteria alters materially both the natural odour of the animal's breath and the smell of the intestinal contents. In young and healthy animals in natural conditions there is some check—it is not easy to say what—upon the putrefactive activities of the omnipresent bacteria. The skin of a healthy young animal has a pleasant odour, and its breath (notably in the case of the cow and the giraffe) is naturally sweet-smelling. The same should be the case, under perfectly healthy conditions, with human beings.

There is one important cause of animal odours and flavours upon which I have not hitherto touched. Many animals acquire an odour or flavour directly from the food upon which they feed. Certain odorous bodies are in the food and are taken up into the blood of the consuming animal unchanged, and are then thrown out by secreting glands on the skin. This is the case with the odorous substance of onions. People do not smell of onions after they have eaten them in consequence of particles of onion remaining in the mouth. The volatile odoriferous matter of the onion is absorbed into the blood. It passes out first through the lungs and later through the small fat-forming glands in the skin. It is difficult to ascertain how far animals derive their odours in this way in a complete state from their food, and how far they chemically construct them afresh by their own activity. No doubt both processes occur; but in plants the odorous bodies are built up entirely by the chemical action of the plant itself upon simple salts of carbonic acid, ammonia and nitrates. Animals can certainly take highly elaborated chemical bodies into their digestive organs without destroying them and absorb them unchanged into the blood and deposit them in the tissues. Thus the canary is made to take up the red colour of cayenne pepper and deposit it in the feathers. Thus the green oysters of Marennes acquire their colour from minute blue plants (diatoms) on which they feed. And thus, too, the canvas-backed ducks of the United States take into their tissues the odorous matter of celery, and our own grouse the flavour of heather, whilst fish-eating birds and whales in this way acquire a fishy taste. So, too, the flounders and the eels of the Thames, and even salmon in muddy rivers, acquire a taste like the smell of river mud. It is probable that many of the odours of animals (but by no means all) are thus derived directly from their food, or are produced by very slight changes of the odorous bodies absorbed in food. Mutton and beef owe their savour in some degree to the scents of the grasses on which sheep and oxen feed. And it is not improbable that the sheep-like smell which the Chinese detect in the European, comes to the latter direct from his general use of the sheep as food.

Plants are the great chemical manufacturers in the world of life, and second to them come our human industrial and scientific chemists. And though we must claim for animals some power of manufacturing distinct odorous bodies from inodorous nutritive matter assimilated by them, it is probable that in many cases the odour which is characteristic of an animal is derived by no very complicated change from odorous bodies existing in its habitual food.

A curious case of a substance valued as perfume by civilised man, and yet coming from a source whence sweet odours would hardly be expected, is that which is known as "ambergris," or "ambre gris" (grey amber). It is still used in the manufacture of esteemed perfumes, and is sold at five guineas the ounce. It is found floating on the surface of the ocean, and is a concretion of imperfectly digested matter from the intestine of a whale—probably the sperm-whale. It is a grey, powdery substance, and in it are embedded innumerable fragments of the horny beaks and sucker-rings of cuttle-fishes—creatures which form the chief food of the sperm-whale and other toothed whales. I have already mentioned above that one of our common cuttle-fishes (the Eledone moschata) has a strong odour of musk, and it is possible that ambergris owes its perfume to the musk-like scent of the cuttle-fish eaten by the whale in whose intestine it is formed. Another "smell" which is extremely mysterious is that produced by two quartz-pebbles, or even two rock-crystals, or two pebbles of flint or of corundum, when rubbed one against the other. A flash of light is seen, and this is accompanied by a very distinct smell, like that given out by burning cotton-wool. It is demonstrated—by careful chemical cleaning before the experiment—that this is not due to the presence of any organic matter on or in the stones or crystals used. It seems to be an exception to the rule that "odours" (as distinct from pungent vapours or gases) are only produced by substances formed by plants or animals. Perhaps that is not so completely a rule as I was inclined to think. It is true that one can distinguish the "smells" of chlorine, of bromine, and of iodine from one another. And there are statements current as to the distinctive smells of metals—though they may possibly be due to the action of the metals on organic matter. In any case it seems, according to our present knowledge, that the smell given out by the rubbing of pieces of silica (quartz, flint, etc.) is due to particles of silica (oxide of silicon) volatilised by the heat of friction, which are capable of acting specifically on the olfactory sense-organ.