Others, again, are both single and double refracting to heat-rays, and it is interesting to note the heat-penetrating value as compared with the refractive indexes of the stone. In the following table will be found the refractive indexes of a selection of single and double refractive stones, the figures for "Light" being taken from a standard list. The second column shows the refractive power of heat, applied to the actual stones, and consisting of a fine pencil blowpipe-flame, one line (the one twelfth part of an inch) in length in each case. This list must be taken as approximate, since in many instances the test has been made on one stone only, without possibility of obtaining an average; and as stones vary considerably, the figures may be raised or lowered slightly, or perhaps even changed in class, because in some stones the least stain or impurity may cause the heat effects to be altered greatly in their character, and even to become singly or doubly refracting, opaque or transparent, to heat-rays, according to the nature of the impurity or to some slight change in the crystalline structure, and so on. _Selection of Singly refracting stones._ _Indexes of Rays of_ LIGHT. HEAT. Fluorspar 1.436 4.10 varies Opal 1.479 2.10 " Spinel 1.726 1.00 Almandine 1.764 1.00 Diamond 2.431 6.11 double _Selection of Doubly refracting stones._ _Indexes of Rays of_ LIGHT. HEAT. Quartz 1.545 4.7 single and double Beryl 1.575 1.0 varies considerably Topaz 1.635 4.1 " " Chrysoberyl 1.765 1.1 " " Ruby 1.949 5.1 single and double The tourmaline has a light-refractive index of 1.63, with a heat index of none, being to heat-rays completely opaque. The refractive index of gypsum is 1.54, but heat none, being opaque. The refractive index of amber is 1.51, but heat none, being opaque. In some of the specimens the gypsum showed a heat-penetration index of 0.001, and amber of 0.056, but mostly not within the third point. In all cases the heat-penetration and refraction were shown by electric recorders. These figures are the average of those obtained from tests made in some cases on several stones of the same kind, and also on isolated specimens. Not only does the power of the stone to conduct heat vary in different stones of the same kind or variety, as already explained, but there is seen a remarkable difference in value, according to the spot on which the heat is applied, so that on one stone there is often seen a conductivity varying between 0.15 to 4.70.