IRON. 143 



5. Sodium phosphate produces precipitate of aluminium 

 phosphate, soluble in acids. 



QUESTIONS. 



231. Mention some varieties of crystallized aluminium oxide found 

 in nature and some silicates containing it. 



232. Give the general formula of an alum, and mention some alums. 



233. Which alum is officinal, how is it made, what are its properties, 

 and what is it used for ? 



234. What is dried alum, and how does it differ from common alum ? 



235. How is aluminium chloride made, and how is the metal obtained 

 from it? 



236. State the properties of aluminium. 



237. What change takes place when ammonium hydrate, and what 

 change when sodium carbonate is added to a solution of alum ? 



238. What is the composition of earthenware, porcelain, and glass ; 

 how and from what materials are they manufactured ? 



239. What is ultramarine ? 



240. Give tests for aluminium compounds. 



25. IRON. 



Fe u = 55.9. Fe 2 vi = 111.8. 



General remarks regarding the metals of the iron group. The 



six metals (Fe, Co, ~N\, Mn, Cr, Zn) belonging to this group are 

 distinguished by forming sulphides (chromium excepted) which 

 are insoluble in water, but soluble in diluted mineral acids; 

 they are, consequently, not precipitated from their neutral or 

 acid solutions by hydrosulphuric acid, but by ammonium sul- 

 phide as sulphides (chromium as hydrate); their oxides, hydrates, 

 carbonates, phosphates, and sulphides are insoluble; their chlo- 

 rides, iodides, bromides, sulphates, and nitrates are soluble in 

 water. 



With the exception of zinc, these metals are strongly mag- 

 netic; they decompose water at a red heat, the oxide being 

 formed and hydrogen liberated; in diluted hydrochloric or sul- 

 phuric acid, they dissolve with formation of chlorides or sul- 

 phates, respectively, and liberation of hydrogen. 



With the exception of zinc, which is bivalent, the metals of 

 the iron group are bivalent in some compounds, trivalent in 



