Page:Handbook of Meteorology.djvu/138

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the fact that dust particles of this dimension are very near to the size of the permanently floating dust motes of the air.

The research work of Dr. John Aitkin, the highest authority on the subject, has shown that clean air contains from 3000 to 5000 visible dust particles per cubic inch. The air of schoolrooms and public buildings with undressed wood floors carries from 60,000 to 80,000 particles which are visible under the high power of a microscope, and an unknown number which can be counted only when amplified in size by the condensation of moisture upon their surface.^[1] Dr. Aitkin found the cleanest air at snow-clad heights in the Alps, and not over the sea, as one might expect.

Electrification.—To the best of knowledge, the invisible dust, both in the stratosphere and the sphere of convection, does not depend on winds for its distribution. The particles themselves behave as do other ionized bodies, and it is not impossible that their suspension in the air is due to electrification.^[2] There seems to be no reason why the ionization of minute dust particles should not occur in the same manner as the ionization of the molecular constituents of the air.

↑ The dust-counter used by Dr. John Aitken consisted of a chamber or receiver, into which a measured portion of air was drawn. The receiver contained a small amount of water—enough to keep the air pretty nearly at saturation. A slight reduction of temperature by means of an air pump causes almost instant condensation. By counting the droplets condensed on a ruled silver plate within the receiver, using a magnifying lens therefor, the number of droplets per cubic centimeter, or per cubic inch, may be estimated. Dr. Aitken obtained the best results when the dust content of the air was small. In practice he therefore mixed the air to be examined with a measured quantity of air made dustless by filtration. A modified dust-counter, the “koniscope” is a more practical instrument, though not so accurate.
↑ A solid of 1 inch cubic measurement, weighing 1 ounce, has 6 square inches of surface. If it be shaved into slices one one-thousandth of an inch in thickness, each slice loses 999 parts of the original weight but only a little more than 4 parts of the original surface. That is, in subdivision, a substance loses weight much more rapidly than surface. The weight of a dust particle one twenty-five thousandth part of an inch in dimension is less than one fifteen trillionth of an ounce. The surface is almost infinitely great in comparison. Now, the electric charge of a dust particle, condensed on its surface, is of the same kind as that of the earth. Therefore they mutually repel. It is only fair to add that the theory of the electrification of dust is not fully substantiated.

[1] The dust-counter used by Dr. John Aitken consisted of a chamber or receiver, into which a measured portion of air was drawn. The receiver contained a small amount of water—enough to keep the air pretty nearly at saturation. A slight reduction of temperature by means of an air pump causes almost instant condensation. By counting the droplets condensed on a ruled silver plate within the receiver, using a magnifying lens therefor, the number of droplets per cubic centimeter, or per cubic inch, may be estimated. Dr. Aitken obtained the best results when the dust content of the air was small. In practice he therefore mixed the air to be examined with a measured quantity of air made dustless by filtration. A modified dust-counter, the “koniscope” is a more practical instrument, though not so accurate.

[2] A solid of 1 inch cubic measurement, weighing 1 ounce, has 6 square inches of surface. If it be shaved into slices one one-thousandth of an inch in thickness, each slice loses 999 parts of the original weight but only a little more than 4 parts of the original surface. That is, in subdivision, a substance loses weight much more rapidly than surface. The weight of a dust particle one twenty-five thousandth part of an inch in dimension is less than one fifteen trillionth of an ounce. The surface is almost infinitely great in comparison. Now, the electric charge of a dust particle, condensed on its surface, is of the same kind as that of the earth. Therefore they mutually repel. It is only fair to add that the theory of the electrification of dust is not fully substantiated.

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