Aitken, a Scottish savant, has shown that no condensation of moisture, as in rain, mist, fog (see For) could occur without nuclei such as dust particles. If the atmosphere were not impregnated with dust there would be no cloud effects, no radiant sunsets, no soft afterglow; the sun would go down instantaneously, the harmonious colorings which lend a halo to the quiet of eventide would never have been. Aitken has also devised an apparatus for gauging the number of dust particles in a given sample of air or gas. He and others have made innumerable tests in all parts of the world. The number of particles varies from a few thousand per cubic inch over oceans and in mountain regions to 50,000,000 and upward in dusty towns. A room near a ceiling has been found to contain 88,000,000 to a cubic inch. Aitken regards that a cigarette smoker sends 4,000,000 particles into the air with every puff. A great amount of solid matter is sent into the air as the result of the imperfect combustion of fuel. The dust fall in English cities is esti mated at from 200 to 2,000 tons per square mile per annum. Tests made in 1913 showed that 195 tons per square mile fell at Sutton in Surrey as compared with 650 per square mile in the East End of London; in the centre of Leeds 242 tons and Leeds Forge 539; Glasgow and Coatbridge 1,330 and 1,939 respectively. Experiments made in 1914 by the Board of Health of the Department of the Seine on the Paris Underground Railway showed that in the metropolitan section of the road the average constitution of the dust embraced 46 per cent metallic iron, 14 per cent iron oxide, 12.1 lime and plaster, 1.12 grease, 12.6 water and organic matter.
But one of the most important and serious questions concerning dust-laden air is the danger it brings to human life as disseminating the bacteria of disease. Nearly one-fourth of
all deaths are due to consumption. Now the expectoration of a consumptive may contain millions of germs. Falling on the sidewalk or carriage-way of a city, it is soon tracked over a large area and gradually mixed with the dust; especially on asphalt pavements, where each wheel acts as a millstone, grinding everything into the finest powder, to be raised by passing vehicles into the air and sent into thousands of healthy lungs. The number of disease bacteria in the air has been calculated by many analyses. Taking 10 litres of air for a basis: in the Boston City Hospital the number of living bacteria was found to be nearly 450, and of molds 225. In a model New York hospital, where everything is supposed to be clean, 12 living germs settled on the disc, and, after sweeping, 226. In a New York tenement-house carpeted room, 75 living bacteria settled on the disc in an exposure of five minutes; after sweeping, 2,700, and mold settled on a plate or disc three and three-quarters inches in diameter. Precautions are now taken in the majority of cities against the peril of dust-spreading disease by constantly flushing the streets and sweeping away all superficial dust into the sewers.
or SMUT, a disease of certain plants, as oats, barley, corn and other cereals. It is caused by a parasite fungi called Ustilago, which causes a swelling that at length becomes a powdery sooty mass. The common forms are Ustilago segetum and U. carbo; that which attacks Indian corn is Ustilago tnaydis. See FUNGI.