The average quantity of all descriptions annually retained for home use in the three years ending in 1793 was 373,782 cwt., while the average quantity consumed during the three years ending in 1829, amounted to 364,156 cwt.; showing an annual decrease in the manufacture of 9626 cwt., notwithstanding the great increase of population and the still greater strides in civilisation made during the interval by all classes of the community. If the quantities produced during equal periods of three years, immediately preceding and following the last and most considerable advance in the rate of duty, are then contrasted with each other, the effect appears in a yet more striking point of view. The annual average quantity made for home use during the three years ending in 1812 was 413,414 cwt., while the average of the three following years ending in 1815 was 264,931 cwt.; showing an immediate falling off of 148,483 cwt., being upwards of 35 per cent. upon the larger quantity; a circumstance which could not fail, among other evils, to bring distress and misery upon a considerable number of operative manufacturers. On the other hand, a diminution in the rate of duty on plate glass was effected on the 5th of July, 1819, it being then lowered from 98s. to 60s. per cwt. As a consequence, the quantity manufactured has since been steadily and progressively increasing. During the three years preceding the abatement, the average quantity annually manufactured was 6209 cwt., yielding a gross revenue of 30,4241., whereas the average quantity made during the three years ending in 1829 amounted to 15,235 cwt., and the revenue produced was 45,7051., being an increase of more than 50 per cent., derived from a rate of duty diminished to the extent of 40 per cent. Could any facts more forcibly point out the pernicious tendency of heavy duties upon articles of domestic manufacture, or more clearly indicate the course which it were wise to follow in remodelling to as great an extent and as quickly as is practicable, this branch of our financial system? It is much to be regretted that any circumstances should have arisen to delay the execution of the expressed intention of the government altogether to remove the duties upon glass. Whenever this measure shall be accomplished, it can hardly fail to induce such an extension of the manufacture as will prove generally beneficial to the community. The abolition of these duties would be accompanied by the still further advantage of removing all those vexatious regulations, and restrictions under which the manufacture is now carried on, and which will cease, as a matter of course, when the article is no longer an object of revenue. It is principally owing to these restrictions that so much foreign glass is now brought into this country in the face of what may be considered an amply protecting duty. Foreign manufacturers are allowed to make any and every article out of that quality of glass which will most cheaply and advantageously answer the end, while our own artists are forbidden to form certain objects, except with more costly materials, which pay the higher rates of duty. Nor is this restriction only commercially wrong, since it forms matter of just complaint on the part of chemists, that they are unable to procure utensils fitted for effecting many of the nicer operations connected with their science, because the due protection of the revenue is thought to require that such utensils shall be formed out of that quality of glass alone which, apart from all considerations of price, is otherwise, from its properties, really unfitted for the purpose. Relaxations are, indeed, sometimes made on this head in particular cases by the commissioners of Excise; but the trouble necessarily attending applications to a public board is greater than can be compensated by the trifling money advantage that can result in each case to the manufacturer, and the interests of science are, consequently, made to suffer. CHAP. II. ON THE VARIOUS INGREDIENTS EMPLOYED IN MAKING GLASS. GLASS ALWAYS COMPOSED OF SILEX WITH ALKALI. DIFFERENT DESCRIPTIONS OF GLASS. SEA-SAND. SODA AND POTASH.PEARL-ASH.- BARILLA. KELP. WOOD-ASHES. NITRE. MANGANESE. ARSENIC. BORAX. LITHARGE.MINIUM. -CHALK. UNDER the general name of glass, chemists comprehend all mineral substances, which, on the application of heat, pass through a state of fusion into hard and brittle masses, and which, if then broken, exhibit a lustrous fracture. Most glasses are transparent also; and the non-existence of this property is generally owing to the presence of some foreign and unessential substance. The glass of commerce that beautiful manufacture to which the generic name is most commonly applied does not include so wide a range of bodies; and is always composed of some siliceous earth, the fusion and vitrification of which has been occasioned by certain alkaline earths, or salts, and sometimes with the aid of metallic oxides. There are five different and distinct qualities of glass manufactured for domestic purposes; Flint glass, or crystal; Crown or German sheet glass; viz. Bottle or common green glass; and Plate glass; the materials and the processes used in making which form the subject of our present enquiry. Before commencing the description of any of the manipulations employed in this interesting manufacture, it will be better to give a general account of the different materials used, and to point out how the particular qualities of glass are influenced by the properties of those various ingredients. Each of the five descriptions contains, in common with the others, two ingredients, which, indeed, are essential to their formation - silex and an alkali. The variations of quality, and distinctive differences observable in glass, principally result from the kind of alkali employed, and its degree of purity, as well as from the addition of other accessary materials; such as nitre, oxide of lead or of manganese, white oxide of arsenic, borax, or chalk. Silex is not equally proper in all its forms for the composition of glass. Sea sand, which consists of spherical grains of quartz, so minute, as to be qualified for the purpose without any preparation except careful washing, is the form wherein silex is most commonly used for the purpose in England. All sea sand is not, indeed, equally applicable to the glass-maker's purpose. That used in this country for making the finer descriptions of ware is usually obtained, either from the port of Lynn, in Norfolk, or from Alum Bay, on the western coast of the Isle of Wight. The best glass was formerly made with common flints, calcined and ground in the manner already described, as used in the manufacture of pottery, and hence the name which it acquired of flint glass. The employment of silex in this form is now wholly discontinued in glasshouses, as it is known that some qualities of sand answer the purpose equally well, while the labour and expense of calcining and grinding are saved by the substitution. Both soda and potash are well adapted to the purpose of making glass. They are used in the form of carbonates; that is, holding carbonic acid in combination with themselves as bases. The acid flies off during the progress of the manufacture, and the result is a compound of silex and alkali. As already stated, the quality of glass is influenced by the degree of purity of the alkali. For making the finest flint glass, pearl-ash, which is potash in a purer form, must be used. This alkali must previously be L still further purified by solution and subsidence, and then evaporating the fluid to dryness. By this purification a loss is sustained, amounting to between 30 and 40 per cent. in the weight of pearl-ash. Coarser kinds of alkali, such as barilla, kelp, or wood-ashes, which are combined with many impurities, are employed for the production of inferior glass. Complete fusion and vitrification are accomplished by these means, the impurities even being of a nature to assist towards the production of these effects. The green colour imparted to glass, is produced by the iron, which is present in a greater or less degree in these coarser alkaline substances. Barilla, when sufficiently cheap, is always chosen preferably to wood-ashes or kelp. The recent abatement of the import duty levied upon this article of commerce will, therefore, probably tend to the increased consumption of barilla in glasshouses. A very small proportion of nitre is used in the composition of glass, to occasion the destruction of any carbonaceous matter which may exist in the ingredients. This salt must be added previous to the fusion of the glass. At a degree of heat much below that of the furnace, nitre will decompose, giving out much oxygen, and maintaining such metallic oxides as may be present in their highest state of oxygenation. It is thus of use in fixing arsenic, the volatile property of which increases as it approaches the metallic state. Oxide of lead, in the form of either litharge or minium, is essential to the making of flint-glass, into the composition of which it enters very largely. This metal acts, in the first place, as a most powerful flux, promoting the fusion of all vitrifiable substances at comparatively low temperatures. It is also permanently beneficial in imparting highly valuable properties to the glass of which it forms a part. This, by its means, is rendered much more dense; has a greater power of refracting rays of light; possesses more tenacity when red-hot, causing it for that reason to be more easily worked; and is rendered more capable of bearing un |