By the examination of other vapours or gases than that which is gaseous water, namely, the vapours of bodies like chloroform and ether, it has been shown that "cloud" forms in a vapour not merely in consequence of the cooling of the vapour, but in consequence of the presence in the air (or in the tube in which the vapour is enclosed for observation) of very fine floating dust particles. They act as centres of attraction and condensation for the vapour particles. When there are no dust particles present clouds do not form readily in cooling vapours, or only at lower temperatures, and in larger mass. Tyndall made some beautiful experiments on this subject, obtaining clouds of great tenuity in vapours enclosed in tubes, which reflected the most vivid blue tints when illuminated by the electric arc-lamp. Later Aitken, of Edinburgh, showed that the "fog" which forms in smoke-ridden towns is due to the condensation of previously invisible watery vapour as "cloud" around the solid floating particles of carbon of the smoke. Aitken further used this property of solid floating particles, namely, that they cause the formation of fog and cloud in vapours–to test the question as to whether the excessively minute odoriferous particles which affect our noses as "smell" are distinct solid floating particles as often supposed, or are of the nature of gas and vapour. He admitted strong perfumes such as musk into tubes containing watery vapour, at such a temperature that the vapour was in a "critical" state–just ready to condense and precipitate as "cloud." If he had admitted fine solid particles such as a minute whiff of smoke, or some "dusty" air–the cloud would have formed. But the admission of the perfume had no such effect Therefore, he concluded that the odoriferous emanations used by him are not distinct particles like those of smoke or dust, but are gaseous.