
CRYSTALLINE STRUCTURE BY COMPRESSION AND TRACTION. 909 
tween its particles; but when the portion of soap is drawn out into a thread, 
these resistances to crystalline arrangement are diminished; elementary prisms, 
or crystals whose length is greater than their breadth, will have a tendency to 
place their greatest length in the line of traction, and all lateral obstruction 
to the play of its natural polarities being to a great extent removed, when the 
substance is drawn into a capillary thread the molecules will have free scope to 
assume their natural crystalline arrangement. 
The application of these views to the powders and particles of hard crystals, 
is not so readily apprehended; but when we consider that the pressure brings 
the molecules of the substance within the sphere of their polarities, and that the 
force of traction reduces the compressed film into separate streaks and lines, like 
the threads of the almond soap, we have reason to conclude, that even in hard 
substances the atoms. when released from their lateral adhesions, and brought 
into narrow lines, will assume the crystalline arrangement. 
In the course of these experiments, I have observed, in some cases where the 
crystalline arrangement was very imperfectly effected, a tendency in the atoms to 
quit their position, as if they were in a state of unnatural constraint, like the par- 
ticles of silex and manganese in certain kinds of glass which experience a slow de- 
composition. If this should prove to be the case, either partially or generally, 
which time only can shew, it will doubtless arise from the non-homologous sides 
of the elementary atoms having come into contact, a condition of the crystalline 
lines perfectly compatible with the existence of neutral and depolarising axes, 
and of the colours of polarised light, provided that the non-homologous sides 
in contact deviate from their proper position, either 90° or 180°. If we cut a plate 
of mica, for example, into two pieces, and combine them by turning one of them 
round 90° or 180’, polarised light transmitted through them perpendicularly, will 
exhibit the same colours as when they were in their natural position, and also 
the same neutral and depolarising axes. If the polarised light is transmitted 
obliquely, the hemitropism of the combination, as we may call it, will be in- 
stantly discovered by the difference of colour of the two plates. 
St LEonarp’s CoLieGce, St ANDREWS, 
February 25, 1853. 
VOL. XX. PART IV. 7M 
