Illustrations of the Huttonian Theory of the Earth

by John Playfair

[73] Newton's Optics, Query 11.

164. The fact of water boiling at a lower temperature under a less compression, is sufficient to justify the supposition, that bodies may be made by pressure to endure extreme heat, without the dissipation of their parts, that is, without evaporation or combustion. A further postulatum is introduced in Dr Hutton's theory, namely, that compound bodies, such as carbonate of lime, when the compression prevents their separation, may admit of fusion, notwithstanding that the fixed part may be infusible when separated from the volatile. This assumption is supported by the analogical fact of the fusion of the carbonate of barytes, as mentioned in the text.

165. In a region where the action of heat was accompanied with such compression as is here supposed, there could be no fire, properly so called, and no combustion; this is admitted by Dr Hutton, and it is therefore a fallacious argument which is brought against his theory, from the impossibility of fire being maintained in the bowels of the earth. This impossibility is precisely what he supposes; and yet Mr Kirwan's arguments are directed, not against the existence of heat in the interior of the earth, but against the existence of burning and inflammation.

After taking notice,[74] that Saussure had succeeded, though with extreme difficulty, in melting a particle of limestone, so small as to be visible only with a microscope, "what (adds he) must have been the heat necessary to melt whole mountains of this matter? Judging by all that we at present know of heat, such a high degree could only be produced by the purest air, acting on an enormous quantity of combustible matter. Now, Ehrman observed, that the combustion of two hundred and eighty cubic inches of air, acting on charcoal, was not able to effect the fusion of one grain of Carrara marble; from whence it is apparent, that all the air in the atmosphere, nor in ten atmospheres, would not melt a single mountain of this substance, of any extent, even if there were a sufficient quantity of inflammable matter for it to act upon. Judging also of subterraneous heat by what we know of that of volcanoes, no such heat exists: the highest they in general produce, is that requisite for the fusion of the volcanic glass called obsidian, which Saussure found not to exceed 115° of Wedgewood; but basaltine, which requires 140° of Wedgewood, is never melted in the lavas of Ætna. How little capable, then, would volcanic heat be to effect the fusion of Carrara marble, which, according to the same excellent author, would require a heat of upwards of 6300° of Wedgewood, if this pyrometer could extend so far? And in what circumstances does Dr Hutton suppose this astonishing heat to have existed, and even still to exist, under the ocean, in the bowels of the earth, where neither a sufficient quantity of pure air, nor of combustible matter, capable of such mighty effects, can, with any appearance of probability, be supposed to exist: and, without these, such degrees of heat cannot even be imagined, without flying into the region of chimeras."

[74] Geol. Essays, p. 453.

166. Now, this reasoning is not applicable to Dr Hutton's hypothesis of subterraneous heat, because it is grounded on experiments, where that very separation of the volatile and fixed parts takes place, which is excluded in that hypothesis. When limestone or marble is exposed to such heat as is here mentioned, or even to heat of a degree vastly inferior, the carbonic gas is expelled, and the body is reduced to pure lime; from the refractory nature of which, as we learn from the fact relative to barytes, mentioned above, no conclusion can be drawn as to the infusibility of the same substance, when combined with the carbonic gas. The Carrara marble may require a heat of 6300° of Wedgewood, to melt it in the open air, where the carbonic gas escapes from it; but under such a pressure as would retain this gas, it cannot be inferred, that it might not melt with the heat of a glass-house furnace. In like manner, it may be true, that two hundred and eighty cubic inches of air, acting on charcoal, cannot effect the fusion of one grain of this marble, after its fixed air is driven off from it; but we cannot from thence draw any inference, applicable to a case where the carbonic gas is retained, and where the action of heat is independent of atmospheric air.

Nothing, therefore, can be more inconclusive than this reasoning, as it proceeds on the supposition, that Dr Hutton's system admits propositions, which in fact it expressly denies.

167. Of the production and maintenance of heat, in circumstances so different from those of ordinary experience, we can hardly be expected to give any explanation; but we are not entitled, merely on that account, to doubt of the existence of such heat. Mr Kirwan thinks otherwise: "Judging," he says, "from all we at present know of heat, such a high degree of it, (as will melt limestone,) could only be produced by the purest air, acting on an enormous quantity of combustible matter. Without these, such degrees of heat cannot even be imagined, without flying into the region of chimeras."[75]

[75] Geol. Essays, p. 454.

Now, in the first place, the high degree of temperature which is here understood, is probably not necessary to the purposes of mineralization, as has just been shown; and, in the second place, it is not fire, in the usual sense of the word, but heat, which is required for that purpose; and there is nothing chimerical in supposing, that nature has the means of producing heat, even in a very great degree, without the assistance of fuel or of vital air. Friction is a source of heat, unlimited, for what we know, in its extent, and so perhaps are other operations, both chemical and mechanical; nor are either combustible substances, or vital air, concerned in the heat thus produced. So also the heat of the sun's rays in the focus of a burning glass, the most intense that is known, is independent of the substances just mentioned; and, though that heat certainly could not calcine a metal, nor even burn a piece of wood, without oxygenous gas, it would doubtless produce as high a temperature in the absence as in the presence of that gas.