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or steam, and carbonic acid. In the long list of gaseous bodies recognised by the chemist, we find very generally some action on the nostrils,-carbonic oxide, sulphurous acid, chlorine, iodine, the nitrous gases, ammonia, sulphuretted and phosphoretted hydrogen, &c., the vapour of muriatic, nitric, and other acids. The singular substance ozone, produced occasionally in the atmosphere, is named from its smell, which is the smell of sulphur, and the odour given forth by electricity. Some of the metals and solid minerals give out an odour, as, for example, the garlic smell of arsenic, and the odour of a piece of quartz when broken. The effluvia of the vegetable kingdom are countless; besides such widely spread products as alcohol and the ethers, a vast number of plants have characteristic odours, usually attaching to their flowers. The animal kingdom also furnishes a variety of odours; some general, as the 'scent of blood,' and others special, as musk, the flavour of the cow, the sheep, the pig. 'All volatile organic compounds,' says Gmelin, 'are odoriferous, and most of them are distinguished by very strong odours; e. g. volatile acids, volatile oils, camphors or stearoptenes, and alcoholic liquids; marsh gas (carburetted hydrogen), and olefiant gas, have but very little odour.'

The pleasant odours, chemically considered, are hydrocarbons; that is, they are composed chiefly of hydrogen and carbon. Such is alcohol and the ethers, eau de Cologne, attar of roses, and the perfumes. Many smells, however, elude investigation from the minuteness of the substance causing them. Thus the vinous flavour is due to a substance which the chemist has been able to separate, being termed the œnanthic ether, but the bouquet of individual wines has not been laid hold of.

The repulsive and disagreeable odours very frequently con

*With regard to carbonic acid, the assertion as to the absence of smell is true of the amount present in the atmosphere; but, collected in mass, this gas has a slightly pungent, somewhat acid odour. This is an important distinction observable in the case of both tastes and smells; some substances yield intense effects in quantities inconceivably minute, while other substances require to act in considerable masses before being sensible in any degree.

tain sulphur.

Sulphuretted hydrogen is one of the most

common of the disgusting class.

The worst smelling substances as yet discovered have arsenic for their base, as will be seen from the following extract. (GREGORY'S Chemistry, p. 382.)

'When acetate of potash is heated along with arsenious acid, a very remarkable liquid is obtained, which is the oxide of a new radical. This liquid, which is spontaneously inflammable, and has a most offensive alliaceous smell, has long been known in an impure state, under the names of liquor of Cadet, and alcarsine. Bunsen, by a long series of the most profound and persevering researches, established its true character as the oxide of the radical kakodyle.' This radical, when obtained, 'is a clear liquid, refracting light strongly. When cooled, it crystallizes in large square prisms, and acquires, when pure, the appearance of ice. Its smell is insupportably offensive, and its vapour is highly poisonous. The two latter characters belong to all the compounds of kakodyle, with hardly an exception.' Protoxide of kakodyle, the chief ingredient in the liquor of Cadet, is most offensive to the smell, and very nauseous to the taste. Chloride of kakodyle is a volatile, horribly fetid liquid, the vapour of which attacks strongly the lining membrane of the nose, and provokes a flow of tears.'

The pungent odours have ammonia for their type. The volatile alkali, nicotine, the element of the snuffs, is an instance. In smelling salts, ammonia is the substance given forth.

Liebig has been able to lay hold of and isolate the substance that gives the odour of roast meat. Burning fat gives forth odours which exemplify the volatile oils specified by Gmelin.

2. The development or production of odours is favoured by a variety of circumstances. Heat, by its volatilizing power, and by promoting decomposition, is the most powerful agent. Light, also, which carries forward the development of the plant, is an odoriferous influence. Hence the abundance and variety of odours in warm and sunny climates, and in the

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summer season. The presence of moisture is often favourable; but the manner of acting of this agency is not always obvious. It may perhaps dissolve solid matters, and thus put them in the way of being volatilized; this may be the cause of the evolution of perfumes after a shower. On the other hand, some flowers are most odorous when dried. Friction is a source of odours; by rubbing two pieces of flint or siliceous rock a smell is given forth; sulphur treated in the same way has a smell. Many of the metals have the same property. Doubtless some ingredient is volatilized by the rubbing action.

3. The diffusion of odours is an interesting point, and has been cleared up by the researches of Professor Graham. Some odours are light, and therefore diffuse rapidly and rise high; as, for example, sulphuretted hydrogen. Such is evidently the character of the aromatic and spice odours; they, by their intensity and diffusibility combined, make themselves felt at great distances. The Spice Islands of the Indian Archipelago are recognised far out at sea. It happens, however, that the sweet odours are remarkably persistent, while the sulphuretted compounds, which are among the most nauseous, are very rapidly destroyed in the atmosphere.

The animal effluvia (excepting sulphuretted hydrogen) are dense gases, and are diffused slowly. They do not rise high in the air. In scenting, a pointer keeps his nose close along the ground, with the view also no doubt of bringing his nose close to the objects touched by the hare. The unwholesome effluvia of the decaying matter laid on the soil is avoided by getting to a moderate height: smells will be felt by a person lying that would not be felt standing, such is the difference between a stratum of eighteen inches and the height of five feet. The danger of lying on the ground in tropical swamps is a matter of fatal experience; swung in a tree fifty feet high, one may pass the night safely. Here diffusibility is one, although not the only circumstance; during the night, the ventilation or upward current from the ground is arrested, and the malaria, being little diffusible or buoyant, settles on the surface.

4. We have next to consider the organ of smell, that is,

M

the nose. This organ consists of, first, the anterior prominent part, composed of bone and cartilage, with muscles which slightly move the latter, and two orifices opening downwards; and secondly, of the two nasal fossæ, in which the olfactory nerves are expanded. The narrow cavities last mentioned are separated one from the other by a partition (the septum of the nose) formed of bone and cartilage; they communicate at the outer side with hollows in the neighbouring bones, and they open backwards into the pharynx through the posterior nares,' or openings. The sensitive surface is a membrane lining the whole of the interior complicated cavities, called the pituitary or Schneiderian membrane. The tortuosity of the passages of the nose gives extent of surface to this membrane, and thereby increases the sensibility of the nose as a whole. I shall quote part of the anatomical description of this sensitive tissue. The cavities of the nose are lined by a mucous membrane of peculiar structure, which, like the membrane that lines the cavity of the tympanum, is almost inseparably united with the periosteum and perichondrium, over which it lies. It belongs, therefore, to the class of fibromucous membranes, and it is highly vascular. Named the pituitary membrane, it is continuous with the skin, through the anterior openings of the nose; with the mucous membrane of the pharynx, through the posterior apertures of the nasal fossa; with the conjunctiva (of the eye), through the nasal duct and lachrymal canals; and with the lining membrane of the several sinuses (hollows) which communicate with the nasal fossæ. The pituitary membrane, however, varies much in thickness, vascularity, and general appearance in these different parts.' With regard also to the distribution of the olfactory nerve on the membrane, there are great differences in the parts, the general fact being that the distribution is most copious in the interior parts of the cavity or those farthest removed from the outer openings. Hence the sensibility must belong mainly to those deeply lodged parts; where there are no nerves there can be no feeling.

The olfactory nerve is the most conspicuous of the nerves of sense; it passes inward to a special ganglion, called the

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olfactory ganglion, which is a prominent object of the brain in all the vertebrate animals, and in the lower orders stands forth as a distinct lobe, or division, of the encephalon.

5. The action of odours on the membrane of the nose has next to be considered. On this subject, as on the action of sapid substances on the tongue, much remains to be known. Nevertheless there are some interesting facts which show that the action is of a chemical nature, or at least depends upon chemical conditions. For the following statements I am indebted to Professor Graham.

Odorous substances in general are such as can be readily acted on by oxygen. For example, sulphuretted hydrogen, one of the most intense of odours, is rapidly decomposed in the air by the action of the oxygen of the atmosphere. In like manner, the hydro-carbons above alluded to as odorous, are all oxidizable, the ethers, alcohol, and the essential oils that make the aromatic perfumes. The gases that have no smell are not acted on by oxygen at common temperatures. The marsh gas, carburetted hydrogen, is a remarkable case in point. This gas has no smell. As a proof of the absence of the oxidizable property, Professor Graham has obtained a quantity of the gas from the deep mines where it had lain for Geological ages, and has found it actually mixed up with free oxygen, which would not have been possible if there had been the smallest tendency for the two to combine. Again, hydrogen has no smell, if obtained in the proper circumstances; now this gas, although combining with oxygen at a sufficiently high temperature, does not combine at any temperature endurable by the human tissues.

It is farther determined, that unless a stream of air con. taining oxygen, pass into the cavities of the nostrils, along with the odoriferous effluvium, no smell is produced. Also, if a current of carbonic acid accompanies an odour the effect. is arrested.

In the third place, certain of the combinations of hydrogen have been actually shown to be decomposed in the act of producing smell. Thus when a small quantity of seleniuretted hydrogen passes through the nose, the metallic selenium is