the air be sufficiently warmed as to become conducting, the body is
rendered more intensely incandescent in a large bulb, obviously
because, under otherwise equal conditions of test, more energy may be
given off from the body when the bulb is large.
At very high degrees of exhaustion, when the matter in the bulb
becomes "radiant," a large bulb has still an advantage, but a
comparatively slight one, over the small bulb.
Finally, at excessively high degrees of exhaustion, which cannot be
reached except by the employment of special means, there seems to be,
beyond a certain and rather small size of vessel, no perceptible
difference in the heating.
These observations were the result of a number of experiments, of
which one, showing the effect of the size of the bulb at a high degree
of exhaustion, may be described and shown here, as it presents a
feature of interest. Three spherical bulbs of 2 inches, 3 inches and 4
inches diameter were taken, and in the centre of each was mounted an
equal length of an ordinary incandescent lamp filament of uniform
thickness. In each bulb the piece of filament was fastened to the
leading-in wire of platinum, contained in a glass stem sealed in the
bulb; care being taken, of course, to make everything as nearly alike
as possible. On each glass stem in the inside of the bulb was slipped
a highly polished tube made of aluminium sheet, which fitted the stem
and was held on it by spring pressure. The function of this aluminium
tube will be explained subsequently. In each bulb an equal length of
filament protruded above the metal tube. It is sufficient to say now
that under these conditions equal lengths of filament of the same
thickness--in other words, bodies of equal bulk--were brought to
incandescence. The three bulbs were sealed to a glass tube, which was
connected to a Sprengel pump. When a high vacuum had been reached, the
glass tube carrying the bulbs was sealed off. A current was then
turned on successively on each bulb, and it was found that the
filaments came to about the same brightness, and, if anything, the
smallest bulb, which was placed midway between the two larger ones,
may have been slightly brighter. This result was expected, for when
either of the bulbs was connected to the coil the luminosity spread
through the other two, hence the three bulbs constituted really one
vessel. When all the three bulbs were connected in multiple arc to the
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