in the year named. This highly-talented officer gave instructions to the Messrs. Rennie for the construction, under the eye of the inventor, of the necessary apparatus and appliances for the purpose.

In

An air-pump of considerable dimensions constituted the main feature of the scheme, and this was formed on a perfectly novel plan. It consisted of a cast-iron cylinder, closely resembling in exterior appearance that of an ordinary land steamengine, but very different in its internal arrangement. The piston of the pump was made up of a series of cast-iron rings, and these were pressed out against the sides of the accuratelybored cylinder by springs of steel. The effect was to make the piston perfectly air-tight, and yet capable of being easily moved upwards and downwards in the cylinder. There were no valves in the piston as there are in those of almost all air-pumps. employed in manufacturing processes. The base of the cylinder was a hollow casting of iron, and so was its cover. these hollow castings the inlet and outlet valves were placed. The upper casting contained sixty-four small apertures, which were covered by small pieces of steel saw-plate, each about two inches long by one inch wide, and fastened by a screw at one end. These delicate springs were in fact the valves. Thirty-two of them were made to open to the atmosphere, and thirty-two to the exhaust or vacuum pipe. The hollow base or bed-plate of the cylinder was furnished in a precisely similar manner. The diameter of the cylinder was three feet six inches, and the length of stroke of the piston three feet. The pump was placed vertically, and immediately below the working-beam of the engine to which the piston-rod was attached.

By this method of construction the air-pump became double-acting, and whether the piston was ascending or descending, it constantly exhausted air from the vacuum tube through the inlet valves, and discharged it through the outlet series. Nothing in the shape of machinery could work more smoothly than did this pump, and this arose mainly from the peculiar character of the valves. The cost of the whole apparatus, with cast-iron exhaust tube, 200 feet in length, ten inches in diameter, and face-jointed, was about £400. This is mentioned for the information more especially of the Pneumatic Despatch Company, who would find the same system of exhaustion-as we think-more effective and less expensive than the centrifugal disc plan which they have adopted at the NorthWestern Railway station for pumping the mail.

It has been said that the exhaust tube was 200 feet in length. This arose from the fact that the engine-house had been erected at that distance from the stamping presses. Instead of being carried underground, as in the Pneumatic

Despatch system, the tube was in this case carried over the roofs of the coining-rooms, and, descending therefrom, was attached to the great vacuum chamber.

The "vacuum chamber" had existed from the period of the erection of the Mint, and was originally devised by Messrs. Boulton and Watt, the well-known engineers of Soho. They had supplied a steam-engine of ten-horse power, and two single-acting air-pumps, each of which discharged air only in its downward stroke, for exhausting the chamber. This cumbrous and comparatively costly, though for its day very ingenious and valuable arrangement, was set aside when the new airpump came into use. It had performed its mission, and was henceforth to be reserved as a duplicate in the event of the derangement, by accident or otherwise, of its modern supplanter. A regulating, or relief valve, and a barometer gauge fitted to the vacuum chamber, enabled the attendant to control and adjust the extent of rarefaction within the latter.

It will now be comprehended that, at all times, when the engine and pump are in action, a vacuum of more or less extent must exist in the chamber. The chamber, it may be explained moreover, is a horizontal tube of iron about fifty feet long, and two feet six inches in diameter. It runs along the floor of the Mint pump-room, in a line parallel to that in which the eight coining presses stand. Arranged along the top of the vacuum chamber, and supported by pipes opening into it, are a series of eight cylinders. These are vertical and fitted with pistons, the rods from which are connected by levers and cranks with the presses. The cylinders are open topped, and consequently their pistons are exposed to the pressure of the atmosphere, once a vacuum exists below them. This is the case when the pneumatic valves within the cylinders open to the vacuum chamber. The air within the cylinders then rushes down to the exhausted tube, the atmospheric column drives the pistons after it to the bottom of the cylinders, and the pistons drag with them the central screws of the coining presses. The instant that the beautifully engraved dies are thus made to come into contact with the discs of gold, the latter receive by the force of impact their impressions. The presses then rebound, carrying with them their pistons. The pneumatic valves again open self-actingly, the dies descend upon new blanks supplied to them by mechanical fingers, another batch of sovereigns is pumped into bright and glorious being, and so long as the great air-pump is exhausting the vacuum chamber, and the presses are fed with blanks, so long the series of minor pumps will proceed with their work, and streams of gold, silver, or bronze coins will flow down from the presses into reservoirs placed below to catch them.

THE DISSOCIATION OF WATER.

THIS term will seem strange to English ears, but perhaps not more so than its equivalent, "la dissociation de l'eau" to the French, and at any rate it seems most advisable to preserve the name given by M. H. St. Claire Deville to the very interesting phenomenon described by him to the French Academy, in a paper of which we proceed to give an account.*

M. Deville commences by stating that if a tolerably rapid current of hydrogen is made to traverse a porous earthen tube, and the gas which escapes is collected, it is found to be, not hydrogen, but in round numbers, oxygen 21, and nitrogen 79. "Thus the hydrogen is dispersed through the atmosphere, and air is absorbed by the porous tube in virtue of endosmose; and in spite of the pressure of some centimetres of water, or mercury, into which the abducting tube is plunged, and which is maintained in the interior of the apparatus." If the porous tube is introduced into an impermeable porcelain tube, shorter than itself, and closed at each end with a cork, through which the porous tube is inserted, a space is enclosed in which any kind of gas can be admitted. For this purpose the corks are pierced so as to admit an exit and entrance tube of glass. The porous tube is similarly provided, and if a current of carbonic acid gas is made to traverse the space between the porous tube and the porcelain tube, while hydrogen is driven through the former, the hydrogen changes its place, and may be inflamed at the exit where the carbonic acid might have been expected, while the porous tube allows nearly pure carbonic acid to escape. These facts, observes M. Deville, are in accordance with the observations of Professor Graham and M. Jamin.

If the preceding apparatus is placed in a furnace supplied with dense fuel, affording a heat of 1100° to 1300° (C), it will suffice to demonstrate the spontaneous decomposition of water, a phenomenon which M. Deville terms dissociation. To accomplish this, vapour of water is passed through the porous tube, instead of the hydrogen in the former experiment, while carbonic acid gas traverses the space between the two tubes. The gases that emerge are collected over a bath containing potash, to absorb the carbonic acid, and received in small glass jars. When the furnace is in activity the tubes yield an explosive mixture of oxygen and hydrogen-the elements of water.

Thus it appears that part of the water is decomposed or "dissociated" in the porous tube, "the hydrogen attracted (appelé) by the carbonic acid in the annular interspace, has traversed the walls of the porous tube, and separated itself by

*Comptes Rendus, Feb. 2, 1863.

the simple action of a filter from the oxygen which remains in the interior tube. A considerable quantity of carbonic acid is attracted in a contrary direction, according to the rule established in the preceding experiments, and mingles with the oxygen." This is the broad explanation which M. Deville gives, but he remarks that the action is in reality more complicated; as, when the hydrogen comes in contact with the heated carbonic acid, some carbonic oxide is formed, and a certain quantity of the latter gas is found to replace the hydrogen. It is also difficult to avoid the escape of some hydrogen, which leaves the oxygen in excess, and the water contains enough air to effect the result. "The carbonic acid deter

mines the separation of the gases by endosmose; but it may also act mechanically." M. Deville adds that he has proved that water heated in a platina tube almost to the fusion point of the metal, reconstitutes itself entirely as it makes its exit, or is not decomposed in a sensible quantity.

In explanation of these facts M. Deville states that the temperature of the combustion of hydrogen in oxygen is not equal to 2500° C, at which point the volume of the gases, estimated at 0° C, is multiplied tenfold, and beyond which the complete decomposition of water takes place. "But this decomposition is accompanied by a considerable absorption of latent heat, to the extent required to keep the molecules of oxygen and hydrogen at a distance beyond the radius of the sphere of their affinity. Thus the decomposition of a body resembles the ebullition of a liquid, the principal characteristic of which is invariableness of temperature under the same pressure." Admitting the comparison between decomposition and ebullition, M. Deville regards "dissociation," or partial decomposition at a temperature below the decomposing point, as resembling the evaporation of liquids below their boiling point. "If," he observes, "you shut up some water in a small vessel, at ordinary temperature, the evaporation is slight, on account of the tension that is produced when vapour is formed; but if you introduce a piece of chloride of calcium, the water evaporates until that substance is saturated, the tension remaining constant all the time." He imagines the carbonic acid to carry off the dissociated gases just as the chloride of calcium absorbs the vapour, and then the process of dissociation, like that of evaporation, is enabled to go on.

JAMAICA SEA-SIDE NOTES.

BY THE HON. RICHARD HILL.

TURTLE.--The chelonians known by our fishermen distinctively as the green turtle and the hawk's-bill turtle, are common on the outside shoals of Old Harbour Bay. I went down to the beach where the turtlers were busy repairing and adjusting their nets preparatory to a start, and learned that the turtle are now (February) numerous, and that they will continue frequenting the shoals here in numbers till the month of July. The hawk's-bill turtle breed here, but the green turtle are migratory visitors. They come, as the fishermen express it, clean, that is free from barnacles, and are the yearly thrown off broods of the older ones to windward. They pass to leeward in successive schools, and only loiter among the marine pastures of Testudinaria, or the turtle-grass, to feed, and then they depart onward and deposit their eggs on the sands of the Central American shoals. The clean condition of their plates, or epidermic scuta, is the evidence that they are the full-grown young of the windward turtles in search of those unfrequented solitudes where alone they are reconciled to deposit their eggs and watch them into life. It would seem that the fields of turtle-grass are not extensive to windward, and that migration is necessary to enable the young generation to subsist. Had the extensive fields of the Testudinaria here been the unfished wildernesses that the Honduras sands are, or the shoals of the Mexican gulf are expected to be, the Chelone Midas would remain and breed, for the bay of Old Harbour is almost one continued pasture. But being pretty well resorted to for fish for the coast markets, the space, great as it is, is not solitary enough for the turtle, and they make it only a passing visit, and go further onward, but always moving socially.

It is in shallow waters, where the islets in the neighbourhood of the continent stand amid marine meadows, and afford occasional stretches of dry sand, that the turtles congregate to breed. The sea in these island clusters is tranquil, and the turtle may be observed browsing there, gathered into flocks, but so watchful and timid that they take flight upon any intrusion. They rise, however, soon to breathe; but if their confidence is not re-established, nothing promises safety but a dash onward into the open sea. Surprised upon land, they are a sure capture. Their power of moving onward is at best but a hasty scumble on shore. Caught and turned upon the back, they flounder and pivot about, but do not regain their fins.

The SNAPPERS, Mesoprions, in one or other of the species,