330 SALTS. 



840. Such paper, if placed in direct 



What effect 



has direct sun- sun light, becomes violet, and then dark 

 l s?n*itivepa- brown, in the course of a few minutes. 

 per? The change is owing to the partial decom- 



position of the chloride of silver. A new substance, 

 of darker color, is then produced ; whether a lower 

 chloride of different shade, or a mixture of metal and 

 chloride, or a compound of oxide and chloride, is not 

 very certainly known. 



841. If a cross or other device cut 



How may cop- 



be pro- irom dark paper, be pressed down upon 

 of sen- sensitive paper, by means of a glass plate, 

 sitive paper ? an d be left to cover it during the exposure 

 to light, the paper will be pro- 

 tected beneath it, and an exact 

 copy of the device thus ob- 

 tained. The most delicate lace 

 may be copied hy the same method. In reproducing 

 engravings by this means, they must be previously 

 rendered translucent, so that the imprinted portions will 

 allow the light to pass. This may he accomplished by 

 waxing them, with the help of a hot iron, or by simple 

 oiling. The dark parts of the engraving appear light, 

 and the light portions dark, in the picture. By copying 

 the copy, a true representation of the original device, 

 called a " positive picture," is obtained. Both the " pos- 

 itive" and " negative" are soon destroyed by the action 

 of light upon the whole sensitive surface. But the 

 means exist for rendering them entirely permanent in 

 any exposure. 



H X Y 



