xxviii INTRODUCTIOK 



The bar drawn through some of the symbols, as in the above, 'ke, ±1, 



Mn, it, &c., indicate that the substances they represent are in the state of a 

 sesquioxide ; or, in other words, that two atoms of the base (represented by 

 the letters) are combined with three atoms of oxygen (represented by the 

 dots). By using the above form of expression, the symbols are rendered 

 much simpler than would be the case if figures and letters were employed 

 to represent the oxygen. 



The letters R and ir are used to denote one or all of those simple sub- 

 stances which can be substituted for each other in a mineral without effecting 

 any essential change in the outward form of the crystal, and which are, 

 therefore, said to be isomorphous with each other (i. e. to possess similar 

 forms). These substances are Iron, Manganese, Lime, and Magnesia. 



Sulphur has, in a few instances, been represented by a dash placed above 

 the symbol ; thus Iron Pyrites, or Bisulphide of Iron, may be represented 



either by FeS^, or by Fe. 



The small figures in the formulae imply that they only refer to the symbol 



which they immediately follow : thus, in 3±lSi^, the small figure ^ placed 



after Si means that it applies only to the Silica ; while the figure 3 placed 

 hefore the formula denotes that it has reference to all the succeeding sym- 

 bols, which, written in full, would then become 3A:l + 3Si^ or 3M + 6 Si. 

 When symbols are joined, it means that they are in a state of chemical 



combination : thus Si:3:l denotes that the silica and alumina are combined 

 in the form of silicate of alumina. 



