SYMPATHETIC INKS. 129 



whilst chloride of potassium remains in solution. If the solutions 

 are sufficiently strong, the precipitate will be so bulky relatively to 

 the amount of fluid present, that the whole mass will apparently 

 set solid, so that the vessel containing it may be turned upside 

 down without any liquid running out. 



Several other pairs of fluids will produce a like result ; thus a 

 solution of silicate of soda may be used along with diluted hydro- 

 chloric acid: if the strengths of the two fluids are properly adjusted, 

 on mixing the two and stirring them together with a stick or glass 

 rod, the whole will speedily set to a firm jelly : this effect is due 

 to the formation of silicic acid, which when liberated in this way 

 does not precipitate as a powder, but gelatinises like glue or jam. 



Expt. 129. To make Invisible Writing Visible. One interest- 

 ing application of the property possessed by certain substances 

 themselves colourless, or nearly so, to form strongly-coloured pre- 

 cipitates on intermixture, is the use of one of a pair of such fluids 

 to write with (using a clean quill pen) on white paper. If care is 

 taken not to scratch the paper, no writing will be visible when the 

 fluid is dry ; but on brushing over the paper lightly with the other 

 fluid, or on dipping the paper into a basin full of the second fluid, 

 the writing becomes visible, owing to the formation of a coloured 

 precipitate in the pores of the paper or adhering to its surface. 

 Thus, if you write with a solution of ferrocyanide of potassium, the 

 pale yellow tint will be so faint as not to be noticeable on most 

 kinds of writing paper, especially if some kind of writing in 

 ordinary black ink be made on the paper, and the invisible writing 

 traced between the lines ; if, after drying, the letters be brushed 

 over with a weak solution of perchloride of iron, the writing will 

 become visible as blue letters ; or if you dip the letter into a basin 

 containing a weak solution of sulphate of copper, the letters will 

 be " developed " in a reddish-chocolate colour. 



Another way of proceeding is to write with a colourless solution 

 of acetate of lead, and then expose the writing to the nasty-smell- 

 ing gas sulphuretted hydrogen, produced in Expt. 13. The letters 

 will appear in brownish-black. 



Scarlet letters may be developed by writing with a solution of 

 iodide of potassium, and developing with solution of "corrosive 

 sublimate" (or perchloride of mercury a strong poison); or the 

 two solutions may be used conversely, the corrosive sublimate to 

 write with and the iodide of potassium as developer; if used in this 

 way the developing solution must not be too strong nor applied in 

 too large a quantity, otherwise the scarlet precipitate at first formed 

 will be again dissolved, and the writing will wholly disappear 

 (Expt. 121). If the writing be done with the corrosive sublimate 



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