426 PHOTOGRAPHS OF ELECTRIC SPARKS IN HOT AND COLD ATR. 
certain of this, however, it would be necessary to be sure of the position of the 
osculating plane at some one of these points.. The simplest mode of doing this 
would have been to obtain simultaneous photographs of the spark from different 
points of view. 
Another thing to be remarked is, that the zig-zag is sometimes directed 
nearly towards the observer, thus giving the appearance of a brighter spot at one 
point of the spark. Specially curious instances of this are to be seen in 2, 3, 6, 
11, and 12. Those in 2 and 12, at least, distinctly indicate this particular origin. 
Some of the others may possibly have a different cause. 
Bifurcation is very common. ‘Thus, in a series of seventeen successive 
sparks (fig. 1)—the camera having been slowly (and, as is clear from the result, 
rather irregularly) moved (by hand) during their discharge—the first (on the 
left), the twelfth, and thirteenth are obviously each in part double. The lower 
spark in 5 presents a magnificent bifurcation. Two very fine instances (in 
successive sparks) are seen in 13. 6 shows a bifurcation, of which one branch 
is very feeble compared with the other. Other good examples occur in the 
upper spark in 7, and the middle one in 8, 
The general result of an examination of these photographs is, that the 
zig-zag appearance depends upon something which heat is capable of removing 
from the air. This is therefore not aqueous vapour,—nor is it very minute 
drops of water, for even falling drops of water were found to produce no 
effect, beyond a mere interruption in the photographed spark (see fig. 12),— 
but is probably organic matter, which, as SCHROEDER and PASTEuR have shown, 
would be as effectually kept out of our apparatus (if once it were got rid of) 
by a plug of cotton wool as by actual combustion. 


