SODIUM 529 



be conducted in china, glass, or similar vessels, because caustic soda 

 attacks these materials, although but slightly. The solution does not 

 crystallise on evaporation, because the solubility of caustic soda when 

 hot is very great, but crystals, containing water of crystallisation may 

 be obtained by cooling. If the evaporation of the alkali be conducted 

 until the specific gravity reaches 1*38, and the liquid is then cooled 

 to 0, transparent crystals appear containing 2NaHO,7H 2 O ; they 

 fuse at +6. 27 If the evaporation be conducted so long as water is dis- 

 engaged, which requires a considerable amount of heat, then, on cooling, 

 the hydroxide, KaHO, solidifies in a semi-transparent crystalline mass, 28 

 which eagerly absorbs moisture and -carbonic anhydride from the air. 29 

 Its specific gravity is 2-13 , 30 it is easily soluble in water, with disen- 

 gagement of a considerable quantity of heat. 31 A saturated solution 

 at the ordinary temperature has a specific gravity of about 1-5, contains 

 about 45 per cent, of sodium hydroxide, and boils at 1 30 ; at 55 water 

 dissolves an equal weight of it. 32 Caustic soda is not only soluble in 



tions expressing the reason for a reaction really add but little to that elementary con- 

 ception of contact which, according to my opinion, consists in the change of motion of the 

 atoms in the molecules under the influence of the substance in contact. In order to 

 represent this clearly it is sufficient, for instance, to imagine that in the sodium carbonate 

 the elements C0 2 move in a circle round the elements Na-jO, but at the points of contact 

 with Fe 2 Oj the motion becomes elliptic with a long axis, and at some distance.from Na 2 O 

 the elements of CO 2 are parted, not having the faculty of attaching themselves to Fe 2 O 3 . 



27 By allowing strong solutions of sodium hydroxide to crystallise in the cold, 

 impurities such as, for instance, sodium sulphate may be separated from them. The 

 fused crystallo-hydrate 2NaHO,7HiO forms a solution having a specific gravity of 1-405. 

 (Hermes). The crystals on dissolving in water produce cold, while NaHO produces 

 heat. Besides which Pickering obtained hydrates with 1, 2, 4, 5, and 7 H 2 O. 



28 In solid caustic soda there is generally an excess of water beyond that required by 

 the formula NaHO. The caustic soda used in laboratories is generally cast in sticks, 

 which are broken into pieces. It must be preserved in carefully closed vessels, because 

 it absorbs water and carbonic anhydride from the air. 



20 By the way it changes in air it is easy to distinguish caustic soda from caustic 

 potash, which in general resembles it. Both alkalis absorb water and carbonic anhydride 

 from the air, but caustic potash forms a deliquescent mass of potassium carbonate, whilst 

 caustic soda forms a dry powdei* of efflorescent salt. 



50 As the molecular weight of NaHO = 40, the volume of its molecule = 40/2-18 = 18-5, 

 which very nearly approaches the volume of a molecule of water. The same rule applies 

 to the compounds of sodium in general for instance, its salts have a molecular volume 

 approaching the volume of the acids from which they are derived. 



51 The molecular quantity of sodium hydroxide (40 grams), on being dissolved in a 

 large mass (200 gram molecules) of water, develops, according to Berthelot 9,780, and 

 according to Thomsen 9,940, heat-units, but at 100 about 18,000 (Berthelot). Solutions 

 of NaHOf wH 2 O, on being mixed with water, evolve heat if they contain less than 6H 2 O, 

 but if more they absorb heat^ 



32 The specific gravity of solutions of sodium hydroxide at 15/4 is given in thejshort 

 table below: 



NaHO, p.c. .5 10 15 20 30 40 



Sp..gr. . . . 1-057 1-113 1-169 1'224 1'831 1-486 



