removed from the spirit-lamp, and after a minute or two, the water began to boil, and was rapidly dissipated in steam. Ammonia and ether were shown, although so exceedingly volatile, to act in the same manner; the ether, however, being decomposed whilst in the vibratory condition, in the same manner as it is by the action of platina wire, forming a peculiar acid. Iodine put upon the heated plate became fluid, and revolved in the same manner as other fluids, no vapors escaping whilst the high temperature of the metal was maintained; but when allowed to cool to the point of dull redness, it was immediately dissipated in violet vapors. The nitrate of ammonia fused on the glowing hot plate, and vibrated with great energy; but on cooling the capsule, the salt entered into vivid combustion. The repulsive action was shown by plunging a lump of silver at a glowing red heat into a glass of water. As long as its bright redness was maintained, there was no ebullition; but as it slowly cooled, boiling took place. In this experiment, it appeared as if the glowing metal formed around itself an atmosphere; and the contiguous surfaces of the water appeared like a silvered plate. The application of the principles involved in these phenomena to the tempering of metals was then explained. If a metal to be tempered is in a highly incandescent state, the necessary hardening will not take place on plunging it into water. It is, therefore, necessary that a certain temperature should be observed. Experiments were made to show that the repulsive power of the spheroidal fluid existed, nor merely between it. and the hot plate, but between it and other fluids. Ether and water thus repelled each other, and water rested on and rolled over turpentine. The bursting of steam-boilers came next under consideration; and it was shown that many serious explosions may be referred to the phenomena under consideration. In a great many cases, the explosions have occurred during the cooling of the boilers after the withdrawal of the fire. An experiment was shown in proof of the view entertained by M. Boutigny. A sphere of copper, fitted with a safety-valve, was heated, and a little water being put into it, it was securely corked up, and withdrawn from the lamp. As long as the metal remained red, everything was quiet; but upon cooling, the cork was blown out with explosive violence. The concluding experiment excited great interest. The production of ice in a vessel at a glowing red heat was a result so anomalous, that every one was desirous of witnessing the phenomenon for himself. It was beautifully performed by M. Boutigny, in the following manner :-A deep platina capsule was brought to a glowing red heat, and at the same moment, liquid sulphureous acid, which had been preserved in the liquid state by a freezing mixture, and some water, were poured into the vessel. The rapid evaporation of the volatile sulphureous acid, which enters into ebullition at the freezing point, produced such an intense degree of cold, that a large lump of ice was immediately formed, and being thrown out of the red-hot vessel, handed round to the company in the Section.

Mr. G. Rennie referred to some experiments of his own, in confirmation of M. Boutigny's views.

ON FIZEAU'S PROCESS OF ETCHING DAGUERRBOTYPE PLATES, and its Application to Objects of Natural History, by Mr. Goadby.-In a Daguerreotype portrait, the black parts of the plate consist of silver, the white of mercury, and the intermediate tint of a mixture of the two, the degree of darkness or light depending upon the excess either of the silver or of the mercury. In converting a Daguerreotype inte an engraved plate, it is necessary to etch away the dark parts and to leave the white untouched. This is done by immersing the plate in a fluid, consisting of dilute nitric acid, nitrous acid, chloride of sodium. and nitrate of potash. The nitric acid is so far diluted, that no decomposition can take place until the mixture is heated, when the chloride of sodium and nitrate of potash are decomposed, and chlo rine and nitrous acid are evolved. These attack and remove the sil ver, or the dark portions of the plate, but have no effect on the mer cury, so that the lights of the picture, being the mercurialized portions of the plate, constitute the etching ground, and effectually defend such portions of the Daguerreotype from the influence of the corroding fluid. After a time, those portions of the plate that have been acted upon by the chlorine, &c., become covered with a protecting coat of the chloride of silver: this must be removed by dilute liquid armo nia, when the biting may be continued by a fresh supply of the mixed acid. Grease and foreign matter must be removed by repeated washings in dilute acid and alkali, and by boiling in caustic potash. These cleansing operations must be repeated after every biting, after washing out the chloride of silver by the ammonia. The plate being thus bitten, but in a slight degree, is to be inked after the ordinary manner of engravers, and allowed to dry; the surface of the plate is then to be thoroughly polished, the ink still remaining in the corroded pot tions of the plate. It is now to be gilded by the electrotype, those parts alone receiving the gold that have been previously polished. The ink is then to be dissolved out of the hollows by potash: the parts that are gilded now constitute the etching ground, instead of the mercury, and the biting may be henceforth continued by nitric acid, in the customary usage of engravers. The plate thus etched, generally requires to be finished by the hand of the engraver, who has the advantage of a perfect, although faint picture to work upon. The amount of labor which he must bestow will depend upon the good ness of the Daguerreotype, and the success of the etching. M. Can det has fully established the successful application of this process to the purpose of illustrating Natural History, by copying from Nature and engraving several delicate and difficult dissections of the lower animals, particularly the nervous system of Aplysia and Tritonia (the latter much magnified,) and the nutrimental organs in situ of a cater pillar. These preparations, together with the engravings of them.

were handed round.

Dr. Carpenter stated, that a similar process had been employed for engraving microscopic objects, the discovery of which was due to Capt. Ibbetson. He exhibited some plates of blood globules, and other microscopic objects published by Dr. Donné, of Paris, which had been procured in this way.

ON A NEW METHOD OF CONVERTING RECTILINEAR INTO ROTARY MOTION, by Dr. Booth.-The object of the communication was to show the applicability of a new species of crank, termed by the inventor the sliding crank, to the steam engine, more especially in those cases where space is an object of primary consideration. One of the most important improvements effected by this motion is, that the distance between the shaft and the top of the cylinder is only onehalf the length of the stroke. Other advantages pointed out in the course of the paper were, that the friction on the sliding parts is nearly insensible; that almost all the parts of the engine have a rotary instead of a reciprocating motion; that all the subsidiary parts of a low-pressure engine are worked with great simplicity; and that in this construction, a longer stroke than in any other of the same dimensions may be introduced, and the expansive principle more fully developed.

Mr. J. Taylor made a few remarks, and observed that the effect of friction on the action of slides, seemed to be in general much overrated. Mr. Fairbairn observed, that the invention, if carried into successful operation, seemed adapted to work an improvement in marine engines, especially, where room was a matter of great importance, by lowering the position of the machinery, which appeared a great desideratum at the present day. He objected, however, to the difficulty of obtaining free access to some parts of the machinery; the raising of the piston cover, for example, on this construction, would be a laborious operation.

NASMYTH'S STEAM HAMMER FOR PILe Driving.-This machine has been described at former meetings of the Association. Dr. Greene now read a letter received from Mr. Nasmyth, dated Devonport, in which it was stated that at the first trial with a part of the machine at the manufactory it drove a pile 14 inches square, and 18 feet in length, 15 feet into the ground with 20 blows of the monkey, the machine then working 70 strokes a minute; the ground was a coarse ground imbedded in a strong tenacious clay, performing this work in 17 seconds. The entire machine is now in full action at Devonport for the embankment to be erected there to keep out the sea, and form a wet dock. He describes it as going far beyond what he had dared even to hope for, and that it is truly laughable to see it stick vast 66 feet piles into the ground as a lady would stick pins into her pin-cushion. The entire of the operations required to be performed on each pile from the time it is floated alongside of the stage until it is embedded in the solid foundation of slate rock is only 44 minutes. The great stage which carries the machine, boiler, workmen, and every thing necessary, trots along on its railway like a wheelbarrow and moves on, the diameter of a pile, the moment it has finished the last. It picks the pile up out of the water, hoists it high in the air, drops it into its exact place, then covers it with the great magic cap, which follows it as it sinks into the ground, then thump goes the monkey on its head, jumping away 70 jumps a minute. At the first stroke the

pile sank 6 feet, its advance gradually diminishing until in the hard ground above the solid slate rock it was reduced to 9 inches. Nothing can better prove the superiority of the principle of this invention, of getting the momentum by a heavy weight moving with smal velocity over the same momentum, as got, on the old principle, by a light weight moving with great velocity, than the state of the heads of the piles as driven by each process. Dr. Greene drew attention to a sketch of two heads of piles, one 56 feet long driven by a monkey of 12 cwt., falling from a great height, and making only one blow in five minutes, and requiring 20 hours to drive it; this, though protected by a hoop of iron, is so split and shattered on the head, that it would require to be re-headed to drive it any further. The other, although 66 feet long, was not even supported by an iron hoop, and the head is as smooth as if it were dressed off with a new plane. It was driven with a hammer 50 cwt. and only 3 feet fall, making 70 blows a minute.

RAILWAY GRADIENTS.-Mr. Fairbairn read a communication, the object of which was to show the importance of economizing the first cost of railways, by introducing steep gradients in difficult districts, whereby the expenses attendant upon tunnels, viaducts, and lofty embankments, would be avoided; whilst the author showed that the desired speed might be obtained by increasing the power of the locomotive. Originally, cylinders only of 10-inch diameter had been used, but at the present time, the engines are furnished with cylinders of 14,16, and 18 inches diameter. The maximum speed which had been originally calculated on, was 10 miles per hour, whereas, at the present time, the ordinary speed on the Great Western, with first-class gradients, is 40 miles. The paper was illustrated by many experiments which had recently been made with regard to gradients on the Manchester and Leeds Railway.

Mr. Whishaw confirmed these views by the results of practical experiments to the extent of nearly 4,000 miles, on nearly all the lines of British railways.

GLOBE OF THE MOON.-Sir J. Herschell exhibited a model of the globe of the moon in relief, expressing the forms and elevations of its mountains as seen in a good telescope. This beautiful and exquisite work he stated to be the performance of a Hanoverian lady, Madame Witte; modelled by her from actual observation through an excellent Fraunhofer telescope, in a small observatory at the top of her own dwelling-house; the selenographical positions and general contours of the principal craters and other leading features being first laid down on the smooth surface from Messrs. Beer and Maedler's micrometrical measures and charts. The diameter of the model is 12 inches S lines (Reinland measure,) or one 10,000,000th part of the moon's actual diameter. The scale of heights is, however, necessarily enlarged to double this amount, as otherwise the relief would be too low for distinctness. The material is a composition of mastic and wax, and the whole is worked out in such perfection of detail as to represent every visible crater and mountain peak-nay, even the minuter lines

of elevation which streak the so-called seas, &c. in their true forms and conventional proportions. In consequence, when properly illuminated, and placed at 30 or 40 feet distance, and viewed through a good telescope, the artificial is scarcely distinguishable from the real moon. The delicacy and precision of the work can only be appreciated by a microscopic examination. In fact, the whole model is stated by Madame Witte to have been executed with the aid of magnifying glasses. Sir J. Herschell accompanied his explanation of this model with several remarks on the physical constitution of the moon in respect of climate, atmosphere, moisture, &c., and compared its surface with the chart of part of Mount Etna, lent him for that purpose by Baron von Waltershausen, and with a drawing of his own of one of the principal craters as seen in his 20-feet reflector-placing the volcanic character of the ring mountains beyond all doubt. By the aid of a large chart by Messrs. Beer and Maehler, several of these, such as Aristarchus, Tycho, Kepler, Copernicus, &c. were pointed out, and their peculiarities described-their places on the model being fixed by the aid of brass circles, representing the moon's equator and meridians. This work, it is understood, will be submitted to the inspection of the Astronomical Society, on the resumption of their meetings in November. Speaking of the climate of the moon, Sir J. Herschell considered as probable the attainment of a very high temperature (far above that of boiling water) by its surface, after exposure to unmitigated and continual sunshine during nearly a whole fortnight. The moon, therefore, when at the full, and for a few days after, must be, in some small degree, a source of heat to the earth; but this heat, being of the nature rather of culinary than of solar heat (as emanating from a body below the temperature of ignition,) will never reach the earth's surface, being arrested and absorbed in the upper strata of an atmosphere where its whole effect will necessarily be expended in the conversion of visible cloud into transparent vapor. The phenomenon of the rapid dissipation of cloud (in moderate weather) soon after the appearance of the full moon (or of a moon so nearly full as to appear round to the unassisted eye,) which he stated himself to have observed on so many occasions as to be fully convinced of the reality of a strong tendency in that direction, seemed to him explicable only on this principle.

ON THE PROJECTION OF A STAR ON THE DARK LIMB OF THE

MOON, just before its Occultation. By Pof. Stevelly. This the Professor considered to be a result of diffraction. Sir Isaac Newton having observed the shadow of a hair placed in a strong beam of sunlight to be broader than the hair itself, was led to investigate the course of a ray as it passed by the edge of a body, like the edge of a knife placed across a hole in a window-shutter, through which a sunbeam is admitted. At some distance the rays proceeded in their usual straight courses; while he found that, at a certain distance, they were bent towards the edge; but the courses of the nearest rays were bent away from the edge, so as to form curves convex towards it. The undulatory theory enabled us to trace these curves, and they were known to be of the nature of the hyperbola,