856 PRINCIPLES OF CHEMISTRY 



<other metals with a layer of nickel (nickel plating). This is done by a 

 process of electro-plating, using a solution of a nickel salt. The 

 colour of cobalt is dark and redder ; it is also ductile, and has a 

 greater tensile resistance than iron. Dilute acids act very slowly oni 

 nickel and cobalt; nitric acid may be considered as the best solvent 1 

 for them. The solutions in every case contain salts corresponding with 

 the ferrous salts that is, the salts CoX 2 , NiX 2 , Correspond with the 

 suboxides of these metals. These salts in their types are similar to the 

 magnesium salts. The salts of nickel when crystallising with water 

 liave a green colour, and form bright green solutions, but in the anhy- 

 drous state they most frequently have a yellow colour. The salts of 

 cobalt are generally rose-coloured, and generally blue when in the 

 anhydrous state. Their aqueous solutions are rose-coloured. Cobaltous 

 'chloride is easily soluble in alcohol, and forms a solution of an intense 

 blue colour. 83 



dilute acids. Only nitric acid dissolves both metals well. Nickel is harder, and fuses at 

 a higher temperature than silver. For castings, a small quantity of magnesium (O'OOl 

 part by weight) is added to nickel to render it more homogeneous (just as aluminium la 

 added to steel). Nickel forms many valuable, alloys. Steel containing 8 p.c. Ni is par- 

 ticularly valuable, its limit of elasticity is higher and its hardness is greater; it is used. 

 for armour plate and other large pieces. The alloys of nickel, especially with copperand 

 zinc (melchior, see later), aluminium and silver^ although used in certain cases, are now 

 replaced by nickel-plated or nickel-deposited goods (deposited by electricity from a 

 solution of the ammonium salts). 



53 The change of colour is dependent in all probability on the combination with 

 .water, or according to others on polymeric transformation. It enables a solution of 

 cobalt chloride to be used as sympathetic ink. If something be written with cobalt 

 chloride on white paper, it will be invisible on account of the feeble colour of the solution, 

 and when dry nothing can be distinguished. If, however, the paper be heated before the 

 fire, the rose-coloured salt will be changed into a less hydrous blue salt, and the writing 

 will become quite visible, but fade away when cool. 



The change of colour which takes place in solutions of CoCl 3 under the influence not 

 only of solution in water or alcohol, but also of a change of temperature, is a character- 

 istic Of all the halogen salts of cobalt. Crystalline iodide of cobalt, CoI 3 6H 2 0, gives a 

 dark red solution between 22 0% and +20; above +20 the solution turns brown and 

 passes from olive to green, from -I- 85 to 820 the solution remains green. According to 

 Etard the change of colour is due to the fact that at first the solution contains the 

 hydrate CoI 2 H 2 0, and that above 85. it contains CoI 2 4H 2 O. These hydrates can be 

 crystallised from the solutions; the former at ordinary temperature and the latter oil 

 beating the solution. The intermediate olive colour of the solutions corresponds to the 

 incipient decomposition of the hexahydraled salt and its passage into CoI 2 4H 2 O. A 

 solution of the hexahydrated chloride of cobalt, CoCl 2 6H 2 O, is rose-coloured between 

 - 22 and -f 25 ; but the colour changes starting from + 25, and passes through all 

 the tints between red and blue right up to 50 ; a true blue solution is only obtained 

 at 65 and remains up 'to 800. This true blue solution contains another hydrate. 

 CoCl 2 2H 2 O. 



The dependence between the solubility of the iodide and chloride of cobalt and 

 he temperature is expressed by two almt>st straight lines corresponding to 'the hexa- 

 and di-hydrates ; th passage of the one into the other hydrate being expressed by a 

 \jrve. The same character ofc phenomena is Seen also in the variation of the vapour 



