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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. 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. 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.