the thermal pack 10 , shown in fig1 , is a generally rectangular container . the container may be made in different shapes as a matter of choice and for special applications . the container 10 is made of supple plastic films forming walls 11 and 12 which have seals 13 , 14 , 15 and 16 closing the corresponding edges to form a liquid tight container . the films may be single ply or co - extrusions of different polymers or laminated films having the same properties or different properties . the walls are formed of materials that will be impervious to each of the ingredients and to the resulting combination . the container may be made using a tubular plastic film which is sealed at the opposite ends . the plastic film may be thermoplastic and the seals may be formed by heat and pressure . if other plastic films are used , the seals may be formed by adhesives or solvents . as illustrated , the container 10 encloses , as one ingredient , a chemical compound 21 which will initiate a thermal reaction when mixed with another ingredient , in the form of a catalyst . preferably , when the chemical compound is mixed with the catalyst a gel - like substance is formed . the gel acts to prolong the thermal effects . as shown in fig1 and 2 , the container 10 encloses another tube having opposing walls 17 and 18 . the walls 17 and 18 are preferably continuous plastic film with end seals 19 and 20 . these films and seals may be formed in the same manner as the films and seals in container 10 . also , the tube may be formed by superimposing film layers and sealing the superposed periphery . the opposing walls 17 and 18 have a structure or are made of material that will rupture before the container 10 , when placed under a compressive load . this insures the integrity of the container 10 and prevents the thermal compound from coming into contact with the user . the catalyst 22 , for the thermal reaction , is enclosed within the inner container until the inner container is ruptured . once the thermal compound and catalyst come into contact with each other , the outer container is kneaded to thoroughly mix the ingredients . as the thermal reaction progresses , there is generated a gaseous by - product . the volume of the gas is released from the interior of the container through aperture 23 and porous membrane 24 . the porous membrane is made from a material that will allow passage of gas , including any air trapped in the interior , but not liquid . the membrane spans the aperture 23 and has a continuous edge seal 25 joining the membrane to the container . the edge seal 25 may be formed by heat and pressure , solvent or adhesives . the material of the membrane may be in the nature of a semipermeable membrane or it can be a microporous nonwoven material . the thermal pack 40 , shown in fig4 , is substantially similar to the thermal pack 10 of fig1 . the thermal pack 40 has a peripheral seal 41 and encloses a catalyst 43 inside the bag , separated from the thermal composition 44 by a frangible wall 52 . the thermal pack has a manual one - way valve 45 which has an annular valve 48 on the internal end of a valve body that reciprocates through a collar 46 sealed into an aperture in the thermal pack wall 42 . the annular valve is seated into an annular recess 49 . as shown in fig5 , the gas by - product of the thermal reaction caused by mixing the catalyst and the thermal composition will act against the annular valve 48 to seat the valve in the recess 49 closing the valve . the gas pressure closing the valve may be overcome by manually depressing valve actuator 47 . the valve body moves displacing the annular valve 48 and opening a vent passageway 50 . the gaseous by - product and any other trapped gases are released through the passageway . as an example of the thermal pack , the outer container may be fabricated from polyethylene film , alone , or laminated with other materials , the thermal compound may be ammonium nitrate , either alone or mixed with other chemicals , disposed inside the outer container . the ammonium nitrate is in the form of dry particles . the inner container may be made of the same plastic composition as the outer container . the inner container may have a weakened portion of a wall or the container may be of a thinner film . the catalyst , enclosed in the inner container , is water . the vent may be a hydrophobic microporous nonwoven plastic material such as tyvek made by dupont co . the vent material will allow gas to pass through while repelling liquid . in use , the thermal pack may be stored indefinitely until the ingredients are mixed . to provide a cold treatment to a patient , the compliant envelope is squeezed or put under compressive pressure to rupture the inner tube . the container is then kneaded to thoroughly mix the chemicals and produce a temperature change . as the chemical endothermic reaction proceeds , gas evolves and escapes from the container through the semipermeable vent leaving the gel . the cold compliant container is then intimately wrapped about the injured part of the patient &# 39 ; s body . it is to be understood that while a certain form of the invention is illustrated , it is not to be limited to the specific form or arrangement of parts herein described and shown . it will be apparent to those skilled in the art that various changes may be made without departing from the scope of the invention and the invention is not to be considered limited to what is shown and described in the specification and drawings .