Bartow—A Wechanical Cause of Homogeneity of Crystals. 579 
in an assemblage consisting of two or more kinds of balls, if the 
different balls expand or contract at different rates when subjected 
to some external change of conditions. 
The behaviour of sulphur is just what one might expect of an 
assemblage of mutually-repellent particles when subjected to such 
changes of conditions as will produce polymorphism belonging to 
class 6 of division 1.1. Thus Lehmann says of this body :—“ Sul- 
phur, as we know, crystallizes from a melted mass, or out of a hot 
solution, in monosymmetric forms which, as the cooling proceeds, 
are transformed more or less rapidly into the rhombic modification, 
and conversely, the latter when heated passes to the monosymmetric 
form, as is perceived by the turbidity which displays itself, or still 
better by the change of colour of a thin lamina in polarized light. 
And it is easy to keep the temperature at such a point that the 
boundary of one or the other modification gives way at the slightest 
change in one or the other direction, .¢., as slight cooling or warm- 
ing occurs.’” 
And, quoting further from the same anthor:—“ If very hot 
melted sulphur is rapidly cooled the viscous modification is obtained. 
In a mass of this kind two kinds of crystals are gradually formed, 
the characteristically bent ones of a yet unknown modification, and 
monosymmetric ones which are more spherolitic. On being touched 
the first are rapidly transformed into the latter. And on being 
heated they disappear first, and therefore have the lower melting 
point. Ifrhombic sulphur is brought in contact with the viscous 
mass the rhombic crystals grow in the form of fine branched and 
bent skeleton crystals; they are, however, quite distinguishable 
from those of the unstable form just mentioned, especially when 
the warming takes place gradually, in which case they grow to 
fine pyramidal groups, while the latter develop to small leaf-like 
erystals.’” 
As another instance resembling the polymorphism of class }, 
division 1, we have ammonium nitrate which presents four forms, 
1 This explanation of the properties of sulphur requires the presence in it of ut 
least two kinds of ultimate parts or particles. If this is regarded as an objection, there 
is the alternative that similar effects may arise from dimorphism due to linkage. (See 
note 1, p. 576). 
* Zeitschr. f. Kryst., 1, p. 112. 3 Tbid., 1, p. 128. 
