emission of calorific force. Flowers which by their color emit the most heat, also emit the most perfume.
The results of the study of the influence of the color of substances on their power of absorbing odors differ a little from these: white, yellow, red, green, and blue absorb odors in a decreasing order, or rather emit them in an increasing one. These colors represent decreasing luminous powers.
Ozone develops the energy of essential oils, and perfumes in turn determine by their oxidation in the air the production of ozone. This is a matter of hygienic significance, for the presence of ozone being favorable to health, we have a means at hand of increasing the supply of it by surrounding ourselves with fragrant substances and flowers.
Heat favors the volatilization of perfumes, and to such an extent that beds of flowers are sometimes inodorous in the bright sunlight which are fragrant in the shade. Some essences need a high temperature for the production of their full effect; while others, to have their delicacy fully appreciated, require the coolness of the evening. This principle may account for appir-fnt differences of tastes among the people of different countries. The odors of many substances are not of equal strength in different climates. Prof. Tyndall believes that there are considerable differences in the absorbing power of different odorous vapors for radiant heat. He perfumed small paper cylinders by dipping them by one end in an aromatic oil, and then placed them in a glass tube, which communicated, through a stop-cock, with a tube in which a vacuum is produced. The air, according as it has been perfumed with one substance or another, discloses to the galvanometer an absorbing power, which, air at the usual pressure being taken as one, varies from thirty for patchouli, to three hundred and seventy-two for anise-seed. These results are, unfortunately, not exact, for no account is taken in them of the tensions of the odorous vapors, which certainly vary, though they are probably of very small absolute value.
Messrs. Nichols and Bailey have compared the smelling powers of men and women. Having made measured solutions of a number of essential oils, a series of flasks was prepared so that the solution in each succeeding one should be only half as strong as that in the preceding one. The flasks were "shuffled," and the subjects of the experiment were called upon to rearrange them in the order of concentration of the solutions. The smelling power of women appeared to be on the whole less delicate than that of the men. The extreme delicacy of the scent of the dog is well known. Mr. Romanes has shown that, by fastening a sheet of paper to the shoes, the odor may be masked, and the dog prevented from following the track of his master; but that a contact with the