CHAMBERS'S INFORMATION FOR THE PEOPLE. 



to growing plants. Phosphate of soda (see p. 

 326) ; borate of soda (borax) (see p. 327). Chloride 

 of potassium, or muriate of potash (KC1), is found 

 in the salt mines of Stassfurth, near Magdeburg. 

 Carbonate of potash (K 2 O,CO 2 ) is prepared from 

 the ashes of plants. The plants contain various salts 

 of potash, with acids containing carbon, hydrogen, 

 and oxygen (organic acids). These when burnt 

 are converted into water and carbonic acid, which 

 go off into the air, and carbonate of potash, which 

 remains in the ash. The ash is then treated with 

 water, which dissolves the soluble salts, the solu- 

 tion is evaporated, and the less soluble salts crys- 

 tallise out (see p. 305), leaving the carbonate of 

 potash in the mother-liquor. This is then evap- 

 orated to dryness, and the solid substance re- 

 maining forms the ' potashes ' of commerce, and 

 consists chiefly of carbonate of potash. Nitrate of 

 potash (K,O,N 2 O 6 or KNO 3 ). Nitre, or saltpetre 

 (see p. 322), is obtained in Bengal from earth con- 

 taining it, by treating the earth with water, and 

 evaporating the solution. It is also formed from 

 nitrate of lime by the action of chloride of potas- 

 sium (CaO,N 2 05+2KCl=K20,N 2 5 +CaCl 2 ). It 

 is used for making gunpowder; see CHEMISTRY 



APPLIED TO THE ARTS. 



Lithium, rubidium, and caesium are compara- 

 tively rare metals, generally resembling sodium 

 and potassium. 



2. Calcium group (or metals of the alkaline 

 earths). The oxides of these metals Lime (CaO) ; 

 Strontia (SrO) ; and Baryta (BaO) are powerful 

 bases, and unite with water to form hydrates 

 soluble in water. Thus, lime, when treated with 

 water, unites with it, giving out a great deal 

 of heat, and forming ' slaked lime.' The solu- 

 tion of slaked lime in water is called ' lime- 

 water.' The normal carbonates and phosphates 

 of this group are insoluble in water. If a cur- 

 rent of carbonic acid is passed into lime-water, a 

 white precipitate of carbonate of lime is formed 

 (CaO,CO 2 or CaCO 3 ). This action enables us to 

 test for carbonic acid in air ; thus, if a jar of air 

 is shaken up with a definite quantity of lime-water, 

 the proportion of carbonic acid present can be 

 approximately estimated by the degree of milkiness 

 produced. If the current of carbonic acid is con- 

 tinued after all the lime has been converted into 

 carbonate, the milkiness gradually disappears, as 

 carbonate of lime is soluble in water containing 

 carbonic acid, probably on account of the forma- 

 tion of a bicarbonate of lime (CaO,CO 2 ,H 2 O,CO 2 ). 

 We cannot obtain this bicarbonate in the solid 

 state, because, when the solution is evaporated or 

 boiled, carbonic acid escapes, and carbonate of 

 lime is precipitated. This solubility of carbonate 

 of lime in water containing carbonic acid is of 

 great practical importance. Water filtering 

 through the soil takes up carbonic acid from 

 decaying vegetable matters, and if it then comes 

 in contact with carbonate of lime (limestone, or 

 chalk), it dissolves it. Water thus charged with 

 bicarbonate of lime, deposits carbonate of lime 

 whenever it is exposed to the air : thus, if such 

 water drops from the roof of a cave or from an 

 overhanging rock, each drop before it falls loses 

 some carbonic acid, and deposits some carbonate 

 of lime on the rock: this deposit, increased by 

 every drop which runs over it, gradually forms a 

 dependent mass of carbonate of lime, hanging down 

 from the rock ; such a mass is called a ' stalactite.' 



328 



But the drops do not lose all their carbonic acid 

 before falling, and therefore do not deposit all 

 their carbonate of lime on the stalactite, so that if 

 they fall upon a rock or upon the floor of the cave, 

 they there, evaporating still more, deposit a further 

 quantity of carbonate of lime ; and this second 

 deposit forms a mass, gradually increasing in 

 height, which is called a ' stalagmite.' As the 

 stalactite grows down, and the stalagmite grows 

 up, they sometimes meet, and form a continuous 

 column, which grows in thickness from the deposits 

 formed by the water, which now runs down the 

 surface of the column. 



When carbonate of lime is heated, it decom- 

 poses into lime and carbonic acid. This is the 

 change effected in the lime-kiln. The lime (CaO) 

 thus obtained is called 'quicklime,' to distinguish 

 it from the hydrate or slaked lime. Common 

 mortar consists of sand, slaked lime, and water. 

 The sand acts merely mechanically to prevent the 

 mortar shrinking ; the hardening of the mortar 

 depends on the slow conversion of the hydrate of 

 lime into carbonate by the action of the carbonic 

 acidoftheair(CaO,H 2 O+CO 2 = CaO,CO 2 +H 2 O). 

 Hydraulic mortar, or cement, consists of slaked 

 lime, sand, burnt clay, and water. Here the sand 

 acts also merely mechanically ; but the burnt clay 

 (which is frequently introduced by using a lime- 

 stone containing clay, so that the two are burnt 

 together) contains silica (SiO 2 ) in a condition 

 such that it can slowly unite with the lime, and 

 form silicate of lime (CaO,SiO 2 ), and it is upon 

 this union that the hardening or setting of hy- 

 draulic mortar or cement depends. Other import- 

 ant salts of lime are : The phosphate (see p. 326) ; 

 the nitrate (see p. 322). The sulphate (CaOSO 3 

 or CaSO 4 ) occurs anhydrous in the mineral an- 

 hydrite, also with two molecules of water of 

 halhydration (see p. 316) as selenite or gypsum. 

 When gypsum is heated to about 500 C. it loses 

 all its water, and is converted into 'plaster of 

 Paris.' The setting of plaster of Paris, when mixed 

 with water, depends on the sulphate of lime taking 

 up the water to form again the halhydrated salt 

 (CaO,SO 3 ,2H 2 O). Sulphate of lime is very spar- 

 ingly soluble in water. Water containing lime 

 salts in solution is ' hard ; ' that is, it requires a 

 greater quantity of soap to form a lather than soft 

 water does (see CHEMISTRY APPLIED TO THE 

 ARTS). The chief lime salts occurring in hard 

 water are the carbonate and the sulphate. The 

 former can be almost entirely removed by boiling, 

 hence the hardness caused by its presence is called 

 removable hardness ; that caused by the sulphate 

 is called permanent hardness. The crust which 

 forms in kettles and boilers in which hard water 

 is boiled, consists usually of both carbonate and 

 sulphate of lime. 



The compounds of barium and strontium re- 

 semble generally those of calcium. Baryta and 

 strontia are more soluble than lime ; their sul- 

 phates are nearly insoluble in water. The metals 

 barium, strontium, and calcium can be obtained 

 by the electrolysis of their fused chlorides. 



3. Metals of the Iron group. Iron forms two 

 basic oxides namely, protoxide of iron or fer- 

 rous oxide (FeO), and sesquioxide, or peroxide, or 

 ferric oxide (Fe 2 O 3 ) ; each of these forms salts 

 with acids, and these two series cf salts are quite 

 different in character from each other. Some 

 metals of the group, like iron, form two basic 



