1 815.] different Rays which compose the Solar Light. 165 



another action of this light still more prompt and energetic. 

 When a mixture of oxymuriatic acid gas and hydrogen gns are 

 exposed to the action of solar light, a detonation takes place, 

 and water and muriatic acid arc formed. These different phe- 

 nomena enabled M. Berard to examine the chemical powers 6f 

 the different rays of the spectrum. By exposing to the different 

 coloured rays, pieces of card impregnated with muriate of silver, 

 or small phials filled with the detonating mixture, he was enabled 

 to judge of the energy of each by the intensity or rapidity of the 

 chemical change which it produced. He found that the chemi- 

 cal intensity was greatest at the violet end of the spectrum, and 

 that it extended, as Ritter and Wollaston had observed, a little 

 beyond that extremity. When he left substances exposed for a 

 certain time to the action of each ray, he observed sensible 

 effects, though with an intensity continually decreasing in the 

 indigo and blue rays. Hence we must consider it as extremely 

 probable, that if he had been able to employ reactives still more 

 sensible, he wouldhave observed analogous effects, but still more 

 feeble, even in the other rays. To show clearly the great dispro- 

 portion which exists in this respect between the energies of the 

 different rays, M. Berard concentrated by means of a lens, all 

 that part of the spectrum which extends from the green to the 

 extreme violet ; and he concentrated, by means of another lens, 

 all that portion which extends from the green to the extremity of 

 the red. This last pencil formed a white point so brilliant that 

 the eyes were scarcely able to endure it; yet the muriate of 

 silver remained more than two hours exposed to this brilliant 

 light without undergoing any sensible alteration. On the other 

 hand, when exposed to the other pencil, which was much less' 

 bright, and less hot, it was blackened in less than six minutes. 

 M. Berard concluded, from this experiment, that the chemical 

 effects produced by light are not solely owing to the heat deve- 

 loped in the body by its combining with the substance of the 

 body; because, on such a supposition, the lac ulty of producing 

 chemical combinations ought to be greatest in those rays which 

 possess the faculty of heating in the greatest perfection ; but 

 perhaps we should find less opposition between these two opi- 

 nions, if we attended to the different results which may be pro- 

 duced by the same agent placed in different circumstances, and 

 it we considered that agents of a nature quite -dissimilar may 

 determine the same combinations when they are employed. 



Such is an epitome of the principal facte which M. Berard 

 ha* established in his memoir. To great accuracy he has united 

 an excel leivt arrangement in his account of his experiments. 

 He has presented the physical properties of the different rays 

 merely as the results of experiments, the hypothetical causes of 

 which he has abstained from inquiring into: and he has always 



