CHAPTER IX.
PHYSICAL PROPERTIES.
G—Heat.
Another method of isolating certain stones is by the action of heat-rays. Remembering our lessons in physics we recall that just as light-rays may be refracted, absorbed, or reflected, according to the media through which they are caused to pass, so do heat-rays possess similar properties. Therefore, if heat-rays are projected through precious stones, or brought to bear on them in some other manner than by simple projection, they will be refracted, absorbed, or reflected by the stones in the same manner as if they were light-rays, and just as certain stones allow light to pass through their substance, whilst others are opaque, so do some stones offer no resistance to the passage of heat-rays, but allow them free movement through the substance, whilst, in other cases, no passage of heat is possible, the stones being as opaque to heat as to light. Indeed, the properties of light and heat are in many ways identical, though the test by heat must in all cases give place to that by light, which latter is by far of the greater importance in the judging and isolation of precious stones. It will readily be understood that in the spectrum the outer or extreme light-rays at each side are more or less bent or diverted, but those nearest the centre are comparatively straight, so that, as before remarked, these central rays are taken as being the standard of light-value. This divergence or refraction is greater in some stones than in others, and to it the diamond, as an example, owes its chief charm. In just such manner do certain stones refract, absorb, or reflect heat; thus amber, gypsum, and the like, are practically opaque to heat-rays, in contrast with those of the nature of fluorspar, rock-salt, &c., which are receptive. Heat passes through these as easily as does light through a diamond, such stones being classed as diathermal (to heat through). So that all diathermal stones are easily permeable by radiant heat, which passes through them exactly as does light through transparent bodies.
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.