1453.  The running together or coalescence of different lines of discharge (1412.) is very beautifully shown in the brush in air.  This point may present a little difficulty to those who are not accustomed to see in every discharge an equal exertion of power in opposite directions, a positive brush being considered by such (perhaps in consequence of the common phrase direction of a current) as indicating a breaking forth in different directions of the original force, rather than a tendency to convergence and union in one line of passage.  But the ordinary case of the brush may be compared, for its illustration, with that in which, by holding the knuckle opposite to highly excited glass, a discharge occurs, the ramifications of a brush then leading from the glass and converging into a spark on the knuckle.  Though a difficult experiment to make, it is possible to obtain discharge between highly excited shell-lac and the excited glass of a machine:  when the discharge passes, it is, from the nature of the charged bodies, brush at each end and spark in the middle, beautifully illustrating that tendency of discharge to facilitate like action, which I have described in a former page (1418.).

1454.  The brush has specific characters in different gases, indicating a relation to the particles of these bodies even in a stronger degree than the spark (1422. 1423.).  This effect is in strong contrast with the non-variation caused by the use of different substances as conductors from which the brushes are to originate.  Thus, using such bodies as wood, card, charcoal, nitre, citric acid, oxalic acid, oxide of lead, chloride of lead, carbonate of potassa, potassa fusa, strong solution of potash, oil of vitriol, sulphur, sulphuret of antimony, and haematite, no variation in the character of the brushes was obtained, except that (dependent upon their effect as better or worse conductors) of causing discharge with more or less readiness and quickness from the machine[A].

[A] Exception must, of course, be made of those cases where the root of the brush, becoming a spark, causes a little diffusion or even decomposition of the matter there, and so gains more or less of a particular colour at that part.

1455.  The following are a few of the effects I observed in different gasses at the positively charged surfaces, and with atmospheres varying in their pressure.  The general effect of rarefaction was the same for all the gases:  at first, sparks passed; these gradually were converted into brushes, which became larger and more distinct in their ramifications, until, upon further rarefaction, the latter began to collapse and draw in upon each other, till they formed a stream across from conductor to conductor:  then a few lateral streams shot out towards the glass of the vessel from the conductors; these became thick and soft in appearance, and were succeeded by the full constant glow which covered the discharging wire.  The phenomena varied with the size of the vessel (1477.), the degree of rarefaction, and the discharge of electricity from the machine.  When the latter was in successive sparks, they were most beautiful, the effect of a spark from a small machine being equal to, and often surpassing, that produced by the constant discharge of a far more powerful one.