Prof. Draper on the Chemical Action of Light. 373 



turn it into cast iron, its subsequent complete malleability seems 

 to disprove this. Spongy platinum did not melt alone, nor even 

 if enclosed in a globule of fused microcosmic salt. AYe may 

 therefore estimate the working power of this lens on a substance 

 placed in its focus as being somewhat above the point of fusion 

 of wrought iron, but lower than the point of fusion of platinum. 



It will be understood that this estimate refers to temperature 

 only. The power of the lens as to light must be enormously 

 greater. The caloritic effect, as we ha\e already seen, is de- 

 pressed by many different causes,— conduction by the support, 

 radiation, &c. These actually bring it down to less than one- 

 sixth of its true amount. But for the light no such effect is 

 exerted ; and the lens, if it were perfectly transparent, and there 

 was no loss by reflexion from its surfeq^s^ would ])roduce '4^f^^ 

 multipljdng effect. ,,('! i,., >i ijcyii 



Witli these preliminary remarks, we may now proceed ^to 

 examine the. chemical effects produced by this lens. ,.,,,,.,[[,.,,1 



Tw^o small glass matrasses, the bulbs of which were about 

 half an inch in diameter, were filled with chlorine-water ; the 

 one being exposed to the direct rays of the sun, and the other 

 to the converging rays of the lens. Decomposition of the w'ater 

 occurred in both, but with far more activity in that placed in 

 the focal pomt. The difference was at once so striking to the 

 eye, that I made no attempt to measure it. It is plain that the 

 greater the quantity of incident light, the more rapid the decom- 

 position ; though, after the first moment of decomposition, the 

 solutions being no longer the same in constitution, the quantities 

 of gas disengaged will not be in,, t-hfisa^n^ proportion as the inci- 

 dent light. ■ ; 



There is no difficulty in effecting the decomposition of water 

 by chlorine under the 'influence of the sun ; but under the same 

 circumstances, iodine and bromine are inadequate to produce 

 such an effect. No change takes place in an aqueous solution 

 of those bodies, even though the exposure should continue for 



manv davs. 



a' solution of bromine in water was prepared, the water being 

 first boiled to expel the air dissolved in it. It was placed in a 

 half-inch matrass, and exposed to the focus of the lens. As 

 the temperature rose rapidly, the water was de])ressed in the 

 bulb by the steam and bromine vapour which occupied the upper 

 part, the bulb being placed uppermost, and the tube dipjjing in 

 a small phial which served as a reservoir. After the exposure had 

 contiimed for two hours and a half, the matrass was rem<)ved 

 and suff(;rcd to cool. There remained uucondensed a little 

 bubble, which measured about yljj cubic inch ; but this was pro- 

 bably nothing more than the atmosphqvic ajr, whjch had found 



