Diathermancy

Diathermancy, that quality of certain substances which renders them capable of transmitting heat-radiation without absorbing it. No substance is perfectly diathermanous or transparent to heat, just as no body is perfectly transparent to light Glass allows of the passage of bright heat, as may be felt in a glass conservatory on a sunny day; but it is athermanous or opaque to dark heat, which renders it difficult for the heat that has entered the conservatory to pass out again. Rock-crystal is remarkably transparent for dark heat; this makes it a useful material for prisms and lenses that are employed to deviate and concentrate dark heat-rays.