Electric fluid in vacuo resembles Aurora.
Mr. Hawksbee very early showed that the electric fluid assumes, in vacuo or in highly rarefied atmosphere, an appearance resembling the Aurora. Mr. Canton contrived an imitation of the Aurora by means of electricity transmitted through the Torricellian vacuum in a long glass tube, and showed that such a tube would continue to display strong flashes of light for 24 hours and longer without fresh excitation.
Experiment with electrical machine and exhausted receiver.
In the ‘Edinburgh Encyclopædia,’ date 1830, is mentioned an experiment in which an electrical machine and air-pump are so disposed that strong sparks pass from the machine to the receiver of the air-pump.
Dr. Franklin’s theory.
As the exhaustion proceeds the electricity forces itself through the receiver in a visible stream, at first of a deep purple colour; “but as the exhaustion advances it changes to blue, and at length to an intense white, with which the whole receiver becomes completely filled.” [It will be noticed that this experiment bears a close resemblance to Prof. Ångström’s exhausted flask referred to later in treating of the spectrum of the Aurora.]
Dr. Franklin gave a different form to the electric theory of the Aurora, supposing that the electricity which is concerned in the phenomenon passes into the Polar regions from the immense quantities of vapour raised into the atmosphere between the tropics (Exper. and Observ. 1769, p. 43).
Mr. Kirwan’s theory.
Mr. Kirwan (Irish Trans. 1788) supposed that the light of the Aurora Borealis and Australis was occasioned by the combustion of inflammable air kindled by electricity.
Mons. Monge’s.