DISRUPTIVE DISCHARGE — GLOW—PRODUCED AND FAVOURED. 135 



of an inch in diameter, with a rounded termination, was rendered positive in free air, 

 it gave fine brushes from the extremity, but occasionally these disappeared, and a 

 quiet phosphorescent continuous glow took their place, covering the whole of the end 

 of the wire, and extending a very small distance from the metal into the air. With 

 a rod 0*2 of an inch in diameter the glow was more readily produced. With still 

 smaller rods, and also with blunt conical points, it occurred still more readily; and 

 with a fine point I could not obtain the brush in free air, but only this glow. The 

 positive glow and the positive star are, in fact, the same. 



1528. Increase of power in the machine tends to produce the glow; for rounded 

 terminations which will give only brushes where the machine is in weak action, will 

 readily give the glow when it is in good order. 



1529. Rarefaction of the air wonderfully favours the glow phenomena. A brass 

 ball, two and a half inches in diameter, being made positively inductric in an air- 

 pump receiver, became covered with glow over an area of two inches in diameter, 

 when the pressure was reduced to 4-4 inches of mercury. By a little adjustment the 

 ball could be covered all over with this light. Using a brass ball 1*25 inches in 

 diameter, and making it inducteously positive by an inductric negative point, the 

 phenomena, at high degrees of rarefaction, were exceedingly beautiful. The glow 

 came over the positive ball, and gradually increased in brightness, until it was at last 

 very luminous ; and it also stood up like a low flame, half an inch or more in height. 

 On touching the sides of the glass jar this lambent flame was affected, assumed a 

 ring form, like a crown on the top of the ball, appeared flexible, and revolved with a 

 comparatively slow motion, i. e. about four or five times in a second. This ring- 

 shape and revolution are beautifully connected with the mechanical currents (1576.) 

 taking place within the receiver. These glows in rarefied air are often highly exalted 

 in beauty by a spark discharge at the conductor (1551. Note). 



1530. To obtain a negative glow in air at common pressures is difficult. I did not 

 procure it on the rod 0*3 of an inch in diameter by my machine, nor on much smaller 

 rods ; and it is questionable as yet, whether, even on fine points, what is called the 

 negative star is a very reduced and minute, but still intermitting brush, or a glow 

 similar to that obtained on a positive point. 



1531. In rarefied air the negative glow can easily be obtained. If the rounded 

 ends of two metal rods, about 0*2 of an inch in diameter, are introduced into a globe 

 or jar (the air within being rarefied), and being opposite to each other, are about four 

 inches apart, the glow can be obtained on both rods, covering not only the ends, but 

 an inch or two of the part behind. On using halls in the air-pump jar, and adjusting 

 the distance and exhaustion, the negative ball could be covered with glow, whether 

 it were the inductric or the inducteous surface. 



1532. When rods are used it is necessary to be aware that, if placed concentrically 

 in the jar or globe, the light on one rod is often reflected by the sides of the vessel on 

 to the other rod, and makes it apparently luminous, when really it is not so. This 



