Heat as an indestructible substance

[Pentcho Valev <pvalev@BAS.BG>]

Bob Sciamanda wrote:

> | In 1824, Carnot published essentially
> | the following argument:
> | Heat is an indestructible substance (calorique).
> | Perpetuum mobile (of the first kind) is impossible.
> | Therefore, all reversible machines working between two given
> temperatures
> | have the SAME efficiency. . . .- Pentcho
>
> Although heat as an indestructible substance was part of the "caloric
> fluid theory", it is not crucial to the referenced Carnot argument.

Carnot draws a straightforward analogy between a heat engine and a
waterwheel so perhaps analysing the latter would make things clearer. The
maximum efficiency of a waterwheel depends on the height of the waterfall
only (this height is analogous to the temperature difference in the heat
engine case). If two waterwheels working under the same waterfall had
different efficiencies, they would be able to perform net lifting of water,
in violation of the first law.
   The condition of indesrtuctibility of water is obviously crucial.
Replace water with a liquid fuel - gasoline - and the conclusion that the
efficiency depends on the height of  the gasolinefall would become false.
That is what happened in mid 19th century. Heat turned out to be fuel-like
- underwent "destruction" in the heat engine and turned into work. Carnot
conclusion (the original version of the second law) bacame just as false as
the conclusion that the efficiency of a gasoline engine depends on the
height of the gasolinefall. However Clausius and Kelvin quickly converted
the failure into an indisputable foundation for the future theory.

Pentcho