SODIUM 515 



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 Berthol let'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, &c.) 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. The crystallo-hydrate of the normal sodium sulphate, 

 Na 2 SO 4 ,10H 2 O, 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. The normal salt fuses at 843 (red heat), and 

 volatilises to a slight extent when very strongly heated, in which case it 

 naturally decomposes with the evolution of S0 3 . At 100 parts of 

 water dissolve 5 parts of the anhydrous salt, 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 at higher temperatures. 6 A concentrated solution 

 at 34 has a composition nearly approacnmg to Na 2 S0 4 -f- 14H 2 0, 



5 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^S, or sodium sulphite, 

 Na 2 SO 3 , &c. When sodium chloride is heated in a mixture of the vapours of water, air, 

 and sulphurous anhydride, sodium sulphate is formed. According to this method (patented 

 by Hargreaves and Robinson), sodium sulphate, Na^SC^, is obtained fromNaCl without the 

 preliminary manufacture of H 2 SO 4 . Lumps of NaCl pressed into bricks are loosely packed 

 into a cylinder and subjected, at a red heat, to the action of steam, air and SO a . Under 

 these conditions, HC1, sulphate, and a certain amount of unaltered NaCl are obtained. 

 This mixture is converted into soda by G-ossage's process (see Note 15) and may have 

 some practical value. 



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

 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 

 solution. Here nothing has been generalised to that extent which would render it 

 possible to predict without previous investigation, if there be no close analogy to help 

 us. Let us state one of these numerous cases: 100 parts of water at 20 dissolve 

 84 parts of potassium nitrate but on the addition of sodium nitrate the solubility of 

 potassium nitrate increases to 48 parts in 10 of water (Carnelley and Thomson). In 

 general, in all cases of which there are accurate observations it appears that the 

 presence of foreign salts changes the properties of any given salt. 



5 The information concerning solubility (Chapter I.) is given according to the deter- 

 minations of G-ay-Lussac, Lovell, and Mulder. 



6 In Chapter L, Note 24, we have already seen that with many other sulphates tbo 



