ODOUR, that quality of certain bodies which excites the sensation of smell. In the Annales de Chimie, Vol. XXI. p. 254, we have a detailed account of certain experiments made by M. Benedict Prevost of Geneva, with a view to render the emanations of odorant bodies perceptible to sight. The account is by much too long for a work like ours; especially as we feel not ourselves inclined to attribute to the experiments all the importance which seems to have been allowed to them by the first class of the French National Institute. We shall therefore state only a few of them, which seem most to favour the author's hypothesis.
1. A concrete odorant substance, laid upon a wet glass or broad saucer, covered with a thin stratum of water, immediately causes the water to recede, so as to form a space of several inches around it.
2. Fragments of concrete odorant matter, or small morsels of paper or cork, impregnated with an odorant liquor, and wiped, being placed on the surface of water, are immediately moved by a very swift rotation. Romieu had made this observation on camphor, and erroneously attributed the effect to electricity. The motion was perceptible even in pieces of camphor of seven or eight gros.
3. An odorant liquor being poured on the water, stops the motion till it is dissipated by evaporation. Fixed oil arrests the motion for a much longer time, and
and until the pellicle it forms on the water is taken off.
4. When the surface of the water is cleaned by a leaf of metal, of paper, or of glass, plunged in and withdrawn successively until the pellicle is removed, the gyratory motion is renewed. If a piece of red wax or of taper be dipped in water, and the drops shaken off into a glass of water containing odorant bodies in motion, the movement will be stopped. The same effect is not produced by metal.
5. A morsel of camphor, plunged to the depth of three or four lines in water, without floating, excites a movement of trepidation in the surrounding water, which repels small bodies in its vicinity, and carries them again to the camphor by starts. The author concludes, that an elastic fluid escapes from the odorant body in the manner of the fire of a fusée or the discharge of fire-arms.
6. When there is a certain proportion between the height of the water and that of the small fragment of camphor, the water is briskly driven off, returns again to the camphor, and again retires, as if by an explosion, the recoil of which often causes the camphor to make part of a revolution on its axis.
7. Camphor evaporates thirty or forty times more speedily when placed upon water, than when entirely surrounded with air.
8. Camphor, during the act of dissipation in the air, preserves its form and its opaque whiteness; upon water it is rounded, and becomes transparent as if it had undergone a kind of fusion. It may be inferred, that this arises from the acquired motion, which causes it to present a greater surface to the air.
9. When small pieces of camphor are plunged in water, the camphor becomes rounded and transparent, does not acquire any motion, and its dissipation is less perceptible than in the air. The concurrence of air and water is therefore necessary to disengage the fluid which is the cause of the motion and total dissipation of odorant bodies.
10. The motion of odorant bodies upon water decays and ceases spontaneously at the end of a certain time; because the water having then contracted a strong smell, the volatilization takes place in all the points of its surface; and the small mass being thus surrounded by the odorant fluid, which is no longer air, dissolves, as in the ordinary odorant fluids, without forming the gaseous jet which is the cause of the motion. The author compares the volatilization of the aromatic substance to a combustion excited by water.
M. Prevost hopes, that these, and other experiments which he explains, will contribute to the theory of odours, which so nearly resembles that of the gases. He does not flatter himself with having exhausted this subject, but considers his discoveries as the means of rendering odour perceptible by water, not only to the sight, but even to the touch, as are likewise the vibrations of sonorous bodies. Men deprived of the sense of smell, and even the blind, according to him, may in this manner distinguish odorant bodies from those which have no smell. "Perhaps (says he) this kind of odoro-scope may, by improvement, become an odorimeter. The exceptions, such for example as that of the cerumen of the ears, which produces much effect on water without being perceptibly odorant, and that of the
fingers when hot or moist, are merely apparent; for if our senses do not in those cases discover odour, those of animals more powerfully energetic, such as the dog, perceive and distinguish individuals by its peculiar character. The odoro-scope may afford the information which is wanting respecting these effluvia. Thus it is that the fat of game, the smell of which is nearly to us imperceptible, is very much so to dogs, and exhibits sensible marks by the odoro-scope."
Professor Venturi of Modena, who heard Prevost's memoir read in the National Institute, had himself made some experiments with camphor kept separately in the air, in the water, and at the surface of the water; whence he deduces, that the most active virtue for dissolving camphor resides at that part where both the air and the water touch the camphor at the same time. Hence he explains why, in like circumstances, camphor evaporates more quickly in a moist than in a dry air; and why the Hollanders use water in their process for subliming this substance.
It might be thought that the camphor was decomposed at the surface of the water; that the water might seize the acidifying part, which renders the camphor concrete; and that the volatile part is dissipated in the atmosphere. The author rejects this notion. He thinks that water with camphor floating on its surface becomes charged with no more than a very small portion: 1. Because in these circumstances the water acquires the same taste and smell of camphor as it obtains when a small quantity of this substance is kept plunged in the same fluid. This water, by exposure to the air, loses the qualities with which it had been charged, and becomes insipid, and without smell. 2. Because when the water is saturated with all it can take up, the dissipation of the camphor continues at its surface as before. 3. Because the aerial emanations of camphor made at the surface of water do themselves crystallize into camphor.
Camphor at the surface of the water does nothing, therefore, but dissolve; and when dissolved at the ordinary temperature of the atmosphere, it is not at first in the state of vapour, as has been thought. It is simply a liquid which extends itself over the surface of water itself; and by this means coming into contact with a great surface of air, it is afterwards absorbed and evaporated. This is proved by the following facts: 1. The solution of camphor at the surface of water is more rapid in proportion to the extent of the surface. In narrow vessels, the section of the column would not be completed in ten days, even though the water might be extremely pure. 2. When the column of camphor has projecting parts, the liquid may be seen issuing by preference from certain points of the column, covering the surface of the water, and driving small floating bodies before it, in the same manner as floating bodies go and return in a basin into which the water of a canal enters with rapidity. 3. If a small piece of camphor, already wetted at one end, be brought near the edge of water contained in a broad saucer, and be made to touch the saucer itself, it deposits a visible liquor, which is oily; and by attaching itself to the saucer, destroys the adhesion between the vessel and the border of the water, so that the water retires on account of the affinity of aggregation, which not being opposed by the attraction of the saucer, causes the water to terminate in a round edge.
edge. If you remove the piece of camphor, the water will not return to its place until the oily fluid is evaporated. 4. In the same manner, when the column of camphor is half immersed in the water, the oily liquor which issues forth destroys the adhesion of the water to the column, and produces a small surrounding cavity. The solution stops, or is retarded for a moment, until the fluid, extending itself over the water, becomes evaporated: the water then returns to its place, and touches the same part of the camphor: the solution begins again, and in this manner the process is effected by alternations of contact and apparent repulsion.
Of these memoirs by Prevost and Venturi, the English reader will find accurate and full translations in the first volume of Nicholson's Philosophical Journal, together with some judicious observations on them by the editor, which we shall take the liberty to adopt. "The philosophical consideration of odorous bodies is somewhat obscured by the old method of generalising, or referring the properties of bodies to some distinct principle or thing supposed capable of being separated from the body itself. Thus the odours of bodies have been supposed to depend on a substance imagined in a loose way to be common to them all and separable from them. Hence the terms, principle of smell, spiritus rector, and even in the modern nomenclature we find aroma. There does not in effect seem to be any more reason to infer the existence of a common principle of smell than of taste. The smell of ammonia is the action of that gas upon the organ of sense; and this odorous invisible matter is exhibited to the sight when combined with an acid gas. But in the same manner as ammonia emanates from water, and leaves most part of that fluid behind, so will the volatile parts of bodies be most eminently productive of this action; and very few, if any, natural bodies will be found which rise totally. The most striking circumstance in the effect is, that an act of such power should be attended with a loss by exhalation which is scarcely to be appreciated by weight, or in any other method during a short interval of time. But we know so little of nervous action, and of other phenomena of electricity, of galvanism (See GALVANISM in this Suppl.), or even of heat, which strongly affect the senses, but elude admeasurement by gravitation, that the difficulty of weighing the effluvia of odorous bodies becomes less astonishing."