GELATINE AND GLYCERINE 225 



battery of six vats in such a manner that the liquor richest 

 in lime salts comes into contact with the bones most recently 

 charged ; the fresh acid thus acts upon the nearly extracted rp 

 bones. The hydrochloric acid used is of 8 to 10 per cent, 

 strength (5 to 7 Be.). Stronger acid is apt to hydrolyze 

 (" rot ") the gelatine, whilst weaker acid takes longer time. 

 The process takes 8 to 10 days, though up to 14 days is 

 sometimes given, and, on the other hand, the process has 

 been occasionally reduced to 4 days. The gelatinous matter 

 undissolved has the shape of the original bone, but is much 

 swollen. When the acid liquor is saturated with lime salt, 

 the liquor is drawn off from below the vats sent to the 

 phosphate precipitation tanks. The phosphate is usually pre- 

 cipitated by adding just sufficient milk of lime to neutralize 

 the hydrochloric acid. The precipitated phosphate is then . /c^ 

 well washed by decantation to remove calcium chloride. It 

 is then drained, and dried at a low temperature. As a large 

 bulk of phosphate is obtained it is often filter-pressed and 

 dried quickly in long revolving chambers through which a 

 current of air is passed. The phosphate is sometimes also 

 precipitated by ammonia. It is then more easily washed and 

 dried, and the ammonium chloride is recovered and may 

 be used to regenerate ammonia, or be sold as a valuable 

 bye-product. Sometimes the acid liquor is not used for 

 making precipitated phosphate, but is evaporated with 

 animal charcoal and silica and then distilled to make phos- 

 phorus, ip ^j 



The next stage is the purification by washing of the 

 gelatinous matter which remains. The vat is filled up with 

 pure cold water and the material allowed to, steep for six or 

 seven hours. The acid and salts remaining diffuse outwards 

 into the water. This is drained off and replaced by fresh 

 water, and the procedure repeated half a dozen times or 

 as often as necessary. The end is said to be determined by 

 the absence of a precipitate on adding silver nitrate to the 

 wash water, or by the absence of any action on blue litmus 

 paper. It will be seen, however, that there are two actions 

 involved, one being the removal of calcium chloride|and the 

 B, 15 



