Heat changes the position of the optical axes of crystals. When applied to a crystal of sulphate of lime, the two optical axes gradually approach to each other and at last coincide; if the heat be continued and increased, the axes open again, but in a direction at right angles to their former position. Thus the force of heat throws every molecule in the body into correlative motion. The angles of all crystals that are not of the octohedral group are changed by heat and vary with the intensity; the difference between the length of the greatest and least optic axes in such crystals diminishes as the temperature is raised, increases when it is lowered, and is constant at a given heat. In Iceland spar heat indirectly affects the doubly refracting power, for the expansion of the crystal in the direction of its axis is accompanied by contraction at right angles to it, which brings the crystal nearer to the cubical form, and consequently diminishes its doubly refracting power. According to the researches of M. Angström, in crystals with different axes of elasticity the velocity of the molecular vibrations is different in different directions when they are heated. In rock crystal and tourmaline the heat radiates from a surface cut parallel to the axis of the crystal; in felspar the radiating surface is at right angles to the symmetrical axis. The optical axes of crystals are also affected by pressure. Doubly refracting crystals with one principal axis acquire two when the pressure is perpendicular to it. The new principal axis coincides with the line of pressure or is at right angles to it according as the crystal is positive or negative, that is, according as the extraordinary ray is refracted to or from the optic axis of the crystal. The colours produced by polarization are affected by compression and dilatation according as the crystal is positive or negative. Sir David Brewster is of opinion that all the properties of double refraction and the gorgeous phenomena of polarization, whether by crystals or produced in various substances permanently or transiently by heat, cold, rapid cooling, compression, dilatation, and induration, are wholly the result of the forces by which the atoms are held together; but these phenomena may rather be said to depend upon a reciprocal action between an irregular molecular structure and the agency of light and heat: which indeed seems to be confirmed by the transit of these two forces through right and left-handed quartz, for there is no reason to believe that there is any difference in the form of the particles in these two crystalline substances.