clarke.] INTRODUCTION. 11 
discovery of new data, to which, however, the formulae themselves may- 
give clews. This sort of uncertainty is peculiarly common among the 
hydrous silicates, and often arises from the difficulty of discriminating 
between water of crystallization, so called, and constitutional hydroxy!. 
If in every silicate analysis the state of hydration were carefully exam- 
ined, the conditions of dehydration being observed at different temper- 
atures, much of this obscurity would be avoided. 
In discriminating betweeu rival formulae, one rule is provisionally 
admissible. Other things being equal, a symmetrical formula is more 
probable than one which is unsymmetrical. Symmetry in a molecule 
conduces to stability; most of the silicates are exceedingly stable; and 
hence symmetry is to be expected. This rule has presumptive value 
only, as an aid to judgment, and can not be held rigidly. It expresses 
a probability, but gives no proof. In a problem like that of the sili- 
cates, however, even a suggestion of this kind may render legitimate 
assistance. 
