John Herapath (30 May 1790 – 24 February 1868) was an English physicist who gave a partial account of the kinetic theory of gases in 1820 though it was neglected by the scientific community at the time. He was the cousin of William Herapath, the chemist and William Bird Herapath, the physician who discovered herapathite.
Herapath's scientific interests started with an attempt to provide a mechanistic explanation for gravity. Motivated by his search for a mechanical explanation of gravitation, he started to consider how a system of colliding particles could give rise to action at a distance. In considering the effect of the high temperatures near the Sun on his gravific particles he was led to a relationship between temperature and particle velocity.
Herapath postulated that the momentum of a particle in a gas is a measure of the absolute temperature of the gas. He used momentum, rather than the kinetic energy on which the later established theory is based, as it seemed to him to avoid some difficulties around whether elastic collisions were possible between indivisible atoms. Apparently ignorant of Daniel Bernoulli's work, he was led to the incorrect, but suggestive, relationship that expresses the product of pressure P and volume V as proportional to the square of his true temperature. The correct relationship is proportional to the absolute temperature, not its square, the error arising from his identification of momentum, rather than energy, with temperature.
He submitted his ideas in a paper to the Royal Society in 1820 where it was peer reviewed by Sir Humphry Davy. Davy had already sympathised with the view that heat was associated with molecular motion rather than with Joseph Black's caloric theory of heat but he rejected Herapath's paper with some coolness, uncomfortable with the implication that there was an absolute zero of temperature at which all motion ceased. Davy may also have had some distaste for the mechanistic Newtonian picture, influenced as he was by the more holistic philosophy of the Romantic movement.
In 1821, Herapath managed to have his paper published in the Annals of Philosophy, a well-read journal that counted Michael Faraday among its regular contributors. However, the paper seems to have attracted little attention other than from James Prescott Joule who presented a short account of the work in 1848, again to little reaction. Meanwhile, Herapath maintained a campaign against Davy and the Royal Society in the correspondence pages of The Times newspaper.
Great Comet of 1831
He discovered the Great Comet of 1831 on 7 January 1831.
In 1835 Herapath became editor of The Railway Magazine, which underwent four changes of name during the boom years of railways to become Herapath's Railway Journal in January 1894. It is now called the Railway Gazette, and is not to be confused with the current Railway Magazine which commenced publication in 1897. This gave him some limited opportunity to publish his scientific ideas. In 1836, he published a calculation of the mean molecular speed in a gas based on his kinetic theory and hence the speed of sound. Joule reproduced his results but is usually incorrectly credited as the originator.
The name changes were -
- Railway magazine May 1835-Feb. 1836
- Railway magazine and annals of science Mar. 1836-Aug. 1839
- Railway magazine and steam navigation journal Mar.-Aug. 1839
- Railway magazine and commercial journal Aug. 17, 1839-Dec. 1840
- Herapath's railway magazine, commercial journal, and scientific review Jan. 1841-Dec. 1842
- Herapath's railway and commercial journal Jan. 1843-Dec. 1845
Revision of theories
John James Waterston
John James Waterston was another contemporaneous Scottish physicist who also worked on the kinetic theory, and whose work was also neglected at the time.
- Hutchison, Keith. "Herapath, John". Oxford Dictionary of National Biography (online ed.). Oxford University Press. doi:10.1093/ref:odnb/13010. (Subscription or UK public library membership required.)
- Herapath, J. (1816), "On the physical properties of gases", Annals of Philosophy, Robert Baldwin: 56–60
- Herapath, J. (1821), "On the Causes, Laws and Phenomena of Heat, Gases, Gravitation", Annals of Philosophy, Baldwin, Cradock, and Joy, 9: 273–293