INK-MAKING. 309 



will be seen ; but draw a sponge over it, wetted 

 with a solution of tin in aqua regia ; the writing 

 will immediately appear of a purple colour. 



3. Write with an infusion of galls, and when 

 you wish the writing to appear, dip it into a 

 solution of green vitriol ; the letters will be black. 



4. Write with diluted sulphuric acid, and nothing 

 will be visible. To render it so, hold it to the fire, 

 and the letters will instantly appear black. 



5. Juice of lemons, or onions, a solution of sal 

 ammoniac, green vitriol, &c. will answer the same 

 purpose, though not so easily, or with so little 

 heat. 



6. Green sympathetic ink. Dissolve cobalt in 

 nitro muriatic acid, and write with the solution. 

 The letters will be invisible till held to the fire, 

 when they will appear green, and will disappear 

 completely again when removed into the cold. In 

 this manner they may be made to appear and dis- 

 appear at pleasure. 



A very pleasant experiment of this kind is to 

 make a drawing representing a winter scene, in 

 which the trees appear void of leaves, and to put 

 the leaves on with this sympathetic ink; then, 

 upon holding the drawing near to the fire, the 

 leaves will begin to appear in all the verdure of 

 spring, and will very much surprise those who are 

 not in the secret. 



7. Blue sympathetic ink. Dissolve cobalt in 

 nitric acid ; precipitate the cobalt by potash ; 

 dissolve this precipitated oxyd of cobalt in acetic 

 acid, and add to the solution one-eighth of com- 

 mon salt. 



This will form a sympathetic ink, that, when 

 cold, will be invisible, but will appear blue by heat. 



X 3 



