Abstract: A flexible container having a substantially planar configuration encloses a first chemical having characteristics for being supercooled to maintain a liquid state even at temperatures below its melting point. A second chemical can be mixed with the first chemical in the container to initiate the crystallization of the first chemical at the substantially constant temperature of the melting point of the first chemical. The container can be enclosed in an insulation envelope to moderate the temperature of the device. The first chemical may include sodium thiosulfate pentahydrate which provides a desirable constant temperature of 48.degree. C. The second chemical may include sodium borate pentahydrate or sodium sulfite which provides the first chemical with a desirable sandy configuration during crystallization in the .alpha. pentahydrate form.