PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 759 



are witlidrawn from there by a pump, which also forces them into 

 the juice. In some places pretty pure carbonic acid is produced 

 by the decomposition of chalk in retorts, by means of superheated 

 steam. This process furnishes the same true pure lime for defeca- 

 tion. The heat must never go above red heat; to prevent the pro- 

 duction of h^'drogen. The retorts used are generally seven feet 

 long and fifteen inches in diameter. Sometimes, though seldom, 

 the carbonic acid of distilleries is collected and employed for the 

 removal of the lime. Also chimney gases are sometimes washed 

 with alkalies and used, but always darken the color of the juice 

 and prevent crystallization to a hard grain. These products of 

 combustion contain 10 to 18 per ct. of pure carbonic gas. For its ap- 

 plication the gas is forced into a closed box or pan filled with juice. 

 At one end this box increased to double its heighth, to give the 

 juice rising space, a sieve being interposed between the pan and 

 this addition, from which a tube carries off the waste gases. On 

 admission of the gas the liquor froths a great deal, and has to be 

 kept down by fat, butter or stirring ; it rises into the additional 

 space. With the decomposition of the saccharate of lime the liquor 

 becomes less coherent, sticky, and ceases to froth. By this sign 

 the conclusion of the process is judged, as also by the quick pre- 

 cipitation of large flakes in a small sample, taken from the pan. 

 As soon as this takes place the access of the gas is shut off, and the 

 liquor heated to the boiling point by means of a steamcoil or jacket. 

 This causes the sediment of carbonate of lime to be less voluminous. 

 It is then withdrawn to settling vats or settled in the saturators, 

 after which the clear liquor is withdrawn into the forefilters, to be 

 entirely purified before admission upon the last filters. This fore- 

 filter consists of a small iron tank, in which a sieve is suspended, 

 filled with boneblack, in the midst of which is a narrow vessel for 

 the collection of the juice, which thence is withdrawn by a syphon. 

 The carbonate of lime, remaining at the bottom of the forefilter, 

 is placed into bags, the juice expressed, and the lime used as 

 manure. The liquor is then lifted into the filter reservoirs by 

 means of montejus, in which every atmosphere of steam pressure 

 lifts the juice twenty-nine feet, consequent upon the density of the 

 juice. In these filter reservoirs the liquor is heated, as in this 

 state it keeps better and permeates the boneblack better, which 

 is used as a filtering medium by being thinner. Boneblack was 

 first suggested for that purpose by Figeur, in 1811, and tried first 

 by Charles Derosne, in 1812. At first only its decolorizing power 



