GALVANISM. 



11 



at the ends of the two wires in the inter- 

 rupted circuit, and bringing their points 

 into contact. When the experiment 

 was tried with the powerful battery of 

 the Royal Institution, already noticed 

 ($ 16.), a bright spark passed between 

 the two points of charcoal, when they 

 came within the distance of the thirtieth 

 or fortieth of an inch ; and immediately 

 afterwards more than halfcof each pen- 

 cil of charcoal, the length of which was 

 one inch, and the diameter one- sixth of 

 an inch, became ignited to whiteness. 

 By withdrawing the points from each 

 other, a constant discharge took place 

 through the heated air, in a space at 

 least equal to four inches, forming an 

 arch of light in the form of a double 

 cone, of considerable breadth, and of 

 the most dazzling brilliancy. This phe- 

 nomenon is represented in fg. 16 ; in 



Fig. 16. 



which W, X, are the conducting wires 

 communicating with the ends of the 

 battery ; C, C, the pieces of charcoal, 

 and A the luminous arch of electrical 

 light, making the passage of electricity 

 through the air. When any substance 

 was introduced into this arch, it in- 

 stantly became ignited : platina melted 

 in it, as wax in the flame of a candle : 

 some of the more refractory substances, 

 as quartz, the sapphire, magnesia, and 

 lime, all entered into fusion : fragments 

 of diamond, and points of charcoal and 

 of plumbago quickly disappeared, and 

 seemed converted into vapour, even 

 when the connection was made in highly 

 rarefied air, and apparentlywithout hav- 

 ing undergone previous fusion. When 

 the pieces of charcoal were placed in 

 the receiver of an air-pump, in propor- 

 tion as the air was abstracted, the dis- 

 tance at which the discharge took place 

 increased : and when the Height of the 

 mercury in the barometrical gage was 

 only one quarter of an inch, the sparks 

 were nearly half an inch in length ; and 

 by then withdrawing the points from 

 each other, the discharge passed through 

 a space of six or seven inches, pro- 

 ducing a most brilliant coruscation of 

 purple light. The whole of the charcoal 



became intensely ignited, and some pla- 

 tina wire attached to it melted with 

 bright scintillations, and fell down in 

 large globules.* A battery of a hundred 

 pair of plates of six inches square will 

 suffice to exhibit these phenomena on a 

 smaller scale. Charcoal, carefully pre- 

 pared from some of the harder woods, 

 such as beech, lignum vitae, or box 

 wood, answers best for these experi- 

 ments. The arched form of the stream 

 of light passing between the two char- 

 coal points is perceptible even when the 

 points are within half an inch of each 

 other. 



The light obtained by voltaic elec- 

 tricity in the manner now described 

 exceeds in intensity any other that art 

 can produce. It often exhibits in suc- 

 cession a variety of the 'prismatic co- 

 lours ; and supplies some of the rays 

 which are deficient in the solar beams. 

 It is so dazzling as to fatigue the eye 

 even by a momentary impression ; and 

 it effaces, by its superior lustre, the 

 light of lamps in an apartment other- 

 wise brilliantly illuminated, and which, 

 on the sudden cessation of the galvanic 

 light, appears for a short time as if left 

 in darkness. It is a light which so 

 nearly emulates the brightness of the 

 sun's rays, as to be applicable for the 

 purpose of illuminating objects in a 

 solar microscope; and even with the 

 magic lantern it has been found capable 

 of exhibiting on a large scale, as was 

 done by Mr. W. Allen in his lectures, 

 all the pleasing and endless variations 

 of the kaleidoscope. 



(28.) The employment of charcoal in 

 these experiments might lead to a sus- 

 picion that the light mighf, in part at 

 least, arise from combustion ; but many 

 circumstances concur to prove that it is 

 quite independent of this cause. During 

 the continuance of the light, although 

 the charcoal be in a state of ignition, yet 

 it suffers but little loss of weight. The 

 light is evolved with equal splendour 

 when the experiment is made in gases 

 that contain no oxygen, such as azote 

 or chlorine, and in which therefore com- 

 bustion could not be maintained : and it 

 is moreover found that during the igni- 

 tion, neither the gas nor the charcoal 

 has undergone any chemical change.f 

 Light from voltaic electricity may also 

 be obtained, though with diminished in- 



_* Davy's Elements of Chemical' Philosophy, p. 



{'Children, Philosophical Transactions for 1815, 

 p: 369, j 



