On the Luminous Discharge of Voltaic Batteries. 547 



No. 187 was then replaced in the circuit, and the same phenomena 

 as already described were obtained. 



I now again avail myself of Dr. Robinson's notes of the experi- 

 ments made on the 5th of August. Tube 190 is of the same dimen- 

 sions as 187 and 196; but instead of the coke balls, it has brass 

 wires attached to the platinum. In this tube (190) the luminous 

 discharge did not appear until the caustic potassa was heated, when 

 most dazzling strata were observed. Dr. Robinson says, — " I had 

 to use a dark-green glass to examine the strata ; as I was observing, 

 the last stratum rolled leisurely away, like a globe of light, from the 

 others, to the negative glow, in which it appeared to dissolve. As 

 the potassa cooled, the strata shrunk up and dissolved at the positive 

 wire, as did the glow ; and when the dark negative reached the 

 point, all luminosity ceased." 



On a subsequent occasion the first discharge in the tube (190) 

 exhibited a fine purple negative glow, with two tawny cloud-like stra- 

 tifications at the positive. This discharge exhibited the fluorescence 

 of the glass tube in a remarkable manner. The circuit was then 

 broken; on again completing it, I was somewhat surprised at 

 finding the luminous discharge was no longer perceptible. I then 

 slightly heated the tube with a spirit-lamp ; the stratified discharge 

 reappeared ; the needle of the galvanometer deflected about 65°, 

 but the leaves of the electroscopes were very slightly affected. 

 On separating the plates of the micrometer-electrometer, sparks 

 passed ; the stratifications in 1 90 became confused, intermingling with 

 each other, and no longer presenting that clearness of definition 

 which I have described. 



With tube 202, which is of the same dimensions, with silver in 

 lieu of brass terminal wires, there was not any discharge from the 

 battery until the potash was heated. At first it presented a fine 

 white glow on the negative wire, then one of a tawny red colour on 

 the positive; and on heat being applied, stratifications, as in 190, 

 were observed. 



The battery was then connected to the wires of tube 219, the same 

 with which the experiments with Daniell's battery were made. This 

 showed the luminous negative as in fig. 3 A, but more brilliant than 

 with the constant battery — producing bright scintillations at the 

 positive, as if particles of the carbon were fused and thrown off. 



By the preceding experiments, I ascertained that a disruptive or 

 spark-discharge could be obtained in air from the nitric-acid, as well 

 as from the water-battery; and that when these discharges were 

 passed through the highly attenuated matter contained in carbonic 

 acid vacua, the same luminous and stratified appearance was pro- 

 duced as by an inductive coil, a proof that whatever may be the 

 cause of the phenomena, it could not arise from any peculiar action 

 of that apparatus. 



With the ordinary arrangement of the voltaic battery, in which 

 the insulation of the cells is disregarded, the luminous discharge is 

 usually obtained by completing the circuit, and then separating the 



