1835.] Purification of Pyroligneovs Acid^ 6fc. 136 



as possible of the oil mixed with tar which swims on the 

 surface is removed mechanically. This is best effected after 

 the acid has been allowed to settle for a day. A cask or 

 large tub, with a double perforated inlaid bottom (einlege- 

 boden) like the alkaline cask of the soap-boiler, is then 

 taken and filled to the height of an inch with straw. This 

 is overlaid by a piece of coarse sack-cloth, which is cut out 

 in the form of the bottom. Over this is placed six inches 

 of moist wood sawings, which are pressed down and smoothed 

 by means of a wooden club. To prevent the sawings from 

 being displaced and from swimming about, they are covered 

 to the depth of four inches with a layer of gravel. A certain 

 quantity of impure acid is then poured into this filtering 

 apparatus which is termed a filtering cask, and passing- 

 through is discharged by a cock at the bottom. The tar is 

 taken up by the gravel. Should the quantity of this be so 

 great as to obstruct the passage of the acid, the sand may 

 be stirred up from above with a ladle. 



The acid which has thus been filtered is now placed in a 

 large cast-iron vessel, which is filled up to within ten inches 

 of the brim. Heat is then applied, and during its action, 

 lime-water previously passed through a hair seive, to free it 

 from foreign particles, is added until the acid is neutralized. 

 The approach to this point may be detected without the 

 use of litmus paper, merely from the colour of the liquid, 

 which becomes darker and passes from a blackish brown into 

 a deep brownish red colour. A great excess of lime is then 

 added. To a bucket of neutralized acid, containing 120 

 pounds (115lbs. troy) half a pound, (-479 lbs. troy) is added 

 in excess. This super-saturation is absolutely necessary ; be- 

 cause the lime combines with a great quantity of empyreu- 

 matic rosin and oily matter in the acid, forming insoluble 

 compounds which are separated first. The next object is 

 to bring the solution to the boiling point, and to avoid its 

 boiling over. When a scum appears, the boiling should be 

 carried on gently, and the matter swimming on the surface 

 removed, as completely as possible, by means of a ladle during 

 the whole process of boiling. The solution in this manner 

 being reduced to one half, is then cooled and clarified in 

 the cask and allowed to stand for 30 or 48 hours. In this 

 state it is called the solution of the first boiling. 



