1-AI.LOOKArilY. 



CTKNoli.UTYUS. 



wbieh *bouW out off one proportion from each of the edge* air, 

 would *vid.ully b* parallel to the plane at /, whose edge* would 

 4* with the diagonals of the primary plane*. It would carry 

 the limit to which we mu.t restrict this |*per, if we were 

 to enter upon a geometrical consideration f 

 three line*, and we shall therefore confine our- 

 selves to this statement, that if, on any square 

 prism, we find a net of plane* truncating it* 

 solid angles, and if we assume the edge* of 

 the**) plaae* to b* respectively parallel to the 

 J Hr""'* f * ne primary planes, the ratio, or 

 comparative length* of the edge* a and r, may 

 D* found, aud thus the distinction between 

 prism* of different heights belonging to different 

 minerals may be ascertained. Crystals be- 

 longing to the other primary form* may 

 grarrally be distinguished, ai we have already stated, by measurement 

 of the angles at which the plane* severally incline to each other. Hut 

 in order to investigate the laws of their respective secondary plane*, 

 we require to know the comparative lengths of the lateral and 

 terminal edge*, which may be found by means analogous to those we 

 have just described. The rhomboid however, whose edges, like those 

 of the cube, are all equal, does not require this preliminary investiga- 

 tion, but the laws of new plane* may be determined from measurement 



. . 



When a plane similar to that shown in fy. 8 occurs on one solid 

 ogle of a crystal, it generally occurs on all the others, establishing 

 what Hauy has termed the Law of Symmetry. But, as we have 

 before stated in reference to the cube, this law is occasionally deviated 

 from by the production of only one-half the symmetrical number of 

 secondary planes on a square prism. This remark also Applies to such 

 other kinds of secondary plane* as we now proceed to describe. 



Besides the plane shown in Jty. 8, there are three other kinds 

 fleeting the solid angles. 



First, such as would cut off one, two, three, or more portions of the 

 >lges a and b, but at the same time some other number from tin- 

 lge e. Thus if one portion be cut from a and one from b. there will 

 be two, three, four, or some other number cut from c ; or if three 

 portions were cut from a and three from 6, either one, two, four, 

 five, or some other number would be cut from e, so that a numerous 

 cries) of plane* of this nature might occur on each solid angle. 



The second kind of planes are those which would cut off an equal 

 number of part* from a and r, but a different number from 6. But 

 in this ca*e there would be two planes on each solid angle, for if wo 

 suppose one plane to cut three part* from a, and three from c, and 

 two from 6, a second plane would also be pro- 

 duced, cutting three parts from 6 and c, and two 

 from a, producing two planes similar to those 

 in jty. 9. 



. of the series of planes of the first kind 

 would have an edge parallel to the diagonal / e, 

 *'y.*; and each of those of the second kind w..i,l.l 

 have edges parallel to the diagonals d / and 

 e / of the same figure. The planes of the third 

 class also occur in pairs, and are such as would 

 be produced by cutting off dissimilar numbers of 

 part* from the three edges, such as two part* 

 from a, three from 4, aud four from c, none of the edges of these new 

 plane* being parallel to any diagonal. 



The secondary planes on the terminal edges may cut off any number 

 of part* from the edge* r and i, and the same, or any other number, 

 from s and e. Thoee on the lateral edges, if they cut unequal 

 portions from and o, and 6 and , will be found to occur in pairs. 

 8i*gl> pUoe* on the lateral edge* are such a* would result from 

 catting a and o, and 6 and PI, equally ; and the secondary planes on 

 the other primary forms are produced by laws analogous to those we 

 have just described. 



The reasons for preferring prisms to octahedrons for the primary 

 lorn* may be thus briefly stated 



We hare already feed that the octahedron derived from the cube 

 night b* taken a* the fundamental or primary figure of that system 

 of cryctaUimtioo. An octahedron derived from the truncation of Uic 

 upper and low* edf** of the square prism, or of its solid angles, by 

 plane* which would internet the terminal plane* parallel to their 

 diagonal*, might b* assumed a* the primary form of this system ; aud 

 octahedrons similarly derived from the other prisms might also be 

 retarded as the primaries of their respective systems. Aud these 

 h ;ures have accordingly bern adopted by Moll*, as the fundamental 

 form* of hi* system of cry.ullography. From the greater simplicity 

 b.wever of derivation which result* from the assumption ,,f .),'.. 

 prism, a* primary form*, and the greater mathematical facilities in 

 determining the relations of the derived to the primary, we have been 

 induced to retain them as the fundamental forms of our system 

 the relation, among these primary and their respective secondary 

 form* are, according to our plan, dependent only upon the proportion* 

 of the primary edge* required to be cut off to produce given second- 

 ary plane*. But in taking the octahedrons a* primaries, Mobs has 

 founded the relations of time to the aooondary figure* upon the 



relative length* of the axes of the derived figures, according to w huh 

 view of derivation the lateral planes of the square prism would he 

 1 a* those of an octahedron with an infinitely long axis, and 

 the end plane* a* those of an octahedron with an infinitely short axis. 

 A nt I for all the various prisms which may occur, octahedrons must 

 first be found, from the infinite prolongation of whose axe* the given 

 prism* may be produced. From the complexity of this method it 

 will probably not extend far beyond the school of its highly ingenious 

 author. 



The exact relations among primary and secondary forms may be 

 determined mathematically, bomctime* from measurement and some- 

 time* from parallelism* between certain edge* of the secondary figure* : 

 am) the mathematical processes may be either those of plane trigono- 

 metry, as applied by Hauy ; or spherical trigonometry, u used by 

 other authors; or analytical geometry, as applied by Professor 

 Win-well in a paper in the 'Phil. Trans.' for 1825 ; or by ret. 

 the planes of the crystal to the surface of a sphere aud deiiutin 

 positions stereographically, as shown in a pafier by Professor Miller, 

 of Cambridge, in the 'Lond. ami Kdiub. I'liil. Mag.' of Feb. 1835. 



Crystallisation and the circumstances under which it lake* place 

 form an interesting subject of inquiry, not only in reaped of the 

 variety of figures under which crystals present themselves, but in 

 relation to much more comprehensive geological investigation!) into 

 the formation of the early crystalline rocks and the various emi 

 crystallised minerals, and into the manner iu which the nun 

 crystalline bodies found ill the metallic and other veins h.i\ - 

 produced. 



From the great length of time during which these natural processes 

 must have been in action, the slowness with which they probably have 

 proceeded, anil the hidden rcce.-ses in which they have taken place, 

 the progress of natural crystallisation can scarcely be said t 

 been ever observed; for the production of saline crystals at the 

 bottom of certain lakes, and even that of iron pyrites, which are 

 said' to have been observed in a progressive state of formation, 

 cannot be regarded as belonging to the class of phenomena we arj 

 contemplating. 



Not having therefore the operation of nature open to our inapt 

 our only sources of information relative to the formation of crystals 

 are those afforded by the processes of artificial crystallisation ; aud 

 here until very recently our experiment) were circumscribed and our 

 views bounded by a very few modes of operation : that of the di 

 of crystals from solution iu some fluid ; their production while 

 gradually cooling from a state of fusion ; and their volatilisation by 

 heat or otherwise. Latterly however, by the aid of that in, 

 agent, electricity, new methods of producing crystals have 

 pursued: much of the darkness iu which the subject had been pre- 

 viously involved has been dispelled, and there can now be little 

 that the phenomena of crystallisation are influenced in a greater or 

 leas degree by electric influence. 



The crystallisation of salts from solution in fluids generally takes 

 place when the solutions are sufficiently evaporated, but the decree of 

 evaporation is very dilferx-nt for different subst ... salu begin 



to crystallise- at the surface very soon after evaporation comm- 

 and others (for example, sugar) must be evaporated to the consistence 

 of a thick syrup before any crystals will bu form d. Hut fluids will 

 generally dissolve more matter than cold ones, aud crystals are 

 frequently produced during the cooling of the hot solution. S,. in- 

 soluble substances however cannot be brought to crystallise ui,,l, r 

 any circumstances hitherto tried; but on the solv< ,tiug a 



thick pasty matter U left, which by further evaporation becomes a 

 hard solid mass. Camphor affords an instance of the formation of 

 crystals by volatilisation. The sides of a bottle containing this body 

 may frequently be observed iucniKted with brilliant crystals. 



The slags of furnaces will frequently be found to contain cr- 

 lised matter; and the common rolls of sulphur when broken will 

 frequently present small cavities lined with thin needle-like crystal*. 



(Anstod, KUnu-ntary liculvyy ; Dana, Manual of Jtmtraiegf.) 



CTENACA'NTHUai a genus of Fossil 1'Ucoid Fishes, from the 

 Mountain Umestoue and OM Hod-Sandstone. (Agassiz.) 



CTKHODA'CTOVA (Oejeau), a genus of Coleopterous Insect* 

 belonging to the section Otodepkaga and sub-section Tntncali/ieunei. 

 It has the following characters : Body but slightly 

 flattened ; thorax longer than broad, truncated post, 

 joint of the palpi almost oval ; three basal joints of lli. i 

 nearly triangular or lirart-ohapod ; claws denticulated beneath. 



It. jean, in his 'Catalogue des Colcoj.teivs,' only enuiucrsto 

 specie* of this genus, all of which are from Guyana. There are 

 however other species kn<Avn. 



C. Chtotutatii is less than half an inch in length, of a blue-black 

 colour above, and brown buuoath ; the thorax is red, and the legs and 

 antenna! are yellowish-red. 



' TKNolUVTYU'S, a genus of Rodent Animals of the family 

 AnietUdm, established by Dr. J. E. Gray. 



Each foot has four toe* only, and an obsolete clawless wart in 

 place of the thumb; claws small and falculated; toes pectinated 

 internally, with small bony appendages. Tail very short aud hairy 

 2 33 



Dental formula : Incisors, j ; Molars, ,pqr. (Gray.) 



