4 M. Berzelius on the Chemical Composition of 
By reducing the Fluo-silicate of Lime into its elements, we 
obtain 
Silex, 
Tesselite. 
52.38 
Lime, 
24.98 
Potash, 
5.27 
Fluoric Acid, 
0.64 
Water, 
16.20 
99.47 
Apophyllite of Uto. Old Method. 
52.13 
52.90 
24.71 
25.21 
5.27 
5.27 
0.82 
... 
16.20 
16.20 
99.13 
99.38 
It appears from the experiments of MM. BonsdorfF and 
Rose, that the Fluoric Acid is often found in small quantities 
in minerals, without having any effect in altering their external 
characters ; but we are ignorant of the kind of combination 
which it forms in these minerals. I have made some attempts 
to discover it, which I shall communicate to you at another op- 
portunity ; and I believe that I have found that the fluoric acid 
combines with the silicates of certain bases, such as potash, lime, 
barytes, without displacing the silex, provided that the acid does 
not exceed what is necessary to neutralise the base, for an excess 
would give rise to the fluo-silicic gas. It follows, therefore, that 
a small quantity of fluoric acid may be absorbed by a silicate of 
potash or lime, without having its constitution sensibly altered. 
The fluo-silicate found in the apophyllites by analysis, does not 
exist as such in the mineral. It is a combination with an excess 
of base, formed by the analytical operations. Besides, I will 
not decide whether the fluoric acid is combined with the silicate of 
lime, or with that of the potash, though the first appears to be 
the most probable, considering the excess of silica combined with 
the potash, the formula which expresses the composition of apo- 
phyllite being KS ® -f- -f 16^^. 
It appears to me, that mineralogical characters drawn from 
optical phenomena presented by transparent crystallised mine- 
rals, may be derived from several different sources. 1. From 
the nature of their elements. 2. From the number of atoms 
of each element, on which depends the crystalline system to 
which the mineral belongs. 3. From accidental mixtures, often 
inconsiderable, which alter, in an essential manner, the transpa- 
rency, the form, and the crystallisation. These are what are 
called in artificial crystals impurities. To these we may add 
