448 ON BURNING GLASSES AND MIRRORS. [MEMOIR XXX. 



suitable support, the substance to be experimented upon. 

 The mass of water kept the temperature down, and in 

 some cases the hot water was removed by an aspirator 

 and cold water introduced below. A spoon could be 

 used when powders were employed of so great a specific 

 gravity as not to drift too high from the focus in the 

 ascending current of hot water. 



In Fig. 96, a a is a matrass filled with water, through 



which come in the converging 

 rays, c. Through the neck at d 

 A ^ a spoon, b, may be passed down 



to the focal point,/. 



The result was, however, the 

 same as before. The focus of a 

 burning-lens cannot cause any 

 chemical change which the un- 



converged sun-rays are incompetent to produce. It mere- 

 ly hastens the effect. 



Upon the whole, we may therefore conclude that it is 

 not the intensity of a beam which determines its decom- 

 posing power, and that we cannot produce greater chem- 

 ical effects by the action of converging mirrors and lenses 

 than we can by the application of the simple sunbeam, 

 continued for an equivalent period of time. 



In estimating the influence of light on different solu- 

 tions, we should constantly bear in mind that the maxi- 

 mum effect is never produced unless complete absorption 

 has taken place. When the color of a solution is pale, 

 it may require considerable thickness before complete 

 absorption is accomplished. Thus if two equal tubes, 

 containing equal quantities of the same solution of chlo- 

 rine in water, be exposed to the rays, they will evolve 

 equal quantities of oxygen gas; but if behind one of 

 them a piece of looking-glass be placed, the effect is im- 

 mediately increased. The rays that have passed through 



