In thermodynamics, perfect refers to a vessel completely devoid of particles, thus being in the form of a ‘perfect’ vacuum.

The word “perfect” originated descriptions of earlier 17th century attempts to make a perfect vacuum, as discussed in the works of Denis Papin and Dutch mathematical physicist Christiaan Huygens, by the ignition or explosion of a substance inside of a piston and cylinder. [1] In this sense, the word “perfect” means that the cylinder had been made perfectly empty of atoms or gas particles, due to the combustion of the explosion, and thus the piston had the largest amount of downward pulling power as the atmosphere pushed down eliminating the newly created empty space.

The term perfect soon began to become synonymous with ‘ideal’, as in an ideal perfect gas or ideal gas.

A combination of these uses may have been in the sense in which in 1849 by Scottish mathematical physicist William Thomson speaks of a ‘perfect thermo-dynamic engine’ as defined by Sadi Carnot in his 1824 treatise Reflections on the Motive Power of Fire. [2] The use by Thomson, however, would not have been completely used in the vacuum sense, but rather in the sense of 'perfect' reversibility of the working substance; although, to note, reversibility was not defined completely until the following decade via Rudolf Clausius.

References
1. (a) Papin, Denis. (1690). “A New Way to Obtain Very Great Motive Powers at Small Cost” (Nova Methodus ad Vires Motrices Validissimas levi Pretio Comparandas). Acta Eruditorum, anno, Aug., pgs. 410-14.
(b) Muirhead, James. (1859). The Life of James Watt, (English translation: Ch. XI, Denys Pain: His memoir of 1690, Section: A New Way to Obtain Very Great Motive Powers at Small Cost”, pgs. 131-42). London: John Murray.
2. Thomson, William. (1849). “An Account of Carnot’s Theory of the Motive Power of Heat – with Numerical Results Deduced from Regnault’s Experiments on Steam”, (127-203) Transactions of the Edinburgh Royal Society, xiv.; Annales de Chime, xxxv. 1852.

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