DISRUPTIVE DISCHARGE — NATUHE OF THE BRUSH. 115 



city, the parts which are luminous being so only because much electricity is passing 

 by them to other parts (1437.) ; just as in a spark discharge the light is greater 

 as more electricity passes, though it has no necessary relation to the quantity re- 

 quired to commence discharge (13/0. 1420.). Hence the form we see in a brush 

 may by no means represent the whole quantity of air electrified ; for an invisible 

 portion, clothing the visible form to a certain depth, may, at the same time, receive 

 its charge. 



1445. Several effects which I have met with in muriatic acid gas tend to make me 

 believe, that that gaseous body allows of a dark discharge. At the same time, it is 

 quite clear from theory, that in some gases, the reverse of this may occur, i. e. that 

 the charging of the air may not extend even so far as the light. We do not know as 

 yet enough of the electric light to be able to state on what it depends, and it is very 

 possible that, when electricity bursts forth into air, all the particles of which are in a 

 state of tension, light may be evolved by such as, being very near to, are not of, those 

 which actually receive a charge at the time. 



1446. The further a brush extends in a gas, the further no doubt is the charge or 

 discharge carried forward ; but this may vary between different gases, and yet the 

 intensity required for the first moment of discharge not vary in the same, but in some 

 other proportion. Thus with respect to nitrogen and muriatic acid gases, the former, 

 as far as my experiments have proceeded, produces far finer and larger brushes than 

 the latter (1458. 1462.), but the intensity required to commence discharge is much 

 higher for the latter than the former (1395.). Here again, therefore, as in many 

 other qualities, specific differences are presented by different gaseous dielectrics, and 

 so prove the special relation of the latter to the act and the phenomena of induction. 



1447. To sum up these considerations respecting the character and condition of 

 the brush, I may state that it is a spark to air; a diffusion of electric force to matter, 

 not by conduction, but disruptive discharge ; a dilute spark which, passing to very 

 badly conducting matter, frequently discharges but a small portion of the power 

 stored up in the conductor ; for as the air charged reacts on the conductor, whilst 

 the conductor, by loss of electricity, sinks in its force, the discharge quickly ceases, 

 until by the dispersion of the charged air and the renewal of the excited conditions of 

 the conductor, circumstances have risen up to their first effective condition, again to 

 cause discharge, and again to fall and rise. 



1448. The brush and spark gradually pass into one another. Making a small ball 

 positive by a good electrical machine with a large prime conductor, and approaching 

 a large uninsulated discharging ball towards it, very beautiful variations from the 

 spark to the brush may be obtained. The drawings of long and powerful sparks, 

 given by Van Marum*, Harris-|-, and others, also indicate the same phenomena. As 

 far as I have observed, whenever the spark has been brushy in air of common press- 



* Description of the Tkylerian machine, vol. i. pp. 28. 32. ; vol. ii. p. 226, &c. 

 t Philosophical Transactions, 1834, p. 243. 



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