the Amalgam used for silvering Mirrors. i 13 



polishing of glass : but the case is not the sanie if the col- 

 cothar o° red oxide of iron retains martial vitriol. This 

 salt, when dissolved in water, is decomposed, and the 

 yellow ochre which results from it penetrates the glass, 

 forms a crust on it, and renders It greasy, dull, and yel- 

 lowish ; a tint which is communicated to the image of the 

 object presented to the mirror. 



Glass when smoothed and polished does not acquire the 

 property of reflecting objects till it has been silvered (as it 

 IS called), an operation effected by means of an amalgam. 

 The tin leaf employed must be of the size of the glass, be- 

 cause, when pieces of that metal are united by means of 

 mercury, they exhibit the appearance of lines. Tin is one 

 of those metallic substances which become soonest oxi- 

 dated by the means of mercury. If there remains a portion 

 of that calx, of a blackish gray colour, on the leaf of tin, it 

 produces a spot or stain in the mirror, and the part where 

 it is cannot reflect objects presented to it : great care, there- 

 fore, is taken in silvering glass to remove the calx of tin 

 from the surface of the amalgam. 



The process is as follows: — The leaf of tin is laid on a 

 very smooth stone table, and 'tnercury being poured over 

 the metal, it is extended over the surface of it by means of 

 a rubber made of bits of cloth. At the same moment the 

 surface of the leaf of tin becomes covered with blackish 

 oxide, which is removed with the rubber. More mercury 

 is then poured over the tin, where it remains at a level to 

 the thickness of more than a line, without running off. The 

 glass is applied in a horizontal direction to the table at one 

 of its extremities, and being pushed forwards it drives be- 

 fore it the oxide of tin which is at the surface of the amal- 

 gam. A number of wdghts are then placed on the glass 

 which floats on the amalgam, in order to press it down. 

 Without this precaution the glass would exhibit the inter- 

 stices of the crystals resulting from the amalgam. These 

 crystals have the form of large square laminae irregularly 

 disposed. 



To obtain leaves of tin, which are sometimes six or seven 

 feet in length, with a proportionate breadth, they are not 

 rolled but hammered. The prepared tin is first cast between 

 two plates of polished iron, or between two smooth stones 

 not of a porous nature, such as thunder stone. Twelve of 

 these plates are placed over each other ; and they are then 

 beat on a stone mass with heavy hammers, one side of 

 which is plain and the other rounded. The plates joined 

 together are first beaten with the latter : whea they become 



Vol. 22. No. 86. July 1805. H extended 



