19. THOMSON’S EXPERIMENTS. GAS AS A CONDUCTOR. VISIBLE INDICATION BY DISCHARGE. _Nature_, Lon., Aug. 23, ’94, p. 409; Jan. 31, ’95, p. 332, and other references cited below. Lec. _Royal Inst. Proc. Brit. Asso._, Aug. 16, ’94. In making comparisons, things of like nature should be considered. Take, for example, gas at .01 m. The number of molecules in such a rarefied atmosphere is comparatively small, while in an electrolyte there are molecules sufficient in number to produce 15,000 lbs. of pressure, if imagined in the gaseous state within the same space. By an experiment and rough calculation, Prof. J. J. Thomson, F.R.S., calculated that the conductivity of a gas estimated _per molecule_ is about 10 million times that of an electrolyte, for example, sulphuric acid. § 14. This is greater than the molecular conductivity of the best conducting metals. The experiment which is illustrated in Fig. IV. was a second experiment which did not serve as a basis for calculation, but exhibited very strikingly to the eye that gases having different pressures have different conductivities. For this apparatus he had two concentric bulbs, as indicated, one being contained within the other. The inner one had air rarefied to the luminous point. The outer one had a vacuum as high as it was practical to make it, and contained in a projection a drop of mercury, which, when heated, would gradually increase the pressure. Two Leyden jars were employed, and their outer coatings were connected to the coil which is seen surrounding the outer bulb, and the inner coatings were connected to the coils of a Wimshurst machine. The operation was as follows: When the mercury was cold, that is, with a high vacuum in the outer compartment, a bright discharge passed through the _inner_ bulb, while the _outer_ bulb was dark. When the mercury was heated, the outer bulb was bright, and the inner one was almost dark. By well-known principles of conductors and non-conductors, the operation was explained by Prof. Thomson, who assumed that the gas in the outer bulb is a conductor; then, at each spark will the alternating current in the coil induce currents of an opposite direction in the gas, which will become luminous, as occurred when the mercury was heated. The currents circulating in the gas act as a shield to the induction of the currents in the inner bulb. However, with the vacuum exceedingly high in the outer bulb, the air therein being a non-conductor comparatively, or for the given E. M. F., does not prevent the discharge through the inner bulb, which becomes, therefore, luminous. He next compared the dielectric power of a gas, a liquid and a solid. He found that the E. M. F. had to be raised, in order to produce the discharge,—higher in the liquid than in the gas, and higher in the solid than in the fluid. § 12. [Illustration: IV. ]