264 On the Production of Crystalline Structure. 



paper, may produce enough of electricity to have some share in 

 giving the same direction to the axes of the particles. 



When a portion of almond soap is placed upon glass, the axes 

 of its particles lie in every direction, and have no tendency to 

 assume the crystalline arrangement. The forces of aggregation 

 emanating from three rectangular axes, are not strong enough 

 to overcome the inertia, as we may call it, arising from the 

 natural quaquavei-sus adhesiveness of the substance, and from the 

 water interposed between 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 capil- 

 lary 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 con- 

 sider that the pressure brings the molecules of the substance 

 within the sphere of their polarities, and that the force of trac- 

 tion 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 particles of 

 silex and manganese in certain kinds of glass which experience 

 a slow decomposition. If this should prove to be the case, either 

 partially or generally, which time only can show, it will doubt- 

 less 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 de- 

 polarizing axes, and of the colours of polarized 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°, polarized light transmitted through 

 them perpendicularly will exhibit the same colours as when they 

 were in their natural position, and also the same neutral and 

 depolarizing axes. If the polarized light is transmitted obliquely, 

 the hemitropism of the combination, as we may call it, will be 

 instantly discovered by the difference of colour of the two plates. 



St. Leonard's College, St. Andrews, 

 February 25, 1853. 



