The most perfect method of utilizing the waste heat hitherto applied is that of the Siemens regenerator, in which the spent gases are made to travel through chambers, known as regenerators or recuperators of heat, containing a quantity of thin firebricks piled into a cellular mass so as to offer a very large heat-absorbing surface, whereby their temperature is very considerably reduced, and they arriveat the chimney at a heat not exceeding 300 or 400 degrees.
This is the cycle employed by Carnot for the establishment of his fundamental principle of reversibility as the criterion of perfect efficiency in a heat engine.
This compound possesses a heat of formation so much lower that electrically it needs but a voltage of 0.9 to decomplose it, and it is easily soluble in the fused sulphides of the alkali metals.