SMELL. — The sense through which we take cognizance of odours.
Of the nature of odorous emanations no thing is certainly known. They are generally supposed to consist of material particles of extreme minuteness, detached from the odor ous body, and dissolved or suspended in the air. This idea derives its chief support from the facts that most odorous substances are volatile, that is, their loss of weight, when exposed to the air, shows that their particles really diffuse thtmselves through it,—that niost strongly odorous substances are ex tremely volatile--and that circumstances which increase the volatility of such substances also augment their odorous powers. These gene ral statements, however, are not without their exceptions. Thus, in the first place, we do not find that many gaseous substances are truly odorous ; the pungent, irritating qualities, by which many of them are distinguished, not being perceived through the sense of smell but through that of touch. Again, although it is true that a great number of volatile liquids are odorous, the strength of their scent bears no constant proportion to their respec tive volatility ; and water, which is so con stantly diffused through the air, has no odor ous property. And with regard to solids, we find that although some of those which are most strongly odorous are also volatile (such as camphor), yet this is not by any means universally the case ; for it has been proved by experiment that no diminution in weight can be ascertained to take place in musk or amber, although they have been freely exposed to the atmosphere for many years, and have imparted their perfume to an almost incalculable volume of air. These considerations have led some philosophers to suppose that odorous emanations are not material, but dynamical : —in other words, that the impressions made upon our olfactory organ do not result from the contact of dif fused particles detached from the odorous body ; but tliat they are effected by a change propagated through the atmosphere or other medium, in the same manner as sound is pro duced by undulations that originate in the sonorous body, and are transmitted onwards, through some material medium, to the organ of hearing. There are strong objections, how ever, to this hypothesis. In the first place,
we find that odours are not perceived unless the air, gas, or liquid in contact with the olfactory surface is, or has been, in direct continuity with the odorous body ; the inter position of any substance which prevents the actual passage of the odoriferous medium being sufficient to prevent the transmission of the odour. This is by no means the case in regard to sound, or to any other agent that is known to be dynamically propagated ; for we find many substances which are capable of conducting these agents, that is, of transmitting their influence through unlimited spaces ; and this may be accomplished in spite of any number of interruptions in their continuity, provided the chain of conducting substances be complete. Thus, sonorous vibrations may be transmitted from air to liquids, from liquids to solids, from air to solids, from solids to air, &e.; and many such changes usually take place, before the vibrations originating in a sonorous body are communicated to the sen tient extremities of the auditory nerve. The same is the case with heat, light, electricity', and other agents whose transmission is be lieved to be dynamical : that it is not the case in regard to odorous emanations must be regarded, therefore, as a powerful argument against the idea of their dynamical nature. Another argument may be derived from the well-known fact, that odorous emanations re quire such a time for their propagation, as corresponds rather with the dyfiesion of the odoriferous medium itself, than with the mere conduction of vibrations. Thus, in a house in which free communication is established throughout by passages, staircases, &c., but in which the course of air is not very direct from one part to another, any strong odour set free in one spot will be gradually diffused through the whole house, the rapidity being governed by the circumstances which favour or obstruct the movement of air. On the other hand, the transmission of sonorous un dulations, which merely throw the air into vibration, is not dependent upon its move ment, and is, indeed, but little influenced by it. This argument is, perhaps, yet more cogent than the former, and may be regarded as conclusive against the dynamical theory of odours.