CHEMISTRY. 



sulphuric acid upon half an equivalent of caustic 

 potash (or, what is the same thing, by the action 

 of two equivalents of sulphuric acid upon one of 

 <austic potash). It is in the case of only some 

 acids that we obtain acid salts : thus, sulphuric, 

 oxalic, tartaric, phosphoric, citric acids form acid 

 salts ; nitric and hydrochloric acids do not As 

 examples of acid salts, we may mention cream of 

 tartar (acid tartrate of potash), salt of sorrel (acid j 

 oxalate of potash), baking-soda (bicarbonate of ' 

 soda), super-phosphate of lime (acid phosphate of j 

 lime). In these, we have salts formed from one 

 equivalent of potash, and two of tartaric acid ; 

 one of potash, and two of oxalic acid ; one of 

 soda, and two of carbonic acid ; one of lime, and 

 three of phosphoric acid. It will be noted that 

 here also we have the proportion of acid and base 

 occurring according to equivalents ; and it will be 

 well to remember, what will be more fully ex- 

 plained afterwards, that it is the acid which deter- 

 mines the formation of acid salts, some acids 

 forming them, others not* Most acid salts have 

 an acid reaction; but where the acid is weak, 

 the reaction may be neutral, or even alkaline. 



Again, we have seen, that if 60 grains (one 

 -equivalent) of acetic acid be treated with m 

 (one equivalent) of oxide of lead, a normal salt 

 (acetate of lead, or sugar of lead) is produced. If 

 this be dissolved in water, and the solution be 

 shaken up with finely divided oxide of lead, it will 

 dissolve 223 grains (two equivalents) more, and 

 form a salt, which is called 'basic' acetate of lead. 

 A basic salt is formed by the action of one equiv- 

 alent of an acid upon more than one equivalent j 

 of base ; just as an acid salt is formed by the 

 action of more than one equivalent of acid upon 

 one of base ; and as it is the acid which deter- 

 mines the formation of acid salts (some acids 

 forming them, others not), so it is the base which 

 determines the formation of basic salts ; some 

 bases, such as oxide of lead, oxide of bismuth, red 

 oxide of iron, forming them ; others, such as 

 caustic potash, soda, oxide of silver, not 



Another class of salts deserves mention here 

 namely, the ' double salts.' These may be re- 

 garded as compounds of two salts with one 

 another. As examples, we may mention common ' 

 alum, consisting of sulphate of potash, and sul- 

 phate of alumina; Rochelle salt, tartrate of 

 potash, and tartrate of soda ; dolomite, carbonate 

 of lime, and carbonate of magnesia. 



We have now considered, pretty fully, the action 

 of acids upon bases ; let us specially turn to the 

 action of acids upon salts, of bases upon salts, and 

 -of different salts upon one another. 



In order to understand the action of acids upon 

 salts, we shall first examine what takes place ! 

 when an equivalent of a base is mixed with an 

 equivalent of each of two acids. It is obvious 

 that one or other of two things may occur 

 First, the base may form a salt with one of 

 the acids, the remaining acid being left as it 

 "was ; or second, the base may be divided into 

 two patts, one forming a salt with part of the 

 one acid, the other with part of the other acid, 

 the rest of both acids being left unacted on. 

 In the first case, we shall have a mixture of one 

 salt and one acid; in the second, a mixture of 



* There are some bases which seldom or never form acid salts, 

 even with those acids which have the greatest tendencies to form 

 :them ; thus, there are no acid salts of oxide of silver. 



two salts and two acids. Thus if one equivalent 

 (in grains) of oxide of lead be mixed with one 

 equivalent (49 grains^ of sulphuric acid, and one 

 equivalent (63 grains) of nitric acid, the whole of 

 the oxide of lead acts on the sulphuric acid, and 

 the nitric acid is left altogether unacted on. 

 Again, if one equivalent (40 grains) of caustic 

 soda be mixed with one equivalent of sulphuric, 

 and one equivalent of nitric acid, the soda 

 acts on both acids, producing sulphate of soda 

 and nitrate of soda, while a part of both acids is 

 left unacted on. Now, the same final result is 

 produced whether the two acids are added at once 

 (as in the cases we have just mentioned) or suc- 

 cessively. In the first case, we might add to the 

 oxide of lead, first, nitric acid, and obtain nitrate 

 of lead; and then to the nitrate of lead add 

 sulphuric acid, when we should obtain sulphate of 

 lead and nitric acid the sulphuric acid turning 

 out (if we may use the phrase) the nitric acid. If, 

 on the other hand, we add the sulphuric acid 

 first, we obtain sulphate of lead, and nitric acid 

 does not act on this ; so that in either case we 

 obtain the same final result sulphate of lead and 

 nitric acid. In the second case, sulphuric acid 

 will partially (at ordinary temperatures) drive out 

 nitric acid from nitrate of soda, and nitric acid 

 will also partially drive out sulphuric acid from 

 sulphate of soda ; in either case, producing a 

 mixture of sulphate of soda and nitrate of soda, 

 sulphuric acid and nitric acid. We have said 'at 

 ordinary temperatures,' and this leads us to a 

 point of great theoretical and practical import- 

 ance. It is obvious that the balance of the mix- 

 ture will be destroyed, that is, the proportion in 

 which the base is divided between the acids will 

 be changed, if a part of one of the acids be taken 

 away. Thus, if, in the last-mentioned case of a 

 mixture of sulphate of soda, nitrate of soda, sul- 

 phuric acid, and nitric acid, we take away some of 

 the nitric acid, a rearrangement will take place ; 

 the soda will be divided between the two acids in 

 proportion to the quantities of them now present, 

 a new quantity of nitric acid will be driven out by 

 sulphuric ; so that, if we continue to remove the 

 nitric acid as it is formed, the nitrate of soda will 

 be entirely decomposed, and nothing but sulphate 

 of soda will be left. Now, nitric acid is much more 

 volatile than sulphuric acid ; therefore, by heating 

 the mixture, we can remove the nitric acid as 

 vapour, leaving the sulphuric acid in the mixture ; 

 and thus, although at ordinary temperatures, below 

 the boiling-point of nitric acid, sulphuric acid only 

 partially decomposes nitrate of soda, at higher 

 temperatures the decomposition is complete, the 

 nitric acid passing off as vapour. It is, of course, 

 not necessary that one of the acids should be 

 removed as vapour; the same result follows, in 

 whatever way it is taken out of the mixture ; thus, 

 it may separate as a solid out of an aqueous solu- 

 tion as, for instance, is f he case when an aqueous 

 solution of borax (borate of soda, the salt formed 

 by the action of soda on boracic acid) is treated 

 with sulphuric acid. Here the sulphuric acid 

 drives out the boracic acid, and as the latter is 

 only sparingly soluble in cold water, a large part 

 of it crystallises out, and the borax is almost 

 wholly decomposed. The consideration of the 

 fact, that, as a general rule, a base is shared 

 between the acids which, so to speak, compete for 

 it ; and that when one acid is partially removed, 



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