84 EXTREME HEAT AND LIGHT PAET i. 



broken gradually, and spark discharges were obtained 

 between them. The vacuum tubes were placed between 

 one of these plates and a pole of the battery ; one of 

 these tubes was 24 inches long, 18 in circumference, 

 and had a circular copper disc 4 inches in diameter on 

 its negative terminal. On completing the circuit, the 

 discharge of the battery passed with a display of mag- 

 nificent strata of dazzling brightness; on separating 

 the plates by the screw, the luminous discharges pre- 

 sented the same appearance as when taken from an 

 induction coil, but brighter. On the copper disc within 

 the vacuum tube, there was a white layer, then a dark 

 space about an inch broad, and then a bluish atmo- 

 sphere curved like the disc, evidently three negative 

 envelopes on a great scale. When the disc was made 

 the positive pole, the effect was feeble. 



In vacuum tubes 6 inches long and 1 inch diameter, 

 with carbon balls on the terminals, the discharge of the 

 nitric acid battery elicits extreme heat. In one of these 

 the discharge presented a stream of light of intolerable 

 brightness, but when viewed through a plate of green 

 glass strata could be seen. This soon changed to a 

 sphere of light on the positive ball, which became red 

 hot, the negative being surrounded by magnificent en- 

 velopes ; with a horse-shoe magnet the positive light was 

 drawn out into strata. The needle of a galvanometer 

 in circuit was violently deflected and the polarity re- 

 versed. When the caustic potash was heated, the dis- 

 charge burst into a sunlike flame, subsequently subsiding 

 into three or four large strata of a cloud-like shape, but 

 intensely bright. This is called the arc discharge : it 

 occurs in vacuum tubes with charcoal balls ; when the 

 potash is heated intensely, dazzling stratifications sud- 

 denly emanate from the positive ball, and powerful 

 chemical action takes place in the battery, after which 

 the discharge ceases. 



