4 LECTURES TO SCIENCE TEACHERS. 



temperatures) which had collected on the parts acted upon by 

 light. 



In June 1839 Daguerre's discovery was announced, and 

 in August of the same year published to the world, and a 

 pension of 6,000 francs per annum given to him by the 

 French Government, whilst at the same time 4,000 francs 

 was allotted to Niepee the Younger, who had succeeded to 

 the partnership with Daguerre after the death of his uncle. 



An outline of the daguerreotype process is as follows : 



A copper plate is silvered by the electro-plating process or 

 any other convenient method, and after very careful cleaning, 

 the silver surface is exposed in the dark to the action of 

 iodine vapours. The iodine combines with the silver, and 

 the metallic surface becomes covered with silver iodide, first 

 canary coloured, then rose, then blue, and so on, the colour 

 being dependent on the thickness of the Layer of silver iodide 

 produced. When canary-coloured it is supposed that the 

 surface is in the best condition for receiving the impact of 

 light. 



It will now be convenient to point out the chemical 

 change that really takes place in the ordinarily employed 

 silver salts when exposed to the action of light. Under 

 certain circumstances, when- subject to its impact, silver 

 iodide (which for our purpose we will call Ag 2 I 2 ) throws 

 off one atom of iodine, and we get subiodide of silver 

 (Ag 2 I), a slightly black body, left behind. Scheele proved 

 "by his experiments that silver chloride (Ag 2 C1 2 ), when acted 

 upon by light, gave off chlorine (Cl), and we now know 

 that the blackened product is sub-chloride of silver (Ag 2 Cl). 



Similarly, silver bromide (Ag 2 Br 2 ) is converted into the 

 sub-bromide (Ag 2 Br). 



Pure and dry silver chloride will change in the light. 

 Pure and dry silver bromide will also change in the light, 

 but not so readily as the chloride. Pure and dry silver 

 iodide is unaffected by light, unless any body which will take 

 up iodine be present ; even moisture will induce the change if 

 the impact of light be prolonged. 



The sensitiveness of both the chloride and bromide is 

 materially increased by the presence of any body which will 

 absorb chlorine and bromine, and in all cases we may lay 

 down the law that the greater its affinity for chlorine, 

 bromine, or iodine, the greater the sensitiveness of the 



