226 ANIMAL PROTEINS 



other the removal of excess acid. The former is the easier, 

 and is almost necessarily brought about by the latter. Hence 

 in some factories the neutralization is brought about, there- 

 fore, by the addition of a certain quantity of soda, or more 

 usually by lime, and the material is sometimes submitted 

 to a veritable liming by which it remains in milk of lime for 

 about three weeks, the lime liquor being renewed several 

 times. The product is finally washed again to remove excess 

 lime. This is carried out in a rotating vessel through which 

 passes a continuous stream of water. If a slightly acid 

 gelatine is required, however, the lime and liming are both 

 superfluous, and the procedure is simply to wash as 

 thoroughly as possible and then to immerse the material in a 

 i per cent, sulphurous acid solution for 3 hours to bleach, 

 and then to proceed with the water extraction or solution of 

 the gelatine. The hydrochloric acid used for these processes 

 should be as pure as possible, and the degreasing as thorough 

 as possible, for, if not, a gelatine with a bad odour is liable to 

 be obtained. 



Instead of using hydrochloric acid for the solution of 

 mineral matter, sulphurous acid is sometimes employed, and 

 has the advantages that its bleaching effect is thereby 

 obtained throughout the process, and that it is recoverable 

 for subsequent use. The Bergmann process, most generally 

 favoured, is described very concisely by Rideal thus : "A 

 sulphurous acid solution is made to circulate over the bones 

 in a series of closed tanks, the solution being continually 

 enriched with sulphurous acid from a cylinder of the liquefied 

 gas. The resulting liquor, containing an acid calcium phos- 

 phate and calcium bisulphite, is heated by steam in a leaden 

 digestor, when the excess of sulphurous acid is liberated and 

 passes back to the tanks, while neutral calcium phosphate 

 and sulphite are precipitated. The latter is decomposed 

 by an equivalent of hydrochloric acid, setting free the 

 remaining sulphurous acid, which is returned to the tanks, 

 leaving calcium chloride in solution, and neutral calcium 

 phosphate in suspension." Not more than 5 per cent, of 

 sulphurous acid is said to be lost in this process, and the 



