What is "diathermancy"

Diathermancy \Di`a*ther"man*cy\, Diathermaneity \Di`a*ther`ma*ne"i*ty\, n. [See Diathermanous.] The property of transmitting radiant heat; the quality of being diathermous.
--Melloni.

n. The condition of being diathermic

Diathermancy (from "dia" through and "thermē" heat) is the property of some fluids that allows rays of light through them without itself being heated. A diathermanous fluid is thus " permeable" by heat.). Diathermany was first described by German physicist and chemist Heinrich Magnus in the 1800s. Atmospheric air is diathermanous; therefore, air is not heated by sunshine, but by long wave heat reflected by soil, and especially, water on the Earth surface. Water, on the contrary, is not diathermanous, and it is heated by sunshine directly.