DETERMINATION OF ALKALIES IN SILICATES. 395 



160° Fah.* 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 per- 

 fectly pure, or if it contain any impurity it will be a trace 

 of carbonate of baryta or strontia, which in no way interferes 

 with its use. 



Sal ammoniac. — To obtain this re-agent in the most con- 

 venient form take some fragments of clean sublimed sal am- 

 moniac, dissolve 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 themselves stir the solution to keep them small ; when 

 half or two thirds of the sal ammoniac is deposited pour off the 

 liquid without waiting for it to cool, throw on a cotton filter, 

 and dry the crystals at the temperature of the atmosphere. In 

 this way sal ammoniac is obtained that can be easily pulverized. 



Vessel for the decomposition. — The ordinary platinum crucible 

 can be used for this purpose, and for many years was employed 

 by me. It was found, however, that while it is the best hitherto 

 contrived both for precision and ease, there was yet a very 

 minute quantity of alkalies lost by volatilization ; and I made 

 further researches to overcome this small loss. This I have 

 successfully accomplished, and for some time I have used an 

 improved form of crucible. The one for half to one gramme of 

 silicate is of the following form and dimensions; viz., an elon- 

 gated, slightly conical'crucible with rounded bottom and cover 

 (either with or without the central wire by which to hold it) ; 

 length, ninety -five millimetres; diameter of opening, twenty - 

 two millimetres; diameter of smaller end just at the turn of 

 the bottom sixteen millimetres ; weight about thirty-five to forty 

 grammes. These are now made by Messrs. Johnson, Matthey 



* This precaution must not be overlooked, as it is desirable to obtain the 

 precipitated carbonate of lime as dense as possible. If the carbonate of am- 

 monia be added to the cold solution, the precipitate, at first gelatinous, will 

 ultimately become much more dense and settle readily ; the same is true 

 if the mixture be heated after the addition of the carbonate ; but in neither 

 case will it be as dense as when the carbonate is added to the hot solution 

 of chloride of calcium. The reaction in the analysis is in no way affected 

 by the form of the carbonate of lime ; but by using the denser form the mix- 

 ture occupies less space in the crucible. 



