386 Prof. Dove on the Electrical Light. 
nosity of a uranium glass occurs with equal vividness with the 
brushes and sparks. I have found no essential difference between 
the luminosity of a Ruhmkorff’s apparatus and that of an elec- 
trical machine, both as regards the sparks and brush in the air, 
and the luminosity in the electrical egg. 
The spark of an electrical machine often appears interrupted 
at one spot by a weaker violet or reddish light. This interrupted 
spot generally lies nearest to the negative end; and by removing 
to an appropriate distance a non-insulated conductor placed near 
the principal conductor, a stream of sparks may easily be obtained 
which appears white at the primary conductor, and coloured at 
the conductor standing near it. This less luminous part is, 
however, very distinctly visible through a red glass, so that it is 
distinct from the light of the brush. 
The preceding experiments, in connexion with the results of 
the prismatic investigation of the spark, appear to me to lead to 
the following conclusion. : 
A wire becoming red-hot by heat is first red, then orange, and 
lastly white, so that it behaves like the combination of light 
which is obtained when a screen is drawn away from the spec- 
trum concealed by it in such a way that the red end first becomes 
visible, and to this the violet is finally added. The increase of 
brilliancy from the slightly luminous brush to the bright spark 
behaves quite otherwise. In this case it is as if the screen 
removed first set free the violet end, and then the other colours. 
This distinction of itself renders it improbable that the phzeno- 
mena of electrical light in the state of less brilliancy can be 
ascribed to a gradually increasing ignition of solid particles. 
They rather resemble the weakly luminous flame of hydrogen, 
which becomes white by solid ignited carbon in the so-called 
gas-flames, or by other solid matters, as in the Drummond hght. 
The true electrical light is produced at great distances in the 
surrounding, isolating, aériform medium, when the latter is 
attenuated. With this coloured light belonging to the strongly 
refrangible part of the spectrum, phenomena of ignition may be 
combined, by particles torn away from the positive and negative 
bodies. If these particles be only at a red heat, the impression 
of a violet light is produced by their mixture with the electric 
light. To this class belong the column of light in the electrical 
egg, and the basal point of the brush, and lastly, the indented 
reddish sparks of an electrical machine, at distances to which a 
white spark does not pass. If particles at a white heat come 
together, the whole is white, as in the sparks of Leyden jars; in 
opposition to the bright light of incandescence, the less strongly 
luminous electric light disappears in the same way as the weak 
bluish lower part in a gas-flame appears black in opposition to 
