298 ANNUAL OF SCIENTIFIC DISCOVERY. 



centric structure ; a slight conformity to this structure being observed, when 

 a bubble of air, or a vacant space, marks the point of commencing deposi- 

 tion. In other cases, a shell in its calcareous composition offers a preferred 

 nucleus, and as it contributes its lime salt, a concentric arrangement may be 

 noticed in the forms resulting, especially after exposing them to heat. 

 Rounded massed once formed become centres, or nuclei of secondary occur- 

 ring aggregates, one central mass being surrounded by spheres attached ; 

 but in all it is easy to read the influence of the tendency of carbonate of lime 

 to crystallize, and the opposition of the finely-divided silt, causing the par- 

 ticles of both to assume forms without straight bounding lines, as the polar- 

 izing force of crystallization is arrested in all directions. 



It may be added that a great number of bodies present rounded forms de- 

 pendent on a modification of this law of restrained crystallization, such as 

 numerous iron ores, bog manganese, and even the more compact forms, 

 where infiltrated solutions, forming part of the material, existed at the 

 moment of aggregation. 



Dr. Hayes also made some remarks on the formation of macle crystals, 

 the true theory of which he stated that he had many years since illustrated 

 by numerous specimens and examples. Starting from the point where 

 Beudant left the subject resting on a supposition, it occurred to him that the 

 law was quite within the scope of a chemical demonstration, which would 

 place this and similar instances of crystallization among known scientific 

 facts. Without entering minutely into the matter, we may take as an ex- 

 ample a salt exerting a strong tendency to crystallize from a hot solution on 

 cooling, ordinary saltpetre. A solution of this salt, in a pure state, 

 slowly cooled, affords solid, six-sided prisms, or the crystals become solid 

 if allowed to rest in the fluid, and we observe nothing but the result of ordi- 

 nary crystallization. If the process has been carefully watched, and it is 

 a most interesting and instructive exhibition, it will be observed that the 

 particles of solid, so soon as they become visible, are rectangular, and that 

 they are polarized. Motion may cause similar poles to approach within the 

 limits of repulsion, when the particle turns and brings its opposite pole in 

 contact, the union taking place at a certain angle, and the frame-work thus 

 laid out becomes closed in by successive layers of polarized particles, form- 

 ing a regular, solid crvstal of a hexagonal form. 



o o * o 



The same operation going on in a solution of the same salt, mixed with a 

 solution of common salt, exhibits for some time the same process of con- 

 struction; but it soon becomes apparent that a solid prismatic crystal will not 

 form, and time does not change the condition of the solution. A frame- 

 work, or skeleton crystal is built up as before, and possibly the interstices may 

 be solidly filled, but there will appear a hexagonal cavity in the centre, repre- 

 senting a considerable part of the volume of the crystal. If we carefully seal 

 this cavity and remove the crystal, we find the fluid contents to be a strong 

 solution of common salt, and the interior of the crystal lias quite finished 

 surfaces. The suggestion at once arises that the crystal, having used in its 

 structure all the saltpetre within reach, has completed its form with a strong 

 solution of common salt. To test the correctness of this supposition, we 



