Prof. Draper on the Chemical Action of Light. 371 



the focus of a lens^ the supply becomes in a given time greater 

 than before, and, the temperature rising, great effects are the 

 necessaiy result. 



But changes brought about by light are in a different jiredi- 

 cament. Here conduction is entirely absent^ as is also loss by 

 currents in the air. The cumulative effects of a long exposure 

 give the same action as a highly concentrated ray furnishes in a 

 brief period of time. In this case, therefore, everything will 

 depend on the absoi-jrtive power of the substance. 



When a piece of polished silver is placed in the focus of a 

 burning-lens, it remains quite cold, because of its high reflecting 

 power ; but if it be blackened, it fuses in an instant. And so 

 with these chemical changes. A body, which, like chlorine, can 

 exert an absorptive action on the ray, becomes modified, and in- 

 duces changes ; but if it has not that property, it will, like oxygen, 

 remain indifferent, and unaffected by the most intense radiation. 



Considering, however, that the calorific effects of the convei'ged 

 solar rays are so striking, we may reasonably inquire whether in 

 like manner the chemical action can be increased. There is a 

 very general impression that the intense radiation of tropical 

 climates accomplishes changes whicli cannot be imitated by the 

 feebler light of higher latitudes ; and perhaps decompositions 

 may be brought about by a large convex lens, which the direct 

 rays of the sun are wholly inadequate to produce. 



We must bear in mind the essential difference just pointed 

 out between the heating effect of such an optical contrivance and 

 its chemical action. When an object is exposed to the focus, it 

 is continually losing its heat by radiation, conduction, and cur- 

 rents in the air. The amount of heat rendered available is also 

 directly as the absorptive, and inversely as the reflective power. 

 In the instance to which allusion has been made, no matter how 

 large a lens may be, a bead of polished silver never melts in its 

 focus, the whole supply of heat being reflected at once ; and in 

 the case of a dark un])olished body, the temperature reached will 

 not alone depend on the size of the lens, but on the radiating, 

 conducting, and absorbent power, as well as on the rapidity of 

 currents in the air, which, sweeping past it, tend to lower its hea 



But with the chemical cft'ects it is altogether different. In 

 these there is no such thing as conduction, and currents in the 

 air exert no action. If, thei'efore, a lens increases the sun's 

 brilliancy sixty times, it ought to produce the same effect by one 

 minute's action that could be given by an exposure of one hour 

 to the naked solar rays ; for tiiere being no cause to abate the 

 effect, the action bec<jmes essentiaily cumulative ; and the office 

 of the lens is merely to produce the sanxe effect in a shorter 

 period of time. 



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