282 
The President read a paper on the luminous phenomena 
produced by the discharge of Ruhmkorff’s induction apparatus 
in vacuo. 
«¢ Although this beautiful experiment has been carefully 
examined by several distinguished philosophers, whose results 
may be found in De Moncel’s pamphlet, I hope I may be ex- 
cused for calling attention to it; as in repeating it I have 
observed some facts which seem to require further elucidation, 
and which I wish to point out as deserving of notice to those 
who possess more perfect apparatus than I command. 
«¢ The Ruhmkorff which I use is of the smaller size, con- 
taining about 6700 feet of fine wire, and excited by one or two 
Grove's cells; its compensator is of thin sheet gutta percha, 
each of whose coated surfaces is 8 square feet ; and the termi- 
nals of the secondary helix are occasionally connected with a 
Leyden jar of 1-25 feet internal coating. The air-pump is a 
make-shift, altered from one of the commonest kind ; but on a 
principle which, with good workmanship, would act well. 
«© 1, When the discharge takes place in the vacuum formed 
by exhausting common air from the receiver, or (as Grove 
calls it) the air vacuum, the appearances are well known. AsI 
observed them, if the terminals are platinum points, 4 inches 
apart, and the gauge 010, a star is formed at the positive one, 
pink, tinged with orange. From this darts a rich stream of 
violet light, crossed with dark bands, which are most distinct 
at its lower extremity ; it seems to revolve on its axis, and is 
enveloped in an elliptic mass of faint yellowish light. It ter- 
minates at 0*7 from the negative point, and about 0°5 is com- 
pletely dark; the negative terminal is wrapt in a coat of bright 
blue light, through which, by an optical delusion, the platmum 
seems red hot: round this is a dark space, then a yellowish enve- 
lope, and round this a purple haze. ‘The appearances are nearly 
the same when the terminals are brass balls ; with the addition 
that the negative ball is covered with green flames, owing to the 
combustion of the brass. Each of these consists of a speck of 
