Before we proceed to describe the experiments under this head, it will be necessary to inform the reader that by the term _vacuum_ which he will frequently meet with in this chapter, we mean such an one as is formed by the action of an air pump, which is a good conductor of electricity. EXPERIMENTS. _The Aurora Borealis._ Take a phial nearly of the shape and size of a Florence flask; fix a stopcock or valve to its neck, and exhaust it of air.—If this phial be rubbed in the usual way to excite electrics, it will appear luminous within, being full of a flashing light, very much resembling the northern lights or aurora borealis. This phial may also be made luminous by presenting one end of it to the prime-conductor, while the other is held in the hand. In this case, the whole cavity of the glass will instantly appear full of a flashing light, which remains in it for some time after the glass has been removed from the prime-conductor. A glass tube exhausted of air in the same manner, and hermetically sealed, may be used instead of this phial, and perhaps with more advantage. The most remarkable circumstance attending this experiment is, that after the phial or tube has been removed from the prime-conductor, and even several hours after the flashing light has ceased, strong flashes will be again visible upon applying the hand. The causes of this phenomenon are two; first, the conducting nature of the vacuum; and second, the charging of the glass; for when one side of the phial is touched with the prime-conductor, the electric fluid communicated to that part on the outside, occasions the natural fluid of the inside surface, to leave its place and pass to the opposite side of the phial, which does not communicate with the electrified conductor; this passing of the fluid through the vacuum occasions the light within, which is more or less subdivided as the vacuum is more or less perfect. That part of the phial which has touched the prime-conductor is actually charged, for its outer surface has acquired an additional quantity of the electric fluid, and the inside has lost part of its natural quantity; but as the outside of the glass has no coating, when it is removed from the prime-conductor and is not in contact with the hand or other conductor, the charged part will be discharged gradually, that is, while its outside surface is communicating its redundant quantity to the contiguous air, the inner surface acquires the electric fluid from the other parts of the phial or tube, and this fluid passing through the vacuum, causes the light which is observed for so long a time. If the phial or tube be grasped with the hand, the discharge will be accelerated, yet it cannot be effected in this way immediately, because the hand cannot touch every part of the glass at once. _The Leyden Vacuum._