568 FOODS AND FOOD ADULTEKANTS. 



tive nutrition, and especially in scrofula, rickets, phthisis, etc. On ac- 

 count of their being an essential constituent of animal tissues there 

 would seem to be some ground for a preference over other forms of 

 powders. The makers of phosphate powdersclaim that the use of such 

 powders restores the phosphoric acid present in the whole grain of 

 wheat, which is largely removed in the bran by milling processes. This 

 claim would have more weight if there were not ample sources of phos- 

 phoric acid in other forms of food, and if the quantity introduced by a 

 baking-powder were not much greater than is required to make up the 

 loss in the bran, and greater than is required by the system, unless in 

 those cases where its therapeutic use is indicated, as in some of the con- 

 ditions of malnutrition given above. 



Acid phosphate of soda is said to have been used in former years as 

 a constituent of baking-powders, but appears to have been entirely 

 superseded by the lime salt. 



ALUM POWDEES. 



In this class the carbonic acid is set free from the bicarbonate by the 

 substitution of sulphuric acid, which combines with the sodium. The 

 sulphuric acid is furnished by some one of the general class of salts 

 known as alums, which are composed of a double sulphate of alumin- 

 ium and an alkali metal. The alum is precipitated as hydrate, while 

 that portion of the sulphuric acid which was combined with it goes to 

 displace the carbonic acid in the bicarbonate. The alkali sulphate of 

 the double salt remains unchanged. 



The alum of commerce is either potash alum, K 2 A1 2 (SO 4 ) 4 . 24H 2 O, or 

 ammonia alum, (NH 4 ) 2 A1 2 (SO 4 ) 4 . 24H 2 O, the one or the other predomi- 

 nating according to the relative cheapness of the alkali salt it con- 

 tains. At the present time nothing but ammonia alum is met with, but 

 at previous periods potash alum was the salt sold exclusively as "alum." 

 Both salts are alike in general appearance and can not be distinguished 

 apart by cursory examination. 



Potash alum may be made directly from some minerals, such as the 

 " alum stone" mined in Italy, which contain all the constituents com- 

 bined. Ammonia alum, however, as well as most potash alum, is made 

 by the combination of the constituents obtained from diiferent sources. 

 The sulphate of alumina is obtained by the action of sulphuric acid 

 upon pure clays, and the sulphate of ammonia from the residue of gas- 

 works. Solutions of the two salts in proper proportions are mixed 

 and the double salt obtained by evaporation and crystallization. 



Crystallized potash or ammonia alum contains twenty-four molecules 

 of water, nearly one-half of its weight. Part of this water is lost at as 

 low a heat as 00 0., and it is driven oif entirely, though slowly, at 100 

 C. " Burnt alum n is simply alum deprived of its water of cryst:illi/.;i- 

 tion, which is generally driven off at about 200 0. Ammonia alum 

 decomposes at 205 C. ; potash alum at a somewhat higher temperature. 



