434 PRINCIPLES OF CHEMISTRY 



Accurate experiments, and more especially the researches of 

 Stas at Brussels, show the proportion in which silver reacts with 

 metallic chlorides. These researches have led -to the determina- 

 tion of the combining weights of silver, sodium, potassium, chlorine, 

 bromine, iodine, and other elements, and are distinguished for their 

 model exactitude, and we will therefore describe them in some detail. 

 As sodium chloride is the chloride most generally used for the pre- 

 cipitation of silver, since it can most easily be obtained in a pure state, 

 we will here cite the quantitative observations made by Stas for show- 

 ing the co-relation between the quantities of chloride .of sodium and 

 silver which react together. In order to obtain perfectly pure sodium 



will be deposited in an even coating ; this, indeed, forms the mode of silver plating by 

 the wet method, which is most often used in practice. A solution of one part of silver 

 nitrate in 80 to 50 parts of 'water, and mixed with a siifficient quantity of a solution of 

 potassium cyanide to redissolve the precipitate of silver cyanide formed, gives a dull 

 coating of silver, but if twice as much water be used the same mixture gives a bright 

 coating. 



.Silver plating in the wet way has now replaced to a considerable extent the old 

 process of dry silvering, because this process, which consists in dissolving .silver in 

 mercury and applying the amalgam to the surface of the objects, and then vaporising 

 the mercury, offers the great disadvantage of the poisonous mercury fumes. Besides 

 these, there is another method of silver plating, based on the direct displacement of 

 silver from its salts by other metals for example, by copper. The copper reduces the 

 silver from its compounds, and the silver separated is deposited upon the copper. Thus 

 a solution of silver chloride in sodium thiosulphate deposits a coating of silver upon a 

 strip of copper immersed in it. It is best for this purpose to take pure 'silver sulphite. 

 This is prepared by mixing a solution of silyer nitrate with an excess of ammonia, and 

 adding a saturated solution of sodium sulphite, and then alcohol, which precipitates 

 silver sulphite from the solution. The latter and its solutions are very easily decomposed 

 by copper. Metallic iron produces the same decomposition, and iron and steel articles 

 may be very readily silver-plated by means of the thiosulphate solution of silver chloride. 

 Indeed, copper and similar metals may even be silver-plated by means of silver chloride ; 

 if the chloride of silver, with a small amount of acid, be rubbed upon the surface of the 

 copper, the latter becomes covered with a coating of silver, which it has reduced. 



Silver plating is not only applicable to metallic objects, but also to glass, china, &c. 

 Glass is silvered for various purposes for example, glass globes silvered internally are 

 used for ornamentation, and have a mirrored surface. Common looking-glass silvered 

 upon one side forms a mirror which is better than the ordinary mercury mirrors, owing 

 to the truer colours of the image due to the whiteness of the silver. For optical in- 

 strumentsfor example, telescopes concave mirrors are now made of silvered glass, 

 which has first been ground and polished into the required form. . The silvering of glass 

 is based on the fact that silver which is reduced from certain solutions deposits itself uni- 

 formly in a perfectly homogeneous and continuous but very thin layer, "forming a bright 

 reflecting surface. Certain organic substances have the property of reducing silver in this 

 form. The best known among these are certain aldehydes : for instance, ordinary 

 acetaldehyde, C 2 H 4 O, which easily oxidises in the air and formff acetic acid, C 2 H 4 O 3 . 

 This oxidation also easily takes place at the expense of silver oxide, when a certain amount 

 of ammonia is added to the mixture. The oxide of silver gives up its oxygen to the 

 aldehyde, and the silver reduced from it is deposited in a metallic state in a uniform 

 bright coating. The same action is produced by certain saccharine substances and 

 certain organic acids, such as tartaric acid, &c. 



