NATURAL PHILOSOPHY. 205 



place of lead ; and by covering it with, a deposit from a silver solution, the 

 impression may be used for stereotyping or electrotyping. 



ON THE COLORED IMPRESSIONS PRODUCED EY THE CHEMICAL 



ACTION OF LIGHT. 



The following communication, by M. Becqueral, is taken from the 

 Comptes Rendus, July, 18-54. 



The chemical action of light has enabled me, as is well known, to render 

 perceptible the electrical effects produced by reactions operating under the 

 influence of luminous rays. On the other hand, it is more than six years 

 since I was led to the observation of this fact, that it was possible to pre- 

 pare a surface which should be chemically capable of taking impressions 

 from light, so that it should become of precisely the same color as the 

 luminous ray which strikes it. The sensitive matter possessing this re- 

 markable property is a chloride of silver, which may be called the violet 

 chloride, containing less chlorine than the white chloride, and, in general, 

 mixed with the latter. The chloride of silver of which I speak being 

 able to be placed in such conditions that it will only be affected between 

 the limits of refrangibility of the rays which are perceptible to the organ 

 of vision, it was important to study attentively in what manner it would 

 behave in the apparatus which I have named the electro-chemical-actinom- 

 eter ; what would be the effects resulting from the action of the different 

 luminous rays whose intensity would be caused to vary within the deter- 

 mined limits ; and, finally, whether it would be 'possible to establish a 

 photometrical method based on different principles from those generally in 

 use. This study I commenced some time since ; but I have found it ne- 

 cessary to reexamine the different circumstances which accompany the 

 preparation of the sensitive matter, and the modifications which heat and 

 light produce, before the luminous rays have given color to it. Such is 

 the aim of the work I have now the honor of presenting. 



In former publications I have described various means by which we can 

 obtain colored impressions on surfaces covered with violet chloride of sil- 

 ver ; but that which gives the best effects consists iii decomposing rapidly, 

 by an electric current, a solution of hydrochloric acid in water, and causing 

 the chlorine to pass to a plate of silver placed at the positive pole of the 

 battery. This process is rendered of easy and certain application, by 

 determining, in each circumstance, and at every minute, the quantity of 

 chlorine which passes over the silver plate. For this purpose we interpose, 

 in the voltaic circuit, a voltameter of water, so that the current which 

 decomposes the hydrochloric acid, and carries the chlorine over the silver, 

 likewise decomposes the acidulated water ; the electro-chemical decompo- 

 sitions taking place in definite proportions, the same volume of chlorine is 

 carried over the silver as there is disengaged of hydrogen gas in the 

 eprouvette, placed above the negative electrode of the voltameter. The 

 other side of the plate is protected by a varnish, so that the chlorine passes 



