CHEMISTRY. 



gas owes this spontaneous inflammability to the 

 presence of a small quantity of the vapour of 

 another compound of phosphorus and hydrogen 

 (PH.,). 



Phosphorus unites with chlorine and also with 

 bromine in two proportions, corresponding to the 

 two oxides namely, PC1 3 ; PC1 5 and PBr 3 ; PBr 6 . 

 The terchloride and terbromide are volatile 

 liquids ; the pentachloride and pentabromide are 

 crystalline solids ; they all undergo decomposition 

 when brought into contact with water, yielding 

 hydrochloric or hydrobromic acid, and phosphor- 

 ous or phosphoric acids : 2PC1 3 + 6H 2 O = 6HC1 

 -f-3H 2 O,P 2 O 3 ;2PCl 5 +6H 2 O = 6HCl-HH 2 O,P 2 O 6 . 

 FLUORINE is an element the compounds of 

 which resemble to a great extent those of chlorine. 

 It occurs in nature combined with metals, forming 

 salts called fluorides. The most important of these 

 are fluorspar (Derbyshire spar) or fluoride of cal- 

 cium (CaF 2 ), and cryolite, which is a compound 

 of fluoride of sodium and fluoride of aluminium 

 (3NaF,AlF 3 ). From these fluorides, hydrofluoric 

 acid (HF) can be obtained by the action of sul- 

 phuric acid : CaF 2 + H 2 SO 4 = CaSO 4 + 2HF. 

 Hydrofluoric acid is a very volatile liquid, dissolv- 

 ing readily in water. It acts very corrosively on 

 animal tissues, and produces severe wounds when 

 applied to the skin. Its most striking chemical 

 character is the way in which it acts upon glass 

 (see Silica), and it is used in the arts for the pur- 

 pose of etching glass. 



SILICON, like carbon, can be obtained in three 

 distinct ' allotropic forms :' two of these are crys- 

 talline, corresponding to diamond and graphite ; 

 and one is amorphous. Silica, the oxide of sili- 

 con (SiO 2 ), corresponds in formula to carbonic 

 acid, and, like it, is an anhydrous acid, uniting 

 with bases to form silicates. It differs, however, 

 very greatly from carbonic acid in physical char- 

 acters. It occurs in nature crystallised as rock- 

 crystal or quartz, and obscurely crystallised in 

 flint and chalcedony. Opal is an amorphous form 

 of silica, usually containing some water. Silica 

 fuses at a very high temperature. When heated 

 with bases, it unites with them, forming silicates. 

 Some of these silicates are of great importance. 

 Thus, felspar (one of the ingredients of granite) is 

 a compound of silicate of alumina and silicate of 

 soda, potash, or lime ; clay is a silicate of alumina ; 

 talc is a silicate of magnesia. Glass is formed of 

 a mixture of silicates ; of these, one must be the 

 silicate of potash or silicate of soda : in crown- 

 glass and plate-glass, the other is silicate of lime. 

 In flint-glass, and the glass which is cut into orna- 

 ments, silicate of lead is present, sometimes in 

 large proportion. Green bottle-glass owes its 

 colour to silicate of iron. 



The silicates of the alkalies (when not com- 

 bined, as in glass, with other silicates) are soluble 

 in water (water-glass), and are decomposed on the 

 addition of an acid. Thus, sulphuric acid acts on 

 silicate of soda, forming sulphate of soda and 

 hydrated silicic acid, which, under some circum- 

 stances, remains dissolved in the water, but readily 

 separates as a jelly. Hydrofluoric acid acts on 

 silica and on the silicates, forming water and 

 gaseous fluoride of silicon (SiF^. On account of 

 this action, hydrofluoric acid is used for etching 

 glass. 



BORON also exists in three forms, correspond- 

 ing to the three forms of carbon and silicon. Its 



oxide is anhydrous boracic acid, B 2 Og. Its most 

 important compound is borax, or borate of soda 

 (Na 2 0,2B 2 3 ). 



SELENIUM and TELLURIUM are comparatively 

 rare elements, forming compounds nearly corre- 

 sponding to those of sulphur. 



METALS. 



The general physical and chemical properties 

 of metals have already been described (p. 312). 

 The characters of the more important metals, and 

 the methods used for extracting them from their 

 ores, are described in the article METALS AND 

 METALLURGY. We shall, therefore, here give 

 only a general sketch of the metals and their 

 compounds. The metals may be divided into 

 several groups, the members of each group having 

 many characters in common. We shall name each 

 group after a metal possessing the common char- 

 acters in a well-marked degree. 



1. The Sodium group comprising Sodium, 

 Potassium, Lithium, Rubidium, and Caesium. 



2. The Calcium group comprising Calcium, 

 Strontium, and Barium. 



3. The Iron group comprising Magnesium, 

 Zinc, Cadmium, Iron, Manganese, Chromium, 

 Nickel, Cobalt, Aluminium, and Uranium. 



4. The Copper group, comprising Copper, Mer- 

 cury, Lead, and Silver. 



5. The Platinum group, comprising Gold, Plat- 

 inum, Palladium, and the rare metals found 

 along with platinum. 



6. The Antimony group, comprising Arsenic, 

 Antimony, Bismuth, Tin, Titanium, Vanadium, 

 Molybdenum, and Tungsten. 



By comparing this list with the table at p. 315, 

 it will be seen that we have not here enumerated 

 all the known metals ; those omitted are either 

 too imperfectly known to admit of classification, 

 or too rare to be of practical importance. 



I. The oxides of the metals of the Sodium group 

 are powerful bases, and unite with water to form 

 soluble hydrates, which cannot be decomposed by 

 heat (Na 2 O,H 2 O ; K 2 O,H 2 O 3 or NaHO ; KHO). 

 The bases themselves and their hydrates are 

 called ' alkalies.' The carbonates and phosphates 

 of the alkalies are soluble in water. The metals 

 themselves can be obtained by electrolysis of their 

 fused chlorides, or by the reduction of their car- 

 bonates by means of charcoal : N^COs -f- 2C 

 = 2Na + 3CO. They decompose water at the 

 ordinary temperature, hydrogen being given off, 

 and the hydrated base remaining in solution : 

 2Na + 2H 2 O = Na 2 O,H 2 O + H^ 



The most important salts of this group of 

 metals are : Chloride of sodium (common salt) 

 (see p. 324) ; sulphate of soda (Glauber's salt) (see 

 p. 324) ; carbonate of soda (Na 2 O,CO 2 ) (wash- 

 ing-soda). This salt is made from sulphate of 

 soda (which is prepared by acting on common 

 salt with sulphuric acid) by heating it with coal 

 and limestone, dissolving out the carbonate of 

 soda thus formed, by means of water, and evaporat- 

 ing the solution. Bicarbonate of soda, baking- 

 soda (Na 2 O,CO2,H 2 O,CO 9 or NaHCOs), is pre- 

 pared by the action of carbonic acid gas on crys- 

 tallised carbonate of soda. Nitrate of soda (Chili 

 saltpetre), Na,O,N 2 O 6 or NaNO 3 , occurs in large 

 quantity in Bolivia, and is used for preparing 

 nitric acid, and also as a manure to supply nitrogen 



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