QUANTITALIVE METHODS OF ANALYSIS 149 



8. Dissolve out the humus with 200 c.c. of 4 per cent, 

 ammonium hydroxid and filter through paper to separate 

 the flocculated clay. 



9. Evaporate 50 c.c. aliquots, dry at 100 C., and weigh. 



10. Ignite the residue and reweigh. 



11. Calculate the humus from the difference in weights 

 between the dried and ignited residues. 



12. Report as percentage of the dry soil. 

 (9) Carbon Dioxid : 



1. Connect a large Erlenmeyer suction flask of about 

 2-liter capacity with a long glass tube by means of a 5o-c.c. 

 pipet bent as shown in Fig. 10. 



2. Fill the glass cylinder about two-thirds full of glass 

 beads and 10 c.c. of normal potassium hydroxid. The 

 beads prevent the carbon dioxid from passing through the 

 alkaline solution too rapidly, and afford much greater 

 surface. The 5o-c.c. pipet prevents suction of the alkali 

 back into the flask. 



3. Place i kilogram of soil in the Erlenmeyer flask. 



4. Connect the 5o-c.c. pipet to the flask and close the 

 ends of the glass tubes by means of clamps. 



5. In order to conduct the carbon dioxid into the potas- 

 sium hydroxid solution draw a current of air slowly through 

 the apparatus for ten to twenty minutes each day. 



6. The current of air should be freed of carbon dioxid by 

 passing through strong alkali or a series of soda-lime tubes. 

 The amount of air may be determined by counting the 

 air bubbles. 



7. After the carbon dioxid has been collected in the 

 potassium hydroxid, disconnect the suction flask and 

 transfer the contents of the glass tower into a 5oo-c.c. 

 Erlenmeyer flask by successive washings with small portions 

 of carbon-dioxid-free water. This is easily accomplished 



