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Rh of odorous material in different parts of the apparatus. Savelieff's method would indeed be fairly satisfactory for clinical purposes if real solutions were used instead of mixtures of ethereal oils and water. It is a great disadvantage, however, to begin an experiment by exhausting the sense-organ with a saturated solution.

Section 2. Control in Zwaardemaker's Olfactometric Method of the Factors which Determine the Intensity of the Stimulus.

Zwaardemaker's measurements of the smell-stimulus are in terms of but one factor of the genetic unit,—viz., in terms of the amount of odorous surface exposed. The time for which different extents of surface are exposed is supposed to be kept constant by the regularity of the movement of the hand which manipulates the odorous cylinder. All of these time-values are so small that their variation may well be disregarded.

In 1890, Henry, a French scientist, instituted in the interests of the perfume industry a modified form of Zwaardemaker's method, and took the time values into account. His instrument differs from Zwaardemaker’s only in the substitution for the odorous cylinder of a porous paper cylinder, hollow, closed at the bottom, and saturated from a surrounding glass reservoir with the fumes of an odorous liquid. The glass inhaling-tube enters from the top, and the subject raises it with a uniform movement while he is making the inspiration required. Stimulus-intensity is reckoned in terms of the surface of the paper cylinder exposed, and of the time which the odorous vapor has had for diffusing into it since the lifting of the inhaling tube. As for this second factor, by which alone Henry’s method differs from Zwaardemaker’s, Passy suggests that the time-rate of evaporation of a liquid under a membrane differs from the time-rate of the same fluid in the open air. Henry supposes that the pressure of vapor on the paper cylinder is constant, but on the contrary, since its surface is wholly covered at the beginning of the experiment and is gradually uncovered as the glass tube is raised, the pressure of vapor will constantly decrease. At any rate, Henry's apparatus will not answer for difference-determinations, as it would render procedure in both directions impossible.

Much more serious in Zwaardemaker's method than any error which may arise from irregularity in the subject's movements is the error due to the adhesion of odorous particles in the glass inhaling-tube, These particles may condense on the sides of the tube or, if the substance is soluble in water, may dissolve in the moisture which forms on the inside during inspiration, A correction can be made for adhesion only for the “minimum perceptible,” and only for a determination taken with the perfectly dry and clean inhaling-tube and a saturated porcelain cylinder. It may be made as follows : Let the length of the inhaling-tube ordinarily used be x and let y be the value of the stimulus-limen as found with it. Then let a