588 APPENDIX 



with the periostracum or " epidermis " of the shell ; enclosing this is a layer show ing a 

 columnar or fibrous structure, Avhich agrees in every respect with the prismatic layer of the 

 shell, while the external layer only consists of finely laminated mother-of-pearl or nacre. 

 Such a pearl, therefore, has the structure of a molluscan shell, but the layers are arranged 

 in the reverse order, periostracum inside and the nacreous layer outside. In some cases the 

 nacreous layer is entirely absent, and the outermost layer is constituted by the prismatic 

 layer or by the periostracum, the pearl then being of a dark brown or black colour, without 

 lustre and, therefore, without value. Not infrequently also there are several nacreous 

 layers, separated from each other by prismatic or conchiolin layers. 



One can easily see the connection between these differences in structure and the 

 processes by which the pearls were formed. If a pearl arises wholly in that region of the 

 mantle which secretes mother-of-pearl, as is generally the case, it will consist wholly of 

 mother-of-pearl substance, the nature of which has been already described. A pearl formed 

 in this way does not, howevei", remain indefinitely in the same position. From one cause or 

 another it may be brought into contact with that part of the mantle which secretes the 

 prismatic layer, when it will become invested with a layer of this description, or with the 

 extreme edge of the mantle, when a layer corresponding to the periostracum of the shell 

 \vill be deposited upon it. If the object, which eventually becomes the nucleus of the 

 pearl, lie first in contact with the edge of the mantle, a layer of material corresponding to 

 the periostracum will first envelop it. If the pearl during its growth moves slowly 

 inwards it becomes coated successively with a prismatic and a nacreous layer, A\'hile if it 

 move back again these same layers will be again laid down, but in the reverse order. In 

 this way is attained great variety in the arrangement of the concentric coats of pearls, this 

 variety depending solely upon the way in which the pearl moved about '\\ithin the shell of 

 the mollusc. 



The pearl is not only identical with mother-of-pearl in structure, but also in chemical 

 composition, hardness, and specific gravity. Like that substance it consists of 

 that modification of calcium carbonate which is known as aragonite. Besides calcium 

 carbonate, there are always present small amounts of other inorganic substances, and, in 

 addition, up to 12 per cent, of conchiolin, the organic material of which the periostracum 

 of the molluscan shell is formed. Layers of conchiolin alternate with layers of calcium 

 carbonate, binding the whole together. The specific gravity of fresh, white, brilliant 

 sea-pearls varies between 2'650 and S'BSG. The hardness is nearly 4 ; it varies somewhat 

 in different pearls, but is always rather less than that of mother-of-pearl. Owing to the 

 admixture of conchiolin, both the hardness and specific gravity of pearls are rather less 

 than those of aragonite. 



As a I'esult of their composition pearls dissolve readily in acid, even in acetic acid, with 

 the evolution of carbon dioxide, which causes a brisk effervescence. Upon this is founded 

 the story that Cleopatra, the Egyptian queen, at a banquet dissolved a priceless pearl in 

 vinegar and drank the solution. In ordinary table-vinegar, however, the acetic acid is too 

 weak to dissolve completely a pearl even of small size in the time that a banquet might be 

 supposed to last, so that the story is probably apocryphal. Moreover, pearls do not dissolve 

 completely in acid, the calcium carbonate only is extracted and the conchiolin left behind 

 as a scaly, soft, somewhat swollen, but still shining mother-of-pearl-like mass, which has 

 the form, size, and colour of the original pearl, but upon which the acid has no further 

 effect. 



Pearls are also affected by perspiration from the skin, and if they are much handled or 

 worn for long in contact with the skin they gradually lose their lustre and much of their 



