438 PRINCIPLES OF CHEMISTRY 



reformation of the salt XX. If it be removed, then, although the 

 quantity of the elements X and X in the mass will be diminished, still, 

 according to Berthollet's law, a certain amount of XX should be again 

 formed. When this substance is again formed, then, owing to its 

 physical properties, it will again pass away ; hence the reaction, in 

 consequence of the physical properties of the resultant substance, is 

 able to proceed to completion notwithstanding the possible weakness of 

 the attraction existing between the elements entering into the compo- 

 sition of the resultant substance XX. Naturally, if the resultant 

 substance is also formed of elements having a considerable degree of 

 affinity, then the complete decomposition is considerably facilitated. 



ISuch a representation of the modus operaitdi of chemical trans- 

 formations is applicable with great clearness to a number of re- 

 actions studied in chemistry, and, what is especially important, the 

 application of this aspect of Berthollet's teaching does not in any way 

 require the determination of the measure of affinity acting between the 

 substances present. For instance, the action of ammonia on solutions 

 of salts ; the displacement, by its means, of basic hydrates soluble in 

 water ; the separation of volatile nitric acid by the aid of non-volatile 

 sulphuric acid, as well as the decomposition of table salt by means of 

 sulphuric acid, when gaseous hydrochloric acid is formed may be 

 taken as examples of reactions which proceed to the end, inasmuch as 

 one of the resultant substances is entirely removed from the sphere of 

 action, but they in no way indicate the measure of affinity. 80 



30 Common salt not only enters into double decomposition with acids but also trith 

 every salt. However, as clearly follows from Berthollet's doctrine, this form of decom- 

 position will only in a few cases render it possible for new metallic chlorides to be ob- 

 tained, because the decomposition will not be carried on to the end unless the metallic- 

 chloride formed separates from the mass of the active substances. Thus, for example. 

 if a solution of common salt be mixed with a solution of magnesium sulphate, double 

 decomposition ensues, but not completely, because all the substances remain in the solu- 

 tion. In this case the decomposition may result in the formation of sodium sulphate and 

 magnesium chloride, substances which are soluble in water; nothing is disrupted, 

 and therefore the decomposition 2NaCl + MgSO t = MgClo-r Xa-jSO, cannot proceed t<> tin- 

 end. However, the sodium sulphate formed in this manner may be separated by free/ing 

 the mixture. Sulphate of sodium is sparingly soluble in the cold, and therefore if a suffi- 

 ciently strong solution of common salt and magnesium sulphate lie taken, the resulting 

 sodium sulphate will separate out in the cold as crystals containing water of crystallisa- 

 tion. The complete separation of the sodium sulphate will naturally not take place, owing 

 to a portion of the salt remaining in the solution in the cold. Nevertheless, this kind ot 

 decomposition is made use of for the preparation of sodium sulphate from the residue- 

 left after the evaporation of sea-water, which contain a mixture of magnesium sulphate 

 and common salt. Such a mixture is encountered at Stassfurt in a natural form. The 

 separation is sometimes carried on by means of artificial cooling by refrigerating 

 machines. It might be said that this form of double decomposition is only accomplished 

 with a change of temperature; but this would not be true, as may be concluded from 

 other analogous cases. Thus, for instance, a solution of copper sulphate is of a blue 



