May 1, 1899.] 



KNOWLEDGE. 



97. 



)^ED MACAZINEl 



STERATWJ 



Founded by RICHARD A. PROCTOR. 



LONDON: MAY 1, 1899. 



CONTENTS. 



Mother-of-Pearl and its Sources. By K. Lydeekek. 

 (Ilhistrafed) 



Electricity as an Exact Science.— 111. Arbitrary Assump- 

 tions and Expressions. Scientific Speculation as 

 opposed to Hysteria. Bv Howaed B. Little 



The Acetylene Industry —II. By Gkoege T. Holloway. 



ASSOC.H.COLI,.SC., J.I.C. 



Clouds. By James QriCE. {Illustrated and Plate)... 



A New Star in Sagittarius. By Edwabd C. Pickeeixs ... 



An Anglo-Saxon 'Story of the Heavens." By E. 



Waltee Maundkh, r.R.A.S 



Letters: Howaed Little; David Flaseey; Chables A. 



■WlTCHELL 



Science Notes 



Notices of Books 



Books Received 



What is a Geological Catastrophe? By X. A. Geaydon 

 The Mycetozoa, and some Questions which they 



Suggest. — III. By the Right lion. Sir Edwabd Fet, 



p.c.L., LL.D., F.E.S., and AsNES Fey 

 Notes on Comets and Meteors. By W. F. Denning, 



P.B.A.S 



Microscopy. By John II. Cooke, f.i.s., f.o.s. 



The Face of the Sky for May. By A. Fowleb, f.b.a.s. 

 Chess Column. By C. D. Locock, b.a. 



97 



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102 

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 105 



106 



108 

 109 

 110 

 112 

 113 



lU 



117 

 118 

 119 

 119 



MOTHER-OF-PEARL AND ITS SOURCES. 



By E. Lydekker. 



OF the various descriptions of shells employed in 

 European manufactures and arts, by far the most 

 important position is held by those yielding the 

 beautiful substance known as mother-of-pearl, so 

 largely used for buttons, knife-handles, inlaying, 

 etc. Forming the inner or " nacreous " layer of the shells 

 of many kinds of molluscs, mother-of-pearl is characterized 

 by its more or less marked iridescent play of colours and its 

 " pearly " nature, being, in fact, in all respects identical in 

 substance with pearls themselves. Unlike the colouring 

 of the outer surfaces of shells, which is due to the deposi- 

 tion of pigment in theii- substance, the iridescence of 

 mother-of-pearl is caused by the mechanical arrangement 

 of the particles of the shell itself, which form a number of 

 extremely minute and delicate ridges and grooves, thus 

 breaking up the light falling upon them into its constituent 

 elements. Pearly lustre is, indeed, precisely analogous to 

 the colours of thin plates or films, and that it is really due 

 solely to mechanical structure is rendered evident by the 



fact that impressions taken from mother-of-pearl in gum 

 or fusible metal exhibit the same play of colours. 



Although the shells of several kinds of molluscs are 

 employed as sources of mother-of-pearl, those of the pearl- 

 oyster" form the staple of the trade. This is due to the 

 fact that in no other shell does the pearly substance form 

 such a thick and flat layer as it does in the pearl-oyster of 

 commerce. As regards intensity and variety in the play 

 of coloui's, the mother-of-pearl of the pearl-oyster is, in 

 reality, much inferior to that yielded by the ear-shells 

 (mentioned in the sequel) ; but for intrinsic beauty the 

 former is perhaps unrivalled. Strictly speaking, the com- 

 mercial pearl-oyster is not an oyster at all — that is to say, 

 it is not included in the same family with the edible oyster. 

 It belongs, however, to the same group of bivalve molluscs 

 as the latter — the true oysters {Ostreuhe), scallops {Pec- 

 tinidte), thorny oysters {Spowhjlitlm), file-shells (Limidce), 

 and pearl-oysters and their allies (Aricidiihc) forming a 

 closely-related assemblage of families. In all the members 

 of this group the gills are folded, the edges of the two flaps 

 of the mantle are quite distinct from one another, and the 

 foot is comparatively small, or even rudimentary. The 

 AvicuUda, or pearl-oyster family, presents the following 

 distinctive features : —The two valves of the shell are more 

 or less unequal, and the vertical or central axis of each is 

 placed very obliquely to the practically toothless and 

 straight hinge-line, the extremities of which are often pro- 

 duced into long wing-like processes. Accordingly, the 

 whole shell is very unsymmetrical, and the attachment of 

 the two valves is formed entirely by the horny ligament. 

 Externally, the shells are more or less rough and scaly, 

 with the annual lines of growth marked by concentric 

 ridges, while internally they are smooth and pearly. A 

 notch beneath the anterior wing marks the position of the 

 byssus, or mooring-rope, secreted by the small foot. In 

 the more typical members of the family, or those properly 

 belonging to the genus Avicula, the shell is comparatively 

 thin, and the wings of the hinge-line, especially the hinder 

 one, are very long and slender. On the other hand, in 

 the true pearl-oysters, the shell often becomes much 

 thickened, and the wings, with the exception of a rudiment 

 of the anterior one, are not developed. Unfortunately, 

 there is a diflFerence of opinion among naturalists how 

 much importance should be attached to these points of 

 distinction, and we consequently find the true pearl- 

 oysters sometimes described as Avicula, and at others as a 

 distinct genus — Melengrina. It is a matter of compara- 

 tively little importance which course is adopted, and. they 

 will here be alluded to under the latter title, as being the 

 one in most general use. 



Of pearl-oysters there are two distinct types. One is 

 represented by the Ceylon pearl-oyster ( Mdsagrhui fucata ), 

 in which the shell is comparatively small, and so thin as 

 to be reckoned of no value m the mother-of-pearl trade, this 

 species being fished only for the sake of its valuable pearls. 

 The commercial pearl-oyster f J/, manjaritiftra / (Fig. 1), 

 which likewise produces pearls, has, on the other hand, 

 a much larger and more massive shell, the two valves of 

 which will not unfrequently weigh as much as two and a 

 half pounds. And it is these shells which yield the great 

 supply of commercial mother-of-pearl. 



Pearl-oysters occur in vast banks in many parts of the 

 Indian Ocean, Bed Sea, Persian Gulf, and the warmer 

 portions of the South Pacific, generally at about twelve 

 fathoms depth. Formerly they were fished by divers, 

 especially women, who worked without any special 

 apparatus, and brought up a mollusc under each arm ; 

 but of late years the diving-dress has come largely 

 into use. 



