to some extent be obviated by ensuring a gradual change
of temperature. This is by no means an easy process, for '
even with care the rise of temperature of the air cannot
be made p-rfeetly uniform, and any slight irregularity gives
rise to sudden fluctuations in the magnified record of growth.
Another ditliculty arises from the radiation of heat-rays
from the sides of the thermal chamber. These rays, I shall
in a dill'erent Paper show, induce a retardation of growth.
The. ell‘ect of rise of temperature in acceleration of growth
is thus antagonism] by the action of thermal radiation.
This trouble may be minimised by having the inner surface
of the thermal chamber of bright polished metal, since the
radiating power of a polished surface is relatively feeble.
'l‘he contrivanee which I employ for ensuring a gradual rise of temperature, consists of a double-walled cylindrical metallic veswl; the plant is placed in the inner chamber, the walls of which are coated with electrically deposited silver and polishel afterwards, and at the bottom of which there. is a little' water. The space between the inner and outer cylinder is tilled with water, in which is immersed a coiled copper pipe. Hot water from a small boiler enters the inlet of the coiled pipe and passes through the outlet at the lower end. The water in the outer cylinder is thus gradually raised by tlow of hot water in the coiled pipe. The rate of llow of hot water, on which the rate of rise of temperature depends, is regulated by a stop-cock. The air of the inner chamber in which the plant is placed, may thus be adjusted for a definite temperature. The small quantity of water in the inner chamber keeps its air in a humid condition, since dry hot air by causing dessication interferes with normal growth.
METHOD OF DISCONTINUOUS OBSERVATION.
Experinwnt 58,—High magnification records are taken for successive periods of ten seconds, for selected temperatures,
