Cambridge | June
My dear Sir,
To-morrow I set out for Manchester2 whence I hope to bring back a supply of specimens3 for you to try their conductive powers whenever you may feel at leisure to make the experiments. I shall return in about ten days or a fortnight.
You ask me whether Prof. Thompson’s4 results on the effect of pressure in resisting solidification of water are not opposed to those which I have obtained with other substances in which the solidification is promoted by pressure.5 In the one case the temperature of fusion6 is lowered in the other it is elevated. According to the explanation which the dynamical theory of heat7 affords of the phenomena, such ought to be the case, because water expands in freezing, whereas the substances I have experimented on contract in becoming solid. The general explanation appears to be something of this kind. Suppose a substance to expand in passing from the solid into the fluid state. Conceive it to be enclosed in a given space and subjected on all sides to a uniform pressure; and suppose moreover that its temperature is such that it is just on the point of liquefaction. Conceive the temperature to be elevated by a small quantity. The substance will begin to melt, and if a sufficient quantity of heat were constantly supplied to it to maintain this increased temperature, the liquefaction would go on till the whole mass became liquid, supposing always the process to be affected by nothing but the temperature. But if we suppose the envelope enclosing the substance to be perfectly rigid so that the space occupied by the substance remains invariable the pressure on all sides will be increased by the tendency of the substances to expand in liquifying. Now the theory tells us that if the volume of any substance be kept constant while its temperature is increased by a small amount, the corresponding increase of pressure cannot exceed a small corresponding amount. Consequently if the process of liquefaction under the conditions above mentioned tends to increase the pressure more than this small amount assigned by the theory, the process will be arrested, while it would necessarily have proceeded in consequence of the supposed increased temperature, had not the pressure been altered. This shows that the temperature of fusion must be increased by pressure when the substance expands in becoming fluid; and similar reasoning would show that that temperature will be diminished by pressure when the substance contracts in becoming fluid. Accurate investigation gives the amounts of the effect thus produced by any assigned pressure in a prepared substance. One object of my experiments is to compare the results of theory and experiment in such substances as are sufficiently tractable to be experimented on.8
Believe me | Yours very truly | W. Hopkins.
RI MS JT/1/TYP/2/607–608
Typescript Only
[12] June [1854]: it looks as if the original typescript stated ‘June 5th ‘56’, but ‘5th 56’ was crossed out and replaced, in pencil with ‘12, 56’. And then, possibly later, ‘54?’ was added as a possibility. The date 12 June 1854 seems the most likely as this letter follows from a previous letter concerning the conductive power of certain specimens that was dated 6 May (see letter 0929).
set out for Manchester: Hopkins travelled to Manchester where he was assisted by James Prescott Joule (1818–89) and William Fairbairn (1789–1874) to conduct experiments on the effects of pressure on the melting point of substances (ODNB) (see also letter 0862).
supply of specimens: these are likely the ‘hard crystalline rocks’ that Hopkins refers to on 6 May (see letter 0929).
Prof. Thompson’s: misspelling of William Thomson, Lord Kelvin.
those which I have obtained … promoted by pressure: Hopkins was involved in research on the effects of pressure on the melting points and conductivities of solids (see n. 2). For further explanation see C. Smith, ‘William Hopkins and the Shaping of Dynamical Geology: 1830–1860’, British Society for the History of Science 22:1 (March 1989), pp. 27–52, on pp. 39–40.
temperature of fusion: this refers to the temperature at which a solid becomes a liquid or a liquid becomes a solid.
dynamical theory of heat: this is a foundational theory of thermodynamics advanced by William Thomson, based largely on the experiments of James Joule concerning the mechanical action of heat (W. Thomson, ‘On the Dynamical Theory of Heat, with numerical results deduced from Mr. Joule’s equivalent of a Thermal Unit, and M. Regnault’s Observations on Steam’, Phil. Mag., 22 (1852), pp. 8–21) (see also letter 0808, n. 5).
One object of my experiments … to be experimented on: see n. 5 and, for example, W. Hopkins ‘An account of some experiments on the effect of pressure on the temperature of fusion of different substances’, Brit. Assoc. Rep. 24 (1854), pp. 57–8. For more detail see C. Smith and M. Norton Wise, Energy and Empire: A Biographical Study of Lord Kelvin (Cambridge: Cambridge University Press, 1989), pp. 552–59.
Please cite as “Tyndall0938,” in Ɛpsilon: The John Tyndall Collection accessed on