What are diathermanous and athermanous substances.

Diathermanous substances: Substances through which heat radiations can pass are called diathermanous substances. e.g. glass,quartz, hydrogen, oxygen, dry air, sodium chloride etc. for these substances t = 1.  

Athermanous substances: Substances through which heat radiation can not pass are called athermanous substances. e.g. wood, water, iron, copper etc For these substances t=0.