Bartow—A Mechanical Cause of Homogeneity of Crystals. 581 
modified sort, transformation commences, but mechanical shaking 
is generally competent to bring about the change.” 
Of other dimorphous substances whose behaviour would appear 
to resemble that which has just been attributed to dimorphous 
assemblages belonging to division 2, Lehmann speaks as follows :— 
“<The rest of the substances belonging here, such as paranitro- 
phenol, occur in an unstable and a stable modification, and the 
change of the former into the latter can be effected, but not 
the converse operation, and the change takes place at a tempera- 
ture which is not definite. Hach of the two modifications has its 
definite melting point, that of the unstable one being always the 
lower.””! 
An interesting instance of a fluid mass having its symmetry 
determined by the symmetry of a portion of the same mass already 
solidified is presented by the familiar fact that from a supersatu- 
rated solution of sodium ammonium racemate, the deposit of sodium- 
ammonium dextro tartrate is produced by contact with a crystal of 
this isomeride, while that of sodium ammonium laevo tartrate is 
brought about by the presence of a crystal of the latter sense.” 
To compare with the conclusion that it is closest-packing which 
is concerned in the arrangement of the particles, we have the fact 
that in cases of polymorphism, greater stability is associated with 
greater density. 
Cleavage. 
With regard to the question as to what will determine the 
direction of rupture of solidified assemblages, a little consideration 
shows that a plane of readiest cleavage, i.e., a plane which is the locus of 
readiest separation, is not necessarily a plane the ties crossing which are 
ties of minimum strength under any given constant conditions. For 
where changes of condition are taking place which affect parts of 
different nature differently, and cause a temporary or permanent 
re-arrangement of them, conditions of sudden strain of the ties 
which bind certain of the parts to one another may arise, and bring 
about a state of things such that the place of readiest rupture is 
1Q. Lehmann ‘‘ Ueber physikalische Isomerie’’; Zeitschr. f. Kryst., 1, p. 131. 
2 Compt. Rend. 63, p. 843. 3 See Lehmann, Zeitschr. f. Kryst.. 1, p. 125- 
