The Applicability of Weber's Law to Smell

Introduction

Progress of the Experimental Proof of Weber's Law. Object and Scope of this Paper. Its connection with Zwaardemaker's Olfactometric Work. Zwaardemaker's Terminology. Defect of All Psychophysical Measurements. The Peculiar Difficulty of Olfactometry, viz., the Indeterminateness of Olfactory Qualities. Zwaardemaker's Classification of Olfactory Qualities. Antecedent Probability of the Applicability of Weber's Law to Smell.

Chapter I. Method

Section 1. Factors which Determine the Intensity of the Smell-Stimulus for the Normal Organ. (I.) The Quantity of Vapor Thrown Off by the Odorous Body. (II.) The Rate of Diffusion of the Odorous Vapor. (III.) The Rate and Manner of Breathing. Compensation of Smells. Direct and Indirect Olfactometric Methods. Methods of Valentin, Fischer and Penzoldt, Dibbits, Ottolenghi, Passy, Frölich, Aronsohn, Savelieff. Section 2. Control in Zwaardemaker's Olfactometric Method of the Factors which Determine the Intensity of the Stimulus. Treatment of the Two Factors of the Genetic Unit. The Method of Henry. Adhesion to the Inhaling-Tube. Control of the Diffusion-Rate of the Vapor. Control of the Manner and Rate of Breathing. Section 3. Anosmia and Hyperosmia. Method of Testing the Openness of the Nasal Passages. The Effect of Exhaustion. Section 4. Psychophysical Methods Employed. An Imperfect Method of Finding the Stimulus-Limen. The Method of Just Noticeable Differences. Intervals between Stimuli and between Determinations. The Method of Minimal Changes. A Modified Form of the Method of Just Noticeable Differences. The Subject's Tendency to Judge in Terms of Hand-Movement. The Method of Right and Wrong Cases. Chapter II. Apparatus and Materials. Section 1. The Standard and the Fluid-Mantle Olfactometers. (I.) The Standard Olfactometer. (II.) The Fluid-Mantle Olfactometer. Section 2. Preparation of Odorous Materials. (I.) Preparation of Odorous Substances Used in Solid (II.) Preparation of Odorous Substances in Solution. Section 3. Other Arrangements and Appliances.

Chapter III. Results

Section 1. The Several Subjects and their Stimulus-Limina. Table I. A Table of Stimulus-Limina. Remarks. Section 2. Results Obtained by the Method of Just Noticeable Differences. Table II. Consecutive Results for One Subject, T. Remarks upon the Constant Sources of Error. Table III. Complete Results for One Solid and One Liquid Substance. Rough Summary of Results by Series. Table IV. Approximate Values of Δr/r Obtained for Pairs of Standard Stimulus-Intensities Sensed under the Same Conditions. Curves Illustrating Parts of Table IV. Lists of the Substances Used by the Several Subjects. Table V. Approximate Values of Δr/r Arranged to Show Variations for Individual Subjects. Table VI. Approximate Values of Δr/r Arranged to Show Variations for Different Substances. Curves Illustrating Part 3 of Table VI. Remarks on Tables V and VI. Some Incidental Exhaustion-Phenomena. Section 3. Results of Other Methods. Table VII. Results of the Modified Form of the Method of Just Noticeable Differences. Table VIII. Results Obtained for Red Rubber by the True Method of Minimal Changes. Table IX. Results Obtained by the Method of Right and Wrong Cases. Table X. Results of a Rough Attempt to Gauge the Applicability of the Method of Right and Wrong Cases to Smell.

Summary and Conclusion.

Introduction.

So long ago as 1834, in a paper entitled "De Tactu," Ernst Heinrich Weber first stated the law which bears his name, the law of psychophysics. Working by the method afterwards by Fechner "the method of just noticeable differences," he had discovered the law in its application to pressure and strain. Before i860 it had been proved to hold also for noise and brightness. Since the establishment of the first psychological laboratory, which occurred in the academic year 1878–9, and, oddly enough, but a few months after Weber's death, the validity of the law for the four sensation qualities mentioned has been over and over again confirmed.

Before 1860 Volkmann, Renz, and Wolf, by the method of minimal changes, had proved its applicability to noise. Bourger, Fechner, and Volkmann, by their "shadow-experiments," and Masson with his rotating disks, had shown, its validity for brightness. Fechner had also established Weber's conclusions in regard to strain by the method of right and wrong cases. In the last twenty years, Tischer, Merkel, and Starke, by the method of minimal changes ; Merkel and Angell by the method of mean gradations ; and Lorenz, Merkel, and Kämpfe, by the method of right and wrong cases, have confirmed for noise intensities the results of Volkmann ; Helmholtz, Aubert, and Kräpelin have established for brightness the results of Masson ; and Merkel, using the method of minimal changes, has also proved the conclusions of Weber and Fechner in regard to strain. In the last six or seven years, the experiments of Merkel by the method of average error have proved the extension of the law to those strain-sensations in terms of which we measure distance by the eye, and the experiments of Schumann by the same method give some indication of its extension to strain-sensations involved in our estimate of intervals between one-half second and three seconds.

In case of the sensation-modalities for which the law has not been proved, and in the case of tone, there are great difficulties in graduating the intensity of the stimulus. Articular sensations, indeed, are not themselves graduated intensively. In the case of tone, the difficulty is mechanical,—that of graduating minutely the objective intensity of simple periodic vibrations. In the case of the two temperature qualities, which are peculiar in depending on different intensities of a stimulus from a physical point of view the same in kind, and in passing into each other through a conscious indifference-point, the extreme adaptability of the so far unknown and inaccessible peripheral organ makes the intensity of the physiological stimulus begin to fall towards the indifference-point upon the application of any new physical stimulus, and thus prevents the phys ical stimulus from being a measure of the physiological. The sensation, moreover, varies in intensity with the extent of surface stimulated and with the weight of the stimulating body. The qualities of taste and smell form manifolds of indefinitely related terms, which must be investigated separately. The intensity of the case of taste, the list is at least closed. In the taste sensation, however, is a function (1) of the degree of saturation of the solution tasted ; (2) of the magnitude of the area excited ; and (3) of the movement, diffusion, and pressure of the substance tasted within the buccal cavity. No very satisfactory way of keeping all but one of these factors constant, while that one is varied, has as yet been found, though the investigations of Camerer, who worked by the method of right and wrong cases, make the law of Weber appear to apply to salt and bitter.

As for the applicability of Weber's law to smell, the object of this paper is to offer a mass of experimental difference-determinations, with a statement of the "checks" or controls to which they must be subjected. This enumeration of possible errors involves a discussion of the essentials of a satisfactory olfactometric method, and a detailed description of the method and apparatus actually employed. The literature of difference-determinations in smell amounts practically to pages 180–181 and pages 188–194 in Die Physiologie des Geruchs of Dr. H. Zwaardemaker, now professor of physiology in the University of Utrecht. The work was translated into German from Dr. Zwaardemaker’s manuscript by Dr. A, Junker von Langegg, and was published in Leipzig in 1895, The experiments to be described are in the main a realization of suggestions of Dr. Zwaardemaker’s, of which some are contained in his book, and some few were made in personal letters, The olfactometric method used was, of course, his. This method was first applied in 1888, and is now familiar in most psychological laboratories. To quote from Science, XV, 44: “Dr. Zwaardemaker of Utrecht has constructed an instrument which he calls an olfactometer. It consists simply of a glass tube, one end of which curves upward to be inserted into the nostril, A shorter movable cylinder made of the odoriferous substance fits over the straight end of this glass tube. In inhaling, no odor is perceived so long as the outer does not project beyond the inner tube. The farther we push forward the outer cylinder, the larger will be the scented surface presented to the inrushing column of air, and the stronger will be the odor perceived.”

We are indebted to Dr. Zwaardemaker for the words “olfactometry” and “olfactometer” (replacing the older “osmometer”), “odorimetry” and “odorimeter.” Olfactometry is that branch of psychophysics which is concerned with the measurement of the keenness of smell. The distinction between the keenness and the delicacy of smell must be kept in mind, On the delicacy of smell depends the discrimination of olfactory qualities. On its keenness depend the bare sensing of odors and the discrimination of them as more or less intense, Odorimetry is a ' side-issue” of olfactometry. It is concerned not with the sense-organ, but with the measurement of the intensity of smell-stimuli considered as objectively as possible. For the unit of keenness of smell, Zwaardemaker uses the word “olfactus,” and for the normal stimulus-limen for each odorous substance he employs the companion word “olfacty.” If, for example, a subject’s stimulus-limen on the olfactometer is 10 mm, when the normal stjmulus-limen used is 5 mm., then his stimulus-limen is two olfacties, and his olfactus 1/2. The olfacty used by olfactometry becomes for each substance the unit of odorimetry. Odorimetry is correlated with photometry and phonometry, Both olfactometry and odorimetry are branches of “olfactology” (to anglicize another word used by Dr. Zwaardemaker). This again is correlated with optics, acoustics and haptics.

The interdependence of olfactometry and odorimetry is not unique. The unit of photometry, i. e., the unit for the measurement of light in the physical sense, is the illuminating power for sensation of the light of some standard candle. “We have no adequate objective method,” writes Prof. Külpe, “of ascertaining the intensity of the non-periodic and aperiodic concussions which form the substrate of simple or complex noises, independently of the statement of the observer whose sensitivity we are testing. The phonometric determination of sound intensities in psychophysical experiments is usually carried out upon a principle similar to that employed in photometry. As the objective stimulus-values in the apparatus employed,—say, elastic balls falling from a measurable height on a resisting plate,—are determined by way of a subjective comparison, the results are purely empirical, valid only for the material used, the special circumstances of the observation, ete.”

The peculiarly unsatisfactory character of the determinations of olfactometry and odorimetry is due chiefly to the fact that olfactory qualities, unlike visual and auditory, are not demarcated. It is true that it is more difficult to keep uniform the duration and extension of smell-stimuli than it is to regulate these attributes for other stimuli, with the possible exceptions of temperature and taste. It is also true that the great gulf of psychophysics, our ignorance of the physiological processes which everywhere link the strictly physical to the psychological, is wider in the cases of temperature, taste and smell, than in the cases of vision, audition, pressure and strain, Yet, at best, the measurements of physics must always be in terms of sensation, and the measurement of sensation must always be in terms of physics.

It seems wise to emphasize at the outset the initial difficulty which makes all quantitative work in smell more or less unsystematic, viz., the indeterminateness of olfactory qualities, It is at present necessary to regard as a simple and separate quality the odor of every substance which from a physical point of view is unmixed ; yet, for several reasons, it is un that there are as many elementary odors as there are simple substances.

One reason is, that it is extremely improbable that either the structure of the fibres or endings, or the snbstance of the olfactory nerve, is differentiated to correspond to the innumerable odorous substances which we encounter; and, on the other hand, it is probable by analogy with other sense-organs, that there are “specific energies” of smell which are limited in number and capable of combination.

A second reason is that we have experimental evidence that the action of the sense organ is differentiated into more and less separable processes. We have sure evidence in the results of exhaustion-experiments, which were first instituted by Frolich and Aronsohn. For example, a subject whose organ is fatigued by the continuous smelling of tincture of iodine can sense ethereal oils and ethers almost or quite as well as ever, oils of lemon, turpentine and cloves but faintly, and common alcohol not at all. We have also evidence of some slight value in the regarded traces of partial anosmia. Unfortunately, very few such cases have been described by persons who took experimental precautions, and such cases as are noted in medical literature fail to show typical anomalies comparable to the uniform phenomena of color-blindness or “tone-islands,” which have played such an important part in the formation of theories of vision and audition.

A third reason is that there are countless instances of smell- fusions in which the components cannot be detected. Nagel intimates that there is no proof of the existence of smell- fusions in which different components can be sensed as differ- ent at the same instant, and points ont that, in this respect, smell-mixtures resemble color-mixtures rather than clangs.

Zwaardemaker, following Arnonsohn and bearing in mind the usages of the perfume trade, holds that only similar odors will mix. He believes, on the basis of his own experiments, that if dissimilar odors of different intensities are mixed, the weeaker odor will cancel part of the odor of the stronger, and will itself be lost, and that if dissimilar odors of the same intensity are mixed, both will disappear or will give but a feeble indeterminate fusion. Zwaardemaker does mot, as alleged by Nagel, adduce his conclusions in regard to mixture as a buttress to his localization and irradiation theory, though he does seek to explain the facts. of mixture and compensation, as he understands them, in harmony with this theory. Nagel, as opposed to Zwaardemaker, believes, on the basis of his own experiments, that any two smells will unite in a mixture which for an instant, at least, will make a simple impression of new quality. He has never found an instance of complete “compensation,” but he agrees with Zwaardemaker that a mixture of several smells is in general weaker than its individual components, and that some combinations of strongly odorous substances are almost odorless. Nagel offers no explanation of the phenomenon of compensation, nor does Zwaardemaker explain it satisfactorily even on the basis of his irradiation-theory. Perhaps it is safe to conclude that most smells will mix. As Nagel suggests, there is no occasion in the perfume trade to mix nauseating or hircine smells with the odors of flowers, spices and resins.

A fourth and final reason for believing that there are not as many simple odors as there are unmixed substances, is that many simple substances have been found by experiment to have composite odors. Chlorphenol and nitrobenzol are good examples of such substances.

Now, if there are a limited number of specific energies of smell, and if most smells are mixed, our ignorance of the elementary smells, and our consequent inability to isolate them, have serious consequences for the value of olfactometric work. This will be clearer if we consider the two methods which are used to discover whether a smell is simple or composite. The method of Passy consists in gradually increasing the dilution of odorous substances, and depends upon the principle that since the stimulus-limina of different odors are different, they must disappear successively as the intensities of the different stimuli are diminished equally. However, it is at least possible that odors which have very different stimulus-limina should have the same difference-limina, The other method is that of exhaustion, and depends on the fact that different odors exhaust the organ with different degrees of rapidity, so that a compound odor, continuously smelled, will alter in quality as first one and then another of its constituents disappears. One may smell continuously the substance to be tested, or may smell it before and after smelling repeatedly an odor very similar in quality. The principle of the method is the same in both cases, The permanency of the mixed odor depends primarily on the equality of rate at which its different constituents fatigue the organ. The more numerous the constituents, the more permanent the quality of the mixture. This fact is well recognized in the perfume industry. Fortunately for the trade, the odor of almost every flower (Sawer mentions jasmine as a unique exception) may be simulated by compounding the odors of other flowers. The odor of violet, for example, is given by a blend of the odors of acacia, rose, Florentine iris, tuberose and almond. The odors of most flowers, again, are possessed by certain chemicals. To the mixture is usually added some substance, such as styrax, amber, or vanilla, which evaporates slowly, and smells strongly enough to compensate parts of the other odors. This is done that quantities of the other odorous substances large enough to allow for evaporation may be put into the solution without raising the intensity of the smell to the neighborhood of the terminal stimulus-intensity. If, now, most smells are mixed, and if mixed smells alter in quality as the organ becomes fatigued, and if different olfactory qualities have not the same limina, then in quantitative work in smell, we are seeking to determine values which are continually changing according to laws which we do not know.

There is no classification of olfactory qualities, which is even provisionally satisfactory from any point of view but a perfumer's. We give to odors the names of the objects which most commonly give rise to them, or to something similar to them. We speak of a “fishy smell” as loosely as Homer, in the days when the terminology of color was in its infancy, spoke of the “wine-hued sea.” Yet the name of an odor is clearly and indisputably applicable only to the smell of that object from which the name is taken,

Giessler's classification of odors may be of value to psychology proper, but is of no value whatever to psychophysics. The most satisfactory method of arriving at a_classification of smells seems to be the method of exhaustion ; but the results so far obtained do not furnish any basis for such a system. Nagel points out as the greatest difficulty in the way that when the organ is fatigued by one smell, its sensitivity does not remain quite unimpaired for one 1arge group of odors, and utterly fail for another group ; but on the contrary, is usually more or less impaired for all odors. Analysis by exhaustion is complicated experimentally by the fact that smells do not fall away steadily, but oscillate at the stimulus-limen, as do mimmal sensations in other departments. In the case of smell this oscillation depends on slight variations in the rate and manner of breathing, as well as on the ordinary ebb and flow of the attention. The apparent “rivalry” of odors is due to this fluctuation at the limen. Moreover, it is only the last component of the mixture to disappear, which is ever really isolated by the exhaustion-process.

Zwaardemaker adopts, with some modification, the old classification of Linnæus, which really has only a subjective basis, though Zwaardemaker attempts, without signal success, to give it a chemical one. On the principle that even a most unsatisfactory system is better than none, some pains have been taken in the experiments to be described to procure smells from as many of Zwaardemaker's classes as possible, and to compare results for representatives of the same class and of different classes. Zwaardemaker's classes of pure olfactory qualities are as follows :

Ethereal smells—including all the fruit odors (a class taken from Lorry).

Aromatic smells—including all such odors as that of camphor, spicy smells, and the odors of anise and lavender, lemon and rose, and almond.

Fragrant smells—including the odors of most flowers, of vanilla, and of such gums as tolu and benzoin.

Ambrosiac smells—including the odor of amber, and all the musk odors.

Alliaceous smells—including the odors of garlic, asafœtida, gum ammoniac, vulcanized India rubber, fish, bromine, chlorine and iodine, etc.

Empyreumatic smells—including the odors of toast, tobacco smoke, pyridin, naphtha, etc., (a class taken from von Haller).

Hircine smells-—including the odors of cheese, sweat, rancid fat, etc., etc.

Virulent smells—including such odors as that of opium, “Odor cimicis,” etc.

Nauseating smells—including the odors of decaying animal matter, of feces and the like.

The pungency of smells is not an olfactory quality, but is due to the excitation of filaments of the trigeminus, which are freely distributed in the Schneiderian membrane. The sensation is more like pressure than smell. When very strong it becomes a tickling, and sneezing ensues. Persons who have congenital or pathological defects of smell are said to have cultivated these sensations by attention to such an extent that they do duty for smells proper, Some smells which are not the flavors of food sensed in expiration, seem to be tastes, as well as smells, For example, we think of the odor of boiling syrup as sweet, and say that curdled milk “smells” sour. This is probably due to early 1as80ciation, which has indissolubly fused certain taste-memories with certain smell-sensations of peripheral origin. It may, however, be due to the entrance of sapid particles through the nose into the pharynx. Smells are often blended with pressure sensations other than pungency and with temperature sensations. It is probable that there is an element of pain in an impression of pungency, while smells often give a “feeling of weight,” pure and simple. Whenever the subjects in these experiments spoke of the heat, taste, pressure, pain, or pungency of an odor, their remarks were carefully noted, on the supposition that such factors in the total impression were disturbing in a quantitative investigation of olfactory qualities proper.

Zwaardemaker's differentiation of the specific energies of smell and localization of their actions on the olfactory mucous membrane is not to our present purpose. We may simply note in passing that he arranges the zones of their operation in horizontal order, since the height to which the air current is carried in the nose makes no difference in the quality of an odor ; and that he rather ingeniously places the nauseating and virulent smells farthest back and closest to the pharynx, in a region where they may excite the reflexes of vomiting and coughing by mere irradiation of nervous excitation without the connecting link of central processes ; puts the hircine and ambrosiac odors in the middle, on account of the connection of hyperaemia of the “corpora cavernosa nasalia” with the blood supply of the generative organs ; and locates the fragrant, aromatic and ethereal smells farthest to the front, since the sneezing-reflex is most easily excited in the anterior portion of the nasal cavities, Nagel's remark that Zwaardemaker's localization-theory leads to “irresolyable contradiction” is not quite clear, but he is certainly right in saying that the theory has no adequate basis. Aside from the lack of experimental evidence, the arrangement of the several zones is too fancifully neat to carry conviction with it ; but Zwaardemaker himself emphasizes the fact that the essential part of his theory is simply the arrangement of the operations of specific energies of smell corresponding more or less exactly to the classification of Linnæus on the olfactory mucous membrane in the order of these classes.

Before bringing these introductory remarks to a close, it may be noted that, aside from any experimental evidence which may be offered, it is probable that Weber's law does apply to such smells, mixed and unmixed, as we daily encounter. In the first place we have the analogy of several other modalities of sensation for believing that the law applies to simple olfactory qualities. In the second place it has never been proved that Weber's law applies merely to unmixed sensations. It has been, neither proved nor disproved for clangs, but many experiences of ordinary life would lead us to believe that it does apply to musical chords as wholes. Thus it may apply to smell-fusions as wholes, and approximately correct difference-determinations may be obtained for these wholes even while their character is gradually altering, Since, in the present state of our knowledge, no one can even pretend to be working with simple olfactory qualities, all difference-determinations in smell must proceed upon the assumption of this possibility. Experimental results must be the only decisive evidence for or against the theory, so that it is needless to discuss it farther in this place.

In the third place the distinction drawn by Passy between “insistent” and “intensive” smells, which is based upon a classification of smells in the popular mind and confirmed by other scientific men, is explained by the supposition that Weber's law applies to smell with different values of Δr/r for different qualities. In Zwaardemaker's language, and in the ordinary language of this paper, the smaller the “minimum perceptible "’ of a substance, the more intense its odor. Passy uses a term, “pouvoir odorant,”—which we may translate “insisteney,”—for “intensity” in our sense. Hesays : “Tout le monde sent que le camphre, le citron, le benzine sont des odeurs for , 'a vanille, l'iris des odeurs faibles,” though vanilla has an insistency one thousand times greater than that of comphor, Besides this subjective basis of distinction between weak odors, however insistent, and strong or intensive odors, he has five objective differentize. (1) Weak smells have vague differences of intensity. For example, vanilla and coumarine soon reach a maximum of intensity which cannot be increased. Greater concentrations simply become unpleasant. (2) Individual differences are more evident for weak smells. (3) The daily variations of sensitivity are more evident for weak smells. (4) Exhaustion has more effect on weak smells. (5) Strong smells hide the weak. In view of the first objective difference, Zwaardemaker explains the subjective difference as follows : As the strength of a sensation is estimated by the number of grades of intensity by which it surpasses the liminal sensation, and as the terminal stimulus is by definition that degree of intensity beyond which increase cannot be shown for our human sense organs with our mechanical appliances, it is obvious that odors with large difference-limina must be subjectively weak, and that subjectively weak odors must have large difference-limina. Thus, the very rapid attainment by some smell-stimuli of the terminal intensity would seem to indicate that Weber's law applied to olfactory qualities, and that the difference-limen differed from quality to quality.

Unfortunately our own experimental results are at variance with the second clause of the theory. They make Weber's law appear to apply to smells as we find them, but show no great variation of Δr/r from substance to substance. The difference-limina even of camphor and vanilline seem much the same. If our figures are accepted as trustworthy, some other explanation than the simple one of Zwaardemaker must be found for the distinction of Passy. May it not be that, for phylogenetic reasons, “intense” smells have more affective value, more of what Miiller calls “Eindringlichkeit,” than have the smells which Passy calls “insistent?” Or may it not be that the smells most useful to human life exhaust the human sense-organ less after many increments than smells less useful do after a few increments, although the increments are relatively equal throughout? The need of some such explanation will be more or less clear as the figures to be offered are more or less convincing.

Section 1. Determination of the intensity of the Smell-Stimulus for the Normal Organ

If all the nervous elements concerned in smell are in a normal condition, and if “‘compensation’’ does not come into play, the intensity of an odor depends on the number of odorous particles in gaseous form which are acting on the olfactory nerve-endings at the time. Perhaps it is safe to say that the intensity is ordinarily a function of the number which are acting on the rod-cells of the olfactory mucous membrane. Whether or not individual rod-cells are subject to cumulative stimulation, we do not know, for we do not know even whether the stimulation is chemical, thermal, or electrical, but we do know that the intensity of the smell seems to depend on the extent of membrane and therefore on the number of rod-cells stimulated,—always supposing that the rod-cells are the olfactory cells proper.

Now the number of odorous particles which act at any given time on the olfactory membrane of the normal nose depends, first, on the quantity of vapor which the fragrant body is throwing off ; secondly, on the rate of the diffusion of this vapor ; and thirdly, on the manner and rate of breathing. Let us consider these facts separately.

I. The Quantity of Vapor Thrown off by the Odorous Body. “Whether” says Zwaardemaker, “odorous particles are set free by evaporation or chemical reaction, the mass of odorous molecules which are given off from a solid body or the surface of a liquid is, ceteris paribus, in compound proportion to the time of exposure and extent of surface exposed. Zwaardemaker has invented a “genetic unit” for the measurement of odor in the physical sense. It is the number of seconds of exposure multiplied by the number of square millimeters of surface exposed. It is unnecessary to say that the genetic unit differs from substance to substance. The “other factors” which must remain equal, if the genetic unit of a given substance is to be constant, are the moisture, weight, and temperature of the atmosphere and the amount of ozone present in it.

That heat and dampness affect the intensity of odors is a matter of common observation. Yellow wax smells twice as strong in summer as in winter. Heat promotes evaporation. Dampness also promotes the vaporization of such solids as are soluble in water, but, on the other hand, retards the diffusion of odorous vapors. The temperature of the laboratory in which smell experiments are in progress should be kept as uniform as possible, and thermometer and barometer readings should be taken whenever the stimulus-limen is determined. Uniformity of temperature was not secured in our own experiments.