244 EXPERIMENTAL INVESTIGATIONS. 



proper time ; and as the plate may at any period be removed 

 from the solution and examined, the first experiment should 

 never exceed twenty-five seconds, when, if not complete, the 

 plate may be again subjected to electrolysis. 



5. The solution to be employed. Here a vast field was 

 open, and still is open to future experimentalists. Admitting 

 the usual explanation of the Daguerreotype, which supposes 

 the light parts to be mercury, and the dark silver, the object 

 was to procure a solution which would attack one of these, 

 and leave the other untouched. If one could be found to 

 attack the silver and not the mercury, so much the better ; as 

 this would give a positive engraving, or one with the lights 

 and shadows, as in nature ; while the converse would give a 

 negative one. Unfortunately, silver and mercury are nearly 

 allied in their electrical relations. I made several experiments 

 with pure silver and mercury, used as the anode of a voltaic 

 combination ; but found that any solution which would act 

 on one acted also on the other. All, then, that could be ex- 

 pected was a difference of action. With the Daguerreotype 

 plates I have used the following : 



Dilute sulphuric acid, dilute hydrochloric acid, solution of 

 sulphate of copper, of potash, and of acetate of lead. The 

 object of using acetate of lead was the following : With this 

 solution, peroxide of lead is precipitated upon the anode ; 

 and, this substance being insoluble in nitric acid, it was hoped 

 that the pure silver parts of the plate, being more closely in- 

 vested with a stratum of peroxide than the mercurialised por- 

 tions, these latter would, when immersed in this menstruum, 

 be attacked, and thus furnish a negative etching. I was also 

 not altogether without hopes of some curious effects, from the 

 colour of the thin films thus thrown down ; here, however, I 

 was disappointed : the colours succeeded each other much as 

 in the steel plate used for the metallochrome, but with in- 

 ferior lustre. On immersion in nitric acid of different degrees 

 of dilution the plates were unequally attacked, and the etching 

 burred and imperfect. Of the other solutions, hydrochloric 

 acid was, after many experiments, fixed on as decidedly the 

 best ; indeed, this I expected, from the strong affinity of 

 chlorine for silver. 



