139.] Other experiments were afterwards made in which mercury and sodium were made to boil in a bent glass tube while raised to a high po- tential by a battery of 50 Leclanché cells. A thick copper wire (Fig. 31) was placed on an insulating stand so that the end of the wire was within the glass tube and surrounded by the vapour of the metal. It was necessary that the wire should not be allowed to touch the tube, because glass at a high tem- perature is a good conductor. It was also necessary to see that the products of combustion from the Bunsen burner did not come in contact with the wire after becoming electrified by the hot tube. Fig. 31.

The wire was connected with the electrometer, but no evidence of con- duction of electricity could be observed, even when the mercury was boiling briskly, and its vapour was being condensed on the wire. But whenever so much mercury had collected on the wire that a drop fell off at the end of theCONDUCTIVITY OF GASES.

wire, there was a deflection of the electrometer because the drop had become charged by induction from the tube and the removal of this charge affected the electrometer. This however was no evidence of conduction through the metallic vapour, but only indicated that the apparatus was in such a state of electrification that any conduction, if it took place, would produce a sensible indication at the electrometer.

It is difficult to reconcile these experiments on the insulating power of hot gases and vapours with the well-known phenomena of the communication of electricity along the stream of heated matter rising from a flame or even from red-hot metal. This stream acts as a powerful conductor of electricity between the flame and bodies placed at a foot or a yard above it where the temperature of the ascending current is much lower than it was in the experiment of the tube and rod.

140.] The whole theory of the electric properties of gases is in a very im- perfect state. According to the kinetic theory of gases, their molecules are in a state of agitation so that they are continually striking against each other. The velocity of this agitation is greater the higher the temperature. It would appear, therefore, that the electric conduction of gases is of the nature of con- vection. At every collision the whole charge of two of the molecules would be equally divided between them, and thus the tendency of the agitation would be to equalize the charges of all the molecules.

But we can hardly admit a theory of this kind when we consider that we have hitherto obtained no evidence of the conduction of electricity through air at the ordinary pressure and temperature under a feeble electromotive force. Whenever a body free from projecting points and sharp edges and charged to a low potential is found to lose its charge, the result can always be traced to conduction through the substance or along the surface of the apparatus which is required to support it. The more perfectly insulating we make this apparatus the more slowly does the electrified body lose its charge, so that it is probable that if we could support an electrified body on a perfectly insulating stand so that it could lose its charge only by conduction through the air, it would never lose its charge.PYRO-ELECTRIC PHENOMENA.

Electric Phenomena of Tourmaline.

141.] Certain crystals of tourmaline and of other minerals possess what may be called Electric Polarity. Suppose a crystal of tourmaline to be at a uniform temperature, and apparently free from electrification on its surface. Let its temperature be now raised, the crystal remaining insulated. One end will be found positively and the other end negatively electrified. Let the sur- face be deprived of this apparent electrification by means of a flame or other- wise; then if the crystal be made still hotter, electrification of the same kind as before will appear, but if the crystal be cooled the end which was positive when the crystal was heated will become negative.

These electrifications are observed at the extremities of the crystallographic axis. Some crystals are terminated by a six-sided pyramid at one end and by a three-sided pyramid at the other. In these the end having the six-sided pyramid becomes positive when the crystal is heated.

Sir W. Thomson supposes every portion of these and other hemihedral crystals to have a definite electric polarity, the intensity of which depends on the temperature. When the surface is passed through a flame, every part of the surface becomes electrified to such an extent as to exactly neutralize, for all external points, the effect of the internal polarity. The crystal then has no external electrical action, nor any tendency to change its mode of electrifi- cation. But if it be heated or cooled the interior polarization of each particle of the crystal is altered, and can no longer be balanced by the superficial electrification, so that there is a resultant external action.

In tourmaline and other pyroelectric crystals it is probable that a state of electric polarization exists, which depends upon temperature, and does not require an external electromotive force to produce it. If the interior of a body were in a state of permanent electric polarisation, the outside would gradually become charged in such a manner as to neutralize the action of the internal electrification for all points outside the body. This external superficial charge could not be detected by any of the ordinary tests, and could not be removed by any of the ordinary methods for discharging superficial electrification. The internal polarization of the substance would therefore never be discovered un- less by some means, such as change of temperature, the amount of the internal polarization could be increased or diminished. The external electrification would then be no longer capable of neutralizing the external effect of the in- ternal polarization, and an apparent electrification would be observed, as in the case of tourmaline.