Chalcedony, had been considered as impossible formations, contradictory to the laws by which Nature acts in the Stalactite process *, yet the primary form of the Carbonate of Lime is nevertheless exhibited by the Stalactites of the Cavern of Antiparos, and the primary form of the Hydrates of Silica by the Stalactites of blue Chalcedony brought from the Hungarian Mines.

E. D. CLARKE.

Cambridge,

Jan. 6, 1821.

*"PIERRES QUARTZEUSES QUI NE CRISTALLISENT PAS. Il y a diverses pierres dans lesquelles la matière quartzeuse domine considérablement, et qui néanmoins ne cristallisent jamais, &c. Tels sont le silex, l' agathe, le cachalon, la calcédoine et ses varietes, l' opale," &c. Patria, Hist. Nat. des Minéraux. Tome II. p. 129. Paris, An. IX. The French author, however, seems afterwards aware that his mode of classification was liable to an exception in the instance of Chalcedony. "La calcedoine (Ibid p. 165.) en génèral ne cristallise pas, non plus que le silex, cependant il y a quelques morceaux où la cristallisation semble n'être pas equivoque."

XIV. On the Application of Hydrogen Gas to produce

a moving Power in Machinery; with a Description of an Engine which is moved by the Pressure of the Atmosphere upon a Vacuum caused by Explosions of Hydrogen Gas and Atmospheric Air.

THERE is scarcely any uniform operation in the Arts which might not be performed with advantage by machinery, if convenient and economical methods could be found for setting such machinery in motion. The extensive application of machinery, therefore, depends much upon the number and various capabilities of the engines which can be employed to produce moving force. Even the most perfect engines at present employed for this purpose, are not capable of being applied universally; but each has a province peculiar to itself, beyond which the use of it cannot be extended with profit or convenience.

Two of the principal moving forces employed in the Arts are Water and Steam. Water has the singular advantage, that it can be made to act at any moment of time without preparation; but can be used only where it is naturally abundant. A steam-engine, on the contrary, may be constructed, at greater

or less expense, in almost any place; but the convenience of it is much diminished by the tedious and laborious preparation which is necessary to bring it into action. A small steam-engine, not exceeding the power of one man, cannot be brought into action in less than half an hour: and a four-horse steam-engine cannot be used under two hours preparation.

These limitations exclude the use of water and steam, as moving forces, in all works which are much interrupted, and discontinued at considerable intervals, and subject to a change of place.

The engine, in which hydrogen gas is employed to produce moving force, was intended to unite two principal advantages of water and steam; so as to be capable of acting in any place, without the delay and labour of preparation. It may be inferior, in some respects, to many engines at present employed; yet it will not be wholly useless, if, together with its own defects, it should be found to possess advantages also peculiar to itself.

The general principle of this engine is founded upon the property, which hydrogen gas mixed with atmospheric air possesses, of exploding upon ignition, so as to produce a large imperfect vacuum. If two and a half measures by bulk of atmospheric air be mixed with one measure of hydrogen, and a flame be applied, the mixed gas will expand into a space rather greater than three times its original bulk. The products of the explosion are, a globule of water, formed by the union of the hydrogen with the oxygen of the atmospheric air, and a quantity of azote, which, in its natural state, (or density 1), constituted .556 of the bulk of the mixed gas. The same quantity of azote is now expanded into a space somewhat greater than three times the original bulk of the mixed gas; that is, into about six times the space which it before occupied its density therefore is about that of the atmosphere being unity.