Page:Collected Physical Papers.djvu/410

390 of means for the exact measurement of the energy absorbed, and the energy stored in photosynthesis. The difficulties have been obviated by the new methods devised for the purpose. The energy absorbed is found by the Calorimetric method. The accuracy of the calorimetric determination was tested independently by results obtained with the highly sensitive Magnetic Radiometer (p. 360). The energy stored was simultaneously found from the volume of oxygen given out by the plant, the carbohydrate factor of which had been very carefully determined. The photosynthetic efficiency of the leaves of Hydrilla is fairly high, being about 7.4 per cent. (p. 336).

The essential difficulty of the investigation arises from the extraordinary slowness of growth, the average rate of which is about $1⁄100,000$ inch per second, a length which is half that of a single wave of sodium light. Even with the magnifying growth recorders hitherto employed, it takes a very long time to detect and measure its rate. For accurate investigations on the effect of a given agent on growth, it is necessary to keep all other variable conditions, such as light and warmth, strictly constant during the whole period of the experiment. We can keep these conditions absolutely constant for only a few minutes at a time. Experiments which require several hours for their completion are, therefore, subject to serious errors which vitiate the results.

The only satisfactory method is one that reduces the period of the experiment to a few minutes; that, however, necessitates the devising of an apparatus for exceptionally