94 THE ANALYSIS OF SOILS. V. 



re-distilled acid of commerce is usually pure enough), heated for 

 some time over the bare flame, care being taken that the soil is 

 completely wetted by the acid and that no dry places are left 

 in the lower part of the flask. When the frothing has ceased 

 (usually in about half an hour) 10 grammes of pure potassium 

 sulphate are added to the flask and about half a gramme to 

 a gramme of anhydrous copper sulphate (easily prepared by 

 heating powdered blue vitriol in a porcelain basin until it 

 becomes quite white) ; the heating is then continued in the 

 draught place, the flask being either placed in an inclined 

 position or a sm*all funnel or watch glass being placed on its 

 neck in order to prevent loss by spirting. The contents of the 

 flask should be kept in constant ebullition, due care being 

 taken to avoid frothing over, which may occur in the early 

 stages of the heating. The flask is best supported upon a 

 retort-stand ring, its neck passing- through a smaller ring, and 

 it is advisable to periodically rotate the flask so that no frag- 

 ments of soil may escape the action of the acid, or rather of 

 the acid potassium sulphate." When all black or brown 

 colouration disappears (generally in about 1^ to 2 hours from 

 the time of starting the heating) it is safe to assume that the 

 reaction is completed. The flask is then allowed to cool, 

 about 50 c.c. or more of distilled water added, and a cork 

 carrying a separating funnel, delivery tube, and inlet tube for 

 steam is inserted, the arrangement being shown in the dia- 

 gram. In the separating funnel is placed about 80 or 90 c.c. 

 of strong caustic potash solution made by dissolving "stick" 

 potash in its own weight of water and boiling the solution for 

 some time to expel any possible ammonia. This solution can 

 be prepared in considerable quantities and kept in a closely 

 corked bottle. 



When the apparatus is fitted up as described, a measured 

 quantity 25 c.c. of decinormal sulphuric acid is run from a 

 pipette into F through G, the cork being loosened to allow of 

 the escape of air. B should be disconnected from A at the 

 rubber joint R and the water in B heated to boiling. In 

 the meantime the 80 or 90 c.c. of potash solution in C are 



* The object of adding the potassium sulphate is to allow the temperature to be 

 raised to a higher point than is possible with sulphuric acid alone. Indeed, the liquid 

 in the flask at the end consists essentially of fused potassium hydrogen sulphate. 



