286 



Prof. G. Wiedemann on the Magnetic 



X, then is 



is to the colloidally dissolved free oxide as 1 — d' 

 m = mQ^l — x) + mi.v. 



Since the magnetism nii of the colloid ferric oxide is only 

 0*2198 of the magnetism m^^ of the oxide in combination with 

 acidj therefore 



■2S(l-^). 



Consequently by determining the magnetism we can directly 

 determine the ratio x : 1 of the quantity of ferric oxide disso- 

 ciated to the total quantity of it in the salt. 



In this manner the magnetism of the unit weight of iron in 

 variously dilute solutions of ferric chloride was observed, with 

 the following results: — 





Series I. 





Series 



XL 



Iron in 10 c. 



c. 





Iron in 10 c. c. 





of solution. 



m,. 



(l-.r)lOO. 



of solution. 



(1-^)1 



g-rm. 







grm. 





1-606 



99-6 



100 



1-491 



100-8 



1-207 



101-7 



100 



1-119 



99-5 



0-806 



98-7 



98-4 



0-747 



98-0 



0-404 



94-8 



97-6 



0-375 



95-1 



0-202 



93-1 



93-4 



0-184 



93-6 



0-138 



91-3 



91-1 



0-129 



91-5 



0-071 



102-1 



88-4 



0-062 



88-8 



In the experiments of series II., 3-1 per cent, more of the 

 ferric oxide than corresponded to equivalence with the acid 

 present (according to the analysis of the most concentrated 

 solution) was already contained in the colloid state. There- 

 fore dissociation in ferric-chloride solutions proceeds slowly 

 with increasing dilution. 



The case is different with neutral solutions oi ferric sulphate 

 (Fe2 (804)3). In them the magnetism of the iron atom is 

 perhaps only 80 per cent, of that of the atom of iron in very 

 acid solutions of ferric chloride. The ratio hardly changes 

 even when the iron contained in the solutions varies from 

 0-567 to 0-073 gram, in 10 cubic centims. From this it is 

 calculated that about 25 per cent, of the salt in the solutions 

 has become dissociated into colloidally dissolved ferric oxide 

 and sulphuric acid, and only 75 per cent, of the total quantity 

 of the ferric oxide and sulphuric acid remain combined with 

 one another. 



If a solution of ammonium iron-alum be investigated as to 

 its magnetism, it is found that the ferric sulphate contained in 



