Page:The American Cyclopædia (1879) Volume XI.djvu/651

MIRROR flow and drop from one corner into bowls. The table is first made perfectly horizontal, and then tin foil is carefully laid over it, cov- ering a greater space than the glass to be coated. A strip of glass is placed along each of three sides of the foil to prevent the mer- cury from flowing off. The metal is then poured from ladles upon the foil till it is near- ly a quarter of an inch deep, and its tendency to flow is checked by its affinity for the tin foil and the mechanical obstruction of the slips of glass. The plate of glass, cleaned with especial care, is dexterously slid on from the open side, and its advancing edge is kept in the mercury, so that no air or floating oxide of the metal or other impurities can get be- tween the glass and the clean surface of the mercury. When exactly in its place, it is held till one edge of the table has been elevated 10 or 12 and the superfluous mercury has run off. Heavy weights are placed on the glass, and it is left for several hours. It is then turned over and placed upon a frame, the side covered with the amalgam, which adheres to it, being uppermost. In this position the amalgam becomes hard, and the plate can then be set on edge; but for several weeks it is necessary to guard against turning it over, as until the amalgam is thoroughly dried the coating is easily injured. Several serious diffi- culties attend this process. The health of the workmen is so affected by the fumes of the mercury that they can rarely follow the busi- ness more than a few years ; for this no rem- edy has been found so effectual as thorough ventilation and the frequent use of sulphur baths. The glass plates are liable to be bro- ken by the weights placed upon them ; and the coating of amalgam is frequently spoiled by the drops of mercury removing portions of it as they trickle down, or by its crystallizing, or by mechanical abrasion. Many methods of silvering have been contrived and patented with the view of obviating these defects, some of which are important. In 1855 a patent was granted in England to Tony Petitjean for a method of precipitating silver, gold, or plati- num upon glass, so as to form a coating upon it, by the use of two solutions, the effect of which when mixed upon the glass is to decom- pose each other. The solutions he employed were different compounds of ammonio-nitrate of silver, tartaric acid, and distilled water; and they were placed upon the plate while this was at the temperature of 150 F. The precipitated silver within 20 minutes covered the glass, to which it adhered ; and the solu- tion being then turned off, all that remained to complete the mirror was to wash the surface, and when dry cover it with a coat of var- nish to protect it from injury. The silvering thus obtained is not so white, and is rarely so free from blemishes, as the amalgam coating. In 1849 Mr. Drayton made known a similar method, an improvement upon a process which he patented in 1843. He employed ammonia MISDEMEANOR 633 1 oz., nitrate of silver 2 oz., water 3 oz., and alcohol 3 oz. ; these, being carefully mixed, were all allowed to stand a few hours, when to each ounce of the liquid was added an ounce of saccharine matter,' as of grape sugar, dissolved in equal portions of spirit and water. Liebig invented a method of coating glass with silver, in which, after the silver coating is laid on, it is covered with a coating of copper precipi- tated upon it by the galvanic current, or is protected by varnish. Silver mirrors are now extensively made in New York. For platini- zing glass, R. Bottger recommends the follow- ing process : Pour rosemary oil upon the dry chloride of platinum in a porcelain dish, and knead it well until all parts are moistened; then rub this up with five times its weight of lavender oil, and leave the liquid a short time to clarify. The objects to be platinized are to be thinly coated with the preparation and afterward heated for a few minutes in a muf- fle or over a Bunsen burner. The brilliancy of aluminum has caused the suggestion of its application to the coating of mirrors ; but no successful experiments have yet been made with it for this purpose. Large mirrors are made in the United States by coating the im- ported plates. The old amalgamation method with tin foil and mercury is preferred to any of the more recent inventions, by reason of the greater whiteness and brilliancy of the reflec- tion and the greater permanence of the coat- ing. For telescopes, philosophical instruments, and lighthouses, various sorts of mirrors are in use, and reference to them may be found under various heads in this work, as BURNING GLASS, FRESNEL, LIGHTHOUSE, OPTICS, SPECULUM, TEL- ESCOPE, &c. Concave mirrors serve to con- centrate the rays of the sun in one point and produce intense heat. MIRZiPORE, a town of British India, in the province and 50 m. E. S. E. of the city of Allah- abad, and 80 m. W. by S. of Benares, on the right bank of the Ganges ; pop. about 80,000. It is the capital of a district containing more than 800,000 inhabitants, is the principal cot- ton market of the province, and is noted for carpet and cotton manufactures. The town contains fine European residences and many Hindoo temples, the latter mostly situated on the bank of the river. It is of comparatively modern origin. MISDEMEANOR. Offences less than treason are, in law, divided into felonies (see FELONY) and misdemeanors. Any crime less than a felony is a misdemeanor. Statutes sometimes declare that the offences which they contem- plate shall be punishable as misdemeanors, but the term applies equally to all those crimes, whether of commission or omission, for which the law has not provided a name. Misdemean- ors are either those which exist at common law, mala in se, or they are those which are created by statute, mala proJiibita. Under the former class, whatever, in the language of Blackstone, mischievously affects the person