PEARL. ESSENCE 27 



compounds are now separated with great efficiency from low-grade 

 ores by flotation with oil. Examples of this selective wetting are 

 given by Bancroft (1921), who discusses this subject briefly as a 

 case of selective adsorption of liquids by solids, citing several refer- 

 ences. For example, kerosene will displace water in contact with 

 metals, but water will displace kerosene from quartz. Alcohol will 

 displace oil in contact with metal, but linseed oil will displace water 

 from white lead. Chloroform appears to be more efficient than 

 ether in removing the crystals, but it and carbon tetrachloride are 

 of such high density that the later separation of the crystals by 

 sedimentation or centrifugation is slow. 



Before this flotation will take place, however, all the protein must 

 be removed from the surfaces of the crystals. This is accomplished 

 as follows : The crude sediment from the scales is washed thoroughly 

 with several changes of fresh water, which is decanted off after sedi- 

 mentation each time, to remove blood and other matter. To each 

 liter of the concentrated sediment from the scale washings are added 

 30 cubic centimeters glacial acetic acid, or its equivalent, and 2.5 

 grams scale pepsin dissolved in water. The mixture is allowed to 

 stand 48 hours or more at room temperature, or a shorter time at a 

 higher temperature, not exceeding 40° C. When digestion is com- 

 plete, ether is added to the digestion mixture, emulsified, and then 

 caused to separate by a gentle rocking of the container. The crystals 

 of guanin pass into the ether, leaving foreign matter in the water. 

 As much water as possible is removed by sedimentation in the dif- 

 ferent steps of the process. The ether causes all the undissolved 

 organic matter to rise in the water and collect immediately under the 

 ether layer. The water should be drawn off from below — in fact, 

 it is important to eliminate as much water as possible at every stage. 



The ether also dissolves a considerable quantity of fat from the 

 mass. The supernatant ether layer, containing the guanin particles, 

 is decanted. It is allowed to stand until the particles settle out, is 

 poured off and replaced by fresh, fat-free, anhydrous ether, again 

 allowed to settle, and the process repeated until the particles are free 

 from fat and water. They are now bright and lustrous and may be 

 transferred by sedimentation or centrifugation to acetone or amyl 

 acetate, concentrated, and the celluloid added to produce the viscous 

 paste. Some dark-colored matter usually floats out with the guanin 

 particles. If suspended in acetone, after complete dehydration with 

 ether, the essence may be freed of this impurity by a brief sedimen- 

 tation in which the dark substance settles first. The supernatant 

 acetone, containing the clean particles, is decanted off. The ether 

 is recovered as far as possible. Used ether, containing water and 

 fat, is dehydrated by lumps of calcium carbide, on which the ether 

 stands for a few hours and from which it is distilled. The thick 

 residue remaining from the crude essence liquor also contains a con- 

 siderable quantity of emulsified or dissolved ether which should be 

 distflled and dehydrated.' 



The particles of guanin should be neither too coarse nor too fine. 

 The finest particles, consisting of minute fragments of crystals, appear, 

 when separated from the coarser ones, chalky white or yellowish, 

 while the larger crystals, such as those from the larger species of fish, 



' For data on the solubility of water in ether, and ether in water, see A, B. Hill, Journaf of the American 

 Chemical Society, Vol. XLV (1923), pp. 1143-1155. 



