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32 to fit tightly when pushed home, To keep the tubes themselves from slipping backwards and forwards in the plugs, we gummed strips of paper to the glass at the edge of the wood. Lumps of these strips will continue to adhere even after many washings. These home-made substitutes for the heavy metal attachments are very serviceable.

We should advise all who purchase the instrument to strengthen the table with metal cross pieces on its under side. The upward warping, which is inevitable, narrows the ways and throws the inhaling-tube out of alignment with the porcelain cylinder. The result is a stiff movement of the rack and pinion on the one hand, and a perpetual breaking of inhaling-tubes on the other. Moreover, if the warping has gone far, the whole table is liable to split. We have also found it necessary to shave the edges of the wooden blocks which carry the shells, and to reduce the friction caused by two spring-brakes placed alongside of the ways. It would be much better if the carrying blocks were moved with cranks, rather than by the milled heads, The exertion necessary to turn the screw and the chafing of the hand by the milling are distracting to the subject's attention. Moreover, the intervals when the experimenter is turning the head to give the stimulus of comparison are undesirably long. Great care must be used in the selection of any oil which is applied to the instrument. We once used clock oil, and afterwards had extreme trouble in eradicating the odor.

The porcelain cylinders for these olfactometers are made by Hooft and Labouchere in Delft, and composed of pure kaolin. They must be kept continually immersed in water, and this must be removed at least daily to minimize the odor of the clay. They must not he dried before they are introduced into their glass coverings. The ends are perfectly smooth, and are glazed for use with the standard olfactometer. The outer and inner surfaces remain porous. All the cylinders used, whether made of porcelain or of the fragrant material itself, have a length of 10 cm., and a bore of 8 mm., 80 as to slide easily along the inhaling-tube, and to cover, in case of the standard olfactometer, the graduated portion of the tube lying beyond the screen. The external diameter,—counting the thickness of the protecting shell of glass, when present,—varies from 24 to 16 mm.

Section 2. Preparation of Odorous Materials.

In Table VI (Chapter III, Section 2) the odorous materials are arranged in their order according to Zwaardemaker’s scheme of olfactory qualities. We shall here describe them in groups according to their mode of preparation. We shall consider first the preparation of the tubes of solid odorous matter, and afterwards discuss the solutions used to saturate the porcelain cylinders.

I. Preparation of Odorous Substances Used in Solid Form. The solid odorous materials from which tubes or hollow cylinders were prepared were vulcanized India rubber, black, red, and gray; cedar, rose-wood and musk-root; Russian leather, yellow wax, parrafin, glycerine soap, mutton-tallow, cocoa-butter and solid oil of mace, asafœtida, gum benzoin, tolu balsam, and a combination of gutta-percha and gum ammoniac in equal parts by weight. Tubes of red and black India rubber, and of gutta-percha and gum ammoniac came with the standard olfactometer from Utrecht. All the other cylinders, and a second tube of gutta-percha and gum ammoniac, were home-made. It is necessary that all such cylinders should be fitted into glass tubes