340 Mr. Children's Summary ViexD of [May^' 



silica and 2 atmns of alumina, and a silicate of lime, in which 

 the quantity of the oxygen in the acid is equal to that in the 

 oxide ; the oxygen in the first salt is, therefore, double the oxy- 

 gen in the second. Now lime contains 2 atoms of oxygen, and 

 silica 3 atoms ; consequently to preserve the equality of oxygen 

 in the two bodies, there must be 3 atoms of lime and 2 atoms of 

 silica. The total quantity of oxygen in this silicate is, therefore, 

 12, and that in the siHcate of alumina24; but in that compound 

 there are only 6 atoms of oxygen ; therefore the salt must con- 

 tain 4 atoms of silicate of alumina. According to this, wc have 



Silicate of alumina 59*47 



Silicate of hme 40-63 



100-00 

 which are composed as follows : 



{Alumina 30-66 

 Silica 28-81 



/Lime 26-13 



\Silica 14-40 



Or, 



Alumina 30-66 



Lime 26-13 



SiHca 43.21 



Hence if the substance operated on was pure, a small portion 

 of lime has, in the analysis, been confounded with the alumina. 



As another example, and one well worthy to follow the pre- 

 ceding, we will take the analysis of a variety of topaz. The 

 results gave 



Oxygen. Atoms. 



Alumina 59 = 27-55 =« 5 



Silica 34 = 17-10 = 3 



Fluoric acid 7 = 5*09 = 1 



100 



We may consider this mineral either as a compound formed 

 by the combination of a double acid (silica and fluoric acid) 

 with alumina ; or as a double salt, consisting of one base 

 united to two different acids ; that is^ as a fluate, and a silicate 

 of alumina. In this instance, the alumina naturally divides 

 itself into two portions, whose quantities of oxygen are 3 and 2. 

 The first portion is combined with a quantity of silica, contain- 

 ing 3 atoms of oxygen, and forms a silicate ; the second is com- 

 bined with a quantity of fluoric acid, whose oxygen is 1. Hence 

 it follows that the oxygen of the first salt is to the oxygen of the 

 second in the ratio of 6 : 3, or 2 : 1. Now fluoric acid contains 

 2 atoms of oxygen, and alumina 3 ; the bi-aluminous fluate must, 



