1839.] 



On Chemical Tests. 



335 



ches in diameter and 3 inches deep— cover the bottom to the depth of |of 

 an inch with a paste of fluate of lime and sulphuric acid. Place in the 

 vessel a shallow platina dish supported by leaden feet and spread thinly 

 therein the siliceous matter to be acted on. Cover the vessel "with a 

 leaden lid and submit it to gentle heat^either in a sand bath, or over 

 a lamp. The mineral will be decomposed in an hour or two. During 

 the operation the powder must be moistened with a few drops of water $ 

 and when finished, concentrated sulphuric acid drop by drop must be 

 added as long as any hi-dro-fluo-silicic acid is given off, using at the same 

 time a gentle heat. Finally the sulphuric acid is to be driven off by 

 continued heat and evaporation to dryness. The dry residue is then to 

 be moistened with muriatic acid, and boiled in water. 



b. Some silicates are soluble in concentrated muriatic acid and form 

 a gelatinous mass ; on adding water, the bases previously combined 

 with the siii ic acid, are dissolved in the state of metallic chlorides, 

 and the free silicic acid remains in the form of delicate flocks. Other 

 silicaies which resist the action of acids, are converted into alkaline 

 silicates soluble in water^ by first fusing them with three times their 

 weight of carbonate of potash in a silver crucible. The fixed mass 

 being heated with concentrated muriatic acid, the silicic acid is preci- 

 pitated as a jelly, and when well washed with water (which may dis- 

 solve it) and the water is evaporated, pure silicic acid remains in the 

 form of a gritty w r hite powder without smell or taste« 



106. SILVER. Muriatic acid free, or in combination., as in common 

 salt, gives a white curdy precipitate in solutions of silver. This preci- 

 pitate turns black in the sun ; and is soluble in ammonia, which distin- 

 guishes it from the precipitate given by muriatic acid in solutions of 

 mercury or of lead. Iron, zinc, and copper precipitate silver from its 

 solutions in a metallic state. The alkalies and their carbonates also 

 act on silver, and so do several other tests, see 11 «, and 82 6. and 

 table of re-agents. 



a. Pure oxide of silver is a greyish brown powder, readily soluble 

 in nitric, and some other acids. The following tests act on solutions of 

 silver. 



b. Potash and ammonia, and carbonate of potash, carbonate of am- 

 monia and oxalic acid produce precipitates, all of which are soluble in 

 ammonia. The colour of the first two is brown ; of the three others 

 white. Phosphate of soda and phosphoric acid give a yellow precipi- 

 tate, also soluble in ammonia, but the solution when this test is used 

 must be neutral. Hydrosulphurets and sulphuretted hydrogen produce, 

 black precipitates. 



