358 MITTLER'S GREEN 



tightly as to deprive it of the elasticity necessary for making it into a mould. This 

 apparatus, being set horizontally, a leaf of tin-foil is spread over it, of sufficient size 

 to cover the surface of the glass ; the tin is brightened with mercury, and then as 

 much of the liquid metal is poured on as a plane mirror requires. The convex glass, 

 well cleaned, is now set down on the cloth, and its own weight, joined to some 

 additional weights, gradually presses down the cloth, and causes it to assume the 

 form of the glass, which thus comes into close contact with the tin submersed iinder 

 the quicksilver. The redundant quicksilver is afterwards drained off by inversion, as 

 in common cases. 



The following recipe has been given for silvering the inside of glass globes :-^-Melt 

 in an iron ladle or a crucible equal parts of tin and lead, adding to the fused alloy 

 one part of bruised bismuth. Stir the mixture well, and pour into it as it cools two 

 parts of dry mercury ; agitating anew, and skimming off the drossy film from the 

 surface of the amalgam. The inside of the glass globe being freed from all adhering 

 dust and humidity, is to be gently heated, while a little of the semi-fluid amalgam is 

 introduced. The liquidity being increased by the slight degree of heat, the metallic 

 coating is applied to all the points of the glass, by turning round the globe in every 

 direction, but so slowly as to favour the adhesion of the alloy. The silvering is not 

 so substantial as that of plane mirrors ; but the form of the vessel, whether a globe, 

 an ovoid, or a cylinder, conceals or palliates the defects by counter-reflection from the 

 opposite surfaces. 



Several processes have been introduced, and some of them patented, for precipi- 

 tating silver on glass. These have not all been ' entirely successful, but the pheno- 

 mena involved are of such an interesting character, that this article would be incom- 

 plete without some notice of them. 



Mr. Drayton patented a process of the following character : A solution of nitrate 

 of silver, rendered neutral by the addition of a little ammonia, was floated over a plate 

 of glass ; or a vessel intended to be silvered was filled with this fluid ; some spirits of 

 wine was mixed with it, and then a small quantity of the oils of cloves aud cassia 

 added. By a complicated action, partly physical and partly chemical, metallic silver 

 was separated from the salt in solution, and precipitated over the entire surface of the 

 glass. The metallic film being of sufficient thickness, the solution was poured off, the 

 coating well washed, dried, and protected from abrasion by a thick varnish or paint 

 laid on the back. The defect in mirrors thus prepared was that small specks appeared 

 in the silver, which became little centres of chemical action ; the silver tarnishing, 

 and circular spots extending from these points ; so that the mirror, either for use or 

 ornament, was ruined. The cause of this may be traced to the compound character 

 of the solutions employed. Nitrate of silver, ammonia, spirits of wine, and essential 

 oils, with water, form a very mechanical mixture, and as the silver fell, it no doubt 

 entangled some of the organic matter, and this, however small, became the starting- 

 point of those stains which eventually destroyed the mirror. Dr. Stenhouse showed 

 that a large number of bodies possess the singular power of precipitating silver from 

 its solution ; amongst others, the following : gum-arabic, starch, salicine, gum-guaia- 

 cum, saccharic acid, aldehyde, oils of pimento, turpentine, or laurel, and especially 

 grape-sugar. 



Mr. Hale Thomson patented a silvering process which involved the use of grape- 

 sugar. A certain portion of grape-sugar is put into a solution of nitrate of silver, 

 rendered as neutral as possible, and a little heat is applied. By this means a beau- 

 tiful film of very pure silver is spread over the glass. By a process analogous to this 

 Foucault proposed to silver reflectors for lighthouses, and for telescopes. A process 

 has also been patented involving the use of tartaric acid as the precipitating ngcnt. 



A method of coating glass with platinum, instead of silver, has been introduced in 

 France, and carried out to some extent by Cresswell and Tavernier. A solution of 

 bichloride of platinum is spread with a fine brush over the surface of the glass, and 

 the metal is precipitated by means of oil of lavender. 



niZSPZCXEIi is arsenical pyrites. See ARSENIC ; PYRITES. 



MXTTXiER'S GRE3BW. This colour has been made in France in limited 

 quantities, for many years past, under the name of emerald green (vert diiiuraude'), 

 by MM. Pannetier and Binet, who kept their process secret. The great beauty of 

 the product induced many to attempt its imitation. But its nature remained un- 

 discovered ; partly, no doubt, because the colour was observed to darken, and to emit 

 steam when heated ; circumstances which gave rise to an impression that it contained 

 organic matter. When, at length, M. Guignet discovered the nature of the article, 

 and the method of its production, ho immediately patented his discovery, and, thus 

 protected, was emboldened to enter upon its manufacture on a great scale, producing 

 tons where the secret system had produced only pounds. 



The preparation of Mittler's green is conducted in tho following manner : 



