reputation in the world; yet, by degrees, the superior value of his labours became appreciated, and he was visited by such men of science as travelled into the neighbourhood of his dwelling. By one of these, a knowledge of his merits was conveyed to M. Frauenhofer, the chief of a celebrated manufactory for optical instruments, established at Benedictbeurn in Bavaria. This gentleman having in consequence obtained some discs of glass made by Guinand, found their quality so satisfactory, that he repaired in person to Brenets, where the artist resided, and engaged him to settle in Bavaria. This was in 1805, when Guinand was upwards of sixty years of age. He continued at Benedictbeurn during nine years, occupied solely in the manufacture of glass, to the great increase of M. Frauenhofer's reputation. Being desirous, at the end of this time, to return to his native land, a pension was granted to him by the establishment, on condition that he should no longer employ himself in making glass, nor disclose his process to any person whatever; a condition which did not long agree with the still active energies of his mind. Believing that by new experiments he could raise his discovery to a yet higher degree of improvement, he obtained the consent of M. Frauenhofer to cancel their subsisting agreement; and relinquishing his pension, once again devoted himself with ardour to his favourite pursuit. M. Guinand lived for seven years after this time (1816), and produced several telescopes of great magnitude, and remarkable for their excellence; it being perhaps not the least extraordinary among the circumstances attending them, that, to use the words of the memoir, whence the foregoing account is drawn, "they have been constructed by an old man upwards of seventy, who himself manufactures the flint and crown glass which he uses in their construction, after having made with his own hands the vitrifying furnace and his crucibles; who, without any mathematical knowledge, devises a graphic method of ascertaining the proportions 8 of the curves that must be given to the lenses, afterwards works and polishes them by means peculiar to himself; and lastly, constructs all the parts of the different mountings, either with joints or on stands, melts and turns the plates, solders the tubes, prepares the wood, and compounds the varnish." Arrangements had been made by the French government for purchasing his secret, when the artist, verging on his eightieth year, died, after a short illness. That secret did not, however, die with him, but is possessed by his son, who continues to labour in the employment so singularly commenced, and so successfully and energetically followed by his father. The name of Frauenhofer, which has been introduced in the foregoing narrative, is one intimately connected with enquiries in the art of making perfect glass. It would be wrong to leave the reader under an impression that the merit of this artist was limited to the single act of patronage extended towards Guinand, and which, although indicative of his discernment as a tradesman, would afford no reason for investing him with any part of the extraordinary merit which truly belonged to his character. Like Guinand, his beginning in life was humble; being indebted solely to the powers of his own mind for the eminence to which he attained. Having occupied the lowest station as an ordinary workman in a great manufacturing establishment, he by the force of his transcendent talents, and in the course of a few years raised himself to the chief direction of its business. During the intervals of labour he acquired a competent knowledge of mathematical science; and devoting himself to the perfection of the refracting telescope, proved that he possessed a truly philosophical and scientific mind. Having soon mastered the theoretical difficulties which presented themselves, he still, however, found all his labours unavailing, through the imperfec tion of the material employed; and set himself to remedy, this evil, by a series of admirable experiments. It might be thought invidious to enquire in what degree his success in these was owing to the previous labours and assistance of Guinand, or how far his discoveries were personal and original. Both produced and left behind them specimens of perfect glass in large pieces; but the public has equally in either case to regret the want of knowledge as to the processes employed for the attainment of an object so desirable. Frauenhofer died in the year 1826, at an early age; a victim, it is said, to unremitting attention bestowed upon an unhealthy employment. Had his life been continued to the same lengthened period as was allotted to his fellow-labourer, what might not the world have expected from one, who so early had burst the chains of ignorance, and overcome the paralysing difficulties of birth and adverse fortune; taking his station during life among the genuine philosophers of the age, and falling, admired, and lamented, and eulogised by the most scientific societies of Europe! The great value of flint glass, from which all perceptible defects are absent, may be imagined from the sketch which has here been given of the efforts made for its production. Very high prices are, in fact, paid for object glasses of a satisfactory quality, which are of any magnitude; while even small fragments of such glass are sought after by opticians with great avidity. A few years ago the director of one of the London glass-houses having made a pot of flint glass for optical purposes, sold this, in the regular course of his business, to a commission merchant, who transmitted it to his correspondent on the Continent. Some months having elapsed thereafter, during which time its possessor had ascertained the true value of his purchase, the manufacturer was surprised a treceiving numerous enquiries on the subject of this lump of glass, on the part of several English opticians. These were anxious to procure portions of a material, the fame of which had reached them from abroad. Upon this, the maker instituted a search, and having succeeded in identifying some fragments, as having formed part of the same melting, was enabled to procure very considerable prices for that upon which he had previously set little or no value, and which had been preserved only through accident. On a yet more recent occasion, information having reached London that a large and superior object glass was on sale in the metropolis of a neighbouring kingdom, one of our most celebrated astronomers hastened across the channel, and while others were chaffering with its possessor about the price, our countryman stept in, and paying at once the full amount demanded, brought off the prize to the great mortification of his competitors. CHAP. XII. ON THE SPECIFIC GRAVITY OF GLASS. IMPORTANCE OF THIS QUALITY. — EXPERIMENTS OF LOYSEL. THE specific gravity of glass is a quality of considerable importance, when the material is required for conversion into the object-glasses of achromatic telescopes, or for the composition of counterfeit gems, although any very minute attention to this point is not considered essential in conducting the commoner processes of the glass-house. Loysel, to whose justly esteemed work on the art of glass-making allusion has been so frequently made in these pages, went through a series of experiments upon the specific gravities of various vitreous bodies, with the view of giving such instructions for the composition of the nicer qualities of glass, as should absolve manufacturers from the necessity of making those preliminary trials upon every occasion, which are attended by much inconvenient delay in the prosecution of extensive operations. Adopting the practical aim of this French author, some of his ingenious formulæ will be here given, toge ther with a statement of the premises whereupon they were founded. The specific gravity of water being expressed by the number 100, that of sand is 263; while soda deprived of all carbonic acid by fusion in the furnace of a glasshouse, is of the specific weight 199, and the same substance, when brought again by cooling to a concrete state, is not heavier than 222. It might, therefore, be sup |