444 ON BURNING GLASSES AND MIRRORS. [MEMOIR XXX. 



black. The specimen employed was cut from a piece of 

 good iron wire ; and though it might be thought that ex- 

 posure on the charcoal would tend to turn it into cast 

 iron, its subsequent complete malleability seems to dis- 

 prove this. Spongy platinum did not melt alone, nor 

 even if enclosed in a globule of fused microcosmic salt. 

 We may therefore estimate the working power of this 

 lens on a substance placed in its focus as somewhat 

 above the point of fusion of wrought iron, and lower 

 than the point of fusion of platinum. This refers to tem- 

 perature only. The power of the lens as to light must 

 be enormously greater. 



We may now examine the chemical effects produced 

 by this lens. 



Two 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, the other to the converging rays of the lens. De- 

 composition of the water occurred in both, but with far 

 more activity in that placed in the focal point. The dif- 

 ference 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 decomposi- 

 tion ; though, after the first moment of action, the solu- 

 tions being no longer the same in constitution, the quan- 

 tities of gas disengaged are no longer proportional to the" 

 incident light. 



There is thus no difficulty in effecting rapidly the de- 

 composition 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. 



A solution of bromine in water was prepared, the wa- 

 ter being first boiled to expel the air contained in it. It 

 was placed in a half-inch matrass (Fig. 93), and exposed 

 to the focus of the lens. As the temperature rose rap- 



