aDiathermanousHaving the property of transmitting radiant heat; diathermal; -- opposed to athermanous.
Century Dictionary and Cyclopedia
diathermanousFreely permeable by heat. The term is specifically applied to certain substances, such as crystalline pieces of rock-salt, etc., which suffer radiant heat to pass through them, much in the same way as transparent or diaphanous bodies allow of the passage of light. See absorption. Also diathermal, diathermic, diathermous.
Webster's Revised Unabridged Dictionary
Gr. to warm through; dia` through + to warm, warm
Rock-salt is diathermanous to heat from nearly all sources.