102 Mr, Mashelyne (m the Crystal Molecule [April 1, 



the planes containing them, for each colour, cross the plane of sym- 

 metry, and the morphological axis is the second mean line, the first 

 mean lines being dispersed for each colour along the plane of symmetry. 



This position is that o^ t3 i^ ' 



Thus far the conformity between crystalline symmetry and the 

 distribution of optical elasticity, would seem sufficiently near to bring 

 the latter into the same category with the elasticity exhibited by the 

 crystals under the solicitation of less subtle forces. But a nearer view 

 of the phenomena goes far to dispel this expectation. 



For all the results that had been previously reviewed, an explana- 

 tion more or less complete may be found in a simple hypothesis ; namely 

 this, that the centres of gravity of the crystal -molecules are always 

 arranged in planes which represent either actual or possible facettes : 

 while the relative distances of molecule from molecule are the same 

 in the same direction, but different for different directions. To disturb a 

 molecule so as to move it out of its plane, would be to destroy the 

 integrity of the molecular system. Magnetic orientation would only indi- 

 cate tlie lines of greater or less distance between the molecules ; increase 

 of thermic intensity (temperature), too, only causes a greater interval 

 between the molecules without altering the crystallographic relations 

 of the planes they lie in — albeit that this increase of interval is 

 different in different directions. These, and, in all probability also, 

 the other as yet less elaborated results of physical experiment, are 

 thus explicable as dependent on the disposition of the mass centres of 

 the molecules, and are so capable of being rendered subordinate to 

 the fundamental law of crystallography. 



But the hypothesis above sketched in outline fails to explain a 

 series of facts which Light reveals to us, and which show that the 

 optical properties of a crystal cannot be directly dependent on the 

 arrangement of the molecules. Among these are the following : — 



Firstly. The action of increased temperature on a crystal, while it 

 alters the volume of the crystal, does so by changing the values of the 

 parameters, changing thereby too the refractive powers of the substance. 

 As such refractive powers vary generally inversely as the amount of 

 elasticity and directly as the density of the light-transmitting medium 

 varies, we might expect the variations in the parameters and 

 refractive indices and also in the axes of elasticity to follow 'pari passu 

 with the changes in the molecular density in different directions as the 

 crystal becomes expanded : nothing of the kind occurs. The axes 

 of optical elasticity vary indeed (in aragonite for instance), but their 

 orientation is quite dissimilar from that of the altered crystallographic 

 axes ; i. c, the directions of the greatest mean and least parameters 

 of the heated crystal, which measure its physical density, entirely differ 

 from the corresponding directions of the greatest mean and least of 

 the new axes of optical elasticity. 



2ndly. The speaker had already alluded to the remarkable fact of the 

 crossing of the planes of the optic axes for different colours in mellate 



