460 



sii.ruih.v. 



SILVER 



SiliiridiP, or CAT-FISHES, a large family of 

 fresh-water hshes, included in the order I'hyo 

 tomi. The skin is naked, or equip|>ed with some 

 bony scutes, never with scales ; the dorsal fin is 

 short and is occasionally absent when present it 

 is inserted almve or in front of the ventrals ; there 

 is generally an adipose tin ; the maxilla is very 

 small ; the barbels are well developed. The family 

 is represented in most parts of the world. In 

 Europe, however, there is only one Siluroid, the 

 Sheat-fish or Shadden (Silurut alanit), which 

 ocean in some eastern and central regions e.g. 

 in the Danube and the Elbe. In size it sometimes 



ri>.vtlitlii 1892. IK97. and 

 1000 In Ihv I'. 8. by J. B. 

 LIppiDOotl CxHupujr. 



Silurua glanis. 



approaches the sturgeon, and is a sluggish but very 

 voracious animal, and lias lieen the subject of 

 many strange stories. In North America most of 

 the common cat-fishes belong to the genus Amiurus ; 

 in tropical America the genus I'imelodns has many 

 representatives (see CAT-FISH). In many species of 

 Arius a marine genus common in the East Indies 

 and on lioth coasts of Central America, the males 

 hatch the c;;gs in their mouths. Species of Doras, 

 common in South America, are remarkable for their 

 habit of travelling from one pond to another. In 

 northern Africa the most remarkable Siluroid is 

 the Electric Cat-fish, Malaptenirus electricus. 



Silver (sym. Ag; equiv. 108; sp. gr. between 

 10-47 and 10'57), one of the very earliest known of 

 the metals, is of a peculiar and" 

 beautiful white, colour, by which 

 it cnn be distinguished from all 

 other metals or alloy, except one or two rare 

 metals, such as lithium and indium, which are 

 seldom seen. Silver is harder than gold, but softer 

 than copper. It takes a very high ]M>lish. and for 

 this reason wa sometimes used for making small 

 mirror* by the ancient Romans. It ranks next to 

 gold in malleability and ductility, the thinnest 

 silver-leaf produced by hammering l>eing only 

 TITO'S on f *n inch thick ; and a wire of the metal can 

 be drawn so fine that a length of 130 yards weighs 

 only one grain. It has been usually stated that 

 silver has a clear ringing sound when struck, but 

 accurate observers, like Karsten and Percy, say 

 that, on the contrary, a bar of the metal emits a 

 dull sound on receiving a blow. Silver conducts 

 heat and electricity letter than any other metal, 

 for which reason it is adopted as the standard 

 represented by 100. Its melting-point has been 

 variously -laled. hut appears to be about 1904 F. 

 (1040 ('.). Silver does not suffer even from long 

 ex]><ure to the atmosphere, except that it readily 

 tarnishes when sulphuretted hydrogen or animal 

 exhalations containing sulphur are present. Both 

 nitric and sulphuric acid dissolve silver ; and aque- 

 ous hydrochloric acid, as well as a solution of com- 

 mon salt, converts the surface of the metal into the 

 chloride of silver. The metal is not attacked by 

 caustic alkalies. 



'* of Stiver. Three compounds of silver 

 with oxygen are known. Aryrntir Oxide or I'm- 

 toxide of Stiver, AgiO, is the host defined of the 

 three. If to an aqueous solution of nitrate of silver 

 lime-water or baryta- water be added, this oxide of 



silver is precipitated. The same brown precipitate 

 is obtained if pure potash or soda lie used as the 

 precipitant. The protoxide of silver when mois- 

 tened altsorbs carbonic acid from the air. It 

 decomposes and loses its oxygen at ,">7- K. (900* 

 ( . . and ignition takes place when it is rubbed in 

 a mortar with sulphide of arsenic or of antimony 

 and other easily oxidisable substances. The otliet 

 two oxides of silver are the A rgentovt Oxide or Sub- 

 oxide, AgtO, and the Peroxide, AgiO* 



Sulphide of Silifr, AgiS. The strong affinity 

 silver has for sulphur is seen from the readiness 

 with which it blackens in an atmosphere con- 

 taining sulphuretted hydrogen. A silver coin 

 can lie easily darkened by rubbing it with sulphuf 

 or by placing it in contact with vulcanised india- 

 rnblier, which contains sulphur. Sulphide of silver 

 can be prepared by melting together silver clippings 

 and sulphur in ft covered crucible. It is also 

 formed when sulphuretted hydrogen or a soluble 

 alkaline sulphide is added to an aqueous solution 

 of a salt of silver, the silver sulphide precipitating 

 as a black powder. The tarnish on silver articles 

 which from their nature cannot be easily rubbed 

 with leather and rouge or brushed can be readily 

 removed by an aqueous solution of cyanide of 

 potassium ; but as this salt is poisonous the 

 article should be afterwards carefully washed. 

 Silversmiths perfectly restore the original white 

 colour to darkened silver objects by heating them 

 in contact with carbonate of soda and nitre. 

 Statuettes and other art objects in silver are 

 frequently ' oxidised,' as it is called. This is really 

 darkening their surface more or less by a film of 

 sulphide produced by immersing them in a hot 

 solution of sulphide of potassium. The prominent 

 parts are then brightened by brushing or other- 

 wise. The native compounds of silver and sulphur 

 (ores) are noticed l>elow. 



Chloride of Silver, AgCl. This salt is prepared 

 by adding to an aqueous solution of the nitrate of 

 silver eitner hydrochloric acid or chloride of sodium 

 (common salt), when a thick, white, curdy pre- 

 cipitate of the chloride is thrown down. The 

 precipitate requires to be washed and dried in the 

 dark, and it is then an anhydrous white powder. 

 When fused and allowed to cool it becomes waxy 

 and hornlike, in which state it is translucent or 

 even transparent in thin plates. In this massive 

 condition it is still sometimes called by (lie old 

 name of horn sili-rr or I mm rnriifn. whether native 

 or artificially prepared. Chloride of silver is very 

 insoluble in water, so that an extremely small 

 proportion of silver may be detected in water by 

 the formation of chloride. It dissolves in ammonia- 

 water and cyanide of potassium or sodium. When 

 a solution of chloride of silver in ammonia-water 

 is boiled a fulminating compound is deposited ; but 

 this can be avoided by evaporating the solution at 

 a gentle heat, by wliich treatment scales of the 

 chloride separate. As commonly prepared, chloride 

 of silver blackens by exposure to daylight. Native 

 chloride of silver is an ini|Mirtant ore of the metal, 

 and is referred to below. So also are the native 

 compounds of silver with bromine nnd iodine. 



Cytniiilr i if Silver, AgCy, is obtained by precipi- 

 tation as a white powder when hydrocyanic ncid or 

 cyanide of potassium is added to an aqueous solu- 

 tion of nitrate of silver. It is insoluble in water, 

 but aqueous solutions of forrocyanide of potassium, 

 hyposulphite of soda, and ammonia and some of 

 its salts dissolve it. It forms double salts with 

 various metallic cyanides, one of these, the ry, nt<>- 

 cyanide of fHitiixxium, being of great sen-ice in 

 Kleetro-nietallnrgy(q.v.). Mr A. Wright, surgeon, 

 Kimiingham, first applied this salt to electroplat- 

 ing, his invention having been bought and patented 

 by the Messrs Elkinton in 1840. 



