206 CHAP. XXIII. PHOTOSYNTIIETIC INCREASE OF WEIGHT 



index, which moves along a scale engraved on a mirror. 

 The left arm of the balance also carries a small pan, to which 

 fragments of aluminium are added till the pointer is adjusted 

 at the zero of the scale. There is a screw-arrangement with 

 two jaws by which the arm of the balance may be clamped 

 (fig. 52). The successive readings of the index are taken 

 as follows. The plant is subjected to a short preliminary 

 exposure to light, after which light is cut off by two opaque 



Fig. 52. The Torsion Balance for Measurement of Increase 

 of Weight of the Living Plant during Photosynthesis 



The rider and the two opaque semi-cylinders are not shown. 



semi-cylinders. The index is now adjusted exactly at 

 zero, and then the plant is exposed to light for about an 

 hour. On account of the increase of weight due to photo- 

 synthesis the index moves upwards, and the exact amount 

 of the increase is found from the previous determination of 

 the value of each division of the scale. Certain precautions 

 have, however, to be taken in obtaining the correct weight. 

 When the plant is exposed to the light of the sky the rate 

 of production of oxygen is relatively slow and the bubbles 

 escape through the cut end ; but under sunlight the rate 



