On Double Refraction. 29 Qi 



arising from the molecules being brought Into closer contact, and the 

 regular double refraction from the variable density impressed upon 

 each elastic molecule, and symmetrically related to the axis of press- 

 ure. The effect thus produced on the resinous mass is precisely the 

 same as what would take place by subjecting elastic spheres to a regu- 

 lar compressing force. The axis of pressure becomes an axis of posi- 

 tive double refraction, and the double refraction increases with the in- 

 clination of the ray to the axis, and becomes a maximum in the equa- 

 tor of the molecules. 



By this view of the preceding facts, we are led to a very simple 

 explanation of the origin and general phenomena of double refraction 

 in regular crystals. That this property is not inherent in the mole- 

 cules themselves may be easily proved. The particles of silex, for 

 example, do not possess it in their separate state. In tabasheer, in 

 many opals, and in melted quartz, there is not the slightest trace of 

 the doubly refracting structure : but when the particles of silex in 

 solution are allowed to combine, in virtue of their polarities or mutual 

 affinities, they then instantly acquire, at the moment of their combi- 

 nation, the property of double refraction, and they retain it while 

 they continue in this state of aggregation. The manner in which 

 this takes place may be easily conceived : a number of elastic mole- 

 cules existing in a state of solution, or in a state of fusion, are kept at 

 such a distance by the fluid in the one case, and by the heat in the 

 other, as to preclude the operation of their mutual affinities ; but 

 when, in the process of evaporation or cooling, any two molecules 

 are brought together by the forces or polarities which produce a crys- 

 talline arrangement, and strongly adhere, they will mutually compress 

 one another, and each will have an axis of double refraction in the 

 directions of the line joining their centres, in the same manner as if 

 they had been compressed by an external force. 



From the phenomena of crystallization and cleavage, it is obvious 

 that the molecules of crystals have several axes of attraction, or lines 

 along which they are most powerfully attracted, and in the direction 

 of which they cohere with different degrees of force. Guided by 

 the indications of hemitrope forms, and supposing the molecules to 

 be spherical or spheroidal, we infer that their axes are three in num- 

 ber and at right angles to each other, and are related in position to 

 the geometrical axis of the primitive form. In like manner the phe- 

 nomena of double refraction are related to the same axis of the primi- 

 tive form, and may be all rigorously calculated by a reference to three 



