Jury 20, 1899] 
NATURE 
285 
surface of the crater grows dark for an instant, then brightens | 
in spots, and finally becomes bright again all over. 
The vaporous arc itself undergoes fewer modifications ; it 
preserves the ordinary characteristics of the silent arc while 
rotating wheels hold possession of the crater, but, when 
humming begins, a green light is seen to issue from the crater, 
and with hissing this becomes enlarged and intensified, till the 
whole centre of the purple core is occupied by a brilliant 
greenish-blue light, as is indicated in Fig. 5. 
ILENT. 
HISSING. 
Fic 4.—Cuarbons :—Positive, 9 mm. Cored ; Negative, 8 mm. Solid. 
Length of Arc. (a) 5 mm., (4) 8 mm. Current (a) 3°5 amperes, (4) 34 
amperes. 
M. Blondel, whose accuracy of observation and originality 
in experiment have added so much to our knowledge of the 
arc, first mentioned a very curious fact about this vapour in 1893 
(The Electrician, 1893, vol. xxxii. p. 170). He noticed that, 
while the arc was silent, the vapour was quite transparent, so 
that, when viewed at a proper angle, the crater could be seen 
through it perfectly; but that, as soon as hissing began, the 
vapour became so opique as to completely hide the crater. 
SILENT. SILENT. 
(6) 
(@) 
Fic. 6.—Carbons :—Positive, 11 mm. Solid; Negative, 9 mm. Solid. 
POINT OF HISSING. 
In any case, however, M. Blondel’s theory of a change from 
| vapour to solid particles in the arc when hissing begins seems to 
me to be hardly tenable, if only for the following reason, When- 
ever we put solid carbon into the arc, such, for instance, asa very 
thin carbon rod, it glows far more brilliantly than the surrounding 
vapour, and hence increases the luminosity of the arc. If, there- 
HISSING. 
Fic. 5 —Carbons :—Positive, 11 mm. Solid ; Negative, 9 mm. Solid. 
Length of Arc, 1.5 mm. Current, 28°5 amperes. 
| fore, hissing is accompanied by a substitution of solid particles. 
for the vapour of the arc, the luminosity of the arc should 
zucrease With hissing. But, M. Blondel mentions, in the same 
article, that the intrinsic brilliancy of the are d¢mznishes when 
hissiug begins, hence the theory of a disruptive discharge of 
solid particles does not appear to cover the facts. 
ON THE 
HISSING. 
(OQ) 
Length of Arc,2mm. Current, (a) 6 amperes, (4) 12 amperes, 
(c) 20 amperes, (@) 30 amperes. 
M. Blondel believes that, when hissing begins, the gentle 
evaporation of the carbon of the crater, which feeds the column 
of vapour in the silent arc, gives place to a disruptive discharge 
of solid particles torn from the crater. 
I have found the opacity of the hissing arc less complete and 
less invariable than M. Blondel, probably because my conditions 
were different from his. Indeed I have often been able to see 
the crater of a hissing arc through the vapour as clearly as if 
the arc were silent. 
NO. 1551, VOL. 60] 
The shafe of the arc also alters when hissing begins. When 
the arc is silent its shape is rounded, and it has an appear- 
ance of great stability; but as soon as hissing occurs, it seems 
| suddenly to dart out from between the carbons and to become 
flattened out, as if under the influence of a centrifugal force act- 
ing at right angles to the common axis of the two carbons. In 
Fig. 5 and in (d@) Fig. 6, this flattened appearance is well 
marked ; and indeed these figures show that every part of the 
vaporous arc itself isinvolved in this flatteaing—the purple core, 
