182 SCIENTIFIC AMUSEMENTS. 



smells of ammonia after shaking, and turns a red litmus paper blue. 

 Now add some solution of chloride of calcium and warm the whole ; 

 a precipitate will be formed which will by and by become 

 granular and sandy ; this consists of tartrate of calcium. Wash 

 this three or four times by decantation (Expt. 120) and then trans- 

 fer the wet mass to a flask, add two or three drops of ammonia solu- 

 tion, and a crystal of solid nitrate of silver the size of a pea, or more 

 if necessary. Now gently warm the flask over a lamp, moving it 

 about so that the pulpy material may flow over the whole of the 

 interior surface in succession; when the temperature rises suffi- 

 ciently, decomposition will take place rapidly and the interior of 

 the flask will be coated with a film of bright silver. Let the flask 

 cool and carefully wash out the interior with a little cold water ; 

 drain mouth downwards, and allow it to stand in a warm room a 

 few hours so as to dry the interior thoroughly. If the silvering has 

 been properly done the flask may be corked and the cork waxed 

 over to exclude air, and the mirror preserved for years ; but if air 

 containing sulphuretted hydrogen gets access to it, the silver will 

 soon be attacked and the mirror spoilt. Some little dexterity and 

 practice is requisite to coat a large vessel thus ; but a small flask 

 (holding from two to six ounces) can be easily silvered after one or 

 two preliminary trials. To clean the silver off a flask partly 

 silvered, but not satisfactorily, put a teaspoonful of diluted nitric 

 acid into the flask and slightly warm it, bringing the acid succes- 

 sively in contact with every part of the interior by turning the 

 flask round ; the film of silver will thus be removed and the flask 

 made fit for another experiment, after well rinsing out with water. 

 In the actual manufacture of mirrors various other chemicals are 

 employed along with silver solutions, the effect in all cases being 

 the " reduction " of metallic silver as a brilliant film. 



Expt. 199. To prepare Spongy Platinum. Metallic platinum 

 can be prepared in the form of a loose powdery substance termed 

 " spongy " platinum by mixing together solutions of platinum chlo- 

 ride and salammoniac (chloride of ammonium) ; combination first 

 takes place between the two materials forming a yellow solid sub- 

 stance sparingly soluble in water, which consequently precipitates : 

 the mixture is made in an evaporating basin, and the whole eva- 

 porated to dryness over a steam bath (Expt. 89). The dry mass 

 is heated red hot in a porcelain crucible, when decomposition takes 

 place, various gases and vapours being evolved, and nothing but 

 spongy platinum being left. 



Expt. 200. To prepare Coal Gas. The ordinary illuminating 

 gas used in our houses is prepared by heating coal in closed 

 vessels, under which circumstances a complicated decomposition 



