436 ON BURNING GLASSES AND MIRRORS. [MEMOIR XXX. 



MEMOIR XXX. 



ON BUENING GLASSES AND MIEROES THEIE HEATING AND 



CHEMICAL EFFECTS. 



Collected and condensed from the Philosophical Magazine, May, 1851; Harper's 

 Monthly Magazine, No. 329. 



CONTENTS : Can concentrated rays produce new chemical decompositions? 

 Effects of amplitude, frequency, and direction of vibration in the 

 ether-waves. Clock lenses for long exposures. Decomposition of water 

 by chlorine. Attempt to decompose it by bromine and iodine. Use of 

 absorbing troughs. Dry silver iodide not decomposed by light. De- 

 compositions under water. Decompositions in a spherical concave. 

 Effect of extraneous mixtures. They do not make collodion more sen- 

 sitive. Antagonization of radiations. Case of electric spark. Effects 

 of polarized light. Attempts to polarize light by an electro-magnet. 

 Mechanical cause of decompositions by light. 



WHEN, many years ago, I commenced an experimental 

 examination of the chemical action of light, I entertained 

 great expectations of what might be accomplished by the 

 use of burning-glasses. It seemed reasonable to suppose 

 that if the direct sun -rays could occasion so many de- 

 compositions, their chemical force would be incompara- 

 bly greater when their brilliancy was exalted by a mirror 

 or a lens. Of the two, a concave metallic mirror should 

 produce a more characteristic effect, since it returns the 

 rays as it receives them, but a special and very impor- 

 tant portion of them is absorbed by the selective action 

 of the lens. 



I had not, however, at that time the means of making 

 these experiments in a satisfactory manner, and, though 

 very much disappointed with the result, postponed the 



