COB 



COB 



used at the rudder's head, and about the 

 pumps at the decks, that no water may 

 go downjthere. 



COATING, in chemistry, is used prin- 

 cipally for the purpose of defending 1 cer- 

 tain vessels from the immediate action of 

 fire ; thus, glass retorts, and the inside 

 of some furnaces, are coated with various 

 compositions. 



COATING, in electricity, means the 

 covering of electric bodies with conduc- 

 tors, or \he 'utter with the former, or, 

 lastly, electrics with other electi'ics. 

 Electrics are coated with conductors 

 for the purpose of communicating to, 

 or removing from their surfaces, the 

 electric fluid in an easy and expeditious 

 manner ; otherwise an electric body, 

 on account of its non-conducting pro- 

 perty, cannot be electrified or deprived 

 of the electric fluid, without touching 

 almost every point of ts surface with an 

 electrified or other body. This coating 

 generally consists of tin-foil, sheet-lead, 

 gi't-paper, gold-leaf, silver-leaf, or other 

 metallic body, either in the form of a thin 

 extended lamina, or in small grains, such 

 as brass filings and leaden shot. The 

 coating may be fastened to the surface of 

 the electric by means of paste, glue, wax, 

 or other adhesive matter. 



COBALT, in chemistry, a metal, when 

 pure, of a white colour, inclining to bluish 

 or steel grey. At the common tempera- 

 ture its specific gravity is more than 8.5. 

 It is attracted by the magnetic needle, 

 and is itself capable of polarity. For 

 fusion it requires nearly the same intensi- 

 ty of heat as cast iron. In a state of oxide, 

 it tinges the saline vitreous fluxes of a 

 deep blue colour. It is soluble in nitro- 

 muriatic acid, and the diluted solution 

 forms a blue sympathetic ink. Cobalt 

 occurs in nature alloyed with other me- 

 tals, and mineralized by oxygen, and 

 by arsenic acid. The white cobalt ore 

 is an alloy of cobalt and arsenic, with 

 a little sulphur, and in some specimens 

 a little iron, the two latter being proba- 

 bly accidental. One variety, analyzed by 

 Klaproth, gave 44 of cobalt, 55.5 of arse- 

 nic, and 0.5 of sulphur Its colour is tin- 

 white, liable, however, to tarnish, and 

 thus to assume a grey or reddish tinge : 

 its lustre is weakly shining and metallic. 



The grey cobalt ore, as it has been 

 named, is an alloy of cobalt with arsenic 

 ancl )ron ; sometimes, also, ;is ba ; been 

 affirmed, with small portions of mckel 

 and bismuth. Its colour is light grey, 

 but very liable to tarn : sh ; its lustre 

 weakly shining and metallic. Exposed 



to the flame of the blow-pipe, it gives 

 an arsenical odour and smoke, but with- 

 out melting: to borax it gives a blue 

 colour, and is reduced to a metallic 

 globule. The native oxide of cobalt oc- 

 curs in a powdery form, or of various 

 degrees of induration, but always dull, 

 and earthy in its fracture, soft, and easily 

 broken. It is also of different colours, 

 from the intermixture of oxide of iron and 

 perhaps other metallic ox'des : whence 

 even species have been formed and dis- 

 tinguished by the names of black cobalt 

 ochre, brown cobaU ochre, and yellow co- 

 balt ochre. Of these the black appears 

 to be the oxide of cobalt in its purest 

 state. They all give a blue colour to 

 glass, or to borax, when fused with it by 

 the blow-pipe. Sometimes also they ex- 

 hale an arsenical odour. The last species 

 is that in which cobalt is mineralized by 

 arsenic acid, the principal variety of 

 which has been named peach-bloom co- 

 balt ore. This name it derives from its 

 colour, which is a beautiful red, simi- 

 lar to that of the peach blossom, passing, 

 however, into other shades of red, and 

 from decomposition into other colours. 

 The ores of cobalt are easily distinguish- 

 ed from all others, by their property of 

 communicating- to borax or to glass, when 

 fust:d with them, a deep blue colour ; and 

 by their solution in nitro-muriauc acid, 

 being a sympathetic ink, lines traced 

 with it on paper not being visible when 

 cold, but becoming visible on exposure 

 to a moderate heat. 



On a large scale, cobalt is extracted 

 from its ores only in the state of an oxide, 

 without being reduced to the metallic 

 form, not only as this reduction is dif- 

 ficult, but also as the metal is not appli- 

 ed to any use. The ore is roasted, by which 

 the sulphur ancl arsenicare expelled, and 

 any fusible metal mixed with it is melted 

 out. The cobalt remains in the state of 

 an impure oxide, named zaflfre. The 

 zafTre of commerce is always mixed with 

 silicious earth ; hence, if exposed to a 

 strong heat, it vitrifies? a glass of a dark 

 blue colour is thus formed, named smalt, 

 which is used on account of its colovir in 

 various arts. It is from the zafTre of 

 commerce that the chemist obtains co- 

 balt ; to obtain it pure, however, is ex- 

 tremely difficult. The common process 

 js ; to mix the zaffre with three times its 

 weight of black flux, a small quantity of 

 oil, and a little sea salt, and expose the 

 mixtiire in a crucible to a strong white 

 heat for some hours. A metallic button 

 is thus obtained, on cooling, at the bot- 



