Page:Collected Physical Papers.djvu/408

388 elsewhere involves a considerable expenditure of energy. The fundamental importance of photosynthesis is, that it is the process by which the plant absorbs the energy it requires, the radiant energy of sunlight, and stores it in the form of latent or potential energy in the process. The energy so stored can readily be set free again and become kinetic by the chemical decomposition of the organic substances, manifesting itself in heat, electric current or movement.

All these changes are effected by the living protoplasm and are the expressions of its physico-chemical reactions. This is made clear by the observation that all the various manifestations of them that have been made accessible to investigation are affected in a similar manner by a given stimulus or change in internal or external conditions.

Automatic Recorder for Photosynthesis.—The estimation of the activity of photosynthesis with water plants from the rate of evolution of oxygen is direct and requires no prolonged chemical analysis. The automatic record by the Electro-magnetic Writer of successive bubbles representing equal volumes of pure oxygen eliminates all personal error of observation. The methbd is so sensitive that records may be obtained from which it is possible to estimate the formation of quantities of carbohydrate as small as the millionth of a gramme (p. 333).

The automatic method of record that has been described, can also be utilised in physico-chemical investigations, such as the determination of the rate of evolution of a gas under controlled conditions of temperature, of concentration, of intensity of light, of catalytic agents and others, either separately or in combination.