IRON, NICKEL, COBALT 499 



late, and the ferricyanide has already been applied to the paper, 

 so that only exposure and washing remain to be done. 



The student may make blue-prints for his own use as follows. 

 Dissolve 10 g. of potassium ferricyanide in 100 c.c. of water, and 

 13 g. of ammonium-ferric citrate in a second 100 c.c. Mix equal 

 volumes of the two solutions and filter if there is any precipitate. 

 Paint evenly over the paper with a clean cameFs-hair brush, 

 dry, and keep in a dark place until required. 



Ink. Writing ink is commonly made by adding ferrous 

 sulphate to an extract of nut-galls. The active constituent of 

 this extract is tannic acid, useful also in dyeing (p. 475) 

 and tanning leather (p. 533). Tannic acid is not, strictly speaking, 

 a single substance, but a mixture of complex phenolic acids (see 

 p. 353). JWrp^is tannate is soluble and almost cnl<">r]f*sa T but 

 is slowly oxidized, when exposed to air, to the insoluble T black 

 ferric tannate. To make the writing visible from the first, a blue 

 or black dye is added to the ink. 



Stains of fresh writing ink, being soluble, can usually be washed 

 out with water, if the latter is used at once. After the oxidation 

 has occurred, the ferric tannate must be reduced again, bv^soak- 

 ing trnp^f. fptjjjiours or longeron VrmjiQnriirn nTnlntr) nnlvjtirrn; 

 and the ink can then be washed out/^Rust stains are often ren- 

 dered soluble by ammonium oxalate also. 



yijjLl COBALT Co AND NICKEL Ni 



.. These metals, like iron, are attracted by a magnet. Cobalt, 

 |t> r\ like iron, has two series of compounds, in which it is bivalent and 

 trivalent, respectively. In its salts, nickel is bivalent only, but 

 the oxide and hydroxide Ni 2 3 and Ni(OH) 3 are also known. 



In olden tunes ores containing cobalt and nickel were frequently 

 mistaken for iron and copper ores, and were treated accordingly. 

 Failure to isolate the expected metals was regarded as due to 

 supernatural influences, hence the word cobalt is derived from the 



