BAKING POWDERS. 569 



Burnt alum is somewhat hygroscopic, but dissolves more slowly in 

 water than the crystallized salt. 



1 have been unable to ascertain in what condition the alum is used for 

 compounding baking-powders. Burnt alum would seem to be the form 

 best adapted for this purpose on accoun t of its slow solubility. Professor 

 Cornwall says this is the form : used, but does not state how he obtained 

 the information ; and he states further that "crystallized alums may be 

 used in connection with burnt alum to secure at first a more rapid escape 

 of carbonic-acid gas." It is probable that the amount of drying given the 

 alum used differs with different manufacturers, but it is not likely that 

 the water of crystallization is entirely driven off. 



The following equation shows the reaction taking place in a baking- 

 powder made with burnt ammonia alum : 



475 504 157 



(NH 4 ) 2 A1 2 (SO 4 ) 4 + 6NaHCO 3 = A1 2 (OH) 6 + 



Sulphate of aluminium Bicarbonate of soda. Hydrate of alnmin- 



and ammonia, or inm. 



"burnt alum." 



426 132 264 



3Na 2 SO 4 + (NH 4 ) 2 SO 4 + 6CO 2 



Sulphate of soda. Sulphate of am- Carbonic dioxide, 



monia. 



If potash alum were used the reaction would be precisely the same 

 with the substitution of potassium for ammonia wherever it occurs in 

 the equation, sulphate of potash being formed instead of sulphate of 

 ammonia. 



A study of the equation will show that 475 grains of burnt alum with 

 504 grains of bicarbonate will produce -2G4 grains of carbonic acid and 

 leave a residue consisting of 426 grains of sulphate of soda, 132 grains 

 of sulphate of ammonia, and 157 grains of hydrate of aluminium, the 

 last named being a precipitate insoluble in water. Sulphate of soda 

 crystallizes with ten molecules of water, so that the total weight of resi- 

 due from the 979 grains of mixed chemicals would be 1,255 grains. If 

 a hydrated alum is used in the powder, the proportion of residue to 

 powder would of course be less, and the proportion of gas evolved 

 would also be less. The character of the residue is seen to be more com- 

 plex than is the case with any of the classes previously discussed, and 

 deserves special attention. The sulphate of soda is similar to other al- 

 kali salts in its physiological action. Sulphate of ammonia is not 

 used therapeutically, but probably has an action similar to that of other 

 ammonia salts, such as the chloride. Professor Cornwall, 2 in his report, 

 speaks as follows concerning this point : 



It is possible, however, that too little attention has been paid to the presence of 

 ammonium salts in the residues from ammonia alum powders. * * * We do know, 

 however, that ammonia salta, in general, are much more irritating and stimulating 

 in their action than the corresponding soda salts, or even than the potash salts. 

 For instance, Still6 and Maisch, speaking of ammonium bromide, state that it has a 



'Report of the Dairy Commissioner of New Jersey, 1888, p. 70. 2 Op. cit., p. 77. 



