SODIOTi CHLORIDE- -BERTHOLLET'S LAWS 427 



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

 with sodium carbonate, Na 2 CO 3 , gives, but not completely, barium 

 carbonate, BaCO 3 (also insoluble), and sodium sulphate, Na 2 S0 4 . If a 

 solution of sodium carbonate acts on precipitated barium sulphate, then 

 the same decomposition is also accomplished (Dulong, Rose), 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 sodium carbonate will have no action whatever 

 on the barium sulphate, because then an equilibrated system, deter- 

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

 carbonate and by the presence of the sodium carbonate and sulphate 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 reverse reactions, producing the barium carbonate 

 by the aid of the sodium carbonate or the barium sulphate by the aid 

 of the sodium sulphate. 



Another most important conception of Berthollet's teaching consists 

 in the existence of a limit of exchange decom%)osition, or in 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 the 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 

 L'XaCl) were taken, then there were produced by exchange sodium 

 sulphate 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 taken 

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



decompositions taking place in dilute solutions where the mass of water is large, its in- 

 fluence, notwithstanding the weakness of affinities, must be great, according to the very 

 spirit of Berthollet's doctrine. 



As explaining the action of the mass of water, the experiments of Pattison Muir (1879) 

 ;ire very instructive. These experiments demonstrate that bismuth chloride is decom- 

 posed the more the greater the relative quantity of water, and the less the mass of hydro- 

 chloric acid forming one of the products of the reaction. 



