Page:Popular Science Monthly Volume 4.djvu/473

Rh As I laid down the paper I asked myself this question, or, rather, I put to myself the same question in another form: "Is there among the sons of men any one who really knows the composition of pure air?"

Still further I queried with myself what answer I should have given to the question had I been one of the applicants for a position on the Board of Health, and it seemed to me that, after stating what almost every school-girl knows about the relative proportions of oxygen and nitrogen, I should have added this codicil: "The question of the composition of pure air is one that is too complicated to admit of an answer." What I have to say this morning on atmospheric electricity and ozone will serve, so far as it goes, to enforce this view.

How the Subject of Atmospheric Electricity and Ozone has been investigated.—During the past quarter of a century regular daily observations of atmospheric electricity have been made in Brussels, Munich, and for the past ten or fifteen years in St. Louis. The difficulties in the study of the subject are very great, but, from the accumulated observations of the different investigators, some few interesting and important general facts have been secured.

Apparatus for studying Atmospheric Electricity; Measuring Apparatus.—Prof. Dellman, of Kreuznach on the Rhine, for several years made three regular observations each day of the atmospheric electricity. The electrometer that he used in these observations is a torsion balance. A small thread of glass going vertically through a glass tube has on its lower extremity a small needle of brass fastened to it. This light brass needle, when influenced by any force, can move over a metallic disk with a graduated scale. Below this light brass needle is another light brass needle, which is fixed and isolated from the metallic disk, and connected with a metallic wire which receives the electrical charge from outside. By means of a micrometer screw the upper needle can be lowered and raised so as to touch the lower needle, or be kept above it.

The whole instrument rests on three iron legs, which can be screwed up and down so as to give it the level required. When the wire outside receives a charge of electricity, it communicates this charge to the lower needle. If, now, the upper needle be lowered and brought in contact with the lower one, it also receives a charge of electricity. But, as like electricities repel each other, the other needle will be at once driven off over the graduated scale. The number of degrees that it is driven will depend on the strength of the charge. To determine whether the electricity is positive or negative, subsequently charge the wire with electricity of known quality. If they are alike—that is, if the first charge be of the same quality as the second—the needle will be repelled still farther; if unlike, the needle will return toward the fixed needle.