The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]