Page:Popular Science Monthly Volume 49.djvu/325

Rh be distinctly traced on its journey to the lightning conductor of the cupola of the Academy of Sciences.

The most interesting part of this photograph is the shadow of the cupola on the wet and foggy heavens. The intensity of the



light produced by the lightning is the cause of this peculiar effect, which I believe has never been obtained before or since. It is analogous with the phenomenon commonly known as the Specter of Brocken.

At a meeting of the Academy of Sciences, held a short time ago, Prof. Zenger read a most interesting paper upon the subject of Electricity as a Vortex Motion, in which he endeavors to prove that electrical discharges, no matter what their origin, produce a vortex motion on matter lying in the electric field. He makes the following experiment: A spark from a Ruhmkorff coil or a Wimshurst machine is discharged within the receiver of an air pump, under which has been placed a test tube containing diluted ammonia and another containing hydrochloric acid. At the instant of the discharge there will be visible eddies or little whirlwinds, rendered visible by the tiny crystals of ammonium chlorid suspended in the air. In circling about they are condensed into peculiar shaped veins which fall upon the plate of the air pump, arranging themselves in somewhat the same manner, says Zenger, as did the débris from the roofs and trees during the cyclone in the valley of Roux in Switzerland—i. e., they form lines of electric force.

Prof. Zenger sends me some very interesting photographs in