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Availability of Heat of Combustion.—Taking the value 1.13 kilogrammetres per kilo-calorie for 1� C. fall of temperature at 100� C., Carnot attempted to estimate the possible performance of a steam-engine receiving heat at 160� C. and rejecting it at 40� C. Assuming the performance to be simply proportional to the temperature fall, the work done for 120� fall would be 134 kilogrammetres per kilo-calorie.

In reality the fraction of the heat of combustion available, even in an ideal engine and apart from practical limitations, is much less than might be inferred from the efficiency formula of the Carnot cycle.

Joule measured the corresponding heats of combustion, and showed that the electromotive force corresponding to a chemical reaction is proportional to the heat of combustion of the electrochemical equivalent.

He also measured the E.M.F. required to decompose water, and showed that when part of the electric energy EC is thus expended in a voltameter, the heat generated is less than the heat of combustion corresponding to EC by a quantity representing the heat of combustion of the decomposed gases.

The problem in the case of an engine using a separate working fluid, like a steam-engine, is to find what must be the temperature θ″ of the boiler in order to obtain the largest possible fraction of the heat of combustion in the form of work.