280 



CHEMISTRY. 



Fig. 103. 



power of the sunlight acts upon the chloride of silver, dark- 

 ening it, except where 

 the threads of the lace 

 prevent it from doing 

 so. The tracings of the 

 lace consist, then, of the 

 chloride of silver un- 

 changed ; while in the 

 dark parts there is me- 

 tallic silver minutely di- 

 vided. In the same way 

 skeletons of leaves, or 

 even the leaves them- 

 selves, may be copied. 

 So, also, we may copy 

 engravings, if we oil 

 them so that the light may shine through the unprinted 

 portions. The dark parts of the engraving will of course 

 be light, and the light 

 parts dark in the copy. 

 This constitutes what is 

 called a " negative ;" and 

 a "positive" or true 

 copy can be obtained by 

 dealing with the " nega- 

 tive" as you do at first 

 with the engraving it- 

 self. In Fig. 103 you 

 have represented a "neg- 

 ative" of a leaf, the dark- 

 est parts of the picture 

 corresponding to the 

 thinnest parts of the 

 leaf, as the light coming through them decomposes the 



Fig. 104. 



