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. It has been found, for example, that over-assiduous efforts at filtering and purifying the air used for ventilation so as to remove all noxious chemical and bacterial ingredients defeat their own end. Such air, although to our artificial tests absolutely clean and pure, seems to the sense of smell to lack freshness. And the nose is right. The tests are wrong. For sojourn in such an atmosphere induces lassitude and torpor of mind, as members of the Houses of Parliament, where this method has been tried, know to their cost—and ours. But albeit so highly sensitive to minute traces, the sense occasionally fails to perceive a highly concentrated odour. For example, every one is aware that a bunch of violets which is filling a room with its fragrance seems when held to the nose to have no smell at all, or at the most to have but a vague, indefinable sort of odour. The effect, as a matter of fact, varies with the perfume employed. Some, like violets, have no smell at all. Others give a different smell when concentrated from what they give when dilute. Muskone, for one, the essential constituent of musk, has an odour of pines when concentrated; and storax, a delightful perfume when dilute, is disagreeable when too powerful, and so on. It is to be noted that the disagreeable character of these last is not due to the mental “cloying” or “sickening” of excessive sweetness; it is a definite odour. Nor is the anosmia for concentrated violets due to the exhaustion of the sense. Heyninx, comparing, as we shall see, olfaction with vision, believes the indefinite odour of concentrated violets to be akin to the absence of colour in white light. But this explanation seems to me to be improbable, since the effect is due not to the combination of a number of odours, as white light is the combination of all the colours of the spectrum, but to the overpowering influence of a single odour.