394 DETERMINATION OF ALKALIES IN SILICATES. 



of the different alkalies from each other, and on the micro- 

 scopic examination of the same, etc. It was proved that, after 

 the caustic alkalies, the most powerful agent to attack silicates 

 at a high temperature is caustic lime, a fact not new to chemists. 

 But for the purpose of accomplishing conveniently by this 

 method a quantitative determination of the alkalies in silicates, 

 certain methods of manipulation and facts with regard to quan- 

 tity of material, admixture, etc., had to be discovered ; and 

 in them resides the success of my process — converting the most 

 difficult parts of the analysis of a silicate into the easiest. 



The methods of analysis by caustic baryta and by means 

 of its carbonate are now no longer used, for various reasons 

 fully detailed by Eose in his Analytical Chemistry. The 

 method still extensively employed is that with hydrofluoric 

 acid, proposed by Berzelius ; and when used with the necessary 

 precautions it has. seemed to decompose all silicates ; still, ac- 

 cording to Eose, there are siliceous compounds that can not be 

 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 caustic 

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

 red heat.f 



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 so- 

 lution ; to it add lime-water, or some milk of lime made from 

 pure lime, until the solution is alkaline to test-paper ; the lime 

 is added to precipitate any magnesia, phosphate of lime, etc., 

 that may have existed in the marble. Filter this solution and 

 precipitate with carbonate of ammonia after heating to at least 



* The process used by Deville in fusing with carbonate of lime is in most 

 cases better than that by hydrofluoric acid, and one that I should use in pref- 

 erence to all others except the one now under notice. 



t Chloride of calcium at a red heat will dissolve more or less caustic 

 lime. 



