^{page 72 note *} Illustrations of the Huttonian Theory, by Mr Professor Playfair, 430.

^{page 81 note *} On one occasion, the importance of this precaution was strongly felt. Having inadvertently introduced a considerable quantity of moisture into a welded barrel, an explosion took place, before the heat had risen to redness, by which, part of the barrel was spread out to a flat plate, and the furnace was blown to pieces. Dr Kennedy, who happened to be present on this occasion, observed, that not with standing this accident, the time might come when we should employ water in these experiments to assist the force of compression. I have since made great use of this valuable suggestion: but he scarcely lived, alas! to see its application; for my first success in this way, took place during his last illness.—I have been exposed to no risk in any other experiment with iron barrels; matters being so arranged, that the strain against them has only commenced in a red heat, in which the metal has been so far softened, as to yield by laceration like a piece of leather.

^{page 82 note *} Eight parts of bismuth, five of lead, and three of tin

^{page 83 note *} Essays of Natural Experiments made in the Academie del Cimento, translated by Waller, London, 1684, page 117. The same in Musschenbroek's Latin translation, Lugd. Bat. 1731, p. 63.

^{page 84 note *} I found it a matter of much difficulty to ascertain the proper quantity of air which ought to be thus inclosed. When the quantity was too great, the result was injured by diminution of elasticity, as I shall have occasion fully to shew here after. When too small, or when, by any accident, the whole of this included air was allowed to escape, the barrel was destroyed.

I hoped to ascertain the bulk of air necessary to give liberty to the expansion of the liquid metal, by measuring the actual quantity expelled by known heats from an open barrel filled with it. But I was surprised to find, that the quantity thus discharged, exceeded in bulk that of the air which, in the same heats, I had confined along with the carbonate and fusible metal in many successful experiments. As the expansion of the liquid does not seem capable of sensible diminution by an opposing force, this fact can only be accounted for by a distention of the barrel. In these experiments, then, the expansive force of the carbonic acid, of the included air, and of the fusible metal, acted in combination against; the barrel, and were yielded to in part by the distention of the barrel, and by the condensation of the included air. My object was to increase the force of this mutual action, by diminishing the quantity of air, and by other devices to be mentioned hereafter. Where so many forces were concerned, the laws of whose variations were unknown, much precision could not be expected, nor is it wonderful, that in attempting to carry the compressing force to the utmost, I should have destroyed barrels innumerable.

^{page 84 note †} I have since constantly used tubes of common porcelain, finding glass much too fusible for this purpose.

^{page 85 note *} The pyrometer—pieces used in these experiments were made under my own eye. Necessity compelled me to undertake this laborious and difficult work, in which I have already so far succeeded as to obtain a set of pieces, which, though far from complete, answer my purpose tolerably well. I had lately an opportunity of comparing my set with that of Mr Wedgwood, at various temperatures, in furnaces of great size and Steadiness. The result has proved, that my pieces agree as well with each other as his, though with my set each temperature is indicated by a different degree of the scale. I have thus been enabled to construct a table, by which my observations have been corrected, so that the temperatures mentioned in this paper are such as would have been indicated by Mr Wedgwood's pieces. By Mr Wedgwood's pieces, I mean those of the only set which has been sold to the public, and by which the melting heat of pure silver is indicated at the 22d degree. I am well aware, that the late Mr Wedgwood, in his Table of Fusibilities, has stated that fusion as taking place at the 28th degree; but I am convinced that his observations must have been made with some set different from that which was afterwards sold.

^{page 87 note *} In many of the following experiments, lead was used in place of the fusible metal, and often with success; but I lost many good results in this way: for the heat required to liquefy the lead, approaches so near to redness, that it is difficult to disengage the cradle without applying a temperature by which the carbonate is injured. I have found it answer well, to surround the cradle and a few inches of the rod, with fusible metal, and to fill the rest of the barrel with lead.

^{page 91 note *} In the same temperature, a mass of the glass of equal bulk would undergo the same change; but it would occupy an hour.

^{page 91 note †} A substance equally efficacious in restraining the penetrating quality of borax, was discovered by another accident. It consists of a mixture of borax and common sand, by which a substance is formed, which, in heat, assumes the state of a very tough paste, and becomes hard and compact on cooling.

^{page 95 note *} See Appendix

^{page 96 note *} I have found, that, in open fire, the entire loss sustained by the carbonate varies in different kinds from 42 to 45·5 Per cent.

^{page 100 note *} I am nevertheless of opinion, that, in some situations, experiments with compression may be carried on with great ease and advantage in such tubes. I allude to the situation of the geologists of France and Germany, who may easily procure, from their own manufactories, tubes of a quality far superior to any thing made for sale in this country.

^{page 111 note *} I was favoured with this account of it by the late Professor Robison.

^{page 115 note *} I measured the capacity of the air-tubes by means of granulated tin, acting as a fine and equal sand. By comparing the weight of this tin with an equal bulk of water. I found that a cubic inch of it weighed 1330.6 grains, and that each grain of it corresponded to 0.00075 of a cubic inch. From these data I was able, with tolerable accuracy, to gage a tube by weighing the tin required to fill it.

^{page 127 note *} The retentive power here ascribed to the procelain tubes, seems not to accord with what was formerly mentioned, of the carbonic acid having been driven through the substance of the tube. But the loss by this means has probably been so small, that the native properties of the carbonate have not been sensibly changed. Or, perhaps, this penetrability may not be so universal as I have been induced to think, by having met with it in all the cafes which I tried. In this doubt, I strenuously recommend a further examination of this subject to gentlemen who have easy access to such procelains as that of Dresden or of Seve.

^{page 135 note *} Turbo terebra, Lin.

^{page 140 note *} This was the size of barrel used in all the following experiments, where the fact is not otherwise expressed.

^{page 157 note *} This topic, however, has of late been much urged against us, and an unfair advantage has been taken of what Mr Playfair has said upon it. What he gave as mere conjecture on a subject: of collateral importance, has been argued upon as the basis and fundamental doctrine of the system.

^{page 158 note *} Edinburgh Transactions, Vol. V. Part, I. p. 60–66.

^{page 162 note *} This situation of things, is similar to what happens when small-coal is moistened, in order to make it cake. The dust, drenched with water, is laid upon the fire, and remains long wet, while the heat below suffers little or no abatement.

^{page 162 note *} Voyage towards the North Pole, p. 142.

^{page 162 note *} Philosophical Transactions, 1751, p. 212.

^{page 163 note *} “On peut donc regarder au moins comme tiès probable, que la prosondeur “moyenne de la mer n'est pas au-deffous de quatre lieues.” de la Place, Hist. de l'Acad. Roy. des Sciences, année 1776.

^{page 168 note *} Phil. Trans. 1777, p. 595.

^{page 170 note *} Saussure, Voyages dans les Alpes, tom. ii. p. 99–104.

^{page 173 note *} This state of viscidity, with its numberless modifications, is deserving of great attention, since it affords a solution of some of the most important geological questions. The mechanical power exerted by some substances, in the act of assuming a crystalline form, is well known. I have seen a set of large and broad crystals of ice, like the blade of a knife, formed in a mass of clay, of such stiffness, that it had just been used to make cups for chemical purposes. In many of my former experiments, I found that a fragment of glass made from whinstone or lava, when placed in a muffle heated to the melting point of silver, assumed a crystalline arrangement, and underwent a complete change of character. During this change, it became soft, so as to yield to the touch of an iron rod; yet retained such stiffness, that, lying untouched in the muffle, it preserved its shape entirely; the sharp angles of its fracture not being in the least blunted.