36 Estimations occurring in Agricultural Analysis [55 



Tube e. This tube must have a plug of cotton wool driven 

 down until it divides the tube into equal portions. One limb 

 is filled with granulated CaCl 2 , and the other with small 

 pieces of pumice which have been soaked in a saturated solu- 

 tion of CuS0 4 , then dried at 200 C. until they have become 

 colourless. 



Tube g is filled with CaCl 2 . The remaining tube (c) is to 

 purify the air which is passed through at the end of the opera- 

 tion. It is filled with soda lime. When all the different pieces 

 of apparatus shown in the figure have been got together, the 

 Liebig's bulbs (d) must be filled about half full of strong sul- 

 phuric acid, and the absorption tubes joined together with india- 

 rubber tubing. Especial care must be taken that the different 

 reagents with which the C0 2 comes in contact after leaving the 

 flask are arranged in the correct order, as it is very easy -to 

 get some of the tubes turned the wrong way round. The 

 correct order is as follows : 



Strong sulphuric acid to dry the gas, and contained in the 

 bulbs d. 



Calcium chloride to complete this drying, and contained in 

 the limb of e next the bulbs. 



Anhydrous copper sulphate and pumice, to absorb any HC1 

 which may come over, and contained in the limb of e next to/ 



Soda lime to absorb the C0 2 , contained in the limb of / 

 next to e. 



Calcium chloride, contained in the limb of / next to g. 

 When the soda lime begins to absorb C0 2 it gets warm and 

 loses a little moisture. This CaCl 2 is to absorb any such 

 moisture. 



Calcium chloride, contained in g, to prevent any moisture 

 from diffusing back into/ 



Having made the absorption apparatus and got it correctly 

 in position, the pipette, b, is fitted into the stopper so that its 



