§ 104. MISCELLANEOUS ESTIMATIONS. 183 



the amount of pliosplioric acid required depends upon 

 the amount of silicates present, and at least 15-20 grms. 

 should be taken for 0.5-1.0 grm. of the substance. The 

 mixture is heated in a platinum dish in an air-bath to 

 190-200° C, and digested five or six hours at this temper- 

 ature, while constantly stirred with a platinum spatula. 

 The resulting mass is boiled several times with water 

 containing 1"| ^ of sodic hydrate, the clear liquid decanted 

 off each time, and the sandy residue itself is finally 

 brought on the filter and washed carefully with acid, alkali, 

 acid again, and finally with water, ignited and weighed. 



MISCELLANEOUS ESTIMATIONS. 



104 • a. Humus. — Weigh out 5-10 grms: of the air- 

 dried soil, pour over it 200 c.c. of water in the flask of 

 the apparatus for determining carbonic acid (§ 60, ^), and 

 30 c.c. of concentrated sulphuric acid ; shake the mixture 

 gently and let it stand some time until it has become 

 quite cold, meanwhile changing the air in the flask sev- 

 eral times by blowing into it, so as to remove all the car- 

 bonic acid expelled from carbonates in the soil by the 

 stronger acid. 



N"ow, put 7-8 grms. of coarsely pulverized potassic di- 

 chromate in the flask (or, better still, 5 grms. of pure 

 chromic acid), or such a quantity that there will be 17 

 parts of chromic acid for one of organic matter, as dc- 

 teraiined, approximately at least, in the beginning, by ig- 

 nition (§ 98, a) ; apply a gentle heat, and proceed to 

 collect the carbonic acid evolved as in § 60, ^, except that 

 a IT tube filled with iron wire should be interposed be- 

 tween the flask and the U tube ff^ to absorb chlorine, 

 and except, also, that no nitric acid need be added to 

 the substance. Towards the close of the operation, boil 

 the contents of the flask five minutes, and finally draw 

 air throuojh in the usual manner. The carbonic acid is 



