356 . MIRRORS 



If the assays be closely watched there can be no loss, for the trial of the pyx in- 

 variably shows the gold coin to err on the side of purity. Therefore, if the Master 

 of the Mint should determine to issue gold of exact standard, he may fairly cover 

 every source of loss, and coin money -without waste of metal. Each grain that is 

 found in excess of the standard upon the pound weight of gold causes a loss of about 

 ISOl. upon each million coined. The moneyers formerly sustained a loss of about 

 700J. for each million coined, such loss being exclusive of melting. This has never 

 been entirely satisfactory; and the loss by coining alone rarely reached so high an 

 amount, although 373. per million would seem to have been determined by some 

 careful experiments as the necessary loss. This gold, it will be easily conceived, was 

 lost by volatilisation, and by other small sources of water in so extensive an operation 

 as that of coining. Although it is perhaps impossible to avoid all loss, it appears to 

 be reduced to a minimum by the great attention that is now paid to every division of 

 the process. G. F. A. 



XVlXRABIIiXS. A genus of plants belonging to the natural order Nyctaginacea. 

 They are known to the French as Belles de nuit. The Peruvian species Mirabilis 

 jalapa, or false jalap, has a purgative root, which was formerly mistaken for the true 

 jalap. M. dichotoma is the well-known garden flower commonly called the ' Marvel 

 of Peru,' or the Fleur de quatres heures of the French, 



MIRB ANE, ESSENCE OF. A fancy name under which M. C. Collas, of Paris, 

 sold nitrobenzol. 



MIRRORS. Under GLASS MANUFACTURE, the process of casting the large plates 

 for mirrors has been described. We have therefore only to describe in this place the 

 preparation of the plate glass and its silvering. 



The smoothing of the plates is effected by the use of moist emery washed to succes- 

 sive degrees of fineness, for the various stages of the operation ; and the polishing 

 process is performed by rubbers of hat-felt and a thin paste of colcothar and water. 

 The colcothar, called also crocus, is red oxide of iron prepared by the ignition of 

 copperas, with grinding and elutriation of the residuum. See COLCOTHAR. 



The last part, the polishing process, is performed by hand. This is managed by 

 females, who slide one plate over another, while a little moistened putty of tin finely 

 levigated is thrown between. 



Large mirror-plates are now the indispensable ornaments of every large and sump- 

 tuous apartment ; they diffuse lustre and gaiety around them, by reflecting the rays of 

 light in a thousand lines, and by multiplying indefinitely the images of objects placed 

 between opposite parallel planes. 



The silvering of plane mirrors consists in applying a layer of tin-foil alloyed with 

 mercury to their posterior surface. The workshop for executing this operation is pro- 

 vided with a great many smooth tables of fine freestone or marble, truly levelled, having 

 round their contour a rising ledge, within which there is a gutter or groove which ter- 

 minates by a slight slope in a spout at one of the corners. These tables rest upon an 

 axis of wood or iron, which runs along the middle of their length ; so that they may 

 be inclined easily into an angle with the horizon of 12 or 13 degrees, by means of a 

 hand-screw fixed below. They are also furnished with brushes, with glass rules, with 

 rolls of woollen stuff, several pieces of flannel, and a great many weights of stone or 

 cast-iron. 



The glass-tinner, standing towards one angle of his table, sweeps and wipes its surface 

 with the greatest care, along the whole surface to be occupied by the mirror-plate ; then 

 taking a sheet of tin-foil adapted to his purpose, he spreads it on the table, and applies 

 it closely with a brush, which removes any folds or wrinkles. The table being hori- 

 zontal, ho pours over the tin a small quantity of quicksilver, and spreads it with a roll 

 of woollen stuff; so that the tin-foil is penetrated and apparently dissolved by the mer- 

 cury. Placing now two rules, to the right and to the left, on the borders of the sheet, he 

 pours on the middle a quantity of mercury sufficient to form everywhere a layer about 

 the thickness of a crown piece ; then removing with a linen rag the oxide or other im- 

 purities, he applies to it the edge of a sheet of paper, and advances it about half an inch. 

 Meanwhile another workman is occupied in drying very nicely the surface of the 

 that is to be silvered, and then hands it to the master workman, who, laying it flat, places 

 its anterior edge first on the table, and then on the slip of paper ; now pushing the glass 

 forwards, he takes care to slide it along so that neither air, nor any coat of oxide on the 

 mercury can remain beneath the plate. When this has reached its position, he fixes it 

 there by a weight applied on its side, and gives the table a gentle slope, to run off all 

 the loose quicksilver by the gutter and spout. At the end of five minutes he covers 

 the mirror with a piece of flannel, and loads it with a great many weights, which are 

 left upon it for 24 hours, under a gradually-increased inclination of the table. By this 

 time the plate is ready to bo taken off the marble table, and laid on a wooden one sloped 

 like a reading-desk, with its under edge resting on the ground, while the upper is 



