660 Scientific Proceedings, Royal Dublin Society. 
mother liquor to evaporate the same two kinds of crystals separated 
and the solution preserved its composition unchanged as is shown 
by No. 6. Such behaviour is, however, only possible if the 
deposited crystals have the same average composition as the 
dissolved matter.* 
In connection with the argument here submitted that the 
conditions competent to produce isomorphism and intercalation of 
similarly-orientated homogeneous assemblages are so little special- 
ized, it is interesting to notice that during recent years a 
considerable number of examples have been recorded of substances 
which form isomorphous mixtures, but are at the same time very 
slightly, or not at all related in a chemical sense.’ 
It is conceivable that in some cases one of two kinds of assem- 
blages entering into a series of intercalated mixtures may always, 
whatever the proportions in which the assemblages are intermixed, 
take the lead in solidifying, and when this is the case, and there is 
some small discrepancy between the forms of internal symmetry of 
the two assemblages, which may or may not affect the angles 
between the planes of centres, we may find that the assemblage 
which thus takes the lead causes some modification of the form of 
the other assemblage, the latter accommodating its symmetry to 
that of the earlier solidified one to which it attaches itself. 
The entire mixture of assemblages will, in such a case, as to 
some of its properties, present a resemblance to the kind of assem- 
blage which thus takes the lead. : 
In this connection a case may be mentioned in which the 
optical properties of a mixed crystal resemble those of one of the 
constituents only, thus furnishing an exception to the laws of 
Mallard and Dufet. The tartrates of ammonium and thallium 
are completely isomorphous, both belong to the mono-symmetric 
system, show nearly identical angles, and crystallize together in all 
proportions. The ammonium salt has a cleavage parallel to the 
basal plane, the optic axes lie in the plane of symmetry, and the 
optic axial angle is 42°38’. The thallium salt, on the other hand, 
shows no cleavage, the optic axial plane is perpendicular to the 
1 Pope’s translation of Fock’s ‘‘ Chemical Crystallography,’’ p. 121. 
2 Ibid. pa Lele 
