24 U. S. BUREAU OF FISHERIES 



vibrating normal to the blades, was probably accurately measured. These 

 measurements gave no evidence of the presence of more than one substance; 

 «=- 1.456, /3=1.8, 7=1.85. 



The relation of crystal shape to the principal indices of refraction accounts for 

 pearly luster in low refracting media in which the crystals may be suspended. 

 No pearly luster could be seen in a liquid of index 1.85. To give the maximum 

 effect, the blades would be nearly parallel to the surface illuminated. The light 

 entering the surface encounters blades, which, on account of decidedly higher 

 refraction and flatness, reflect more strongly in some directions than in others. 



CHEMICAL PROPERTIES 



Guanin, 2-amino, 6-oxypurin, has the composition C5H5N5O, or 



HN 



a derivative of purin, and belongs to the group known as the purin 

 bases. In the living animal it occurs combined with other groups 

 in nucleic acids, which in turn are combined in the complex nucleo- 

 proteins found in cell nuclei. Guanin is insoluble in water, alcohol, 

 ether, chloroform, ethyl or amyl acetate, acetaldehyde, or any 

 neutral solvent, acetic acid, dilute or glacial, acetic anhydride, 

 formic, lactic, or solutions of salicylic or citric acid. Ammonium 

 hydroxide solutions containing 1, 3, or 5 per cent NH3 dissolve 9, 15, 

 and 19 milligrams of guanin, respectively. Hot ammonia solutions 

 dissolved relatively more (Wuln, 1893). In supersaturated am- 

 monia, especially if hot, it is still more soluble, and from this solu- 

 tion, on spontaneous evaporation, crystallizes out in needles or 

 plates (Drechsel, 1881). It is soluble in dilute mineral acids and in 

 solutions of sodium or potassium hydroxide. On neutralization of 

 the acid solutions with ammonia, amorphous guanin is precipitated. 

 It is decomposed by strong oxidizing agents, potassium permanganate 

 or chlorate, or chlorine, and is converted into xanthine by nitrous 

 acid. It forms compounds with many acids, acid radicals, organic 

 groups, and inorganic salts (see Wulfi', 1893). When pure, guanin 

 or pearl essence should leave no residue if ignited on platinum foil. 

 It is not changed by heating in water to 250° C. 



Guanin was synthesized by Emil Fischer (1897) from trichloropurin. 

 The purin was prepared from ammonium urate. It has been sug- 

 gested that pearl essence may be made synthetically. Perhaps it 

 may be, but preparation of guanin of the delicate crystalline lorm 

 necessary for pearl essence appears to present such difficulties as to 

 indicate continued dependence on the natural supply from fish 

 scales, especially since there appears to be no real scarcity of the 

 latter, once the collection of it is organized. 



When guanin crystals are suspended in a nonaqueous liquid like 

 ether, that itself is not miscible in all proportions with water, the 

 liquid must be strictly anhydrous. If a trace of water is dissolved 

 in the ether, the crystals refuse to subside into a compact silvery 

 mass, but hold apart from each other in a feathery aggregation. If 

 more water is present they will flocculate in large masses which 

 become impossible again to break up. Perhaps under these condi- 

 tions they acquire an electrical charge, but this has not been investi- 



