VOLATILE MATERIALS IN SILICATES. 301 



at a red heat ; a tube containing chloride of calcium 

 is arranged to receive the water. This method is not 

 always adopted, because it is frequently possible to 

 determine other things with the water, but is some- 

 times useful. In case there is only water, as in the 

 zeolite, the ignition and loss of weight indicate the 

 amount of water. 



When there is any evolution of volatile materials at 

 a high temperature, these are, as stated, fluorides. 

 There are a large number of fluorides, but we shall 

 consider only those which may be expelled by calcina- 

 tion; the fluoride of silicium, the fluoride of boron, 

 and the alkaline fluorides. 



All the fluosilicates may be decomposed by heat, 

 and all the fluorides, mixed with a sufficient quantity 

 of silica, are changed into fluoride of silicium ; there- 

 fore the fluoride of silicium may be determined at once, 

 and it will be easy to determine the other fluorides. 



When a substance contains fluoride of silicium in a 

 large quantity, topaz for example, and when it is desir- 

 able to collect and determine this fluoride, the following 

 method may be used : take three platinum crucibles, 

 a large, medium, and small one. In the small one, 

 which has been weighed, the material to be ignited is 

 placed and weighed ; over the small crucible, covered 

 with its lid, the medium crucible is inverted so as to 

 form a cap, and finally the two crucibles thus arranged 

 are placed in the large crucible ; the whole are weighed 

 together in such a way that the weight of the appa- 

 ratus, less that of t]ne topaz, may be ascertained. Then 

 pour between the last two crucibles a certain quantity 

 of carbonate of lime and weigh it. This gives the 

 weight of the apparatus, the topaz, and the carbonate of 

 lime. 



The whole is heated to a red heat, the carbonate of 

 lime is reduced to quicklime, and it is heated for a 

 long time over the large lamp until the fluoride of 

 26 



