8 Organic Acids in the Examination of Minerals. 



It is evident that a solution of citric acid effects the decora- 

 position of mineral carbonates with sufficient ease to render it 

 extremely useful where hydrochloric acid can not be conveni- 

 ently employed. The organic acid acts, however, somewhat 

 more slowly ; in fact, time appears to be an important factor iu 

 studying these reactions. On some minerals the organic acid 

 has at first no effect, and an appreciable time elapses before 

 bubbles of carbonic anhydride appear, indicating the decom- 

 position of the mineral ; this is especially noticeable with 

 dolomite and azurite. The time of incipient action varies also 

 with the acid employed.* 



Owing to the viscosity of a concentrated solution of citric 

 acid, the liberated gas has difficulty in escaping, and often 

 remains attached in bubbles to the powder until sufficiently 

 large bubbles have formed to ascend through the syrupy liquid .; 

 this, of course, has reference to examinations made in the 

 cold. Moreover, the precaution was observed of distinguishing 

 between the escape of bubbles of air entangled by the pow- 

 dered mineral and a true liberation of carbonic anhydride. 

 These minute details are mentioned, because observation of 

 them is necessary to obtain the same results as those recorded 

 in the first part of this section. Magnesite and siderite were 

 repeatedly tested, and always refused to effervesce with a cold 

 solution of citric acid, a reaction which distinguishes them from 

 other carbonates. 



5. In making this investigation we have constantly borne 

 in mind the possible employment of the methods in the field, 

 and have conducted the tests in the simplest manner with that 

 end in view. Since it is seldom convenient in field work to 

 obtain the minerals in fine powder, an examination was made 

 of the action of citric acid on the massive carbonates ; and by 



* The interesting subject of the Velocity of Chemical Reactions has recently >>een 

 investigated in certain cases by Boguski and Kajander (Beriehte d. dentschen Chem. Ges. 

 Berlin, ix, lb'46, and x, 34). They find that when nitric, hydrochloric, and hydrobromic 

 acids, of the same concentration, act upon marble, the Telocity of evolution of carbonic 

 anhydride is inversely proportional to their molecular weights. Cf. American Journal 

 of Science, [3] xii. p. 299. It will be interesting to ascertain whether this law holds good 

 with organic acids. 



