PEARL ESSENCE 23 



they adsorb coloring matters from solution and thus become ''off 

 color." Suspended in ether or amyl acetate, they adsorb and hold 

 tenaciously to any fat present. When cleaned, they can be sus- 

 pended in any liquid that does not dissolve them. 



The guanin particles, when reduced to a dry powder, have a smooth, 

 soapy feel between the fingers, like that of talcum powder. When 

 properly cleaned they are presumably colorless and transparent, 

 but when viewed by transmitted light under the microscope they are 

 often slightly yellowish, probably because of adsorbed impurities on 

 their surfaces. A drop of the essence dried on white paper is usually 

 brownish yellow. 



To understand the beautiful and peculiar appearance of pearl 

 essence, one may visualize very thin light blades, floating in a liquid 

 and turning over and over like narrow strips of paper falling in air, 

 their narrow surfaces throwing brilliant flashes of light in all directions 

 and at the same time giving a play of colors like soap bubbles do. 

 The light coming from them is doubly refracted and polarized, and 

 when it passes successively through two of these crystals it is broken 

 into colors by a twisting of the polarized beam. 



The crystals show their maximum luster when they are oriented 

 parallel to each other. This condition occurs when a current is set 

 up in the liquid containing them. If the crystal happens to lie 

 across stream, the velocity gradient perpendicular to the direction of 

 flow causes one end of it to move faster than the other, and it is there- 

 fore steered around until its long axis is parallel to the direction of 

 flow of the stream or current. This accounts for the whirly effect in 

 the liquid essence. It also indicates that in applying lacquers the 

 maximum effect will be obtained if the fluid is caused to flow. In 

 imitation pearl, advantage is taken of these facts to brush patterns of 

 various kinds into the film. If the coat is applied as a uniform film 

 with the crystals pointing promiscuously in all directions, the effect 

 will be a metallic or dull pearly luster. 



OPTICAL AND CRYSTALLOGRAPHIC PROPERTIES 



The writer submitted specimens of pearl essence from shad, sus- 

 pended in ether and monobromonaphthalene, to Dr. H. E. Merwin 

 of the Geophysical Laboratory of the Carnegie Institution of Wash- 

 ington, to whom he is indebted for the following statement of the 

 optical and crystallographic properties of the guanin crystals: 



The materials most studied had been kept in turpentine and in methylene 

 iodide, but they were found to be closely similar to or identical with samples 

 from ether or bromonaphthalene. 



The preparations consisted of minute crystalline blades of rather uniform size 

 of about 0.10 by 0.02 by 0.001 miUimeters. Flatwise the blades showed no 

 appreciable double refraction, but when tilted the double refraction was evi- 

 dently strong; and on blades tilted sufficiently to show definite extinction, angles 

 of extinction were as high as 30°, but practically parallel extinction was observed 

 also. Edgewise, extinction was parallel. Thus the possibility that two sub- 

 stances were present — one orthorhombic and the other monoclinic — had to be 

 considered. No definite interference figures were observable. The indices of 

 refraction were observed by the immersion method, but the blades were so thin 

 that the index ^, which had to be measured on flatwise blades, could be found 

 only roughly; 7, vibrating lengthwise, was so high that no known immersion 

 liquid could be found which may not have slightly attacked the crystal;^ a, 



5 The liquid finally used was arsenic sulphide, dissolved by heating in methylene iodide. In a liquid of 

 the same index, containing methylene iodide, sulphur, and the iodides of tin, arsenic, and antimony, 

 the crystals appeared decidedly higher. 



