270 J. L. Smith — Determination of the Alkalies in Silicates. 



posed by Berzeliiis ; and when used with the necessary precau- 

 tions, it has seemed to decompose all silicates ; still, according to 

 Eose, there are siliceous compounds that cannot he completely 

 decomposed by hydrofluoric acid.* 



Dismissing all criticism, I at once proceed to the method 

 which is the subject of this article, viz : the Decomposition of 

 Silicates by ignition with Carbonate of Lime and Sal-ammoniac. 

 A mixture of carbonate of lime and sal-ammoniac is used in 

 the decomposition simply for the purpose of bringing the caus- 

 tic lime to act, in a most thorough manner, upon the silicates at 

 red heatf 



Pure carbonate of lime. — The first requisite is pure carbonate 

 of lime. This is made in my laboratory, as follows : Take as 

 good marble (calcite) as can be conveniently found, and dissolve it 

 in hydrochloric acid (it is not necessary that the acid be perfectly 

 pure), add an excess of the marble and warm the solution ; to 

 it add lime water or some milk of lime made from pure hme, 

 until the solution is alkaline to test-paper ; the hme is added 

 to precipitate any magnesia, ^phosphate of lime, &c., that may 

 have existed in the marble. Filter this solution and precipitate 

 with carbonate of ammonia, after heating to at least 160° F.| 

 The carbonate of lime thus precipitated is to be thrown on a 

 filter and well washed with distilled water. Thus prepared, 

 the carbonate of lime is a dense powder and perfectly pure, or 

 if it contain any impurity, it will be a trace of carbonate of ba- 

 ryta or strontia, which in no way interferes with its use. 



Sal-ammoniac— l^o obtain this reagent in the most convenient 

 form, take some fragments of clean sublimed sal-ammomac, dis- 

 solve them in water with a gentle heat, filter, evaporate the 

 filtrate over a steam bath or a sand bath, or by means of any 

 other convenient gentle heat, and as the crystals deposit them- 

 selves, stir the solution to keep them small ; when half or two- 

 thirds of the sal-ammoniac is deposited, pour off the liquid with- 

 out waiting for it to cool, throw on a cotton filter, and dry the 

 tals at the temperature of the atmosphere. In this way, 

 immoniac is obtained that can be easily pulverized. 



Vessel f<yr the decomposition.— The ordinary platinum cruciDie 



1 be used for this purpose, and for many years was employea d> 



The process used by Deville, in fusing with carbonate of lime, is in ^^ ^ 

 ter than that by hydrofluoric acid, and one that I should use in preteren 



Chloride of calcium at a red heat will dissolve more or less '^"^j^'^g^precipi- 

 id carbonate of Ume as dense^as possible. If the carbonate of ammoma^ 

 ed to the cold solution, the precipitate, at first gelatinous, will .ultimately ^^^ 

 r.?"^.'l™.«Ifi-- -^ _setele readily ; the «ame is true jf the f ixt^^^dense ^ 



^: 



