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Another important constituent is the so-called dust, the presence of which must have been noticed for ages. It is a highly complex mixture of inorganic, lifeless organic, and living organic matter. The first named originates partly from natural causes, such as wind and volcanic activity. Thus the writers have in their possession sand found at Las Palmas, which had been blown over 100 miles of sea from the Great Sahara Desert. During the great eruption of Krakatoa in 1883 vast quantities of dust were hurled into the air, and for several years afterwards the sunsets were remarkable for their glow, indicating that the particles of dust continued to float in the atmosphere for a proportionately long time. In a spectroscopic analysis of dust Hartley has found a large number of inorganic substances that must have travelled from many parts of the earth very wide distances apart. Among the inorganic particles are minute crystals, for example of sodium chloride which owe their origin to sea spray, and may be carried inland for hundreds of miles; sodium sulphate particles, probably formed by the interaction of the sodium chloride with sulphurous acid vapour and subsequent oxidation, are also present, and cause the ready crystallisation of supersaturated solutions of sodium sulphate when exposed to the air. A good deal of atmospheric dust is of artificial production, due to the combustion of coal, to factories, etc., thus a massive bank of smoke in London has been seen to rise to a height of several thousand feet and be carried away in a sunlight-obscuring trail fifty miles in length. Knecht has examined Manchester soot and finds in it some 50 per cent, of substances that are not carbon, such as salts of ammonium, calcium sulphate, and the like. Even the purest atmospheres contain particles of dust, and Aitken doubts if a perfectly dust- free atmosphere can exist. It is rare to find air with fewer than 100 particles of dust per c.c. In London it may rise to 100,000 or even 150,000 particles per c.c. Fridlander found 2000 particles per c.c. of air on crossing the Atlantic Ocean, on the Mediterranean Sea 875, the lowest results being obtained on the Pacific and Indian Oceans, the particles numbering 245 and 243 respectively. Similarly, Melander has studied the dust of Swiss air, but never found fewer than 300 particles per c.c.

In the following table are given some of the data obtained by Owens on the Norfolk Coast in a series of tests during August 1921: -

Examination of Air at Holme, Norfolk.

Date. August 1921.Number of Particles per c.c. of Air.Average Diameter. (Microns).Remarks.
131500.5Fine dark-coloured dust. No crystals.
183100.5 to 0.7Very uniform in size. No crystals.
201450.5Some crystals.
253040.5All rounded. Some perfect spheres.

It is interesting in this connection to recall the experiments of Lehmann and his colleagues who demonstrated quite recently that when a dust-laden air is breathed through the nose, approximately 40 per cent, of the dust remains in the system, either in the lungs or stomach, the remaining 60 per cent, being either respired or retained by the nose and mouth. If, however, the subject breathes through the mouth some 80 per cent, is retained by the body. This illustrates the importance of breathing through the nose, but it also illustrates the necessity of reducing the dust in the atmospheres of public rooms and dwellings to the smallest possible amount, as no matter how careful the subject is, some of the dust finds its way into the system.

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