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 investigations in this direction. "The results connected with the different conditions of positive and negative discharge," he wrote, "will have a far greater influence on the philosophy of electrical science than we at present imagine."

Twenty more years, however, passed before another notable advance was made. That a subject so full of promise should progress so slowly may appear strange; but one reason at any rate is to be found in the incapacity of the air-pumps then in use to rarefy gases to the degree required for effective study of the negative glow. The invention of Geissler's mercurial air-pump in 1855 did much to remove this difficulty; and it was in Geissler's exhausted tubes that Julius Plücker, of Bonn, studied the discharge three years later.

It had been shown by Sir Humphrey Davy in 1821 that one form of electric discharge—namely, the arc between carbon poles—is deflected when a magnet is brought near to it. Plücker now performed a similar experiment with the vacuum discharge, and observed a similar deflexion. But the most interesting of his results were obtained by examining the behaviour of the negative glow in the magnetic field; when the negative electrode was reduced to a single point, the whole of the negative light became concentrated along the line of magnetic force passing through this point. In other words, the negative glow disposed itself as if it were constituted of flexible chains of iron filings attached at one end to the cathode.

Plücker noticed that when the cathode was of platinum, small particles were torn off it and deposited on the walls of the glass bulb. "It is most natural," he wrote, "to imagine that the magnetic light is formed by the incandescence of these platinum particles as they are torn from the negative electrode."

He likewise observed that during the discharge the walls of