38 
Proceedings of the Royal Irish Academy. 
meta-silicates of the dyad metals, the two silicates Ca'^ Mg^' Al''\ Sij 
Oi2, and J^a'a AI2 Si^ O12 ; and alnminous angite, in addition to the 
meta-silicates of the dyad metals, the silicate Mg AI2 Si Og. The latter 
may be considered as the magnesian part of the first of the two silicates 
of alumina in aluminous hornblendes. 
I shall first consider the relationship of the two groups, which the 
formula of Dr. Tschermak would more or less disconnect. Meta- silicic 
acid and meta-silicates, like met a-pho spheric acid and the meta-phos- 
phates, readily form condensed molecules without the loss of atoms. 
The series of condensed bodies thus formed are, therefore, polymeric, or 
simple multiples of the first or type acid or salt. They are merely a 
number of similar molecules, riveted together into more complex, but 
still similar molecules. All the members of a polymeric series of acids, 
or of salts of the same base, are doubtless strictly isomorphic the 
members of a condensed series of salts containing difi'erent isomorphic 
bases must also be isomorphic within certain limits. If the typical salt 
of any polymeric series of salts of the same base be dimorphic, the dimor- 
phism should extend to the whole series. In the case of condensed salts 
containing difi'erent bases, a special case of dimorphism may arise, 
which has not been heretofore noticed. Let us suppose the basic ele- 
ments in such a series of condensed salts to be the dyad metals, mag- 
nesium, iron (ferrosum), manganese, and calcium, and the salts to be 
meta-silicates. The forms of the molecules of magnesic and ferrous 
meta-silicates must more nearly resemble each other than either of 
them does the molecule of calcic meta- silicate. At least, this is 
probable from the analogy of the magnesic and ferrous sulphates, which 
crystallize with seven molecules of water, while the molecule of calcic 
sulphate takes only two molecules of water. Meta-silicate of manga- 
nese should form the link between the magnesian and ferrous salts on 
the one hand, and the calcic one on the other, if we may argue from 
the sulphate, which can crystallize with seven molecules of water, and 
with two. We should find this difference manifest itself in the angles 
of the forms of the salts, according as magnesium or calcium pre- 
dominates. That is, the condensed salts containing much magnesium 
should crystallize in a different crystalline series from those containing 
a larger amount of lime. Although slight variations in the values of 
the angles and modifications of angles or edges of the crystals always 
accompany changes in the chemical composition of bodies, the forms of 
one crystalline series never graduate into another. The limits of 
variation of the values of the angles of the figures of a series are very 
small, while the limits of change of composition consistent with the 
maintenance of the forms of a given crystalline series are very con- 
siderable. Thus, the form of ferrous sulphate remains unaltered, save 
in some slight variations of the angles, and in the character of the 
modifications, which of course belong also to the crystalline series of 
* If exceptions to this rule exist, their study would throw much light on the 
structure of condensed molecules. 
