680 Dr. A. E. H. Tutton [March 14, 



It has thus l^een proved experimentally from the morphological 

 side that a crystal has a definite structure, and that its unit " bricks " 

 or " cells " have definite and measurable dimensions. But the fact 

 is equally well demonstrated optically. We have only to pass a beam 

 of light through a 60° prism of a non-cubic crystal, for instance, 

 quartz, to see at once the radical difference of effect from that given 

 by glass or other non-crystallized substance. For instead of the 

 usual single spectrum produced l)y glass the quartz prism refracts two 

 distinct spectra, unless it happens to have been cut so that the light 

 traverses the unique direction of single refraction, coincident with the 

 axis of the natural quartz crystallographic prism, which is also the 

 axis of trigonal (threefold) symmetry. The quartz prism used in 

 the experiment is cut at right angles to this direction, so that the axis 

 and refracting edge of the cut prism are parallel to the natural axis, 

 and the separation of the two spectra, corresponding to the two 

 refractive indices of quartz, is thus at a maximum. Moreover, on 

 placing a Nicol prism in the path of the refracted rays, we observe 

 that the light producing the two spectra is oppositely polarized, one 

 spectrum extinguishing when the Nicol has its vibration plane 

 vertical, and the other when the Xicol is rotated so that the vibration 

 plane is brought into the horizontal position. The crystal thus 

 possesses a structure, which is capable of separating a beam of 

 ordinary light into Iwo beams, having definite and perpendicularly 

 different vibration directions. 



Again, we see proof of structure if we cut a plate out of the 

 crystal and examine it in a converging beam of polarized light, 

 especially if the crystal, say one of calcite, be cut perpendicularly to 

 the singular axis (or to the bisectrix of the two sucli axes in the cases 

 of biaxial crystals) of single refraction. A beautiful interference 

 figure is produced, composed of spectrum-coloured rings and a black 

 cross, that is, a figure symmetrical about the axis of trigonal symmetry 

 and of single refraction. This evidence of structure can be most 

 wonderfully reproduced by glass, if we strain the glass by heating and 

 rapid cooling about a cylindrical axis, but an ordinary unstrained 

 piece of glass affords no such effect at all. It is clear, therefore, that 

 the calcite crystal has a symmetrical structure about the axis of single 

 refraction. Similarly, the beautiful biaxial interference figure ex- 

 hibited on the screen, afforded by a plate of rhombic potassium 

 nitrate, is symmetrical about the centre of the double-looped figure 

 of spectrum-coloured lemniscates. 



Sufficient evidence will now have been brought forward that a 

 ci-ystal is endowed with a definitely organized structure. In the 

 crystal of a pure substance we are dealing with a chemical element or 

 compound, and if with the latter it may be of any grade of com- 

 plexity, from a very simple binary compound to a most highly 

 complicated one composed of a large number of atoms. If the 

 crystal be that of an element the structure is obviously composed of 



