Behaviour of Chemical Compounds. 287 



tlie alum, quite independently of the presence of tlie sulphate 

 of ammonium J is dissociated, as when by itself, to the amount 

 of 25 per cent. It is consequently in the highest degree pro- 

 bable that the iron-alum in the solution is completely decom- 

 posed into alkaline and ferric sulphates. 



In a solution of ferric nitrate which in 10 cubic centims. 

 contained 0*266 gram of iron, and to one eq. of Fe2 O3 0*73 eq. 

 of 3 ^2 O5 (being therefore basic), according to the magnetic 

 investigation about 32 per cent, of the total amount of iron 

 was dissolved as colloid ferric oxide. This proportion was 

 increased to only 36 per cent, on diluting the solution to three 

 times its original quantity. 



Solutions oi ferric acetate exhibit a dissociation far stronger 

 still. In a neutral solution containing 0*182 gram, of iron in 

 10 cubic centims. nearly 70 per cent, of the total ferric oxide 

 w^as in the colloid state ; in neutral solutions of ferric tartrate 

 about 57 per cent, of the salt is dissociated, and so forth. 



19. Comhining-propovtions of Acids and Bases. — If to one 

 equivalent of ferric oxide different quantities of dilute sulphuric 

 acid be put, with increase of the latter continually greater 

 quantities of ferric oxide enter into combination, as may be 

 calculated direct from the magnetism of the unit of weight of 

 iron in the solutions. 



If we write neutral ferric sulphate Fe2, 3 S04 = feO SO3, 

 and if the ratio of the equivalents fe and SO3 is as stated in the 

 following Table, the percentages of the dissociated ferric oxide 

 are those specified under x. Dividing the values 100— .a?; 

 consequently the percentages of ferric oxide which have com- 

 bined with the acid to form salt, by the total number of equi- 

 valents of acid employed in the experiment, Ave obtain the 

 values specified under ?/, which therefore indicate the number 

 of equivalents of ferric oxide taken into combination by each 

 equivalent of acid in the solution, thus: — - 



Iron contained iu ^^ 



10 cub. centims. 

 gram. 



0-343 100 



0-339 



0-334 ......... 



0-329 



0-325 



0-315 



0-304 



0-293 



I. 







S03. 



(t. 



y- 



73-4 



58-7 



0-56 



85-1 



37-9 



0-73 



98-6 



26-9 



0-74 



111-5 



14-5 



0-76 



122-8 



12-2 



0-71 



146-6 



8-3 



0-63 



182-8 



5-9 



0-51 



219-6 



1-9 



0-45 



