Bartow—A Mechanical Cause of Homogeneity of Crystals. 657 
And again, if the properties of an assemblage are influenced 
by the mere presence of another assemblage intermixed with it, 
the priority in crystallization of one of the assemblages, which has 
just been referred to, may depend on the proportions in which they 
are roughly intermixed, or the same cause may in some other 
way determine which of two dimorphous forms one of them shall 
assume, and consequently whether they shall be isomorphous or not. 
It has just been said that the proportions in which the con- 
stituents of different kinds are added to a solid surface growing in 
a mixed collection of isomorphous assemblages, will depend, not 
only on the proportions in which they occur in the mixture, but 
also on the relative rates at which the different kinds of assem- 
blage are prepared to solidify at any given point. It may be 
added that the greater any discrepancy of form between two 
assemblages which are nevertheless practically isomorphous and 
capable of close-packed intercalation as above explained, the more 
subject to fluctuation will be the conditions of solidification of a 
mixed mass of them as the proportions of the two kinds present 
are changed. 
For we have already concluded that in a solidifying assemblage 
the place of maximum tranquillity is the place of readiest solidifi- 
cation,’ and if the assemblages intercalated do not fit together 
very well, the places of their junction will be places of less tran- 
quillity, and the effect of these on each kind of assemblage present 
will vary according to the proportions in which they occur. 
Suppose that we take a series of mixtures composed of different 
proportions of two isomorphous assemblages which are capable of 
solidifying together, but between whose forms there is some con- 
siderable discrepancy, the series being arranged so that succeeding 
terms contain more and more of one constituent, and less and less 
of the other, one assemblage unmixed standing at one end of the 
series, and the other assemblage also unmixed standing at the 
other end. 
It is then evident that when we examine the behaviour of the 
combinations forming such a series near the solidifying point, we 
can discriminate at least two classes of cases :— 
1 See pp. 565 and 623. 
