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CRYSTALLOGRAPHY. 



Crystallo. 

 graphy. 



Linnaeus 



Rome' de 

 Lisle. 



Ohystallography is that branch of science which 

 treats of the forms and structure of crystals. 



The word crystal («£uim*;\A«? ) among the Greeks, 

 signified ice. It was likewise applied to what we at 

 present call rock ciystal, or quartz, because it was the 

 opinion of the Greek philosophers that this mineral was 

 merely water frozen by the cold, and converted into 

 stone. Hence they conceived that rock crystal occur- 

 red only in high mountains, where the temperature is 

 always low. But the word crystal in the English and 

 other modern languages has a very different significa- 

 tion. It is a name given to all those regular figures (usu- 

 ally bounded by plain surfaces) which a great variety 

 of bodies assume ; thus, what in common language is 

 called sugar-candy, is nothing else than sugar crystal- 

 lized. These crystals, when examined, will be found 

 *o be four-sided or six-sided prisms, terminated at the 

 ends by dihedral or trihedral summits. In like manner, 

 saltpetre usually occurs crystallized in six-sided prisms, 

 and common salt in regular cubes. Epsom salt occurs in 

 four-sided prisms, with square bases, and alum in re- 

 gular octahedrons, while the figure of the garnet is a 

 dodecahedron with rhomboidal faces. At first the term 

 crystal was confined to those regular bodies which have 

 a certain degree of transparency, and therefore was 

 conceived to be peculiar to saline bodies. But at pre- 

 sent the idea includes nothing more than regularity of 

 shape, and it belongs to many bodies not in the least 

 degree analogous to salts in their nature. 



The remarks made upon crystals by the ancients are 

 not entitled to any attention : Nor are the researches of 

 Huygens, Newton, &c. respecting the form of calca- 

 reous spar, intimately connected with oar subject. The 

 first person that attempted to form a catalogue of the 

 different crystalline forms was Linnaeus, and he made 

 it the basis of his classification of minerals. His clas- 

 sification was very incomplete, and his arrangement er- 

 roneous, and in many cases absurd ; yet it deserves to 

 be mentioned, because it drew the attention of minera- 

 logists to the subject, and constituted, in some mea- 

 sure, the beginning of the study. Rome de Lisle stu- 

 died the subject in a very different manner. He form- 

 ed a very numerous collection of crystallized minerals, 

 examined the shape of each crystal with particular care, 

 measured all the angles, and compared the relative sizes 

 of the different faces. Now it happens that the same 

 mineral very frequently occurs in a variety of shapes. 

 Thus calcareous spar sometimes assumes the form of a 

 rhomboidal prism, sometimes of a six-sided prism, some- 

 times of a dodecahedron, not to mention a great va- 

 riety of other forms common to this mineral. Fluor 

 spar occurs sometimes in cubes, sometimes in octahe- 

 drons. Quartz is sometimes crystallized in rhomboidal 

 prisms, differing little from a cube, sometimes in six- 

 sided prisms terminated by six-sided pyramids. To 

 account for this variety of forms, Rome de Lisle assum- 

 ed some particular form, generally the most simple he 

 could find, which he considered as the natural shape 

 of the mineral ; and he shewed how all the other crys- 

 talline shapes of the species were derived from this pri- 

 mitive form, by a number of supposed truncations of its 

 edges, or angles, or both. Thus the cube, when its 



right angles are sufficiently truncated, is converted in- Crystalli- 

 te an octahedron. graphy. 



This method of Rome de Lisle was very ingenious. s -^\"**' 

 It served to connect together all the different crystalline 

 forms of the same mineral, and familiarised the minds 

 of mineralogists with the forms of crystals, and thus 

 very much promoted the progress of mineralogy. His 

 method was adopted by Werner, the celebrated Pro- Weruee 

 fessor of Mineralogy at Freyberg. It is affirmed by 

 the pupils of Werner, that the method of Rome de Lisle 

 was in fact the method of Werner, communicated to the 

 French mineralogist in the course of a correspondence 

 between them. But as during the 40 years that have 

 elapsed since the first publication of Rome de Lisle's 

 book, no such claim has ever been publicly made by 

 Werner, it is obvious that such assertions are entitled 

 to no attention, and that Rome de Lisle has a right to 

 the full credit of his method. 



But this method, though ingenious and useful, was 

 entirely arbitrary. No reason could be assigned for the 

 various and capricious truncations to which the primi- 

 tive form was subjected. Nor could any conjecture be 

 formed of the number of crystalline forms which were 

 likely to occur in the same species, far less of their 

 forms and angles. A great and fundamental improve- Ber-muit. 

 ment was made by Bergman. The first edition of his 

 Essay was published in the year 1773, the very year 

 in which the first edition of Rome de Lisle's Crystallo- 

 graphy appeared. As we have never had an opportu- 

 nity of seeing this first edition, we do not know how 

 much it differs from the seoond edition, published in 

 the year 1780, in the second volume of his Opuscula, 

 He observed that the secondary forms of crystals al- 

 ways contain a nucleus of a determinate shape, which 

 may be obtained from them by a skilful dissection. 

 Thus calcareous spar, whatever be its shape, always 

 yields a nucleus which is a rhomboidal prism with de- 

 terminate angles. He shewed how all the secondary 

 forms may originate from this primitive one, by the ad- 

 dition of slices of certain determinate shapes to each 

 of its faces. According to Bergman, it was Assessor 

 J. G. Gahn who first observed the rhomboidal nucleus 

 of calcareous spar. We have been informed that it had 

 been detected before by Mr Keir of Birmingham, who 

 mentioned his discovery in his English translation of 

 Macquer's Chemical Dictionary. We have not had it 

 in our power to consult this translation since we were 

 informed of the circumstance, and therefore we cannot 

 say how far Mr Keir is entitled to the merit of the dis- 

 covery. 



This idea of Bergman was taken up by the Abbe Abbe 

 Hauy, or rather indeed the same idea occurred to him Hauy. 

 before he had any information of what had been done 

 by Bergman. For he informs us that his first disserta- 

 tions on the subject had been presented to the Academy 

 of Sciences before he had any information of Berg- 

 man's Dissertation, and that this dissertation was com- 

 municated to him by the Academy as likely to interest 

 him, because it treated of the same subject with his 

 own. Be that as it may, the subject has been prose- 

 cuted ever since by the Abbe Hauy, with the most in- 

 defatigable industry, and brought by bin to an urs 



