FLINT GLASS, or CRYSTAL. A species of glass which derives its name from flint, bemuse that substance was formerly employed in its manufacture. It is very extensively used for domestic purposes ; but is chiefly interesting to the philosopher on account of the pro-. perty which it possesses of causing a greater dispersion of the rays of light which pass through a prism or lens form ed of it than any other of the vitreous compounds. This property renders it invaluable in the manufacture of the ob ject glasses of telescopes and micro scopes ; for by combining a concave lens of flint glass with one or two convex lenses of crown glass, which possesses a much less dispersive power, a compound lens is formed, in which the prismatic colors arising from a simple refraction are destroyed, and the lens rendered achro matic. This construction of object glasses was first discovered by a Mr. Hall, a coun try gentleman in Worcestershire, about 1729; but the discovery was forgotten, and no farther notice taken of it for near ly 80 years, when it was again brought to light by John Dollond, after a long-con tinued course of experiments under taken for the purpose of perfecting the telescope. It is, however, very difficult to prepare flint glass fit for the purposes of achromatic telescopes. This difficulty arises not from the want of sufficient dis persive power in the substance, but from the want of purity or homogeneity ; the slightest impurity or inequality of com position in the glass giving use to a streaked or imperfect image by reason of the unequal refraction of the nays. The composition of pure flint glass long re mained a secret in the family of the Dol londs, and its manufacture formed a very profitable article of exportation ; for till about the beginning of the present cen, tury, no flint glass of good quality was made on the Continent. Of late years,
however, a great change has taken place in this respect, and glass of the best quality has been manufactured, both in France and Germany, in much larger masses than the English artists have yet succeeded in obtaining. This result has been mainly produced by the experimen tal researches of D'Artigues, Fraunhofcr, Cauchoix, Guinand, and Korner. For merly, an object-glass exceeding five inches in diameter could scarcely be pro duced. Fraunhofersucceeded in making them of nine, and even twelve inches. The object-glass of the large parallactic telescope belonging to Sir James South, at Campden Hill, was manufactured by Cauchoix ; it exceeds twelve inches, and is throughout of the utmost purity. The exact proportion of the ingredients which enter into these choice specimens is not known, and probably their excellence de pends in part on some accidental circum stances in the preparation. Korner pro duced some of his best specimens by employing the following ingredients : 100 parts of quartz, first treated with mu riatic acid; 80 parts of litharge, or red lead ; and 80 parts of the bitartrate of potash. Flint glass for commonpur poses is usually made of 120 parts of fine white sand, 40 parts of well purified pearl ash, 35 parts litharge or minium, 13 parts nitre, and a small quantity of the black oxide of manganese ; the latter ingredient being used to correct the green color occasioned by the presence of oxide of iron in the sand.