PEARL ESSENCE 19 



bility in mineral acids, and its characteristic crystallization. These 

 and its other properties led him to refer it to the guanin of Unger. 



Voit (1863 and 1865), at the instance of Professor Siebold, investi- 

 gated this substance and agreed with Barreswil as to its identity, 

 though his conclusion was reached independently, and he gives 

 numerous other properties and reactions to support that conclusion. 

 Referring also to the production of a compound of barium and guanin 

 by Strecker (Annalen der Chem.ie und Pharmacie, Band CVIII, 

 1861. p. 154), Voit cites properties that led him to believe the lus- 

 trous particles are a compound of guanin and lime. This idea 

 persists in the reference books to-day in spite of Bethe's (1895) 

 definite finding that it is pure guanin. Voit's experience with the 

 synthetic production of "guanin-hme" led him to speculate that 

 some day the artificial preparation of pearl essence might be realized. 



It will be noticed that up to this time the analytical procedure 

 consisted of trying various reactions for guanin and in making various 

 qualitative tests for calcium and other mineral substances. Bethe 

 (1895) was first to undertake the solution of the problem quanti- 

 tatively. He carefully prepared and purified the crystals, dissolved 

 them in hydrochloric acid, filtered the solution, and precipitated a 

 highly purified guanin with ammonia that had a percentage com- 

 position agreeing very closely with the theoretical. The amount 

 of calcium and other mineral impurities was found to vary with the 

 method of preparation, a fact that led him to conclude that they were 

 not an essential part of the compound. His work bears all the marks 

 of accuracy and finality, and the lustrous particles must now be 

 regarded as pure crystalline guanin. 



RECENT HISTORY 



With the outbreak of the World War in 1914, the European supply 

 of pearl essence was cut off, and attention was directed from all 

 sides to the possibility of obtaining a supply of it in the United States. 

 While the ablette or bleak does not occur in this country, other fishes 

 were found that yielded a satisfactory product, and an industry grew 

 rapidly and has now reached large proportions. Improved methods, 

 aided by machinery, are followed, and the recent application of pearl 

 essence in the manufacture of imitation mother-of-pearl celluloid 

 products has greatly increased the demand and consequent supply. 



SOURCES AND DISTRIBUTION OF GUANIN AMONG ANIMALS 



PRODUCTION OF GUANIN Bit ANIMALS 



Mention has already been made of the work of Bethe in demon- 

 strating that pearl essence is pure cr3^stalline guanin. Guanin is one 

 of the end products of protein metabolism in the animal body. It is 

 a derivative of nucleic acid and occurs in all nucleated cell tissues. 

 The biological significance of its occurrence in the form of deposits 

 in animals is interesting and important. Occurrences in the higher 

 animals of more than very small amounts of such end products of 

 metabolism as urea, uric acid, creatin, xanthin, taurin, and guanin 

 are unusual and these substances are indicative of pathological con- 

 ditions when present in any considerable quantity; but, perhaps be- 

 cause of less efficient organs of elunination, these and similar chemical 



