as distinguished from inter-molecular energy still operating? Dr. Percy in the volume 'On Fuel, &c.' (p. 54) of his 'Metallurgy,' states on the authority of Fournet that 'the same silicates are more fusible in the vitreous than in the crystalline state,' also that "when devitrified glass is heated it does not soften before melting, but passes suddenly into the liquid state." He has also observed acetic acid crystals retaining their form permanently at temperatures at which the same acid when liquefied would not again crystallize. Now since we have no reason for supposing that there is any difference in the amount of heat latent in a given mass of a liquefied silicate, whether liquefied by the fusion of the vitreous or crystalline form, it follows that the excess of the quantity of heat applied to liquefy a given mass of a given silicate (or mixture of silicates) when crystalline over that required to liquefy the same mass when in the vitreous state must represent the latent heat of vitrification. This seems to furnish direct proof of the existence of such latent heat, and to suggest a method for its measurement.

*

In the light of the theory put forward in my paper on 'Dissociation and Contact-Action' (Chemical News,' vol. liv, No. 1402), as well as from considerations already put forward in this note, it seems reasonable to regard this residual latent heat as intra-molecular (atomic) kinetic energy. On the principle of 'conservation of energy' it is represented by the equivalent of work done in keeping down the atoms of the individual molecules in a lower (less stable) state of combination, giving freer play to atomic forces. A consideration of the known properties of artificial glass throws some additional light upon the subject.

6

(a) In the dry way. The tension or strain to which unannealed glass owes its brittleness is illustrated in a remarkable degree by the disruption of a Rupert's Drop,' when the point is broken off, from the extremely unstable molecular condition in which the surface particles have solidified. The devitrification of glass and its conversion into 'Réaumur's porcelain,' by heating the mass strongly for a considerable time embedded in sand or gypsum, was known a century and a half ago. There have been various speculations (by Dumas and others) as to what the precise nature of this change may be, but they have now little more than a historical interest. Among later workers Benrath has found that glass which contains more silica than is represented by the formula M,Si,O7 readily becomes devitrified. Leydolt maintains the existence of obscure crystals+ in ordinary glass, which have formed in a super-saturated non-aqueous solution, since a crystalline texture can be detected by the microscope on the surface of all melted unpolished glass after contact with strong hydrofluoric acid, and washing with weaker mineral acids. Péligot states that the melting point of devitrified glass is higher than that of the vitrified portion; a fact which again suggests the idea of latent heat of vitrification.

*Corrigenda in that paper:

p. 179, col. 2, line 21 from bottom, for 'variations' read 'vibrations.' p. 180, col. 2, line 29 from bottom, for 'solubility' read 'stability.'

+ Probably mere crystallites or belonites as in the case of sulphur described above. It is very probable that these forms are developed (as Prof. Bonney has suggested to me) by the action of the strong acid upon the glass, since this is a mixture of silicates.

lization out of supersaturated solutions-which abound in the Magnesian Limestones of Durham, occurring sometimes nearly a foot in diameter, while smaller ones often make up the entire rock-mass of whole beds of limestone, containing 98 per cent. of carbonate of lime.*

Of all the changes which we should call metatropic those of vitrification and devitrification of rock (as, e.g., obsidian and tachylite) are by far the most complicated. It is pretty generally recognized that the former of these is the result of solidification by rapid cooling. Much obscurity however still hangs over the latter phenomenon; but perhaps the consideration of a few known chemical facts may go a little way towards penetrating it, and will lead us to recognise a change of molecular structure as the essential part of the process of devitrification; though the physicist and the chemist pronounce with one voice our ignorance of the actual constitution of the molecules of solid bodies. Two of the best-known instances of allotropy are those of the elements sulphur and phosphorus.† Both these bodies may undergo such a metatropic change as to assume an allotropic modification, which we are justified in calling the vitreous condition.' I have for some years used this term with reference to them, and observe that this use of it is gradually finding its way into text-books of chemistry. In both cases the vitreous condition is induced by rapid cooling; common or vitreous phosphorus, by the arrangement employed for casting it into sticks; sulphur, when poured in a molten state at temperatures not far below its boiling point into cold water. The translucent flexible needle-shaped prisms which are formed when sulphur solidifies in the dry way at the higher temperature of its fusing point (115°C) must be regarded as a vitreous form, so far as their internal texture goes, since they are quite isotropic. That in the vitreous condition of both these bodies the molecular structure is not the most permanent or stable appears from several considerations. Vitreous sulphur (both in the plastic and in the prismatic state) is known to assume the crystalline state in forms of the ortho-rhombic system in a few days, the translucent mass becoming opaque (devitrified) as the result of crystallization.

*

They must not be confounded with concretionary dolomitic 'mudstones' found in other parts of the same rocks, on the weathered surface of which a laminated structure can be easily traced. In the summer of 1886, to the surprise of Mr. Howse, of the Newcastle Museum, I split some of these with the hammer, and disclosed casts of Axinus within them.

+ Carbon, another allotropic element, is omitted here, as it is not known in the glassy or isotropic modification; but we do know that in its crystalline state (Diamond, Sp. Gr. 3.5) its density is greater than in the amorphous state (Charcoal, Sp. Gr. 1·82) (cf. Note L, App. ii.).

They are in fact crystallites.

Further there is found both in flowers of sulphur and in vitreous sulphur an amorphous electro-positive form of the element (insoluble in CS, and other solvents in which normal crystalline sulphur readily dissolves), and this amorphous sulphur is found in greater proportion as the sulphur has been more strongly and continuously heated and more suddenly cooled. This may be well compared with the amorphous dirt' which the microscope reveals in the vitreous forms of volcanic rocks; so that this may perhaps be regarded as a concomitant product of vitrification. Amorphous insoluble sulphur is also formed on the surface of the mass when molten sulphur is allowed to congeal in the presence of strong sunlight, just as vitreous phosphorus is partly converted (with the assumption of various shades of red) into amorphous phosphorus by the same physical agent. Prolonged action of heat too, as is well known, converts the whole of a mass of vitreous phosphorus in the red amorphous variety; while the same result is effected by heat in much less time, if a trace of iodine or phosphoric iodide is present. Amorphous sulphur however ultimately assumes the normal crystalline form, as does also vitreous sulphur; and the change is accompanied in both cases by the liberation of heat, the sudden crystallization of vitreous (plastic) sulphur being accompanied by a sudden rise of temperature from 93°C to 110°C, when the experiment is properly conducted. (See App. i, a.)

We may say then with Prof. Wislicenus* that "the orthorhombic modification of sulphur is at ordinary temperatures the most stable, into which all others pass spontaneously; native sulphur occurs accordingly without exception in this form." Selenium undergoes a similar series of metatropic changes. Less is known about the crystallization of phosphorus, but vitreous phosphorus is well known to form a crystalline sublimate, when kept for some time in the dark in an exhausted and hermetically-sealed glass tube. In my own work too I have obtained evidence of the spontaneous transformation of the other allotropic forms of phosphorus into a crystalline sublimate. We may then regard the crystalline condition of both sulphur and phosphorus as the most stable, as that towards which there is a constant strain or struggle in both the other two molecular states.† This comes out too when we consider their specific gravities; both these bodies have the lowest specific gravity in the vitreous, and the

* Lehrbuch der Anorganischen Chemie,' § 236.

† See Appendix i. Such a molecular strain as is here suggested may, it seems, occur and produce its optical effect in the mineral, altogether independently of any incipient deformation due to external pressure; an important fact to bear in mind in microscopic petrology.

highest specific gravity in the crystalline state.* The thermal phenomena point the same way. The rapid transition of sulphur from the vitreous to the permanent crystalline state is accompanied with liberation of heat-as has been noticed before; and the heat of combustion of vitreous phosphorus is known to be considerably greater than that of the other modifications of the element, being about one-eighth greater than that of the red amorphous variety.+ Both these facts seem to point to such a thing as latent heat of vitrification, which will be seen at once to be an important factor in devitrification generally. The work done by it seems to consist in keeping down the molecular architecture of the body to a simple primitive form; more highly complex and more permanent molecules being built up when the latent heat is set free.

We need not confine our attention to the consideration of known metatropic facts in elementary bodies, since a vitreous condition is assumed under certain physical conditions by such compound bodies as arsenious oxide (As2O), metaphosphoric acid (HPO), silica (Si O2), and borax (Na, B, 0, + 10 H2 0). We must glance briefly at these.

4

(1.) Arsenious oxide in the vitreous or glassy state is well known, and is produced in the refining process by the condensation of the vapour upon the upper heated zones of the iron retort which is used in the process. In time this glassy mass becomes turbid and porcelain-like, as the crystalline texture is developed within it. This assumption of the crystalline form by glassy arsenic may be made to take place so rapidly by the application of a drop of HCl to a saturated solution of glassy arsenic as to develope a beautiful phosphorescent light, through the rapid liberation of the latent heat of vitrification. The quantity of heat set free by one equivalent of arsenic (As, O1 = 198) has been found to amount to as much as 2,652 calories in passing from the vitreous to the crystalline state.‡

(2.) Vitreous phosphoric acid (metaphosphate, H PO). This body is best obtained as a transparent solid isotropic mass ("glasige Phosphorsäure ") by calcining the ortho-acid in a platinum crucible, and expelling in this way two out of the three equivalents of the water of constitution of the ortho-acid.

Wislicenus. op. cit., § 285 (1 vitreous P. about 6,400 calories).

Cryst.

2.07

2.34

gm. of red P. produces 5,592, 1 gm. of

See Wislicenus, op. cit. (§ 319.) "Beim Uebergange der krystallinischen in die amorphe Modification wird nämlich Wärme latent, welche bei der entgegengesetzten Umwandlung in Form von Licht und Wärme wieder frei wird. Diese Wärmemenge beträcht auf die der Formel As2 Og entsprechende Menge (198 Theile) 2,652 Wärmeeinheiten."