MARKING INK. 181 



compounds of silver are decomposed by exposure to light, which 

 circumstance is the basis of the majority of the processes employed 

 in photography. Precipitate some chloride of silver by adding 

 some nitrate of silver solution to brine (Expt. 120), and wash the 

 precipitate once or twice by decantatioii. Place the washed silver 

 chloride in sunlight for some time, when it will become blackened 

 through decomposition. The presence of animal or vegetable 

 matter in contact with the silver compound greatly promotes the 

 action and setting free of metallic silver in the form of black 

 particles ; thus if a piece of paper be soaked in brine and dried, 

 and then written on with a quill pen dipped in a solution of silver 

 nitrate, it will become charged with particles of silver chloride pre- 

 cipitated in its pores by double decomposition wherever the pen has 

 touched it ; on exposing the prepared paper to light it will blacken 

 much more quickly than pure silver chloride alone, speedily render- 

 ing the writing visible. 



Expt. 197. Marking Ink for Linen, &c. Write with a clean 

 quill pen dipped in a solution of nitrate of silver on the linen to 

 be marked, having previously soaked the spot in salt water and 

 dried it again. Chloride of silver will be formed where the pen 

 passes, as in the paper in the preceding experiment ; on exposure 

 to sunlight the chloride of silver will be decomposed, and metallic 

 silver deposited in the pores of the linen so as to mark it per- 

 manently. 



Ordinary marking inks are usually solutions of nitrate of silver 

 or some other silver compound coloured with sap-green, indigo, 

 &c., to make the writing legible at first. In use, the decomposi- 

 tion is generally brought about by means of heat rather than light, 

 a hot flat-iron being placed over the writing, which produces the 

 same result more rapidly. India-rubber stamps and types made 

 of metal that will not act chemically on silver compounds 

 (unlike copper, Expt. 134) are often conveniently used instead of 

 quill pens ; if the metal be acted upon, the silver will be more or 

 less precipitated on the type by the action, and the liquid will not 

 communicate the decomposable silver compound to the linen, so 

 that only a comparatively faint mark will be produced, which will 

 not bear frequent washing. 



Expt. 198. To Silver a Hollow Glass Vessel internally. The 

 action of heat in decomposing silver compounds so as to precipitate 

 metallic silver is made use of for the purpose of coating hollow 

 glass vessels internally with silver in such a fashion as to form a 

 bright glistening surface, converting the vessel into a mirror. 

 One way of proceeding is as follows: Dissolve a small quantity of 

 tartar ic acid in water and add ammonia solution until the liquid 



