oned by the difficulty of perfectly fusing substances of such different density as the materials employed. The materials, being properly prepared, are thrown at intervals into a crucible of Stourbridge clay, which will hold about 1600 lbs. weight of glass when fused. The mouth of the crucible is then covered with a double stopper, but not luted, to permit the escape of the moisture remaining in the materials, as well as the carbonic acid gas and excess of oxygen. It requires from 50 to 60 hours application of a rapid, intense, and equal heat, to effect the perfect fusion of the materials and to drive off the gas; during which time the unfused particles and excess of salts are skimmed off as they rise to the surface. The progress of fusion cannot be watched, nor can any mechanical means for blending the material during fusion be resorted to, lest the intensity of heat requisite for the production of a perfectly homogeneous glass should be diminished, the quality of the product being influenced by any inattention on the part of the fireman, as well as by the state of the atmosphere or of the wind. It has been ascertained, that there is a certain point or crisis of fusion at which the melted metal must be kept to insure a glass fit for optical purposes, and even when that point be attained, and the crucible shall furnish proper glass during several hours, should there be such diminution of heat as to require the furnace to be closed, the remainder of the metal in the crucible becomes curdy and full of striæ, and thus unfit for use. It is the same with the glass made for the flat bore tubes for thermometers, which are never annealed, because the smoke of the annealing furnace would render the interior of the bore unfit for the reception of the mercury. These tubes will only bear the heat of the blow-pipe when they are made from a metal which has been produced under all the favorable circumstances before described. It is, therefore, to be inferred, that the most homogeneous and perfect flint glass can only be produced by exposure to an intense and equable degree of heat, and that any excess or diminution of that heat is injurious to its quality.

The English method of manufacturing the flint plate for optical purposes, is thus described:-About 7 lbs. weight of the metal

is taken in a ladle of a conical shape from the pot, at the proper point of fusion, and then blown into a hollow cylinder, cut open, and flattened into a sheet of glass of about 14 inches by 20, and varying in thickness from ths to 4th of an inch. This plate is afterwards annealed, and in this state goes into the hands of the optician, who cuts and grinds it into the requisite form. When a glass furnace is about to be put out, whole pots of metal are sometimes suffered to remain in it, and cool gradually. The crucibles being destroyed, pieces of glass may be cloven from the mass of metal, softened by heat, and made to assume the requsite form, and then ground. It is believed that the excellent glasses made by Frauenhoffer, and other manufacturers on the continent, are produced by some such means. On attempting to cut glass ware, it is easily perceived if it be sufficiently annealed; if not, the ware is put into tepid water, which is heated, and kept at the boiling point during several hours; it is then suffered to become gradually cold. This method is more efficacious than re-annealing by the ordinary means. A piece of unannealed barometer tube of 40 inches in length, being heated and quickly cooled, contracted onlyth of an inch, whereas a similar piece, annealed by the usual means, contracted nearly th of an inch. Unannealed flint glass, being heated and suddenly cooled in water, exhibits the appearance of a mass of crystals; it is thence inferred that the process of annealing renders the glass more compact and solid; it thus becomes incapable of polarization.

Flint glass being remarkably elastic, has caused it to be used for chronometers. To prove its elasticity, a hollow ball of unannealed glass of 3 inches diameter, weighing about 16 ounces, was dropped, when cold, from a height of 7 feet upon a stone floor; it rebounded uninjured about 3 feet, but broke on falling to the ground after the rebound. Similar balls, both at a bright and a low red heat, were dropped from the same height, and both broke immediately without any rebound; thus demonstrating that its elasticity only exists while cold. Glass being sometimes deteriorated in the process of re-heating, not only in colour, but in its faculty of welding, by the sulphur

P 2

existing in the coal or coke used in the furnaces, this is prevented by occasionally throwing about a quart of cold water on the fire; the explosive vapour thus raised, carries off the sulphureous gas.

The process of annealing has the remarkable property of carrying off from the glass the reddish tint imparted to it by manganese; and in large masses, not only the reddish tint disappears, but the glass sometimes becomes green or blue, probably by the action of the sulphureous acid gas from the coke. The reddish tint will however return, and the greenish one disappear, should the annealed glass be afterwards re-heated or re-melted. Should the pot crack during fusion, and the flame or smoke come in contact with the melted metal, a green tint and abundance of dense striæ will be the consequence. Such an accident can only be repaired, if the crack be accessible, by throwing cold water on the exuding metal, which thus becomes gradually cooled, and itself forms a lute, so as to enable the process of melting to be continued. Long experience has shown that the best fuel for melting glass in the furnaces, is oven-burnt coke mixed with a small quantity of screened coal.

Mr. Pellatt illustrated the preceding paper by specimens of glass exhibiting peculiar effects of crystallization; among them were cylindrical solid pieces of flint glass, which, from being suddenly cooled by plunging them into water, had the interior entirely dislocated, and were merely held together by the exterior coating; portions of tubes showing the same effect; a portion of a vase of white glass dipped into blue glass of a greater densityin cooling, the interior white glass appeared to be crushed by the contraction of the exterior coating; a similar vase of white and blue glass of more equal density had cooled, and bore cutting without cracking; a mass of optical glass, exhibiting striæ, specks, and imperfections; which, together with the modes of manufacture, he explained.

In answer to several questions, Mr. Pellatt was not aware of any attempt having been made to cut the bulb of Prince Rupert's drops: he believed the peculiar property of the bursting of these

drops or tears, on the end being broken, arose from a crack suddenly commencing and extending itself rapidly throughout the mass, causing the dislocation of the particles. Flint glass is seldom sufficiently fluid to make these drops; they are generally made from glass which does not contain lead.

Alluding to the use of plate glass in Nasmyth's Pneumatic Mirror, he observed that, owing to the absence of lead, plate glass was purer and more homogeneous than flint glass, and the equality of thickness produced by grinding and polishing enabled the curve caused by the pressure of the atmosphere to be very regular.

The use of coke as a fuel, by the regularity of its combustion, assists materially in producing good results, and prevents the injury which frequently arises from a difference in the heating powers of various coals: unfortunately, the form of the furnaces causes the greatest heat to be in the centre, thus acting most powerfully upon the backs of the pots, instead of being equally distributed around them, which would be more desirable and would insure better results.

Mr. Pellatt still continued to use nine parts of coke mingled with one part of small coal in preference to any other fuel. He had abandoned the use of gas coke, and now purchased small coal at a low price, which he converted into a moderately-hard coke, rather less dense than that used for smelting iron. In the north of England, a charge of coal generally remained in the oven during 48 hours; in London, only 36 hours; he made lighter charges and coked them in 24 hours. He still found the calorific effect of 8 or 9 lbs. of coke to be equal to that of 12 lbs. of coal; in his ovens, 20 cwt. of coal produced 14 cwt. of coke.

Mr. Parkes inquired, which was the best method of annealing tubes for water gauges on boilers? He generally used those prepared by Mr. Adie, of Liverpool, who annealed them by placing them in cold water and gradually raising the temperature to the boiling point, at which it was kept for 24 hours; yet, in spite of these precautions, which generally were successful, he had seen twelve of these tubes break in one day, while an ap

parently ill-made tube had lasted six weeks. He found thin tubes last longer than thick ones. He was in the habit of removing the stains of bog water from his boiler guages by scouring them with emery; when reheated, they invariably broke; after many experiments, he tried the use of acid, which answered perfectly, and no tubes were subsequently broken.

Mr. Pellatt recommended boiling as a safe and good mode of annealing all kinds of glass; in the ordinary method of annealing, thick and thin ware is often subjected to the same process, and remains in the leer for the same period: this would account for the superior duration of the thin tubes. He attributed the fracture of the tubes to the tension of the exterior coating and the vibration caused by the process of cleaning: this effect was so well known that old tube could scarcely be sold, as it generally broke in cleaning.

Mr. Hawkins observed, that tubes almost invariably broke in merely removing dust from the inside, whether it was done by rubbing with a tight packing or by slightly wiping it out. In some experiments on the production of carbonic acid gas, he used glass tubes of of an inch internal diameter and of an inch thick: they bore a pressure of 100 atmospheres. Some wrought-iron tubes into which holes of of an inch diameter were drilled and pieces of glass inserted, bore a pressure of 600 atmospheres.

List of Patents

Granted by the French Government from the 1st of July to the 31st of October, 1839.-(continued.)

PATENTS FOR FIVE YEARS.

To Wiss, of Paris, represented in Paris by M. Perpigna, advocate of the French and Foreign Office for Patents, 2, ter: Rue Choiseul, for improvements in boots and gaiters,

Roumestan, of Paris, represented in Paris by M. Perpigna, advocate, for improvements in binding.

Fournels, represented in Paris by M. Perpigna, advocate, for a smoke consuming apparatus.