288 SILICATES. 



the operations of calcination or not. The substance 

 is introduced in small fragments into a weighed cruci- 

 ble, after which the whole is weighed, then placed 

 over a gas lamp for several minutes, in order to eva- 

 porate the water and see if there is any loss indicated 

 by the balance. 



It is heated -until the weight is constant, and then 

 taken from the smaller lamp and placed over the larger 

 one, Fig. 45 ; at this temperature it may be fused, 

 decrepitated, change its color, all of which should be 

 carefully noted; when it is certain that the mineral 

 does not lose any more in weight we may proceed to 

 the analysis. 



The silicate is decomposed by the means of lime ;* 

 there should be the least possible amount of it added, 

 still, it is necessary to employ such a quantity that in 

 pulverizing the glass obtained and treating it with 

 acids the silica will take the gelatinous form. 



To decompose bottle glass it is necessary to add 

 from 10 to 20 per cent, of carbonate of lime, window 

 glass a little more of it. Wollastonite, amphibole, and 

 pyroxenes 35 per cent, of their weight ; feldspar re- 

 quires 55 per cent., and some substances containing 

 a large proportion of alumina and silica, as disthene, 

 require 75 per cent. 



As a general rule the quantity of lime to be added 

 is in proportion to the amount of silica contained in 



* [To prepare it, white marble is dissolved in nitric acid, evapo- 

 rated to dryness, ignited in a platinum crucible until the nitrate 

 begins to decompose, and caustic lime is formed on the surface. 

 It is then treated with distilled water and the thick liquid boiled 

 for some time. It is then filtered, and, when cold, an excess of 

 concentrated carbonate of ammonia is added. This is decanted 

 and washed for some time with warm water over a funnel covered 

 with a piece of cotton cloth. If there remains any nitrate of am- 

 mouia in the carbonate of lime, it will form nitrate of lime during 

 the desiccation or at the commencement of the calcination, and 

 the loss of weight which is thus caused in the carbonate will be 

 an error.] 



