Prof. Draper on the Chemical Action of Light. 377 



blue it undergoes no change. The action sCtiini^ to be the same 

 as is caused in this substance by ignition. " ' 



Protoxide of manganese absorbs oxygen, and becomes of i 

 ruddy black; in the blue focus it turns black. . ,. i,. 

 Green oxide of chromium is -wholly unchanged. ' .,' , V. ... 

 Black sulphuret of mercury instantly volatilizefe'^thja nis^mg 

 noise. ■ •.'!!'■, ''"'■''' '-''".j.^ '^,(rp 



Prussian blue becomes of a reddish -bro^vh in either focus. 

 On reviewing these experiments, it appears that, with the ex- 

 ception of the case of the cyanide of mercury, all the effects are 

 such as we should expect from a high temperature; nor does 

 the light seem to exert any special action, though concentrated 

 to so enormous a degree. Even allowing for the high tempera- 

 ture Avhich must necessarily be reached, it must not be forgotten 

 that the heat is all the time acting at a disadvantage. Cooling 

 causes are in operation, so that the actual calorific eflfect is not 

 one-sixth of what it should be ; but the luminous agent encoun- 

 ters no such reduction. If, then, the increase of intensity of the 

 light facilitated chemical decomposition, we might certainly expect 

 to see oxides of all kinds completely reduced, or instantly brought 

 to the metallic state. We might also expect that compound sub- 

 stances which could resist the force of the ordinary ray would 

 be broken down by the intense illumination of the focal point of 

 the burning lens, and that that instrument would do, under 

 proper regulation, what the voltaic pile has done, — effect decom- 

 positions which have never been effected before. 



The case of the cyanide of mercury is perhaps an apparent 

 rather than a real exception ; for a sharp heat, suddenly applied, 

 will often produce results chfferent from those given by a more 

 moderate temperature. -.-- 



To reduce the calorific effect as' ^ar ife possible, and give every 

 advantage to the condensed luminous focus, I received the cone 

 of rays coming fi-om a large burning-lens on a glass globe, six 

 inches in diameter, filled with water. This increased the con- 

 vcrgency of the rays, and brought them more quickly to a focal 

 point. Then, through the neck of the globe, was introduced to 

 the focus the instrument containing the substance experimented 

 upon, a small matrass, spoon, &c. The cooling effect of so 

 large a mass of water kept the temperature of the substance 

 down ; and in some cases, where a prolonged exposure was re- 

 quired, by means of tubes suitably bent, the hot water was re- 

 moved by an aspirator, and cold water introduced below. A 

 spoon could be used where powders were (;mi)loyed of so great 

 a specific gravity as not to thift too high from the focus in the 

 ascending current of hot water. 



The result of all these investigations is, however, the same. 



