1525.  It seems very possible that the remarkable difference recently observed and described by my friend Professor Daniell[A], namely, that when a zinc and a copper ball, the same in size, were placed respectively in copper and zinc spheres, also the same in size, and excited by electrolytes or dielectrics of the same strength and nature, the zinc ball far surpassed the zinc sphere in action, may also be connected with these phenomena; for it is not difficult to conceive how the polarity of the particles shall be affected by the circumstance of the positive surface, namely the zinc, being the larger or the smaller of the two inclosing the electrolyte.  It is even possible, that with different electrolytes or dielectrics the ratio may be considerably varied, or in some cases even inverted.

  [A] Philosophical Transactions, 1838, p. 47.

* * * * *

Glow discharge.

1526.  That form of disruptive discharge which appears as a glow (1359. 1405.), is very peculiar and beautiful:  it seems to depend on a quick and almost continuous charging of the air close to, and in contact with, the conductor.

1527. Diminution of the charging surface will produce it.  Thus, when a rod 0.3 of an inch in diameter, with a rounded termination, was rendered positive in free air, it gave fine brushes from the extremity, but occasionally these disappeared, and a quiet phosphorescent continuous glow took their place, covering the whole of the end of the wire, and extending a very small distance from the metal into the air.  With a rod 0.2 of an inch in diameter the glow was more readily produced.  With still smaller rods, and also with blunt conical points, it occurred still more readily; and with a fine point I could not obtain the brush in free air, but only this glow.  The positive glow and the positive star are, in fact, the same.

1528. Increase of power in the machine tends to produce the glow; for rounded terminations which will give only brushes when the machine is in weak action, will readily give the glow when it is in good order.

1529. Rarefaction of the air wonderfully favours the glow phenomena.  A brass ball, two and a half inches in diameter, being made positively inductric in an air-pump receiver, became covered with glow over an area of two inches in diameter, when the pressure was reduced to 4.4 inches of mercury.  By a little adjustment the ball could be covered all over with this light.  Using a brass ball 1.25 inches in diameter, and making it inducteously positive by an inductric negative point, the phenomena, at high degrees of rarefaction, were exceedingly beautiful.  The glow came over the positive ball, and gradually increased in brightness, until it was at last very luminous; and it also stood up like a low flame, half an inch or more in height.  On touching the sides of the glass jar this lambent flame was affected, assumed