874 



CHEMISTRY. 



[MKBCUKY. 



nult in an economical manner, hare already been re- 

 ferred to in the article on Electro-Metallurgy. 



The nalu, \o., it silver, are of considerable importance. 

 In fact, at the preeent time, they have become an exten- 

 sive article of commerce, for the purpose* of photography. 

 Their * in thU respect have already been fully de- 

 embed under the head of Photography, in the ration on 

 Light ; to which we raurt direct pur reader* for infor- 

 mation on that iutorvating application of optical ami 



SUrer combine* with oxygen to form three oxidea. 

 The tub-oxide, Ag O, U of little importance. The pro- 

 toxide, Ag O, U the bate of the aalU of the metal, and it 

 produced by adding a solution of potass or lime to the 

 solution of the nitrate of silver ; when a brown-coloured 

 powder U produced. The peroxide, Ag O,, is the result 

 of electric action in bad-conducting solutions of the 

 nitrate : it* production often interferes with the regu- 

 larity of electro-plating processes. The chloride, Ag Cl, 

 has been frequently referred to in our previous pages ; 

 both under the head of Photography, and as an illus- 

 tration of chemical changes. The cyanide of silver has 

 also been fully described, as employed in electro-plating, 

 and with respect to its constitution and manufacture. 

 The iodide and bromide of silver have no interest except 

 in connection with photography ; and their uses and 

 nature in that respect have been described in connection 

 with that subject 



The nitrate of silver, as we have already remarked, is 

 a substance of great importance. Besides its uses in 

 igraphy, it is employed in medicine in a fused state, 

 as the " lunar caustic" of the shops ; and most kinds of 

 ink, used for marking linen, have this salt as their basis. 

 Nitrate of silver is produced by dissolving pure silver, 

 by heat, in dilute nitric acid ; from this solution it may 

 be readily crystallised. Its solutions turn black on ex- 

 posure to linht, if they contain any animal matter ; hence 

 their employment as hair-dyes. The sulphate of silver 

 is produced by boiling silver with strong sulphuric acid. 

 The carbonate results from a mixture of carbonate of 

 soda with nitrate of silver, both being in solution. The 

 acetate may bo similarly produced oy substituting the 

 acetate of soda. We have mentioned that polished sil- 

 ver readily tarnishes when exposed to the action of 

 sulphur ; hence plate, spoons, Arc , soon lose their 

 polish ; and for a similar reason, silver spoons, if not 

 gilded, become black on being used with eggs or mus- 

 tard ; each of which contains sulphur as one of their con- 

 stituent*. 



Fulminating silver is a compound of the metal with 

 nitrogen and hydrogen ; and it is produced by adding 

 alcohol to silver dissolved in strong nitric acid, when 

 a powder is produced, which explodes violently on the 

 slightest friction. We shall describe the process more 

 fully when speaking of the fulminate of mercury. An 

 explosive comjiound of silver is produced by adding 

 lic|iiid ammonia to the oxide of silver ; forming what is 

 termed the ammouiuretted oxide. This, however, is not 

 a fulminate of the metal. 



MERCURY. 



Tuts well-known metal fonns an exception to the general 

 rale that such bodies are solid, inasmuch, as at ordinary 

 temperatures, it is in a fluid condition. In the Arctic 

 regions, however, it is frequently observed in the solid 

 state, owing to the abstraction of its latent heat by 

 von* of the intense cold prevalent in those latitudes. 

 I 'ii the readiness with which this metal moves 

 about, it has acquired the name of " quicksilver," by 

 which it U generally known in the art* and in oommeroe. 

 It* specific gravity is about 13 '6 ; it* symbol, Ug ; and 

 UM equivalent -'lo 



Our chief sources of mercury are South America and 

 Kjin, from which countries it is imported in wrought- 

 inni Imitle*. It is obtained from its ores by the process 

 <>f ,i-. -i i , !!!. just as spirit* or water are distil!.,! 

 by UM application of heat The oxides of mercury are, 

 in fact, readily decomposed by heat, the attraction be- 



tween tlio metal and the oxygen being comparatively 

 slight It boils at a temperature of about (ifXr Fall, nml 

 it* vapour has a highly expansive power. This 

 circumstance affords a ready means of separating 

 it from various impurities with which it is often con- 

 taminated. The mercury is introduced into a retort, 

 and a moderate heat is applied. The quicksilver is by 

 this |il:in \i 'Utilised, and condenses into small globules, 

 which run into a receiver. The impurities, such as lead, 

 tin, and bismuth, are left behind in the body of the 

 retort, and are so separated from the mercury. 



The uses of the metal are very numerous. In Brazil, 

 California, Australia, and other countries in which gold- 

 sands abound, mercury is used to dissolve out the fine 

 grains, which are too small to be seen by the naked eye. 

 The amalgam of the gold so formed is then introduced 

 into a retort, and heat is applied. The mercury distils 

 over, and leaves the gold pure. The metal silver is often 

 separated from its ores by similar means, and the pro- 

 cess is termed "amalgamation." There are many gold 

 mines in several parts of the world, which would be 

 utterly valueless but for this singular property of quick- 

 silver. 



We may here caution our readers to be careful lest 

 any gold or silver ornament or coin should come in 

 contact with the metal, for it soaks into them just as 

 water enters the pores of loaf-sugar. They are thereby 

 rendered very brittle ; and if exposed to any mechanical 

 action, are sure to break. In the case of mercury being 

 absorbed by either of the metals named, the best plan is 

 to place them on a piece of iron over a common fire, by 

 which the mercury will be at once driven off, and the 

 article restored to its old state. 



Quicksilver is largely employed for "silvering" look- 

 ing-glasses. A sheet of plate-glass, previously well 

 cleaned, is placed on a board having a slight inclina- 

 tion. A piece of tin-foil is then pressed on to the face 

 of the glass, am. over this some quicksilver is spread 

 by means of a hare's foot. A sheet of stout paper is 

 then placed on the foil ; and on this heavy weights are 

 put for the purpose of driving off any superfluity of 

 mercury. After a day or twos time the weights are 

 removed, when it is found that the amalgam has ad- 

 hered to the glass, and has so afforded it a reflecting 

 surface. 



As we have already mentioned, amalgams are formed 

 by mixing mercury and other metals together. The 

 amalgams of gold and silver are used for plating pur- 

 poses. The metal to be plated is coated with the atnal- 

 gam, and afterwards exposed to heat, by which the 

 mercury is driven off, and the gold or silver left on tho 

 surface. This process is called water-gilding and plutiii','. 

 It has, however, gone much out of use since the intro- 

 duction of the electrotype process. An amalgam of 

 silver U employed by dentists for the purpose of tilling 

 hollow teeth. 



The metal is also much used in making barometers 

 and thermometers. It is of great value for the former 

 purpose, owing to its fluidity and great specific gravity. 

 Indeed, a column of mercury of a height of thirty 

 inches, balances one of water thirty-two feet high. In 

 thermometers, it shows tho increase of temperature from 

 very intense cold to its boiling point, by a gradual and 

 almost regular expansion more so, in fact, than any 

 other liquid with which we are acquainted. 



The combinations of mercury are both numerous and 

 valuable, and of considerable use in the arts, mi 'ilic.ine, 

 and commerce. It forms two definite combin.-itioiis with 

 oxygen. The sub. or black oxide, U produced by adding 

 a solution of potash or soda to those of the sub-nitrate 

 or sub-chloride of the metal, and is so obtained as a 

 d.-irk-brown or black powder. The protoxide is pro- 

 duced by adding the solutions of the alkalies or alkaline 

 earths to those of chloride of mercury, or corrosive 

 mi Mi mate, as it is frequently termed. It is of a red 

 colour; and the oxygen may be separated from thu 

 metal by distillation, when the latter passes over in a 

 free state. Mercury resembles the metals we have pre- 

 viously described in this respect Tho protoxide may , 



