6 LECTURES TO SCIENCE TEACHERS. 



some body is present which can take up the liberated iodine. 

 What have we here, in this Daguerrean process, to do so ? 

 "We have the metallic silver, for recollect that the iodide is 

 only on the outer surface of the plate. Now I could demon- 

 strate to you the fact that this silver is necessary had I the 

 time. I might have repeated an old experiment, and, having 

 silvered a similar glass plate, and converted the whole of 

 the delicate metallic layer into silver iodide, have then shown 

 its insensitiveness to light, owing to there being nothing 

 to combine with the iodine, which it is anxious to liberate. 

 I will show you by-and-bye another experiment which will 

 illustrate the necessity of an absorbent. 



Goddard, a countryman of ours, discovered that by treat- 

 ing the silver plate with bromine after the iodide had been 

 formed, the exposure in the camera necessary to form a 

 mercury-condensing image was shortened from minutes to 

 seconds. Perhaps this was the greatest of all improvements 

 in the daguerreotype process, as it rendered it thoroughly 

 practicable. 



In 1834, whilst Daguerre was working in France on the 

 production of sun pictures, Fox Talbot, a gentleman whom 

 I am glad to say is still living, began experimenting 

 with silver chloride, pursuing the same line of thought as 

 Sir H. Davy, and in June 1839 (six months earlier than 

 the publication of Daguerre's process) he read a paper at the 

 Royal Society on photogenic drawing. This photogenic 

 drawing is really the same photographic printing process that 

 we employ now. Talbot impregnated writing-paper with 

 common salt or sodium chloride, and when dry treated it 

 with washes of silver nitrate, the result being to produce 

 silver chloride 1 in the paper with a little pure silver nitrate 

 ready to take up the chlorine which the darkening chloride 

 would liberate. Ferns, leaves, lace, &c., he copied by this 

 method ; more than rivalling the draughtsman in accuracy 

 and rapidity. 



Let us suppose that one of the objects to be copied was 

 a piece of black lace. When the lace was laid on the paper 

 those parts beneath the cotton or thread would remain white 

 whilst the ground would be blackened. The paper on the 

 removal of the lace would represent the lace as white on a 

 black ground. This picture Talbot termed a negative picture, 

 i NaCl + AgN0 3 = AgCl + NaN0 3 . 



