Smallest.  Greatest.  Mean.
 _ | Air, s and S, pos. 0.60 0.79 0.695 |_Air, s and S, neg. 0.59 0.68 0.635 _ | Oxygen, s and S, pos. 0.41 0.60 0.505 |_Oxygen, s and S, neg. 0.50 0.52 0.510 _ | Nitrogen, s and S, pos. 0.55 0.68 0.615 |_Nitrogen, s and S, neg. 0.59 0.70 0.645 _ | Hydrogen, s and S, pos. 0.30 0.44 0.370 |_Hydrogen, s and S, neg. 0.25 0.30 0.275 _ | Carbonic acid, s and S, pos. 0.56 0.72 0.640 |_Carbonic acid, s and S, neg. 0.58 0.60 0.590 _ | Olefiant gas, s and S, pos. 0.64 0.86 0.750 |_Olefiant gas, s and S, neg. 0.69 0.77 0.730 _ | Coal gas, s and S, pos. 0.37 0.61 0.490 |_Coal gas, s and S, neg. 0.47 0.58 0.525 _ | Muriatic acid gas, s and S, pos. 0.89 1.32 1.105 |_Muriatic acid gas, s and S, neg. 0.67 0.75 0.710

1389.  The above results were all obtained at one time.  On other occasions other experiments were made, which gave generally the same results as to order, though not as to numbers.  Thus: 

Hydrogen, s and S, pos.          0.23       0.57       0.400
Carbonic acid, s and S, pos.     0.51       1.05       0.780
Olefiant gas, s and S, pos.      0.66       1.27       0.965

I did not notice the difference of the barometer on the days of experiment[A].

  [A] Similar experiments in different gases are described at 1507.
  1508.—­Dec. 1838.

1390.  One would have expected only two distances, one for each interval, for which the discharge might happen either at one or the other; and that the least alteration of either would immediately cause one to predominate constantly over the other.  But that under common circumstances is not the case.  With air in the receiver, the variation amounted to 0.2 of an inch nearly on the smaller interval of 0.6, and with muriatic acid gas, the variation was above 0.4 on the smaller interval of 0.9.  Why is it that when a fixed interval (the one in the receiver) will pass a spark that cannot go across 0.6 of air at one time, it will immediately after, and apparently under exactly similar circumstances, not pass a spark that can go across 0.8 of air?

1391.  It is probable that part of this variation will be traced to particles of dust in the air drawn into and about the circuit (1568.).  I believe also that part depends upon a variable charged condition of the surface of the glass vessel a.  That the whole of the effect is not traceable to the influence of circumstances in the vessel a, may be deduced from the fact, that when sparks occur between balls in free air they frequently are not straight, and often pass otherwise than by the shortest distance.  These variations in air itself, and at different parts of the very same balls, show the presence and influence of circumstances which are calculated to produce effects of the kind now under consideration.