to build a practical heat pump if the reaction is too slow too much metal would be required the metal should provide a reaction time of about 20 minutes with a 6 minute maximum preferred and a 5 minute maximum most preferred the specific heat of the material is an important factor because the material must cycle between its high temperature and its low temperature the specific heat is at most 0 20 kcal kg u00b0 c preferable 0 15 kcal kg u00b0 c or less and most preferably no higher than 0 10 kcal kg u00b0 c the maximum pressure of the operating pressure range is also important as it affects the amount of container material required in the apparatus a high pressure requires a thick walled container which is less desirable than a thin walled container with presently available construction metals the maximum pressure to be experienced in the apparatus should not be higher than 60 absolute atmospheres preferable not higher than 50 absolute atmospheres and most preferable not higher than 40 absolute atmospheres of course if higher strength metals are developed then these maximum pressures could be increased several factors effect the rates of hydrogen metal reaction besides the nature of the.