CHEMISTRY APPLIED TO THE ARTS. 



the sulphur from the solution of the oxidised waste 

 by means of hydrochloric acid. 



Caustic Soda is formed from soda-ash by dis- 

 solving it in water, and adding slaked lime, when 

 carbonate of lime falls as an insoluble white 

 powder, leaving caustic soda in solution. This 

 solution is either used as such, or boiled down to a 

 solid mass. The action is : 



NagO.CO, 4- CaO,H,O = Na,O,H a O + CaO,CO, or 

 NajCOs + CaH 2 O, = 2NaHO + CaCOg. 

 Carbonate Slaked Lime. Caustic Soda. Carbonate 

 of Soda. of Lime. 



Caustic soda is also often prepared direct from 

 the ' black ball,' which, for this purpose, is made 

 from a mixture containing a larger quantity of coal 

 than is used for preparing soda-ash. 



Bleaching-powder. Slaked lime readily absorbs 

 chlorine gas, forming a substance called ' chloride 

 of lime,' or bleaching-powder. This substance 

 may be regarded as a mixture, or compound 

 of chloride of calcium (CaCl^, hypochlorite of 

 lime (CaO,Cl 2 O or CaCl 2 O 2 ), and slaked lime 

 (CaO,H 2 O). Exposed to the air, it gives off 

 hypochlorous acid, the hypochlorite of lime being 

 decomposed by the carbonic acid of the air. 

 When treated with sulphuric acid, sulphate of 

 lime is formed ; and if only so much acid is added 

 as is required to neutralise the slaked lime, and 

 decompose the hypochlorite, hypochlorous acid is 

 given off; but if a larger quantity of acid is added, 

 the chloride of calcium is also decomposed, and 

 the hydrochloric acid produced in this decomposi- 

 tion acts on the hypochlorous acid, and produces 

 chlorine : 



2HC1 + C1 3 O = H 2 O + 2C1 8 . 



Other strong acids act in the same way. 



BLEACHING. 



The bleaching of vegetable or animal fibre, 

 yarn, or cloth, consists in the destruction or 

 removal of the colouring-matter naturally accom- 

 panying the fibre, so as to leave the latter white. 

 Along with this, we may conveniently consider 

 the discharging that is, removal or destruction of 

 colouring-matter artificially added to the fibre. 

 In bleaching vegetable fibres, or the yarn or cloth 

 made from them, two distinct things have to be 

 attended to first, the removal of fatty or oily 

 matters, either originally present, or introduced in 

 the processes of spinning or weaving ; and second, 

 the destruction of those colouring-matters which 

 are not removed along with the oil. The removal 

 of the oil is effected by boiling with alkalies by 

 which a soap is formed (see p. 349), which is 

 washed out with water. This washing removes 

 along with the soap the dirt which the oil kept 

 fixed to the fibre. There still, however, remains 

 .a yellowish colouring-matter, which must be 

 destroyed if the cloth is to be rendered quite 

 white. This may be done either by the old pro- 

 cess of grass-bleaching, in which the cloth is 

 spread in a moist state upon grass, and exposed 

 for several days to the action of the air, or by 

 means of wet chlorine or hypochlorous acid. In 

 either case, the colouring-matter is destroyed by 

 oxidation. In grass-bleaching it is probable that 

 the oxidation of the colouring-matter is caused by 

 the ozone (see CHEMISTRY) that is present in the 

 air. 



In order to understand how chlorine along with 

 water can act as an oxidising agent, we must 

 recollect that chlorine and hydrogen readily enter 

 into combination to form hydrochloric acid. If a 

 solution of chlorine gas in water is exposed to 

 light, the chlorine slowly displaces the oxygen 

 hydrochloric acid is formed, and oxygen is set 

 free. Now, this decomposition of water takes 

 place only slowly, and only when the mixture is 

 exposed to bright light, if no oxidisable substance 

 is present. But very many oxidisable substances, 

 even such as are not easily oxidised by means of 

 free oxygen, assist the chlorine to decompose the 

 water ; so that, the constituents of the water being, 

 so to speak, pulled in opposite directions, the 

 chlorine takes the hydrogen, forming hydrochloric 

 acid, and the oxidisable substance is oxidised. 

 It is in this way the chlorine bleaches, and it will 

 be at once seen that water is essential in the pro- 

 cess. This can easily be shewn by placing a 

 piece of cloth, dyed with a vegetable colour tur- 

 key-red, for instance in a jar of chlorine gas ; if 

 part of the cloth be wet with water, and part left 

 dry, it will be found that, in a few minutes, the 

 wet parts have become white, while the dry parts 

 remain red. Hypochlorous acid acts also as a 

 bleaching agent, by oxidising the colouring-matter, 

 Cl.jO + H 2 O becoming 2HC1 + O 2 the oxygen 

 not being given off as gas, but acting upon the 

 colouring-matter. Most vegetable colouring-mat- 

 ters are converted by such oxidation into colourless 

 substances ; or, at all events, are rendered soluble 

 in water, so that they can be washed away. 



In practice, cotton cloth, after having been 

 boiled in alkaline solutions, and washed, is soaked 

 in a very dilute solution of bleaching-powder, and 

 then ' soured ' that is, passed through water con- 

 taining a small quantity of acid (sulphuric or 

 hydrochloric). The acid, acting on the bleaching- 

 powder, produces chlorine, which bleaches the 

 cloth in the way explained above. The excess of 

 chlorine, or hypochlorous acid, must be got rid of, 

 as, if left long in contact with the cloth, it injures 

 the fibre. This is done by means of washing with 

 water, or by treating the cloth with what is techni- 

 cally called an 'antichlor' that is, some sub- 

 stance which will convert the chlorine into a 

 harmless compound. Sulphite of soda and hypo- 

 sulphite of soda are the substances mostly used 

 for this purpose. They are oxidised by the moist 

 chlorine, the chlorine becoming hydrochloric acid, 

 which can be neutralised by an alkali and washed 

 out. 



Chlorine and hypochlorous acid are not only 

 used to bleach cloth or vegetable fibre, but also to 

 discharge colours with which the cloth has been 

 dyed. If this discharging is confined to definite 

 parts of the cloth, a white pattern can be produced 

 on a coloured ground. This is effected by printing 

 the pattern on the cloth with a mixture of tartaric 

 or citric acid and gum. The cloth is then passed 

 through a dilute solution of bleaching-powder, 

 when the acid sets free chlorine upon the parts 

 printed ; and by the oxidising action explained 

 above, the colour is discharged from these 

 parts. 



Animal fibres (silk, wool) cannot be bleached by 

 means of chlorine, as that substance exerts a de- 

 structive action upon them. The only materials 

 used in bleaching wool and silk are water, soap, 

 carbonate of soda, and sulphurous acid. 



3 9 



