15. GORDON’S EXPERIMENT. DUST PARTICLES HASTEN DISCHARGE. _Gordon_, Vol. II. Other experimenters had investigated the phenomena of the electric spark with different densities of the dielectric by a spark produced by a frictional or an influence machine, or, in a few cases, by powerful batteries without coils, while Gordon claims to be the first to carry out these experiments with an induction coil. He observed that when the discharging limit was nearly reached, small circumstances, such as a grain of dust or a rusting of the terminal by a former discharge, would cause the discharge to take place at a lower E. M. F., which should be allowed for. 16. KELVIN’S EXPERIMENT. _Proc. R. So._, 1860. _Encyclo. Brit._, Art. Elect. He used as the terminals, two plates. One of them was perfectly plane, while the other had a curvature of a very long radius. The object of this arrangement was to obtain a definite length of spark for each discharge. The plates were gradually moved away until the spark would no longer pass, and the reading of the distance was noted. The law which he found cannot well be expressed in the form of a rule or principle, because it is of a rather intricate nature, but a discovery resulted, namely in the case where the distance was greater, the dielectric strength was smaller for respective distances of .00254 and .535 cm. Many theoretical considerations in reference to this matter have been presented, notably that of Maxwell in his treatise on Electricity and Magnetism, Vol. I. 17. CAILLETET’S EXPERIMENT. SPARK. PENETRATING POWER. HIGH PRESSURES. INCREASED DIELECTRIC STRENGTH. _Mascart_, Vol. I. He experimented with dry gas up as high as pressures of 700 lbs. per sq. inch. He found that the dielectric strength continues to increase with increase of pressure. He used about 15 volts in the primary and a powerful induction coil. The dielectric strength was so great that at the maximum pressure named above, the spark would not pass between the electrodes when only .05 mm. apart. § 25 and 11, near end. 18. FARADAY’S EXPERIMENT. DISCHARGES IN DIFFERENT CHEMICAL GASES VARIABLY RESISTED. _Exper. Res. Phil. Trans._, Se. XII., Jan. ’36. Faraday passed on from the consideration of the effect of pressure, temperature, etc., and wondered whether there would be any difference in the law according to what gas was used. He arranged apparatus so that he could know, with air as a standard, whether another gas had a greater or less dielectric power. (Cavendish before him had noticed a difference.) He tabulated the results. They exhibited the following facts, namely that gas, when employed as dielectrics, depend for their power upon their chemical nature. § 10. Hydrochloric acid gas was found to have three times the dielectric strength of hydrogen, and more than twice that of oxygen, nitrogen or air; therefore the law did not follow that of specific gravities nor atomic weights. See also De la Rue, _Proc. Royal So._, XXVI., p. 227.