26 CHAP. IV. AUTOMATIC RECORD OF EVOLUTION OF OXYGEN 



A full description has now been given of the method 

 which enables us to obtain automatic record of the rate 

 of evolution of equal volumes of oxygen. This rate affords 

 a direct measure of the photosynthetic activity. In the 

 study of induced variation of photosynthesis in a particu- 

 lar plant (the physiological condition of which is constant) 

 by change of one factor at a time, it is not necessary, as 

 already stated, to determine the absolute but the relative 

 variation. The experiments may be completed in the 

 course of a few minutes, during which all the factors but 

 one can be kept constant. The intensity of light can be 

 maintained uniform by devices which will be presently 

 described ; maintenance of constant temperature offers no 

 special difficulty ; the C0 2 -content of the water is also 

 constant at the concentration of about 8 mg. per ioo c.c. 



Among these factors, the securing of a constant in- 

 tensity of light is a problem of considerable difficulty. The 

 ideal requirements are : (i) that the radiant should be a 

 point ; and (2) that the light should be cold, not containing 

 a large proportion of heat-rays. 



When the luminous source is a point, it is easy, by 

 means of a lens, to obtain either a parallel or a divergent 

 beam of light. This is impossible when the source of light 

 is a luminous surface, such as a gas-mantle or the large 

 spiral of an incandescent electric bulb. The condition of 

 the radiant being a point is specially necessary in investi- 

 gations on the effects of different intensities of light ; for 

 the varying intensities can in this case be accurately de- 

 termined by measuring the distance from the radiant, 

 the intensity varying inversely as the square of the distance 

 from the source of light. The arc-lamp, though it gives 

 a strong light from a small source, has the drawback that 

 the intensity cannot be maintained absolutely constant ; 

 moreover it gives out a large quantity of heat. The pro- 

 duction of heat either by an incandescent mantle or by 

 an arc-lamp introduces various complications ; for the 

 temperature of the experimental room itself is continuously 



