“This is the Maida Vale ’bus,” was the contemptuous reply. “I knows it by the smell o’ musk.”
The inexhaustible capacity of the olfactory organ, to which we alluded above, is by no means its only marvel. It is also of the most wonderful delicacy, equalling, even if it does not surpass, in this respect, the sensitiveness of the eye to light.
This property of the smell-organ has been scientifically estimated. There are many ways of doing so, that by means of Zwaardemaker’s olfactometer being perhaps the most popular:
“This consists of two tubes that slide one within the other, and so shaped that one end of the inner tube may be applied to the nostril. The odorous material is carried on the inner surface of the outer tube. When the inner tube, which is graduated, is slipped into the outer one so as to cover completely its inner face, and air is drawn into the nostril through the tube, the odorous surface, being covered, gives out no particles, and no odour is perceived. By adjusting the inner tube in relation to the outer one, whereby more or less of the odorous surface is exposed, a point can be found where minimum stimulation occurs. The amount of odorous substance delivered under these circumstances to the air current has been designated by Zwaardemaker as an olfactie, the unit of olfactory stimulation. Having determined for a given substance the area necessary for the delivery of one olfactie, doubling that surface by an appropriate movement of the inner tube will produce a stimulus of two olfacties, and so forth. Thus a graded series of measured olfactory stimuli can easily be obtained. Further, by using outer tubes carrying different odorous substances various comparisons can be instituted as measured in olfacties” (Parker).
Instruments more elaborate and of greater accuracy have, as a matter of fact, been devised and used, but they need not detain us.
The results obtained by these and other methods of determining the minimum stimulus of olfaction are certainly astonishing, and reveal as nothing else can the delicate acuteness of the sense.
Fischer and Penzoldt found that they could plainly smell one milligram of chlorphenol evaporated in a room of 230 cubic metres capacity. This is equivalent to 1/230,000,000 of a milligram to each cubic centimetre of air, or, assuming 50 cubic centimetres of air as the minimum needed for olfaction, the amount of chlorphenol capable of exciting sensation is 1/4,600,000 of the thousandth part of a gram—approximately 1/276,000,000 of a grain!
Many other odours have been similarly tested, and although there is much numerical discrepancy in the records made by different observers, all agree as to the extreme delicacy of the sense. (For vanillin and mercaptan, see p. [39].)
Those experiments and estimations explain how it comes about that many odours (musk, for example) may go on giving off their scent until they part with the whole of it without undergoing any appreciable loss of weight.
Thus there is no chemical test known to us so delicate as olfaction.