The Philosophical Review/Volume 1/Summary: Parry - Sur quelques minimums perceptibles d'odeur
The odorous substances used were not complex natural 'essences,' but bodies which were chemically well defined. Expressing the threshold values in millionths of a gramme, P. found for camphor 5, for ether 1, for citral 0.5, for vanilla 0.005. The odoriferous power of an odor is to be distinguished from its intensity: the former is inversely as the threshold value; the latter is determined by the relative amount of a substance required to mask the odor of another. Thus, when threshold amounts of citral and vanilla are put together in a flask, the vanilla alone is perceived; but if the citral is increased tenfold in weight, the vanilla would have to be increased one hundred times to be perceptible. The intense substances correspond to Beaunis' odors [short reaction time]; and the powerful substances to Beaunis' perfumes [long reaction time]. The 'minimum perceptible' varies with different individuals, and with the same individual at different times.