June 24, 1880] 



NATURE 



175 



The authors proceed to describe the appearance of the 

 arc with terminals of various forms at different distances 

 and with various pressures. It wa3 found that the Hght 

 emitted by different parts of the arc was not of the same 

 intensity throughout, and that from the first there was a 

 tendencv to break up into distinct entities, as shown at 

 A, P., c, b, E, F in the diagram, Fig. 4, which only indi- 



cates the central bright portion of the arc, this never 

 quite reached the negative terminal, near which there was 

 always the well-known dark discharge. 



As the pressure was diminished the arc widened out, 

 until at last the entire surface of the negative disk was 

 covered with a luminous halo, and the discharge took up 

 a stratified appearance. 



The appearances presented by the arc in air, hydrogen, 

 and carbonic acid are illustrated by copies of photographs 

 and drawings in mezzotint, as shown in the Plate. 



The arc in air bet-ween two points at various distances 



and pressures with a constant nnmbcr of cells. Temp. 



127 C". The references are to the Plate 



11,000 cells, distance o'54. inch, pressure atmospheric, 

 current o'o::456W; the total resistance of battery and 

 arc was found to be 461,500 ohms, that of the arc, by sub- 

 stituting wire resistance, 27,550; whence the potential 

 between the terminals was 657 cells. The appearance of 

 the arc is shown in the plate. Fig. 20 ; it exhibits clearly the 

 tendency to break up into luminous entities ; the photo- 

 graph of which this is a copy is nearly full size, and was 

 obtained in twenty seconds. All the other copies of photo- 

 graphs are on a reduced scale. As the batteries were 

 undergoing the annual overhauling, the number of cells, 

 some being re.iioved from time , to time, was somewhat 

 less in the following experiments, namely, 10,940. 



Fig. I, from a photograph obtained in 10 seconds. — 

 Pressure atmospheric 748'6 mm., 985,000 M, distance 

 o'58 inch, current not observed, no depression of the 

 mercury in the gauge was noticed ; indeed, it will be seen 

 that at the higher pressures the depression is generally 

 less than at the lower, up to a certain point. 



Fig. 2, from a photograph in 15 seconds. — Distance 

 o'5S X 2 = ri6 inch, pressure 294^9 mm., 388,026 M, 

 current o'o28Si W, depression 16 mm., total pressure 

 294'9 -+- 16 = 3 io'9 ; ratio of increased to normal pressure 

 as i'o54 to I. It will be observed that the central spindle 

 has become bifurcated about midway. 



Fig. 3, from a photograph in 15 seconds. — Distance 

 X 3 = i'74 inch, pressure I9r3 mm., 251,711 M, current 

 O'04o6o W, depression 17 mm., total pressure 2oS'3mm. ; 

 ratio of increased pressure roSg. The bifurcation is 

 apparent in this also. 



Fig. 4, in which the central spindle is broken up into 

 several luminosities. — Distance X 4 = 2'32 inches, pres- 

 sure i42'6 mm., 187,631 M, current 0*04474 W, depres- 

 sion 19 mm., total pressure i6i'6 mm. ; ratio of increased 

 pressure ri33. 



Fig. 5, from a photograph in 15 seconds ; in this the 

 central spindle is split up into bright entities connected 

 by less bright portions. — Distance X 5 = 2"9 inches, 

 pressure ii2'6mm. 148,157 M) current o'o345 9 W, depres- 

 sion 19 mm., total pressure 131 '6; ratio of increased 

 pressure ri69. 



Fig. 6, from a photograph in 1 5 seconds. The luminous 

 entities still seen, but are less marked. — Distance X 6 = 

 3*48 inches, pressure 99^4 mm., 1 30,789 M, current o'0307i 

 W, depression 21 mm., total pressure I20'4 mm. ; ratio of 

 increased pressure r2ii. 



Fig. 7, from a photograph in 15 seconds. — Distance 

 X 7 = 4"o6 inches, pressure 859 mm., 1 13,026 M. current 

 o'o3259 W, depression 22 mm., total pressure lo/'gmm. ; 

 ratio of increased pressure x'256. The central spindle is 

 divided into two luminosities, with a tendency to form 

 a third near the negative. 



Fig. 8, from a photograph in 15 seconds. — Distance 

 X 8 = 4-64 inches, pressure 71-6 mm., 94,210 M, current 

 o'02693 W, depression 22 mm., total pressure 93'6 mm. ; 

 ratio of increased pressure i'307. The central spindle 

 nearly of the same character as Fig 7. 



Fig. 9, from a photograph in 15 seconds — Distance 

 X 9 = 5-22 inches, pressure 65^5 mm., 86, 184 M, current 

 o'02693 W, depression 22 mm., total pressure 87-5 mm. ; 

 ratio of increased pressure i'336. The bright entities 

 show a tendency to break up into less bright portions. 



Fig. 10, from a photograph in 15 seconds. — Distance 

 X 10 = 5'8 inches, pressure 64-4 mm., 84,737 M) current 

 0-0307 1 W, depression 20 mm., total pressure 84 '4 mm.; 

 ratio of increased pressure r3io. The arc resembles that 

 seen in Fig. 9. 



The appearance of the arc between disks in hydrogen 

 at the various pressures used in determining the potential 

 necessary to produce a discharge, is represented in the 

 plate. Figs. 11-19. 



M Cells Seconds 



Fig. II at pressure of 1S.6S4. with 633, from a photograph taken in so 



„ 12 ,, 5S.634 ,, 1200 „ ,, 50 



,, 13 ,, 141,974 ,. 2400 ,, ,, 5*3 



,, 14 ,, 252.368 ,, 3600 ,, ,, 50 



,, 15 ,. 386,316 ,, 4800 „ ,, 20 



., i5 „ 558.816 „ 6300 „ „ 4 



,, 17 „ 55S.816 „ 6300 „ .. I 



,, iS ,, 651,316 „ 77«o .. •■ 5 



The arc in hydrogen between two points. Temp. i6°*2 C. 

 10,940 cells > 



Distance 075 inch, pressure 745 mm., 980,263 M, current 

 001575 W, the appearance is represented in the plate. 

 Figs. 21 and 22, the first copied from a photograph 

 obtained in 5 seconds, the second in 15 seconds. The 

 central spindle breaks into a brush-like form towards the 

 negative, there is then a dark interval between it and the 

 glow on the negative. 



Fig. 26, from a drawing. — Distance 09 inch, pressure 

 745 mm., 980,263 M, the discharge passed intermittently, 

 so that the current could not be read off on the galvano- 

 meter. The appearance is represented full size. 



Figs. 23, 27, 28.— Distance 09 X 2, i -8 inch, pressure 

 385-6 mm., 507,368 M, current 0-01575 W, depression) 14 

 mm., total pressure 399-6 mm., ratio of increased pressure 

 1-04. Fig. 23 is fro.n a photograph in 13 seconds. The 

 distance between the brush-like termination of the central 

 spindle and the glow on the negative has relatively 

 increased. Figs. 27 and 28 are other representations 

 copied from drawings. 



Fig. 24, from a photograph in 15 seconds. — Distance 

 X 3, 2-7 inches, pressure 317-8 mm ,418,158 M, current 

 0-00580 W, depression 13 mm., total pressure 3308 mm., 



