22 U. S. BUREAU OF FISHERIES 



PROPERTIES OF PEARL ESSENCE 



Pearl essence is a suspension of the natural crystals of guanin in a 

 liquid, ammoniated water, amyl acetate, acetone, or other liquid. 

 One commercial form of it is a highly concentrated suspension of the 

 crystals or particles in a thick lacquer of celluloid in amyl acetate. 

 The properties given are chiefly those of the crystalline guanin. 



PHYSICAL PROPERTIES 



The natural crystals of guanin obtained from fish scales are usually 

 much broken up in the process of manufacture. While the size of 

 crystals varies somewhat on a single fish, it is in some measure pro- 

 portional to the size of the fish from which the crystals come, those 

 from the shad and whitefish, for example, being much larger than 

 those from the alewife or sardine herring. While the crystalline 

 form appears to be similar in different American species of fish, one 



Fig. 1.— Crystals and fragments of guanin from pearl 

 essence derived from shad. Magnification approximately 

 100 diameters 



Specimen of pearl essence of Japanese origin has come to the writer's 

 notice in which the crystals appear to be very thin needles rather 

 than blades. Those from the shad have dimensions of about 0.1 by 

 0.02 by 0.001 millimeters. Such a crystal would have a volume of 

 0.000000002 cubic centimeter, a surface of 0.0000424 square centi- 

 meter, and a weight of 0.000000032 gram. The specific gravity of 

 the crystals is somewhat in excess of 1.6, since they sink slowly in 

 carbon tetrachloride. One gram of the crystals has an aggregate sur- 

 face of about 12,575 square centimeters — somewhat more than a 

 square yard — and contains about 312,500,000 individual crystals. 

 Tlie large amount of surface accounts for some of the properties of 

 the essence, for the crystals adsorb substances from solution and 

 hold them on their surfaces. They appear to hold albumen, which 

 fact would account for the difficulty in transferring them from 

 water to nonaqueous liquids that precipitate albumen. Likewise, 



