April 1, 1891.] 



KNOWLEDGE. 



67 



direct to the ocean by the wind, they may probably 

 flourish on some remote island. Necessarily such seeds 

 must be well protected against outer injury, whether from 

 friction or from the action of salt water. Dr. Hooker 

 found in his examination of a large number of the floras of 

 islands that the LeguminosiP, to which order the Pea, 

 the Vetch, and the Bean belong, contained more species 

 common to other parts of the world than any other 

 order. The preponderance was due to the characteristic 

 form of the fruit, its strong pericarp being well adapted 

 for preserving the seed, whilst its shape enabled it to float 

 on the water. 



Of course, it is only fruits of a certain lightness 

 and buoyancy that admit of transport by wind and water. 

 Those that are too heavy for this method of distribution, 

 such as pulpy fruits, depend upon the agency of animal 

 life for dispersion. There can hardly be any doubt that 

 the bright, succulent, and edible coverings of fruits are 

 specially intended to attract the attention of birds and 

 other animals. In order to produce this attractive exterior 

 we find not only the ovary — afterwards the pericarp proper 

 — specially developed, as in the Orange or the Grape, but 

 various other parts of the flower. 



Thus, by structural developments adapted to take ad- 

 vantage of the means of transport existing in the forces 

 of wind and of water, and in the surrounding animal life 

 — afibrding another proof of the interdependence which 

 exists in Nature — have plants been spread over the Earth. 

 " The real difficulty," says Sir Charles Lyell, " which must 

 present itself to everyone who contemplates the present 

 geographical distribution of species, is the small number 

 of exceptions to the rule of the non-intermixture of 

 different groups of plants. Why have they not, suiJiJosing 

 them to have been ever so distinct originally, become 

 more blended and compounded together in the lapse of 



CALCITE AND ARAGONITE IN SHELLS. 



By Vaughan Cornish, B.Sc, F.C.S. 



IF different materials have the same chemical compo- 

 position, they are generally regarded as being 

 essentially identical, and are looked >ipon as varieties 

 of the same substance. Carbonate of Ume is a 

 familiar example ; chalk, limestone, marble, Ice- 

 land spar, and aragonite are all composed of carbonate of 

 lime, and are spoken of as different varieties of carbonate 

 of lime. This does not, however, represent the view of 

 the mineralogist. Chemical composition is only one 

 among several criterions considered in the definition of a 

 mineral species. It often happens that minerals differing 

 fundamentally in their crystalline form, and in the in- 

 ternal structure which is connected with the external 

 form, have the same chemical composition. These are 

 regarded by the mineralogist as distinct species, notwith- 

 standing the identity of chemical composition. Funda- 

 nu'Utal difference of crystalline form is the basis of 

 differentiation among minerals of which the composition 

 is identical. We will endeavour to make clear the 

 principles on which it is decided whether a difference of 

 form is to be regarded as fundamental. In mineralogical 

 collections, ranged side by side with the well-known rliomb 

 of Iceland spar, may be seen a vast variety of crystalline 

 forms of carbonate of lime. In general appearance they 

 dift'ei' greatly from one another ; yet the mineralogist will 

 tell one that there is no fiiiiiliniitntul difference in these 

 forms, whicli are all intimately related to that of the 

 rhomholu'dron. This relationship is a matter of geome- 

 try, which we must he content to state merely in a 



general manner. A crystal is a body bounded by certain 

 plane surfaces — the faces of the crystal. The arrange- 

 ment of a system of planes is best understood by con- 

 sidering how their position is related to three lines 

 intersecting at a point, these lines being termed aaxa. A 

 crystalline form is defined, first, by the position of these 

 lines ; secondly, by the manner in which the position of 

 the planes is related to that of the lines. All the various 

 forms which, in the mineralogical collection, are placed in 

 proximity to the rhomb of Iceland spar, are forms which 

 can be built up on the same system of axes, and the 

 positions of the planes or faces with respect to these axes 

 are all related to one another according to a simple 

 geometrical law. All these specimens are, therefore, 

 reckoned as belonging to one mineral species, to which the 

 name calciU' is given. 



In the next compartment of the mineralogical collection 

 will be found another set of crystals, also composed of 

 carbonate of lime. These, however, are members of 

 another mineral species known as (iiai/diiite. Their forms 

 are related among themselves by a simple geometrical law, 

 but the plan of construction is radically different from that 

 of the calcite forms. The system of planes representing 

 the faces of the crystals cannot be built up on the same 

 three axes as those of the calcite forms, and the law con- 

 necting the positions of the planes themselves is different 

 in the two cases. 



For the mineralogist, carbonate of hme comprises two 

 mineral species — calcite and aragonite — and all the difl'e- 

 rent varieties are classed under one of these two names. 



The question may be asked. Is there not something 

 fanciful in basing a classification of material substances 

 merely on certain abstract ideas of symmetry"? Calcite and 

 aragonite are composed of the same kinds of stuff' or 

 matter ; if our minds were not gifted or encumbered with 

 abstract notions about symmetry, should we discover any 

 difference of properties between these two mineralogical 

 species '? As a matter of fact, the properties of crystalline 

 substances furnish a striking example of the real and inti- 

 mate relation of oui- ideas of symmetry with the actual con- 

 stitution and properties of matter. Every property of a 

 crystalline substance is related to the particular symmetry 

 of its form. Thus, take the case of the action of the sub- 

 stance upon liglit. A ray of light is affected in the same 

 way by its passage through any crystal of calcite : all speci- 

 mens of aragonite behave alike in their action on light ; but 

 the mode of action is entirely different in the case of 

 aragonite from that of calcite. For exhibiting these 

 differences of behaviour in the most distinct manner an 

 elaborate instrument is employed, the stauroscope, in 

 which iKihiriu'il light is used. We cannot enter here 

 into the method of using the instrument, but in most 

 text-books of mineralogy will be found plates showing the 

 different intcrt'firmr phenomena afforded by a crystal of 

 calcite and one of aragonite. .\ fragment of a crystal 

 shows the phenomena characteristic of its species, which 

 do not depend upon the specimen possessing crystalline 

 faces. The action upon liglit depends upon the actual 

 structure of the material of which the crystal is composed, 

 which, as the above example shows, is intimately related 

 with the crystalline form. We see, then, that the phy- 

 sical properties justify the mineralogist in sorting the 

 varied crystalline forms of carbonate of lime into two 

 classes, and in characterizhig every member of each class 

 by one of two names — the names of two mineral species. 



We have said that <ill varieties of carbonate of lime 



are classed as calcite or aragonite, and many of these 



varieties do not show crystalline form at all, as, for 



I instance, limesti ne and marble. What data justify the 



