SODIIM 507 



same salt lies at the bottom of several lakes (an area of about 10 

 square kilometres) in the Kouban district near Batalpaschinsk, and 

 here its working has been commenced (1887). In Spain, near Aran- 

 goulz and Madrid, mineral sodium sulphate has likewise been found, 

 and is already being worked. 



The methods of obtaining salts by means of double decomposition 

 from others already prepared are so general, that in describing a given 

 salt there is no necessity to enumerate the cases hitherto observed of 

 its being formed through various double decompositions. 3 The possi- 

 bility of this occurrence ought to be foreseen according to Berthollet's 

 doctrine from the properties of the salt in question. On this account 

 it is important to know the properties of salts ; all the more so because 

 up to the present time those very properties (solubility, formation of 

 crystallo-hydrates, volatility, tfec.) which may be made use of for sepa- 

 rating them from other salts have not been generalised. 4 These pro- 

 perties as yet remain subjects for investigation, and are rarely to be 

 foreseen. 



Sodium sulphate very easily parts with water, and may be obtained 

 in an anhydrous state if it be carefully heated until the weight re- 

 mains constant ; but if heated further, it partly loses the elements of 

 sulphuric anhydride. It fuses at 861 (red heat), and volatilises in a 

 small proportion when very strongly heated, in which case it naturally 

 decomposes. At in 100 parts of water, 5 parts of the anhydrous 

 salt dissolve, at 10 9 parts, at 20 19 -4, at 30 40, and at 34 55 parts, 

 the same being the case in the presence of an excess of crystals of 

 Na 2 SO 4 ,10H 2 O. 5 At 34 the latter fuses, and the solubility decreases 



salt-cake, in mineralogy tlienardite. Crystalline decahydrated salt is termed in mine- 

 ralogy mirabilite. On fusing it, the monohydrate NaaSC^BLjO is obtained, together 

 with a supersaturated solution. 



3 The salts may be obtained not only by methods of substitution of various kinds, but 

 also by many other combinations. Thus sodium sulphate may be formed from sodium 

 oxide, and sulphuric anhydride by oxidising sodium sulphide, Na 2 S, sodium sulphite, 

 Na^SOs, &c. When sodium chloride is heated in a mixture of the vapours of water, air, 

 and sulphuric anhydride, sodium sulphate is formed. 



4 Many observations have been made, but little general information has been extracted 

 from particular cases. In addition to which, the properties of a given salt are changed 

 by the presence of other salts. This takes place not only in virtue of mutual decomposi- 

 tion or formation of double salts capable of separate existence, but is determined by the 

 influence which some salts exert on others, or by forces similar to those which act during 

 decomposition. Here, nothing has been generalised to that extent which would render 

 it possible to foretell uninvestigated examples. Let us state one of these numerous 

 cases : 100 parts of water at 20 dissolve 34 parts of potassium nitrate, but on the addition 

 of sodium nitrate the solubility of potassium nitrate increases to 48 parts in 100 of water 

 (Carnelley and Thomson). In general, in all cases of which there are accurate observa- 

 tions, it appears that the presence of foreign salts changes the property of any given 

 salt. 



5 The information concerning solubility (Chapter I. p. 72) is given according to the 

 determinations of Gay-Lussac, Lovell, and Mulder. 



