398 DETERMINATION OE ALKALIES IN SILICATES. 



lime slacked into a h} r drate, and some of the lime combined 

 with the silica and other ingredients of the silicate in an im- 

 palpable form. In solution there is an excess of chloride of 

 calcium formed in the operation, and all the alkalies originally 

 contained in the mineral, as chlorides. All that now remains 

 to be done is to filter and separate the lime as carbonate, and 

 nothing is left but the chlorides of the alkalies. To do this 

 proceed as follows: throw the contents of the capsule on a 

 filter, the best size of which for the quantity above specified 

 is one three to three and a half inches in diameter; wash well, 

 to do which requires about two hundred centimetres of water; 

 the washing is executed rapidly. The contents of the filter 

 (except in those cases where the amount of the mineral is very 

 small, and there is no more for the estimation of the other 

 constituents) are of no use, unless it be desired to heat again 

 to see if any alkali still remains in it. 



The filtrate contains in solution all the alkalies of the min- 

 eral, together with some chloride of calcium and caustic lime; 

 to this solution, after it has been thrown into a platinum or 

 porcelain capsule, is added a solution of pure carbonate of am- 

 monia (an amount equal to about one and a half grammes is 

 required). This precipitates all the lime as carbonate ; it is 

 not, however, filtered immediately, but is evaporated over a 

 water-bath to about forty centimetres, and to this is added 

 again a little carbonate of ammonia and a few drops of caustic 

 ammonia to precipitate the little lime that is redissolved by the 

 action of the sal ammoniac on the carbonate of lime; filter on a 

 small filter (two-inch), which is readily and thoroughly washed 

 with but little water, and the filtrate allowed to run into a small 

 beaker. In this filtrate are all the alkalies, as chlorides and a 

 little sal ammoniac. Add a drop of carbonate of ammonia to 

 make sure that no lime is present. Evaporate over a water- 

 bath in a tared platinum dish, in which the alkalies are to be 

 weighed; the capsule used is about sixty centimetres' capacity, 

 and during the evaporation is never filled to more than two 

 thirds of its capacity. 



After the filtrate has been evaporated to dryness the bottom 

 of the dish is dried, and on a proper support heated very gently 

 by a Bunsen flame to drive off the little sal ammoniac. It is 

 well to cover the capsule with a piece of thin platinum to pre- 



