O. N. Rood on the Electric Spark. 369 
the “atmosphere” consists simply of heated — we should 
expect merely a general and ill-defined blackening of the sensi- 
tive plate, which would probably be much alike at the two 
electrodes. 
The following experiments will serve to show that in point of 
fact the most remarkable difference does indeed exist between 
the positive and negative figures generated by the movable at- 
mosphere; that these figures are for the most part definite and 
delicate in outline, and filled with details that strongly suggest 
the idea that they are produced either by many discharges of 
varying intensity, or by a continuous discharge subject to con- 
stant and often tolerably regular fluctuations. These results 
serve therefore to a considerable extent to confirm the views set 
forth by Mr. Rijke. 
Mode of Excperimenting.—The apparatus was arranged as be- 
fore, a metallic point or brass ball being used as electrode, and 
€ spark was generated while a current of air traversed the sur- 
face of the sensitive plate. It was found in practice that blow- 
mass of details quite invisible before. — the brass ball is 
positive electrode, figures similar to fig. 14, are pro- 
duced, consisting of a s ia 
Which is generated — >_> 
the electrode, while the at- 
mosphere produces the long Gn od 
tail, wich is often filled * 
with complicated cross 
markings, as it were interwoven. Fig. 15 was produced by 
Temoving the electrode to 15. 
¥s in., and here it can be 
Seen that the movable atmos- 
phere starts from a central 
Star corresponding to the 
inner ring in fig. 4, while the 
external star of fig. 15 also corresponds to that seen in fig. 4 
With a spark ,4, in. long, figures like 16 (only partly = 4, 
rawn,) are produced. Here we have a star corres- 
: setae to that whichis produced by 16. 
_ Mictional electricity, with the luminous 
atmosphere emanating from near its Pe, 
Centre. The same may be said of fig. 
17, obtained with a spark one inch in : : 
tength. Fig. 18 was produced by a point, the spark being 7 inch. 
Ax. Jour. Sct.—Szconp Szrtes, Vou. XXXVIII, No. 114. -, 1864, 
47 
