336 CONSIDERATION OF CARbON COMPOUNDS. 



Tartar emetic is soluble in water, insoluble in alcohol ; it has a 

 sweet, afterward disagreeable metallic taste. 



Action of certain organic acids upon certain metallic oxides. 

 The solution of a ferric salt (or certain other metallic salts) is pre- 

 cipitated by alkali hydroxides, a salt of the alkali and ferric hydroxide 

 being formed. When a sufficient quantity of either tartaric, citric, 

 oxalic, or various other organic acids has been added previously to 

 the iron solution (or to certain other metallic solutions) no such pre- 

 cipitate is produced by the alkali hydroxides, because organic salts 

 or double salts are formed which are soluble, and from which the 

 metallic hydroxides are not precipitated by alkali hydroxides. Upon 

 evaporation no crystals (of the organic salt) form, and in order to 

 obtain the compounds in a dry state, the liquid, after being evaporated 

 to the consistence of a syrup, is spread on glass plates which are 

 exposed to a temperature not exceeding 60 C. (140 F.), when 

 brown, green, or yellowish-green, amorphous, shining, transparent 

 scales are formed, which are the scale compounds of the U. S. P. 



Instead of obtaining these compounds, as stated above, by adding 

 the organic acids (or their salts) to the inorganic salts, they are more 

 generally obtained by dissolving the freshly precipitated metallic 

 hydroxide in the organic acid. 



The true chemical constitution of many of these scale compounds 

 has as yet not been determined with certainty. 



Of official scale compounds containing tartaric acid may be men- 

 tioned the iron and ammonium tartraie, and the iron and potassium 

 tartrate. The first compound is obtained by dissolving freshly 

 precipitated ferric hydroxide in a solution of ammonium acid tartrate, 

 the second by dissolving ferric hydroxide in potassium acid tartrate. 

 The clear solutions, after having been sufficiently evaporated, are 

 dried, as mentioned above, on glass plates. 



Citric acid, Acidum citricum, H 3 C 6 H 5 O 7 .H 2 O = 210. Citric 

 acid is a tribasic acid containing three atoms of hydrogen replaceable 

 by metals ; its constitution may be expressed by the graphic formula : 



OH 



C 3 H 4 



^ 



X C0 2 H 



Citric acid is found in the juices of many fruits (strawberry, rasp- 

 berry, currant, cherry, etc.), and in other parts of plants. It is 



