SODIUM CHLORIDE -BERTHOLLErS LAWS 437 



Barium sulphate, BaS0 4 , which is insoluble in water, when fused 

 \7ith sodium carbonate,. Na 2 CO 3 , gives, but not completely, barium 

 carbonate, BaCO 3 , (also insoluble), and sodium sulphate, Na 2 SO v If a 

 solution of sodium carbonate acts on precipitated barium sulphate, 

 the same decomposition is also effected (Dulong, Kose), but it is 

 restricted by a limit and requires time. A mixture of sodium carbonate 

 and sulphate is obtained in the solution and a mixture of barium carbo- 

 nate and sulphate in the precipitate. If the solution be decanted off 

 and a fresh solution of sodium carbonate be poured over the precipitate, 

 then a fresh portion of the barium sulphate passes into barium carbonate, 

 and so by increasing the mass of sodium carbonate it is possible to 

 entirely convert the barium sulphate into barium carbonate. If a 

 definite quantity of sodium sulphate be added to the solution of sodium 

 carbonate, then the latter will have no action whatever on 

 the barium sulphate, because then a system in equilibrium deter- 

 mined by 'the reverse action of the sodium sulphate on the barium 

 carbonate and by the presence of both sodium carbonate and sulphete in 

 the solution, is at once arrived at. On the other hand, if the mass of 

 the sodium sulphate in the solution be great, then the barium carbonate 

 is reconverted into sulphate until a definite state of equilibrium is 

 attained between the two opposite reactions, producing barium carbonate 

 by the action of the sodium carbonate and barium sulphate by the action 

 of the sodium sulphate. 



Another most important principle of Berthollet's teaching is 

 the existence of a limit of exchange decomposition^ or the attain- 

 ment of a state of equilibrium. In this respect the determinations of 

 Malaguti (1857) are historically the most important. He took a 

 mixture of solutions of equivalent quantities of two salts, MX and 

 NY, and judged the amount of the resulting exchange from the 

 composition of the precipitate produced by the addition of alcohol. 

 When, for example, zinc sulphate and sodium chloride (ZnSO 4 and 

 2NaCl) were taken, there were produced by exchange sodium sul- 

 phate and zinc chloride. A mixture of zinc sulphate and sodium 

 sulphate was precipitated by an excess of alcohol, and it appeared 

 from the composition of the precipitate that 72 per cent, of the salts 

 iaken had been decomposed. When, however, a mixture of solutions 

 of sodium sulphate and zinc chloride was taken, the precipitate pre- 

 sented the same composition as before that- is, about 28 per cent, of .the 

 salts taken had been subjected to decomposition. In a similar expeHU 

 tnent with a mixture of sodium chloride and magnesium sulphate, 

 SNaCl + MgSO 4 or MgCl 2 -f Na 2 SO 4 , about half of the metals under*, 

 went the decomposition, which may be expressed by the equation 



