38.2 Prof. Draper on the Chemical Action of Light. 



bi'ief action of a move powerful one, no matter how intense it 

 may have been. But though the fact of a decomposition depends 

 on some other quaUty of a ray than its intensity, the rate of de- 

 composition is directly proportional thereto, a brilliant beam 

 being more efficient than a feeble one. 



The actiny ray is the absorbed ray ; and whatever can increase 

 absorption will increase the energy of effect. A changeable 

 medium may have its absorbent quality so reduced by dilution 

 with water, or admixture with other bodies, that even a feeble 

 ray can pass through it, losing but little and bringing about little 

 change. But if the medium be concentrated, so that its absoi-pt- 

 ive power is at a maximum, the first layers will completely absorb 

 from a beam its acting rays, and the amount of decomposition 

 mil now depend on the intensity of the ray ; for the more bril- 

 liant it is, the deeper it can penetrate before losing by absorption 

 all its power. 



In estimating the influence of light on different solutions, we 

 should constantly bear in mind that the maximum of effect is 

 never jjroduced unless complete absorption has taken place, 

 ^^lien the colour of a solution is pale, it may require considerable 

 thickness before complete absorption is accomplished. Thus if 

 two equal tubes, containing equal quantities of a solution of 

 chlorine in water, are exposed to the rays, they will evolve equal 

 quantities of oxygen gas ; but if behind one of tliem a piece of 

 looking-glass is placed, the eff"ect is immediately increased. The 

 ray which has passed the solution and produced its eff'ect is com- 

 pelled to cross it again, and thus act once more. The following 

 illustrations are examples of the kind. 



Two small matrasses of equal size, containing chlorine-waterj 

 were exposed to the rays of the sun ; behind one of them a con- 

 cave hemispherical metallic mirror was placed, so that the light 

 which had crossed the solution was compelled to cross it again ; 

 the amount of (jxygen set free in this matrass was about one- 

 fourth greater than in the other. 



The same was I'epeated, the exposure being to the sky-light 

 instead of the sun-rays, and the quantity of oxygen set free in 

 the two matrasses was as 18 to 55. 



It might be supposed that a part of this increased effect is 

 due to the rise of temperature from the mirror obstructing ra- 

 diation. To exert a cooling action, the following modification 

 was therefore tried. In a glass jar filled quite full of quicksilver^ 

 a half-inch bulb containing chlorine-water was placed in such a 

 way, that a small portion of its surface, about one-sixth of an inch 

 in tUameter, projected above the surface of the liquid metal. On 

 this part the solar focus from a lens was thrown. The rays 

 therefore gained access t(j the interior of the bulb, and were 



