3-t ALUMINA-CHROME-ALUM. 



The precipitated hydrate of alumina is well washed, 

 for which purpose hot water is to be preferred, and 

 strongly ignited in order to expel the water. 



The water contained in the alum is determined by 

 loss. It may also be estimated directly by carefully 

 exposing the salt for a very long time to a gradually 

 increasing heat, which must finally be raised to dull 

 redness. 



17. IRON- AMMONIA- ALUM.* 

 NH 4 0, S0 3 ; Fe 2 3 , 3 S0 3 + 24 HO. 



At a strong red heat, this salt is entirely decom- 

 posed, leaving pure sesquioxide of iron. 



The determination of ammonia is effected as in No. 

 5 ; that of sulphuric acid according to No. 3. 



In order to control the determination of the sesqui- 

 oxide of iron, another portion of the salt is dissolved 

 in water, and the sesquioxide precipitated by ammonia. 

 The precipitated hydrate is washed, dried, and ignited. 



18. ALUMINA-CHEOME-ALUM.t 

 KO,S0 3 ; 



The sulphuric acid is precipitated by chloride of 

 barium as in No. 3. 



* Powdered red or brown iron-stone is digested with concen- 

 trated sulphuric acid ; the white sulphate thus produced is dis- 

 solved in water, the solution mixed with sulphate of ammonia, 

 filtered and allowed to crystallize. 



f Three parts of finely-powdered bichromate of potassa are mixed 

 with 15 parts of water, and 1 part of concentrated sulphuric acid 

 is gradually added, so that no evolution of heat may ensue; sul- 

 phurous acid gas is then passed through the solution, which is 



