16 Organic Acids in the Examination of Minerals. 



could not be determined. The crystals are generally com- 

 paratively short and stout, being quite different from the 

 hydro-barium tartrate, which is almost capillary in character. 

 Some of the larger crystals of formate of lead are thick and 

 cloudy, apparently from the presence of microlites and fluid 

 cavities. 



Calcium citrate, from citric acid and calcite, had no crystal- 

 line appearance, being in the form of very minute masses, 

 and is probably amorphous as thus obtained." 



METALS AND ORGANIC ACIDS. 



11. That citric and tartaric acids dissolve iron and zinc, 

 with evolution of hydrogen gas, is a well-known fact, stated 

 in many handbooks of chemistry.* 



Crommydis has recently taken advantage of the solubility 

 of zinc in oxalic acid to prepare glycollic acid (HoC 2 H,0 3 ), 

 a reaction in which nascent hydrogen plays an important 

 part.t 



In repeating and extending these experiments we have 

 made the following observations : Iron, zinc, and magnesium 

 dissolve readily in cold saturated solutions of citric, tartaric, 

 oxalic, and malic acids, as well as with formic (Sp. Gr. = 1.060) 

 and acetic acids (Sp. Gr. = 1.037), evolving hydrogen more or 

 less freely ; on heating, the action becomes violent. Magne- 

 sium is attacked by citric and other acids violently, the 

 liquid becoming much heated. 



A cold-saturated solution of citric acid, diluted with half 

 its volume of water, attacks zinc slowly in the cold ; on boil- 

 ing, hydrogen comes off freely and continues to do so after 

 cooling for a long time ; the disengagement of gas being slow 

 but steady, and under favorable conditions lasting for twenty- 

 four hours. If the solution becomes very concentrated, an 

 insoluble citrate of zinc precipitates, soluble, however, in 

 water. 



Tartaric acid acts on zinc feebly in the cold ; on boiling, 

 solution ensues, and at the same time the hot liquid becomes 



* Cf. HandwOrterbuch der Chemie, article " Citronsaure." 

 t Bull. soc. chim., xxvii, p. 3, 1877. 



