regularly cryslalUzed Bodies- 145 



systems of principal lines of elasticity similar to each other ; 

 but the extreme facility with which carbonate of lime cleaves, 

 permits us to discover a peculiarity which does not appear in 

 rock crystal." 



" It is known that the rhomb of carbonate of lime is often 

 susceptible of mechanical divisiofi, in directions parallel to its 

 diagonal planes ; but as these planes cut one another perpen- 

 dicularly in pairs, the intersections of each pair with the rhom- 

 boidal faces of the crystal form the great and the small diagonal 

 of each face, so that if we imagine a plane which turns round 

 the great diagonal, it ought always to remain perpendicular 

 to the supernumerary joint which passes through the small one. 

 From this it follows^, that if we cut a series of plates round 

 this same line, their structure, considered in the direction of 

 their plane, will be different in different directions at right angles 

 to each other, whence arises the production of nodal lines 

 crossed at right angles, as in plates cut round one of the axes of 

 elasticity, in bodies where these axes are rectangular. We 

 may therefore conclude, that rock crystal possesses, like car- 

 bonate of lime, supernumerary planes of cleavage parallel to the 

 diagonal planes of its primitive rhomboid, and that it is to the 

 existence of these supernumerary joints that we must ascribe 

 the principal peculiarities of the elastic state of this substance. 



" The only marked difference which appears to exist between 

 these two minerals is, that in the carbonate the small diago- 

 nal of the rhomboidal face is the axis of least elasticity, while 

 in quartz it is the axis of greatest elasticity. This result is 

 curious, as the former is a crystal with negative double refrac- 

 tion, and the latter with positive double refraction. 



'' The preceding researches are doubtless far from forming a 

 complete work on the elastic state of rock crystal and carbo- 

 nate of Ume ; nevertheless we hope, that they will be sufficient 

 to show, that the mode of experiment we have used may yet 

 become a powerful means of studying the structure of solid 

 bodies, whether regularly or irregularly crystallized. The 

 relation for example, which exists between the modes of divi- 

 sion, and the primitive form of crystals, allows us to hope, 

 that by means of sonorous vibrations we may determine the 

 primitive form of certain substances, which do not admit of 



NEW SERIES. VOL. I. NO. I. JULY 1829. K 



