55 



ball is rendered positively inductric; and that a similar difference;, 

 though to a less extent, is observable, when the smaller ball is ren- 

 dered negative. The smaller ball, rendered positive, gives also a 

 much longer spark than when it is rendered negative ; in which 

 latter case, however, it affords, at equal distances, a luminous brush 

 of greater size, and gives it much more readily than when positive. 

 In order to ascertain the relative degrees of charge which the balls 

 acquire before the occurrence of the discharge, the author employed 

 an apparatus attached to the insulated conductor of the electrical 

 machine, and also to the conductor connected with the discharging 

 train, and consequently uninsulated, consisting, on each side, of a 

 rod branching out in the form of a fork, and terminating, at one of 

 its extremities in a large ball, and at the other in a small one ; the 

 position of the forks being capable of adjustment, so that the large 

 iball of each rod might be brought exactly opposite to the small one 

 of the other : and the distances between each pair admitted of being 

 regulated at pleasure, until the discharges through each interval 

 were rendered apparently equal to one another. From numerous 

 experiments made with this instrument, the author concludes that 

 when two conducting surfaces of small but equal size, are placed in 

 air, and electrified, the one positively and the other negatively, a 

 discharge takes place at a lower tension from the latter than from 

 the former ; but that, when a discharge does occur, a greater quan- 

 tity of electricity passes at each discharge from the positive, than 

 from the negative surface. Experiments of a similar nature were 

 made in gases of different kinds, by enclosing them in an apparatus 

 constructed on the same plan as the former one, but capable of act- 

 ing in a receiver, from which the air could be exhausted, and the 

 particular gas, whose powers in modifying the electric discharges 

 were to be ascertained, could be introduced in its place. The 

 results of various trials are given in a table, from which it appears 

 that different gases restrain the discharge in very different degrees. 

 The discharge from the small ball, through nitrogen and hydrogen 

 gases, most readily takes place when the charge is positive ; and 

 through oxygen, carbonic acid, and coal gas, when it is negative. 



The author next directs his attention to the peculiar luminous 

 phenomena attending the disruptive electrical discharge, which he 

 terms a glow, and which appears to depend on a quick, and almost 

 instantaneous charge given to the air in the immediate vicinity, and 

 in contact with the charged conductor ; and he enters into a de- 

 tailed account of the circumstances by which it is influenced, and 

 its production favoured ; such as diminution of the charging sur- 

 face, increase in the power of the machine, rarefaction of the sur- 

 rounding air, and the particular species of electricity concerned. 

 The relations which the glow, the brush, and the spark bear to one 

 another, as well as the steps of transition between each are minutely 

 investigated; and the conclusion is deduced that the glow is in its 

 nature exactly the same as the luminous part of a brush or ramifi- 

 cation, namely, a charge of air ; the only difference being that the 

 glow has a continuous appearance from the constant renewal of the 

 same action in the same place, whereas the ramification is occa- 



