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.