Mayer entirely ignored the grand fundamental principle laid down by Sadi Carnot - that nothing can be concluded as to the relation between heat and work from an experiment in which the working substance is left at the end of an operation in a different physical state from that in which it was at the commencement.
Just as the working substance which alternately takes in and gives out heat in the steam-engine is water (converted during a part of the action into steam), so in the air-engine it is air.
The practical drawbacks to employing air as the working substance of a heat-engine are so great that its use has been very limited.
One of the chief practical objections to air-engines is the great bulk of the working substance in relation to the amount of heat that is utilized in the working of the engine.
They differ from it, however, in the fact that their working substance is not air, but a mixture of gases - a necessary consequence of internal combustion.
