SILVER MERCURY. 331 



Silver is 'the whitest of all metals, and takes the highest polish ; 

 it is the best conductor of heat and electricity, and melts at about 

 1000 C. (1832 F.); it is univalent, and forms but one series of salts; 

 it is not affected by the oxygen of the air at any temperature, but is 

 readily acted upon by traces of hydrogen sulphide, which forms a 

 black film of sulphide upon the surface of metallic silver. Hydro- 

 chloric acid scarcely acts on silver, nitric and sulphuric acids dis- 

 solve it. 



While many of the non-metallic elements have long been known to exist in 

 allotropic forms, none of the metals had been obtained in such a condition 

 until quite recently, when it was shown that silver is capable of assuming a 

 number of allotropic modifications. These are obtained chiefly by precipi- 

 tating silver from solutions by different reducing agents. While normal silver 

 is white, the allotropic forms have distinct colors blue, bluish-green, red, pur- 

 ple, yellow and differ also in many other respects. Thus they are converted 

 into silver chloride by highly diluted hydrochloric acid, which does not act 

 on common silver; they are soluble in ammonia water, and act as reducing 

 agents upon a number of substances, such as permanganates, ferricyanides, 

 etc. Allotropic silver can be converted into the common form by dift 

 ferent forms of energy for instance, by heat, electricity, and the action of 

 strong acids. 



This allotropic form of silver is known as colloidal silver, and its solution in 

 water is not a true, but a colloidal solution. Such a solution has the same freez- 

 ing- and boiling-point as water itself, and it has been shown that the colloid is 

 simply suspended in the liquid, although it is in too fine a state of division to 

 be retained by a filter-paper. Colloidal solutions of silver, gold, or platinum 

 result when electric discharges pass between wires of these metals held under 

 water. 



Collargol (Argentum Crede) is a preparation of colloidal (soluble) 

 silver, said to contain 85.87 per cent, of silver and a small amount of albumin. 

 It is bluish-black, scale-like, soluble in 20 parts of water. Albumin is added 

 to prevent precipitation of silver from its solution by acids and salts, or by 

 heating. It is incompatible with the usual silver reagents. 



Collargol Ointment (Unguentum Crede") is an ointment containing 15 per 

 cent, of collargol, of a dark, bluish-gray color. 



Silver is too soft for use as coin or silverware, and, therefore, is 

 alloyed with from 5 to 25 per cent, of copper, which causes it to be- 

 come harder, and consequently gives it more resistance to the wear 

 and tear by friction. 



Pure silver may be obtained by dissolving silver coin in nitric acid, 

 when a blue solution, containing the nitrates of copper and silver, is 

 formed. By the addition of sodium chloride to the solution a white 

 curdy precipitate of silver chloride, AgCl, forms, while cupric nitrate 



