I have already noticed an important fact for which we are indebted to Mr. James Thomson, and have referred to the original communications on the subject. I shall here place the physical circumstances connected with this fact before my reader in the manner which I deem most likely to interest him.
INFLUENCE OF PRESSURE ON BOILING POINT.
When a liquid is heated, the attraction of the molecules operates against the action of the heat, which tends to tear them asunder. At a certain point the force of heat triumphs, the cohesion is overcome, and the liquid boils. But supposing we assist the attraction of the molecules by applying an external pressure, the difficulty of tearing them asunder will be increased; more heat will be required for this purpose; and hence we say that the boiling point of the liquid has been elevated by the pressure.
INFLUENCE OF PRESSURE ON FUSING POINT.
If molten sulphur be poured into a bullet-mould, it will be found on cooling to contract, so as to leave a large hollow space in the middle of each sphere. Cast musket-bullets are thus always found to possess a small cavity within them produced by the contraction of the lead. Conceive the bullet placed within its mould and the latter heated; to produce fusion it is necessary that the sulphur or the lead should swell. Here, as in the case of the heated water, the tendency to expand is opposed by the attraction of the molecules; with a certain amount of heat however this attraction is overcome and the solid melts. But suppose we assist the molecular attraction by a suitable force applied externally, a greater amount of heat than before will be necessary to tear them asunder; and hence we say that the fusing point has been elevated by the pressure. This fact has been experimentally established by Messrs. Hopkins and Fairbairn, who applied to spermaceti and other substances pressures so great as to raise their points of fusion a considerable number of degrees.
Let us now consider the case of the metal bismuth. If the molten metal be poured into a bullet-mould it will expand on solidifying. I have myself filled a strong cast-iron bottle with the metal, and found its expansion on cooling sufficiently great to split the bottle from neck to bottom. Hence, in order to fuse the bismuth the substance must contract; and it is manifest that an external pressure which tends to squeeze the molecules more closely together here assists the heat instead of opposing it. Hence, to fuse bismuth under great pressure, a less amount of heat will be required than when the pressure is removed; or, in other words, the fusing point of bismuth is lowered by the pressure. Now, in passing from the solid to the liquid state, ice, like bismuth, contracts, and if the contraction be promoted by external pressure, as shown by the Messrs. Thomson, a less amount of heat suffices to liquefy it.
EXPERIMENTS.
These remarks will enable us to understand a singular effect first obtained by myself at the close of 1856 or in January 1857, noticed at the time in the 'Proceedings of the Royal Society,' and afterwards fully described in a paper presented to the Society in December of that year. A cylinder of clear ice two inches high and an inch in diameter was placed between two slabs of box-wood, and subjected to a gradual pressure. I watched the ice in a direction perpendicular to its length, and saw cloudy lines drawing themselves across it. As the pressure continued, these lines augmented in numbers, until finally the prism presented the appearance of a crystal of gypsum whose planes of cleavage had been forced out of optical contact. When looked at obliquely it was found that the lines were merely the sections of flat dim surfaces, which lay like laminæ one over the other throughout the length of the prism. [Fig. 50] represents the prism as it appeared when looked at in a direction perpendicular to its axis; [Fig. 51] shows the appearance when viewed obliquely.[A]
