DETERMINATION BY TORSION BALANCE 207 



of evolution may become so greatly increased that the vent 

 at the cut end of the stem is not large enough for the escape 

 of all the oxygen. Small bubbles are. thus forced out from 

 the edges of the leaves and remain attached, thus producing 

 a false buoyancy by which the weight of the sprig under 

 water is reduced. The difficulty is, however, completely 

 removed by slowly raising the plant above the water by 

 means of the hook from which the plant is suspended, the 

 balance being clamped in the meanwhile. The pressure of 

 the water-column being removed, the small bubbles burst 

 and escape through the film of water. The hook is replaced 

 in its proper place and the balance is released. 



We can ascertain the value of each division of the scale 

 by removing the rider, weighing 3 -5 mg., from the right arm 

 of the balance. This was found to produce a downward 

 movement of the index through 38 divisions of the scale. 

 An up-movement of the index through one division of the 



scale thus indicates an increase of weight of ^-^ or • 092 mg. 



With finer and longer torsional wire, it would not be difficult 

 to construct a micro-balance by which a very small fraction 

 of a milligram could be determined. The sensitiveness of 

 the balance used is, however, quite sufficient for our present 

 purpose. 



I will give several results obtained with the Torsion 

 Balance. The specimen was exposed to sunlight for two 

 successive periods of one hour each. Exposure to sunlight 

 from 3 to 4 p.m. gave rise to an up-movement of the index 

 through 6 divisions of the scale. The increase of weight 

 produced in one hour by photosynthesis is thus 6 X 0-092 

 = 0-552 mg. 



The experiment was repeated for the next hour from 

 4 to 5 p.m. As the evening was approaching the intensity 

 of sunlight was undergoing a rapid diminution. The move- 

 ment of the index was now 4 divisions, and the increase of 

 weight for the second hour is 



4 X 0-092 = 0-368 mg. 



