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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 perfectly uniform, and any slight irregularity gives rise to sudden fluctuations in the magnified record of growth. Another difficulty arises from the radiation of heat-rays from the sides of the thermal chamber. These rays, I shall in a different Paper show, induce a retardation of growth. The. effect of rise of temperature in acceleration of growth is thus antagonised 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.

The contrivance which I employ for ensuring a gradual rise of temperature, consists of a double-walled cylindrical metallic vessel; the plant is placed in the inner chamber, the walls of which are coated with electrically deposited silver and polished afterwards, and at the bottom of which there is a little water. The space between the inner and outer cylinder is filled 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 flow of hot water in the coiled pipe. The rate of flow 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.

Experiment 58.—High magnification records are taken for successive periods of ten seconds, for selected temperatures,