Leonard James Spencer, 1911 Encyclopædia Britannica, Volume 13… (1911)
“ In the figure, the faces at the upper end of the crystal are the basal plane k and the domes o, p, l, m, whilst at the lower end there are only the four faces of the pyramid P. Connected with this polarity of the crystals is their pyroelectric character—when a crystal is subjected to changes of temperature it becomes positively electrified at one end and negatively at the opposite end. There are perfect cleavages parallel to the prism faces (d in the figure) . Crystals are usually colourless, sometimes yellowish or greenish, and transparent; they have vitreous lustre. ”
