On Material Molecules and the Etherial Medium. 251 



an elliptic cylinder the same as a biaxal crystal, these phe- 

 nomena ceasing as soon as the glass becomes uniformly 

 heated. If a cylinder of glass, which has been heated to 

 redness, be rapidly cooled at the exterior, the glass becomes 

 permanently doubly-refracting, the position of the molecules 

 being in this case permanently altered. 



These, and the phenomena produced in crystals by the 

 combined effects of heat and pressure, may all be accounted 

 for by the different changes in different directions, which 

 take place in the position of the material molecules by the 

 action of these causes. 



The next question is, whether the optical properties of 

 crystals are determinated solely by the mass of the mole- 

 cules and their arrangement in space, or are they dependent 

 on the nature of the molecule, that is, on its chemical con- 

 stitution, or the kind of matter of which it is built up ? 



The principal facts bearing on this point, which at first 

 sight appear to show that such is the case, were observed 

 by De Senarmont, who found that a change in the compo- 

 sition of the molecule, produced by isomorphous replace- 

 ment, alters the angle between the optic axes. But the 

 change thus produced may, in reality, be due to the altera- 

 tion in the arrangement, and to the change of mass of the 

 molecules, consequent on isomorphous replacement. It may 

 be objected, that such a supposition is contrary to the very 

 meaning of isomorphism, which asserts that isomorphous 

 substances can replace each other in all proportions, without 

 any alteration in the form of the crystal, that is, in the 

 arrangement of the molecules. It has, however, never been 

 established, that the angles between the faces of crystals 

 remain unaltered by isomorphous replacement ; on the con- 

 trary, it appears to be very probable that this is not the case, 

 from the very fact that isomorphous substances differ, in 

 some cases considerably, in their " angular elements," as 

 jetermined by the angles between the faces of crystals. 

 In reality, there is no such thing as absolute isomorphism. 

 In the well-know T n case, for instance, of the isomorphism of 

 the carbonates of calcium, lead, strontium, and barium, in 

 the minerals aragonite, cerussite, strontianite, and witherite, 



VOL. in. 2 K 



