346 METALS AND THEIR COMBINATIONS. 



type of compound is illustrated in the case of the sodium compounds, NaCl.- 

 HgCl 2 or NaHgCl 3 , and 2NaCl.HgCl 2 or Na 2 HgCl 4 : 



NaHgCl 3 Z Na* -f HgCL,'; 

 Na 2 HgCl 4 ^ 2Na* + HgCl/'. 



But there is considerable dissociation also into Hg* ions and Cl' ions. Hence 

 with not too much sodium chloride present and in very dilute solution, the 

 mercuric chloride tablets containing sodium or ammonium chloride do not 

 lose materially in germicidal power, since a relatively large proportion of the 

 ions are the active Hg* ions. 



The complex potassium mercuric iodide, K 2 HgI 4 , gives K* ions and Hgl/ 7 

 ions, but very few Hg* * ions. Hence the failure of some reagents, as alkalies 

 and carbonates, to give a precipitate in a solution of the compound. 



32. ARSENIC. 

 As = 74.4. 



General remarks regarding 1 the metals of the arsenic group. 

 The metals belonging to either of the five groups, considered hereto- 

 fore, show much resemblance -to each other in their chemical prop- 

 erties, and consequently in their combinations. This is much less 

 the case among the six metals (As, Sb, Sn, Au, Pt, Mo) which are 

 classed together in this group. In fact, the chief resemblance which 

 unites these metals is the insolubility of their sulphides in dilute 

 acids and the solubility of these sulphides in ammonium sulphide 

 (or alkali hydroxides), with which they form soluble double com- 

 pounds ; the oxides have also a tendency to form acids. In most 

 other respects no general resemblance exists between these metals. 

 On the other hand, arsenic and antimony have many properties in 

 common, and resemble in many respects the non-metallic elements 

 phosphorus and nitrogen, as may be shown by a comparison of their 

 hydrides, oxides, acids, and chlorides. 



QUESTIONS. How is silver obtained from the native ores, and how may it 

 be prepared from silver coin? State of silver nitrate: its composition, mode 

 of preparation, properties, and names by which it is known. Give analytical 

 reactions for silver. How is mercury found in nature ; how is it obtained from 

 the native ore ; what are its physical and chemical properties? Mention the 

 three oxides of mercury; how are they made, what is their composition, what 

 is their color and solubility ? State of the two chlorides of mercury : their 

 names, composition, mode of preparation, solubility, color, and other proper- 

 ties. Mention the same of the two iodides, as above, for the chlorides. State 

 the difference between mercuric sulphate, basic mercuric sulphate, and mer- 

 curous sulphate. What is formed when ammonium hydroxide, calcium hydrox- 

 ide, potassium or sodium hydroxide is added to either mercurous or mercuric 

 chloride ? Give tests answering for any mercury compound, and tests by which 

 mercuric compounds may be distinguished from mercurous compounds. 



