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Rh a bell or makes an electro-magnetic writer inscribe successive dots on a revolving drum (fig. 104). The

automatic method eliminates all personal errors of observation; it is so extremely sensitive that it is possible to indirectly measure by its means, a deposit of carbohydrate as minute as a millionth of a gram.

The following example illustrates the practical working of the apparatus. The plant with the apparatus is so placed as to face the northern light; the bell rings each time it has absorbed a certain amount of CO2. If a person now stands obstructing the light, the assimilation is slowed down and the bell now strikes at longer intervals. When strong sunlight is thrown on the plant, the successive strokes on the bell become greatly