§ 109. 3IAEL. 207 



latter, and, while the effervescence continues, let the flask 

 lie in an inclined position, with the end of the test-tube 

 partly stopping up its mouth. When the effervescence 

 lias ceased, blow air into the flask to remove the carbonic 

 acid gas, weigh the whole again, and count the loss as car- 

 bonic acid ; the results wall be within 0.25° 1^ of the truth. 



c. Lime and Magnesia. — ^Digest 2-3 grms. of the well- 

 pulverized marl with dilute hydrochloric acid, and exam- 

 ine the solution more particularly for lime and magnesia, 

 as under b, Scheme lY., § 94. 



d. Phosphoric acid and the alkalies. — Allow 300 c.c. 

 of concentrated hydrochloric acid (Sp. Gr. = 1.15) to act 

 on 100 grms. of the well-pulverized marl in a large flask, 

 for 48 hours at common temperatures, with frequent agi- 

 tation; (or, take 120 grms. of soil and 360 c.c. of acid, if 

 ferric oxide and alumina are to be determined, and esti- 

 mate them in M, of the solution obtained ; this determina- 

 tion, however, will not generally be important). 



Decant the solution from the insoluble residue, dilute 

 with some water, filter, put the insoluble residue on the 

 filter, and wash it first with cold and afterwards with hot 

 water. Evaporate the filtrate to dryness, and remove 

 silicic acid. 



Examine the filtrate from the silica as under h, in 

 Scheme I, § 94. 



If the marl contains a large proportion of clay, some 

 of the phosphoric acid may remain undissolved by the 

 cold concentrated acid. In this case, boil about 50 grms. 

 of the marl one hour with 150 c.c. of concentrated acid, 

 filter, eliminate silica, and proceed to determine phosj^horic 

 acid, as above. 



The insoluble residue, left after treatment of the marl 

 with cold acid, and composed of clay and sand, is dried 

 in the air-bath, left for a considerable time exposed to the 

 air, and weighed. If from this we subtract the weight 

 of coarse sand, obtained by the rough method in the 



