Popular Science Monthly/Volume 21/May 1882/On the Diffusion of Odors
THE following paragraph is similar to others I have occasionally seen going the rounds of the papers for the last twenty-five or thirty years:
More than thirty-six years ago I announced, in some lectures I was then engaged in delivering, that there were some facts in the phenomena of odors and the sense of smell that were incompatible with the effluvia or diffusion-of particles theory; and I suggested an explanation based on the idea of a vibration or wave-motion, and an "odoriferous ether" analogous to, if not identical with, that of the luminiferous ether.
In the year 1863, in a letter to Professor Tyndall, I submitted the thought to him. After quoting some passages from his book, "Heat a Mode of Motion," upon the subject of odors, I wrote as follows: "I would respectfully ask if, in the consideration of, or in the course of, experiments upon this subject, it has ever occurred to you that odor might be as essentially "a mode of motion" as heat, light, or sound?. . . The seemingly unlimited generation of odoriferous particles (?) by certain substances, without sensible diminution of bulk or weight, first led to the conception that, however copiously odoriferous particles of matter were disseminated through the atmosphere, the odorous property itself was as purely a specific variety of motion as the undulations of the luminiferous ether. That this must be the explanation of the action of the odor-generating force for a part of its route to the human sensorium seems to be incontrovertible, for it is hardly conceivable that the material particles should actually penetrate the membrane and force their way, as moving bodies, through the pulpy tissue of the nerves to the seat of sensation; but that through that portion of their career, at least, their power is propagated by wave-like motions analogous to those of heat and sound."
Professor Tyndall did me the honor to answer my letter, but not to indorse my view, except in a very faint and qualified manner. Nevertheless, reflection and added experience have only gone to confirm me in the correctness of it, and I venture to predict that before many years it will be as much an accepted fact of science as the undulatory, luminiferous-ether theory now is.
In the case given above the entire space of the chamber is thoroughly impregnated with the perfume as much as if it were an absolute solid of odor. And yet these "particles," so profusely diffused through the room, are wafted away, and their places supplied by new emissions from the undiminished "grain," "many thousands of times: every year without appreciable "sensible diminution of its volume or weight," or pungency. This is an obvious impossibility upon any theory of molecular Or atomic diffusion. The assumption of immense diffusibility and vastness of inter-particular spaces would only enhance the difficulty, for the odor spans the spaces—is as absolutely continuous as if the particles were in actual contact. That is, in the given space, the chamber, anywhere within the limits of the odor, there is no place where it is not. This actio in distans implies ethereal motion—vibration—between the particles.
According to this view the odoriferous bodies, or their molecules, have no more to do (in the sense of physical impact) in producing the sensation of smell than a luminous body—a candle or the sun—has to do (by impact) with the sensation of light. There is corporeal impact or touch in neither case. Of course, with each molecule as a center of activity, the effect will be more pronounced at the immediate surface (as with all radiant energies) than at any distance. And, undoubtedly, particles of disintegrating, odorous matter are often brought in contact with the Schneiderian membrane; but the sensation of that impact, if there be any, would be of touch, not of smell, as surely as that, from that point of contact to the sensorium, the effect or influence is conveyed by a vibration—a wave-motion in the "fluid" of the nerve-duct—as the undulations of the luminiferous ether are propagated along the course of the optic nerve to the seat of sensation, where they are translated into light and color. But, if, for any portion of the distance between the internal sense and the fragrant body, the odor, like light, is but a motion, it is safe to assume it for all. The analogy of this mode of odors to that of light and sound is something in its favor.