saucer covered with a thin layer of water, the water immediately retires, sometimes for several centimetres, before the odorous substance. The laws of the diffusion of liquids may be summarized by saying that the rapidity depends on the nature of the substance, increases in proportion to the degree of concentration of the solution, and augments as the temperature rises. Graham's dialyzer is based on the very feeble diffusibility of certain substances, like the gums, and the great diffusibility of certain crystalline substances, like salt. It is simply a vessel, the bottom of which is formed of a leaf of parchment paper, that lets the diffusible substances pass into the water around it and holds the others.
The diffusion of gases and vapors, which is more important in questions of smell, is subject to laws which have been only approximately determined. A glass tube about a metre long is used, divided perpendicularly to its length by a thin metallic partition, which can be made to slide between two perforated glasses. A gas is introduced into each of the separated halves of the tube; the supply-cocks are closed, the partition is lifted out, and the two halves of the tube are put in communication; a half-hour later the partition is shut, and the gaseous mixture contained in each of the compartments is analyzed. Mr. Loschmidt has in this way found the mathematical rule for the measure of the diffusion of different gases, one within the other.
The volatility of a liquid is expressed by the weight of that liquid which evaporates per second and per square millimetre at a given temperature. All that is known of it is that this weight is proportioned to the excess of the maximum tension of the vapor at that temperature over the tension which it has in the air; and this weight varies inversely as the atmospheric pressure according to a law special for each liquid. Evaporation may, therefore, give us valuable information concerning the purity of the odor, and spare us, in many cases, the delicate problem of determining the maximum tension which is so important a characteristic of substances. A special apparatus has been devised for the rapid measurement of volatility.
Tables have been prepared showing the relative volatility of different perfumes, of the substances used for adulterating them, and of the adulterations, by means of which a convenient method is afforded for the detection of frauds.
The influence of different physical forces on the disengagement of odor has been studied; and possible relations between the colors of flowers and the intensity of their perfumes have been inquired into. It has been found that white flowers represent the largest number of odoriferous species, and after them come red, yellow, green, and blue. The order corresponds with that of the