402 AGRICULTURAL ANALYSIS 



bon dioxid is then passed through the apparatus until a moder- 

 ate stream of bubbles rise in the mercury trough. The stop- 

 cock is left in this position, and the admission of gas is con- 

 trolled by the pinch-cock. The bath of calcium chlorid is so 

 adjusted as to cause the bulb retort to be almost entirely sub- 

 merged, and the temperature of the bath is kept at 130 to 140. 

 Small quantities of water are next admitted into the bulb and 

 expelled as steam in the current of carbon dioxid, the supply 

 of this gas being shut off before the evaporation is entirely com- 

 pleted, so as to leave as little carbon dioxid as possible in the 

 apparatus. Previous to very delicate experiments it is advisable 

 to introduce through the funnel-tube a small quantity of potas- 

 sium nitrate, ferrous chlorid, and hydrochloric acid, rinsing the 

 tube with the latter reagent. .Any trace of oxygen remaining 

 in the apparatus is then consumed by the nitric oxid formed ; 

 and after boiling to dryness and driving out the nitric acid with 

 carbon dioxid, the apparatus is in a perfect condition for a quan- 

 titative experiment. 



351. Preparation of the Materials to be Analyzed. According 

 to Warington, soil extracts may be used without other preparation 

 than concentration. 



Vegetable juices which coagulate when heated require to be 

 boiled and filtered or else evaporated to a thin sirup, treated, 

 with alcohol, and filtered. A clear solution being thus obtained, 

 it is concentrated over a water bath to a minimum volume in a 

 beaker of small size. As soon as cool, it is mixed with one 

 cubic centimeter of a cold saturated solution of ferrous chlorid 

 and one cubic centimeter of hydrochloric acid, both reagents 

 having been boiled and cooled immediately before use. 



In mixing with the reagents, care must be taken that bubbles 

 of air are not entangled, which is apt to occur with viscid extracts. 



The quantity of ferrous chlorid mentioned is amply sufficient 

 for most extracts, but it is well to use two cubic centimeters 

 in the first experiment, the presence of a considerable excess of 

 ferrous chlorid in the retort being thus insured. With bulky 

 vegetable extracts more ferrous chlorid should be employed. 

 To the sirup from 20 grams of mangel-wurzel sap, five cubic 



