THE METHOD OF FLOTATION 167 



(1) ' Only light of wave-length between 770 fiu and 300 /xfi 

 conduces to assimilating activity in green plants ; they are 

 approximately the same rays that are visible to us. 



(2) ' The assimilatory effect of different rays is unequal, 

 but still not in such a way that some only are active whilst 

 others lying beyond these are quite inactive. If we express 

 the wave-length by abscissae and the activity of assimilation 

 by ordinates, we obtain a curve which does not in the least 

 correspond with the curve expressing the energy of sunlight 

 obtained by Langley.' * 



The object of the following investigation is to obtain 

 quantitative values of the relative effectiveness of the 

 different rays of the spectrum. I employed for this purpose 

 two independent methods, the Method of Flotation and the 

 Method of the Bubbler. The principle of the latter has 

 already been explained. I now describe in detail the 

 Method of Flotation, which will be found to possess features 

 which are novel. 



A short length of Hydrilla plant floats in a beaker of 

 water : on attaching a thin platinum wire to the young 

 rosette of leaves at the upper end of the plant, the apex of 

 the stem sinks down so that the cut end is uppermost. 

 This end of the stem is then attached by means of a single 

 waxed cocoon-thread to one arm of the Torsion Balance to 

 be presently described. Adjustment is made (by adding a 

 small weight to the pan p) such that the stem sinks about 

 1 cm. below the surface of the water in the beaker (fig. 46)- 



The balance consists of a light lever of aluminium, 

 through which passes a thin torsion wire of phosphor-bronze 

 used for galvanometer suspension. When a weight is 

 placed on the right arm of the balance a torsion is produced, 

 and the left arm is raised. The weight on the right arm is 

 measured by the equivalent weight which has to be placed 

 on the left arm in order to cause the balance to return to 

 the zero position. This return to zero is more accurately 

 determined by means of a beam of light reflected from a 



1 Jost, Plant Physiology (English edition), p. 127. 



