So far as they involve distinct manufacturing processes, these varicties of glass will be here noticed in the above order. Such of the divisions as result from the application of special methods of ornamentation, and as come under the head of art glass, do not fall within the scope of this article. CROWN GLASS.-This, with sheet or cylinder glass, forms all ordinary blown window glass. Both varieties are precisely the same in composition, being a mixture of sodic and calcic silicates, and differ only in the manner in which the sheets of finished glass are produced. The raw materials employed for this and all other kinds of glass vary within rather wide limits, and, as already explained, the form in which the sodic and calcic compounds are used may also be varied. The following composition of batch for window glass must therefore be regarded as only one out of very many mixtures in use :

Sand, purified..........

100 parts Chalk, or limestone..................... 85 to 40 Sulphate of soda.............................. 40 to 45 Cullet.

To these materials a minute proportion of white arsenic and peroxide of manganese, as bleaching agents, may also be added. Crown glass was, in the early part of the present century, the only form of window glass made in Great Britain, and consequently it was generally recognized as English window glass, having been manufactured only on a very limited scale in any other country. Since the introduction of sheet-glass making, the crown-glass industry has steadily declined, and now its manufacture may be regarded as practically a thing of the past, not more than one or two crown fur. naces being in operation. Seeing that it possesses little more than an historical interest, it is now unnecessary to enter into much detail as to the processes employed in the manufacture of crown glass. The metal being brought to a proper condition for working, the "gatherer" dips into the pot of metal an iron pipe or tube, & or 7 feet in length, of the shape shown in fig. 6, heated at that end which up the glass, and, by turning it gently round, gathers about 1b

takes

FIG, 6.-Blowing Tube.

of liquid glass on the end of it. Having allowed this to cool for a little, he again dips the rod into the pot, and gathers an additional quantity of from 2 to 3 lb. This is also permitted to cool as before, when the operation of dipping is again repeated, and a sufficient quantity of metal, from 9 to 10 lb weight, is "gathered," to form what is technically called a table or sheet of glass. The rod, thus loaded, is held for a few seconds in a perpendicular position, that the metal may distri. bute itself equally on all sides, and that it may, by its own weight, be lengthened out beyond the rod. The operator then moulds the metal into a regular form, by rolling it on a smooth iron plate, called the "marver," a term corrupted from the French word marbre. He then blows strongly through the tube, and thus causes the red-hot mass of glass to swell out into a hollow pear-shaped vessel. The tube with the elongated sphere of glass at the end of it is then handed to the "blower," who heats it a second and third time at the furnace, pressing the end, between each blowing, against the bullion bar, so called from the part thus pressed forming the centre of the sheet or "bull's eye," and by the dexterous management of this operation, the glass is brought into a somewhat spherical form. The blower now heats a third time at the bottoming hole," and blows the metal into a fullsized flattened spheroid. When this part of the process has been completed, and the glass has been allowed to cool a little, it is rested on the "casher box," and an iron rod, called a "pontil" or punty rod, on which a little hot metal has been previously gathered, is applied to the flattened side, exactly opposite the tube, which is detached by touching it with a piece of iron, dipped beforehand in cold water, leaving a circular hole in the glass of about 2 inches diameter. Taking hold of the punty rod, the workman presents the glass to another part of the furnace called the "nose hole," where the aperture made by its separation from the tube is now presented and kept until it has become sufficiently ductile to fit it for the operation of the flashing furnace. Whilst here, it is turned dexterously round, slowly at first, and afterwards with increasing rapidity; and the glass yielding to the centrifugal force, the aperture just mentioned becomes enlarged. The workman, taking great care to preserve, by a regu. lar motion, the circular figure of the glass, proceeds to whirl it round with increasing velocity, until the aperture suddenly flies open with a loud ruffling noise, which has been aptly compared to the unfurl. ing of a flag in a strong breeze; and the glass becomes a circular plaue or sheet, of 4 feet diameter, of equal thickness throughout, except at the point called the bullion or bull's eye, where it is attached to the iron rod. The sheet of glass, now fully expanded, is moved round with a moderate velocity until it is sufficiently cool to retain its form. It is carried to the mouth of the kiln or annealing arch, where it is rested on a bed of sand and de.

tached from the punty rod by a shears. The sheet or table is then lifted on a wide pronged fork, called a faucet, and put into the arch wise, and supported by iron frames to prevent their bending. From to be tempered, where it is ranged with many others set up edge 400 to 600 tables are placed in one kiln. A sketch of the interior of a crown-glass house, during the progress of these operations, has been given in Plate VI., fig 2. The kiln having been clayed up, the fire is permitted to die out, and the heat diminished as gradually as possible. When the glass is properly annealed, and sufficiently cold to admit of its being handled, it is withdrawn from the oven after the removal of the wall built into the front of the arch, and is then quite ready for use. made will cut into slabs 30 inches across, from which squared pieces The largest sized tables of crown glass measuring 38 by 24 or 35 by 25 inches may be obtained.

SHEET GLASS, as already mentioned, is the same in composition as crown glass, which it has now entirely supplanted. The success of sheet glass is due principally to the fact that it can be produced in sheets of much greater dimensions than is possible in the case of crown glass; it is free from the sharp distorting striæ and waves common in crown class; there is no loss of glass as there is with the bull's eye of crown; and modern improvements effected in the manufacturing process leave little distinction in brilliancy of surface between the two qualities. Sheet glass is made on the greatest scale in Austria, Germany, and Belgium, and it was long distinguished in the British market as German sheet glass. In 1832 Chance of Birmingham and subsequently Hartley & Co. of Sunderland introduced the manufacture into England, and in the hands of these firms, ee well as of others who followed in their footsteps, the industry prospered and developed, till it has now attained dimensions equal to those it has reached in most of the Continental nations, where the art was long established before it came into use in England. Sheet-glass making involves two principal operations,--(1) the blowing of the cylinder, and (2) the opening, flattening, or spreading of the glass. The structure and internal arrangements of the melting furnace is practically the same as in the case of own glass. The ordinary type of oblong furnace usually contains 10 pots-5 in each side of the fire-grate each pot being of a capacity of about 1 ton or 22 cwt. of metal. Radiating from the work-holes, and raised about 7 feet above the floor level, or a correspondingly deep sunk pit, are ten long stages with an open space between each sufficient to allow the workman to swing about his long tube freely in forming the elongated cylinder of glass. Fig. 7 is a ground plan of common sheet-glass furnace and staging of planks c, at the extremities d of which are placed a tub of water and a wooden moulding-block. Instead, however, of having these stages erected in front of the melting furnace, it is now a common practice to gather and block the glass at the melting furnace, and to blow it in front of a separate oblong reheating or blowing furnace, from each opening of which the wooden stage runs out over a pit excavated to the depth of 7 feet or thereby. Common bricks may be used for the construction of this reheating furnace, as the heat required in it is by. no means intense.

Blowing. The charge or batch requires about 16 hours to melt, and other 8 hours are consumed in cooling it to FIG. 7.-Plan of Sheet-Glass Furnace. the working consistency. When the metal is ready for working, the workmen take their stations, each having his own pot and stage and also an assistant, and commence making the cylinders. After gathering the quantity of metal required (which on an average amounts to 20 lb), the workman places it in a horizontal position in the large hollow of a wooden block (fig. 8), which has been hollowed so that, when the workman turns the metal, it shall form it into a solid cylindrical mass. In the meantime, the assistant, with a sponge in his hand, and a bucket of water by his side, lets a fine stream of water run into the block, which keeps the wood from burning, and also gives a brilliancy to the surface of the glass. The water, the moment it comes in contact with the glass, is raised to the boiling point, and in that state does no injury to the metal; but it is only when the metal is at a high temperature that such is the case; for, whenever the glass is cooled to a certain degree, it immediately cracks upon coming in contact with water. When the workman perceives that the mass of metal is sufficiently formed and cooled (fig. 9), he raises the pipe to his mouth at an angle of about 75 degrees, and