basis for the comparison of individuals with reference to their keenness of smell, and of substances with reference to their value for the sense, and thus may indirectly lead to some knowledge of the greatest degree of dilution in which an odorous substance can be detected.[1]
The method which Valentin invented in 1848 may be called classical, since it is mentioned in most of the standard text books of physiology. It was direct, and consisted in taking a certain volume of odorous gas and mingling it with a hundred volumes of air, taking a certain volume of the mixture and mingling this again with a hundred volumes of fresh air, and so on until the last mixture gave a just discernible odor. Valentin varied his procedure by allowing the vaporization of smaller and smaller quantities of a highly concentrated solution of an odorous substance in a definite amount of air, or by mingling smaller and smaller quantities of it with a mass of water of a given volume.[2] It is plain that a certain amount of the odorous substance must adhere to the vessel in which such a mixture is contained, so that the amount of odorous substance taken away from the receptacle for a new admixture will never be so large as the ratio of the gas or liquid removed to the whole volume would indicate, and that this error must increase as the experiment proceeds. As for the use of highly concentrated solutions, it involves two serious disadvantages, the blunting of the sense by exhaustion and the adhesion of odorous particles to obſects in the laboratory.[3]
The invention of no other direct olfactrometric method is recorded before that of the method employed by Fischer and Penzoldt in 1887. Avoiding Valentin's progressive dilutions, these investigators sought to determine how much mercaptan and how much chlorphenol must be introduced into the whole mass of air in a laboratory of a certain size in order to give au odor just noticeable to a person entering the room. The walls of the laboratory were perfectly smooth, the floor was of stone, and the equal distribution of the odorous gas for all parts of the room was secured by the motion of fans. The solutions were scattered with a fine spray.[4] Unfortunately, these solutions were alcoholic.
In the same year H. C. Dibbits arrived at a partial determination of the stimulus-limen for the odor of acetic acid. Acetate of zinc is decomposed in the presence of water, and an insoluble basic salt and free acetic acid are formed. Dibbits, during the course of sixteen hours, allowed 60 litres of damp air to pass over a mass of salt which had been freed from water of crystallization, found the loss of weight to be 16.8 mg,, and calculated the proportion that the weight of the acetic acid set free must bear to this loss of weight to be 10⁄7. As 24 mg. of acetic acid must have been communicated to 6o 1. of air, and as the odor was discernible in this air, the stimulus-limen of acetic acid must lie under 0.4 mg. per litre.[5] While the methods of Tischer and Penzoldt and of Dibbits are comparatively accurate, it is obvious that they are impracticable for difference-determinations.
A method employed in 1889 by Ottolenghi for testing the olfactory
sensitivity of criminals is a modified form of Valentin's, and is essentially the same as the method recommended by Passy in 1892.
