1392.  When a spark had passed at either interval, then, generally, more tended to appear at the same interval, as if a preparation had been made for the passing of the latter sparks.  So also on continuing to work the machine quickly the sparks generally followed at the same place.  This effect is probably due in part to the warmth of the air heated by the preceding spark, in part to dust, and I suspect in part, to something unperceived as yet in the circumstances of discharge.

1393.  A very remarkable difference, which is constant in its direction, occurs when the electricity communicated to the balls s and S is changed from positive to negative, or in the contrary direction.  It is that the range of variation is always greater when the small bulls are positive than when they are negative.  This is exhibited in the following Table, drawn from the former experiments.

Pos.       Neg. 
In Air the range was       0.19      0.09
Oxygen                  0.19      0.02
Nitrogen                0.18      0.11
Hydrogen                0.14      0.05
Carbonic acid           0.16      0.02
Olefiant gas            0.22      0.08
Coal gas                0.24      0.12
Muriatic acid           0.43      0.08

I have no doubt these numbers require considerable correction, but the general result is striking, and the differences in several cases very great.

* * * * *

1394.  Though, in consequence of the variation of the striking distance (1386.), the interval in air fails to be a measure, as yet, of the insulating or resisting power of the gas in the vessel, yet we may for present purposes take the mean interval as representing in some degree that power.  On examining these mean intervals as they are given in the third column (1388.), it will be very evident, that gases, when employed as dielectrics, have peculiar electrical relations to insulation, and therefore to induction, very distinct from such as might be supposed to depend upon their mere physical qualities of specific gravity or pressure.

1395.  First, it is clear that at the same pressure they are not alike, the difference being as great as 37 and 110.  When the small balls are charged positively, and with the same surfaces and the same pressure, muriatic acid gas has three times the insulating or restraining power (1362.) of hydrogen gas, and nearly twice that of oxygen, nitrogen, or air.

1396.  Yet it is evident that the difference is not due to specific gravity, for though hydrogen is the lowest, and therefore lower than oxygen, oxygen is much beneath nitrogen, or olefiant gas; and carbonic acid gas, though considerably heavier than olefiant gas or muriatic acid gas, is lower than either.  Oxygen as a heavy, and olefiant as a light gas, are in strong contrast with each other; and if we may reason of olefiant gas from Harris’s results with air (1365.), then it might be rarefied to two-thirds its usual density, or to a specific gravity of 9.3 (hydrogen being 1), and having neither the same density nor pressure as oxygen, would have equal insulating powers with it, or equal tendency to resist discharge.