CHAP. XI. ON THE PRINCIPAL DEFECTS OBSERVABLE IN GLASS. THREADS -RENDER STRIE RENDER GLASS UNFIT FOR OPTICAL PURPOSES.- THE principal defects observable in manufactured glass, are striæ, threads, tears, and knots. These, when they occur to any extent, all impair its beauty, and some of them injure its actual quality. Although it is not difficult to attain such an amount of proficiency in the manufacture as will preserve the materials from these evils in their extreme degree, yet, altogether to avoid their occurrence, and to obtain glass of a perfect quality, is a task that long, and with only doubtful success, has engaged the thoughts and labours of men devoted to scientific pursuits. The difficulties that attend the attainment of this object are sufficiently proved by the fact that, during ten years, one of the most considerable and most scientific opticians in London has been disappointed in his efforts to procure a disc of flint glass only five inches in diameter, sufficiently fitted, by the absence of defects, to be employed in the construction of a tele scope. Striæ are undulating appearances, perfectly vitrified, and equally transparent with any other part of the glass they do not occasion any roughness or inequality in the surface, but result from a want of congruity in the composition of the particles which make up the substance: in other words, the structure is not perfectly homogeneous; and although each different portion may be altogether good in itself, and the whole mass, if made up of any one of these portions, would be equally perfect in itself, yet, the whole acting without any uniformity, the rays of light in passing through them are bent or refracted differently, and the objects beyond appear distorted. This condition must exist to a considerable extent to be easily discernible by the naked eye, or detrimental to the quality of glass, when applied to the more ordinary purposes of use or ornament; but glass striated in a scarcely perceptible degree, is yet wholly inapplicable to the construction of optical instruments, where the objects they are intended to present to the eye will be many times magnified; and where, consequently, every defect or distortion that accompanies their transmission through the glass will be equally enlarged. The end proposed in the employment of these philosophical instruments, is the minutely accurate examination of distant or very diminutive objects; and this purpose it is evident must be completely frustrated, by the defect here described. The name of threads is usually given to fibrous appearances in the body of the glass, which result from the vitrification of clay. Their colour is greener than that of the rest of the glass. Threads, if existing in great numbers, render the material extremely fragile; and the same effect is produced, if, although fewer in number, the threads are individually larger. The cause of this increased brittleness is, that the dilatation and contraction at different temperatures, of glass which results from the fusion of clay, differ from those of glass made with siliceous sand; for which reason, each in turn exerts a hurtful influence upon the other. Tears are, perhaps, the greatest defect that can be found in glass. They are in fact an exaggeration of the imperfection last described, and usually proceed from the fusion and vitrification of portions of the clay that forms the arch of the furnace, and which are suffered to drop into the pots, and to float in the glass while in its state of fusion. Wherever these tears exist, the material is brittle in a very high degree, so as frequently to crack, without any apparent cause, by the mere effect of the unequal expansion just described, which accident is more likely to occur in proportion as the drops are nearer to the surface. This defect is one of so serious a nature, that it is usual, on discovering its existence, at once to throw aside the glass as useless. In places where, as is frequently the case in England, covered crucibles are employed, this accident is in a great degree avoided. Three kinds of knots are observable in glass; one of these arises from the aggregation of several imperfectly vitrified grains of sand. Another is owing to some portions of glass-gall not having been removed during the refining; and the third kind is produced by any small parts of the crucible or of the furnace which, having been abraded by the rubbing of the tools or other accidental circumstance, have fallen into the glass. Small bubbles are frequently seen abundantly spread throughout the substance of the glass. These indicate an imperfect degree of refining, and proceed from the disengagement of gas which occurs during the process of vitrification. Their presence announces that the glass has not been sufficiently fluid in the course of its refining to allow of their dispersion. This may happen through one of two causes, either that a sufficient amount of fluxing material has not been used with the sand, or that the fire has not been sufficiently intense for the due liquefaction of the compound. These bubbles are chiefly objectionable on account of their unsightly appearance, and do not really deteriorate the quality of the glass even for optical purposes. In this case each bubble acts as a small convex lens, rapidly turning aside the rays which strike against it, and occasioning a diminution of light in proportion to its area. But when these bubbles are even numerous, the sum of their united areas will amount to only a small proportion of the whole surface of the glass; and the loss of light will be inconsiderable.* It thus appears that the principal object to be sought after in the manufacture of perfectly homogeneous glass is, to avoid those variations in the composition and specific gravity of its different parts, which occasion] the striated appearance described above. To enter minutely, and at length, into a consideration of the means that have been proposed and adopted with a view to remedy this considerable evil, would present little that is amusing to the general reader; while those persons who feel any particular interest in the subject, or whose taste for scientific research leads them to admire the detail of well considered and ably conducted plans for the mastery of a difficult operation, may gratify themselves by consulting Mr. Faraday's truly valuable paper already referred to, and which will be found comprised in the Philosophical Transactions for the year 1830. Some exceedingly favourable specimens of glass for optical purposes have lately been prepared by Mr. Green, the proprietor of the Stangate Glass-house; a gentleman whose personal attention has been unintermittingly given during many years to all the practical operations and details of an extensive establishment. Mr. Green is far from asserting that in what has been accomplished he has arrived at any certainty in the solution of this difficult problem, and feels that at most he has hitherto made only an approach to it; while, however, it is such an approach as justifies the hope, that, through continued thought and exertion, a still greater and more important degree of perfection may be attained. * Mr. Faraday, Bakerian Lect.; Phil. Trans. 1830, p. 7. The circumstances which attended the long-continued and laborious investigations on this subject of another and a very extraordinary man, are, in themselves, so curious and interesting, and seem likely to be followed by such important consequences, to at least one branch of the art,' that a treatise on the manufacture of glass might be justly charged with incompleteness, if it did not furnish at least a sketch of those circumstances. The following account is condensed from a memoir, read at a sitting of the Society of Physics and Natural History of Geneva, on the 19th of February, 1823, as given in the nineteenth volume of the Quarterly Journal of Science, published in London in the year 1825. The late M. Guinand was born in an inconsiderable village, among the mountains of Neufchâtel in Switzerland. His father was by trade a joiner, and must have been in very indifferent circumstances, as his son was called upon to assist him when only ten years old, and without having acquired more than a very imperfect knowledge of the first rudiments of learning; a deficiency which was never afterwards supplied, as M. Guinand always read with difficulty, and wrote very imperfectly. He must even, at this early period, have been a lad of considerable talent, and of a disposition that urged him to the exertion requisite for raising his condition in society. We find him, when between thirteen and fourteen years old, having quitted the employment of a joiner for that of a cabinet-maker, chiefly engaged in making cases for clocks. At this period he acquired from an acquaintance some knowledge of the art of casting and working in metals, of which knowledge he afterwards availed himself by adopting, when twenty years of age, the occupation of a watch-case maker, the manufacture of watches forming a very considerable branch of industry in that part of the country. At the house of a person for whom he then worked, M. J. Droz, the constructor of several automaton figures, which forty years ago made the tour of Europe, young Guinand enjoyed an opportunity of seeing for the first |