306 



ASSIMILATION. 



ice, in order to appreciabty lower the temperature of the water. 

 At a certain point it will be observed that uo bubbles are given 

 off, and their evolution does not begin again until the water be- 

 comes warm. 



(6) Examine, at the close of the series of simple experiments, 

 some of the leaves with iodine solution, for the detection of 

 starch. Even with no precaution the chlorophyll granules will 

 reveal the presence of a considerable amount of the first visible 

 product of assimilation, namel}-, starch. Lastlj-, keep a second 

 uninjured spray of the same plant in the light for a time, and 

 then in darkness for a day or two, after which examine it for 

 starch ; probably after this lapse of time no starch can be de- 

 tected, for although it has been made in the light, in darkness it 

 has been consumed in the various activities of the plant. 



815. According to the accepted theory, light consists of waves 

 which are set in motion in a tenuous elastic medium termed the 

 ether. The existence of this medium is made known to us only 

 by the phenomena which light itself presents ; but, having as- 

 sumed its existence, the phenomena of light can be explained. 

 The tenuitj' of this medium, which fills all space, far exceeds 

 that of any known gas, and its elasticit}' is far higher than that 

 of an^' known elastic solid. In it a luminous body sets in mo- 

 tion undulations which produce upon the retina the sensation of 

 light ; upon differences in the amplitude and the duration ' of 

 these undulations depend differences in the intensitj^ and the 

 color of the light which reaches the eye.'' 



1 The terms just employed, namely, amplitude and duration, seem hardly 

 applicable to waves of such incredible minuteness and velocity as those named 

 in the following table : — 



'^ "The intensity of the luminous impression must depend upon the force 

 of the atomic blows which are transmitted to the optic nerves, and it is also 

 evident that this force must be proportional to the square of the velocity of 

 the oscillating atoms, or, what amounts to the same thing, to the square of the 

 amplitude of the oscillation ; assuming, of course, that the oscillations are 

 isochronous. The connection of color with the time of oscillation is not so 



