a flexible thermal pack to be intimately applied to a patient &# 39 ; s body is in the form of a plastic container enclosing a chemical compound capable of an endo - or exo - thermic reaction when intermixed with a catalyst . the plastic container has a vent in one wall covered by a semipermeable membrane for passage of gas but not liquid . also enclosed in the container is another frangible container housing a suitable catalyst for the compound .