~Sodium thiosulphate~ (_hyposulphite of soda or "hypo"_) (Na_{2}S_{2}O_{3}.5H_{2}O). This salt, commonly called sodium hyposulphite, or merely hypo, is made by boiling a solution of sodium sulphite with sulphur: Na_{2}SO_{3} + S = Na_{2}S_{2}O_{3}. It is used in photography and in the bleaching industry, to absorb the excess of chlorine which is left upon the bleached fabrics. ~Thio compounds.~ The prefix "thio" means sulphur. It is used to designate substances which may be regarded as derived from oxygen compounds by replacing the whole or a part of their oxygen with sulphur. The thiosulphates may be regarded as sulphates in which one atom of oxygen has been replaced by an atom of sulphur. This may be seen by comparing the formula Na_{2}SO_{4} (sodium sulphate) with the formula Na_{2}S_{2}O_{3} (sodium thiosulphate). ~Sodium carbonate~ (_sal soda_)(Na_{2}CO_{3}.10H_{2}O). There are two different methods now employed in the manufacture of this important substance. 1. _Le Blanc process._ This older process involves several distinct reactions, as shown in the following equations. (a) Sodium chloride is first converted into sodium sulphate: 2NaCl + H_{2}SO_{4} = Na_{2}SO_{4} + 2HCl. (b) The sodium sulphate is next reduced to sulphide by heating it with carbon: Na_{2}SO_{4} + 2C = Na_{2}S + 2CO_{2}. (c) The sodium sulphide is then heated with calcium carbonate, when double decomposition takes place: Na_{2}S + CaCO_{3} = CaS + Na_{2}CO_{3}. ~Technical preparation of sodium carbonate.~ In a manufacturing plant the last two reactions take place in one process. Sodium sulphate, coal, and powdered limestone are heated together to a rather high temperature. The coal reduces the sulphate to sulphide, which in turn reacts upon the calcium carbonate. Some limestone is decomposed by the heat, forming calcium oxide. When treated with water the calcium oxide is changed into hydroxide, and this prevents the water from decomposing the insoluble calcium sulphide. The crude product of the process is a hard black cake called black ash. On digesting this mass with water the sodium carbonate passes into solution. The pure carbonate is obtained by evaporation of this solution, crystallizing from it in crystals of the formula Na_{2}CO_{3}.10H_{2}O. Since over 60% of this salt is water, the crystals are sometimes heated until it is driven off. The product is called calcined soda, and is, of course, more valuable than the crystallized salt. 2. _Solvay process._ This more modern process depends upon the reactions represented in the equations NaCl + NH_{4}HCO_{3} = NaHCO_{3} + NH_{4}Cl, 2NaHCO_{3} = Na_{2}CO_{3} + H_{2}O + CO_{2}.