DUST

solid matter in a fine state of division. Spores of plants, bacteria, ., are found in the atmosphere; but, in general, organic particles are numerous only over thickly populated districts. Inorganic particles are derived from various sources. Where the soil is dry, dust is whirled aloft by the winds, this cause giving rise to the great sand-storms of tropical desert regions. Volcanoes in eruption eject large quantities of dust. It is estimated that millions of meteors are encountered by the earth per day. Most of these are exce ively minute. They are speedily disintegrated, and generally entirely reduced to dust at high levels. Evaporation is almost always proceeding over seas and oceans, and from foam thrown up and swept along by the winds the di olved salts are liberated as solid particles. Again vast quantities of dust are produced in the consumption of fuel. The late Dr. John Aitken, F.R.8., of Falkirk, Stir lings hire, contrived a means of gauging the dust contents of the atmosphere. This consists of a 20 DUTCH AUCTION 17 DUTCH METAL gla box about a centimetre in, result on reaching his minimum the thickne . Two pieces of wet filter paper inside serve to keep the contained air damp. The bottom of the box is a micrometer plate, divided rectangularly in millimetres. It can be examined from above by a lens. AN air-pump can withdraw definite volumes of air as desired. When the air is partially withdrawn, the expansion of the remainder preduces cooling. The dust particles form nuclei for condensation of the vapour. They are thus precipitated on the plate and counted, leaving the air dust-free. A measured quantity of the air to be tested is next drawn in and shaken up. Further operation of the airpump causes its expansion, and the deposition of its dust particles, which can then be counted. Dr. Aitken found the proportion of dust on Ben Nevis to vary at different times from under 100 particles to over 14,000 per | cubic centimetre. Over oceans the numbers were from about 500 on the Indian to 2,000 on the Atlantic; but over cities 100,000 per cubic centimetre are frequently present. A puff of cigarette smoke was estimated to contain 4,000,000,000 particles. Many phenomena are connected with the existence of dust in the atmosphere. As a result of Dr. Aitken’s discoveries the belief largely prevailed that the formation of fog, of rain, and other varieties of precipitation, was nece arily dependent on dust particles as nuclei of condensation. Though they certainly function to a preponderating extent, it has been shown that gaseous particles can act similarly, particularly when air is ionized. Dust is the main cause of the scattering of the sun’s rays which produces twilight, the blue of the sky, the gorgeous red and golden hues of sunrise and sunset, and the purple lights of advancing dusk. After the great Krakatoan eruptions of 1883, dust was carried in the upper atmosphere several times round the earth, and caused extraordinary colour efiects. To a le er degree similar phenomena followed the West Indian eruptions of 1902. The unusual sunle ne of the summer of 1912 was attributed to dust expelled in the preceding great eruptions at Katmai, Alaska.