440 ON BURNING GLASSES AND MIRRORS. [MEMOIR XXX. 



This idea is favored by what we find in the case of heat. 

 The burning-glass has long had celebrity in that respect, 

 and in former times was the most powerful means of 

 reaching a high temperature. 



The effect of the glass is due to the rapidity with 

 which it can supply caloric, contrasted with the loss by 

 conduction, radiation, etc. Thus an object of any kind 

 exposed to the sun receives heat at a certain rate; but 

 it is simultaneously experiencing a loss by conduction, 

 radiation, and currents in the air. Exposed to the focus 

 of a lens, the supply becomes, in a given time, greater 

 than before ; and the temperature rising, great effects are 

 the necessary result. 



But changes brought about by light are in a different 

 predicament. 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 con- 

 centrated ray furnishes in a brief period of time. In this 

 case, therefore, everything will depend on the absorptive 

 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 blackened, it melts in an 

 instant. And so with chemical changes. A body which, 

 like chlorine, can exert an absorptive action on the ray 

 becomes modified, and induces changes; but if, like oxy- 

 gen, it has not that property, it will remain indifferent 

 and unaffected by the most intense radiation. 



Considering, however, that the calorific effects of the 

 converged 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 which cannot be imitated by the feebler light 

 of higher latitudes, and perhaps decompositions may be 



