1473.  The character of the negative brush is not affected by the chemical nature of the substances of the conductors (1454.), but only by their possession of the conducting power in a greater or smaller degree.

1474.  Rarefaction of common air about a negative ball or blunt point facilitated the development of the negative brush, the effect being, I think, greater than on a positive brush, though great on both.  Extensive ramifications could be obtained from a ball or end electrified negatively to the plate of the air-pump on which the jar containing it stood.

1475.  A very important variation of the relative forms and conditions of the positive and negative brush takes place on varying the dielectric in which they are produced.  The difference is so very great that it points to a specific relation of this form of discharge to the particular gas in which it takes place, and opposes the idea that gases are but obstructions to the discharge, acting one like another and merely in proportion to their pressure (1377.).

1476.  In air, the superiority of the positive brush is well known (1467. 1472.).  In nitrogen, it is as great or even greater than in air (1458.).  In hydrogen, the positive brush loses a part of its superiority, not being so good as in nitrogen or air; whilst the negative brush does not seem injured (1459.).  In oxygen, the positive brush is compressed and poor (1457); whilst the negative did not become less:  the two were so alike that the eye frequently could not tell one from the other, and this similarity continued when the oxygen was gradually rarefied.  In coal gas, the brushes are difficult of production as compared to nitrogen (1460.), and the positive not much superior to the negative in its character, either at common or low pressures.  In carbonic acid gas, this approximation of character also occurred.  In muriatic acid gas, the positive brush was very little better than the negative, and both difficult to produce (1462.) as compared with the facility in nitrogen or air.

1477.  These experiments were made with rods of brass about a quarter of an inch thick having rounded ends, these being opposed in a glass globe 7 inches in diameter, containing the gas to be experimented with.  The electric machine was used to communicate directly, sometimes the positive, and sometimes the negative state, to the rod in connection with it.

1478.  Thus we see that, notwithstanding there is a general difference in favour of the superiority of the positive brush over the negative, that difference is at its maximum in nitrogen and air; whilst in carbonic acid, muriatic acid, coal gas, and oxygen, it diminishes, and at last almost disappears.  So that in this particular effect, as in all others yet examined, the evidence is in favour of that view which refers the results to a direct relation of the electric forces with the molecules of the matter concerned in the action (1421. 1423. 1463.).  Even when special phenomena arise under the operation of the general law, the theory adopted seems fully competent to meet the case.