378 Prof. Draper on the Chemical Action of Light. 



Putting aside those cases in which the change is obviously 

 brought about by the high temperature, I have not found that 

 the focus of the burning-lens can cause any change which the 

 uncouverged sun-rays alone ai-e incompetent to produce. Its 

 action is merely to hasten the effect. 



As everything shows that the activity of a ray in causing de- 

 composition depends on the absorption exerted by tlic changing 

 medium, I investigated the circumstances particularly in the case 

 of a solution of the pcroxalate of iron, a substance remarkably 

 well-suited for such inquiries. 



In a tube half an inch in diameter, solution of pei'oxalate of 

 iron Avas coniined over mercmy, and surrounded on all sides by 

 another portion contained in a wider tube, so that no ray could 

 reach the former except after having traversed the latter, under- 

 going its absorptive action. I found that no decomposition oc- 

 curred in the inner tube until the surrounding liquid was so 

 far advanced in change as to be sensibly passing into the colour- 

 less state. 



This therefore proves, that when a solution of peroxalate of 

 iron is submitted to the sun, commencing with the exterior, it is 

 acted upon in succession to the centre, and that a ray which has 

 suffered absorption can no longer bring about change. 



In reality, it does not require a special experiment like the 

 foregoing to satisfy us of the truth of this statement ; we have 

 only to observe in a solution of this peroxalate the part from 

 which bubbles of carbonic acid escape. This will be found to be 

 uniformly on the side nearest to the light, and the parts further 

 back are affected in due succession. 



Whatever can increase the absoi-ptive power of a body will 

 increase the action of a ray upon it. It is for this reason that 

 the peroxalate of iron decomposes more freely when its tempera- 

 ture is raised. The solution then turns brown, and a greater 

 absorbent action on the incident ray is exerted ; or, if the heat 

 declines, and the liquid becomes paler, the lowering of its tint 

 is at once accompanied with a diminution in the action of the 

 light, and a correspondingly smaller quantity of carbonic acid is 

 set free. 



Adniixture with other substances which can change the tint 

 gives rise to results of the same kind. Thus, when the peroxa- 

 late is rendered acid by mixture with oxalic acid, its colour from 

 being brown-yellow becomes grass-green. If a comparative trial 

 be made of such a sample, it will be found that with loss of 

 depth of tint there lias been a corresponding diminution of sen- 

 sitiveness, and much less carbonic acid is set free from the pale 

 acid solution, than from that the tint of which is deep. 



This principle seems to be of general application in all cases 



