ON MICROSCOPIC DISCOVERY. 



582 



The fantastic and picturesque formations so 

 commonly observed on the windows in the 

 winter season, will, if closely examined, be 

 found to consist of a series of branches dis- 

 posed under the uniform angle of sixty de- 

 grees ; and all the irregular variety is merely 

 the result of numerous small impediments to 

 the freezing process. 



An almost infinite variety of configuration j 

 may be observed in the flakes of falling snow, 

 all however thus far resembling each other, j 

 that they uniformly consist of six branches ' 

 radiating from a centre. The side shoots are 

 also disposed according to the general prin- 

 ciple, and in all the peculiarities of figure we 

 discover the angle of sixty degrees determin- 

 ing the formation. Plate 35, figs. 50 and 51, 

 are two snow flakes. In fig. 50 we observe \ 

 the branchings from the centre, and the side 

 shoots disposed according to the general law. 

 The hexagonal figures terminating the main 

 branches are explained thus : the two sides 

 connecting with the branch are lateral shoots 

 from it, the two next are lateral shoots from 

 the former two, and the other two sides are 

 similar shoots from the second two. And it 

 is ia this manner by a little consideration of 

 the figure, that we are enabled, in almost every 

 instance, to trace out a uniform principle of 

 conformation. In fig. 51 we see the six 

 branches and the lateral shoots as before, and 

 the leaf-like outline of the figure is nothing 

 more than the natural result of the lateral 

 shoots near to the centre pressing upon each 

 other, and the central ones extending farther 

 out than those more recently put forth. 



Congelation and crystallization, though 

 closely resembling each other, are distinct 

 operations. Congelation fixes the whole fluid 

 mass ; but the process of crystallization causes 

 the particles of mineral bodies, when sus- 

 pended in fluid or separated by fusion, to 

 assemble in regular figures, agreeably to 

 ascertained principles of formation. 



Notwithstanding the variety of figure observ- 

 able in crystals, their primitive forms are few, 

 such as the cube, parallelopipedon, &c. ; and 

 all eccentric formations arise from a peculiar 

 arrangement of their laminae on the faces of 

 the primitive crystal. Dr Hooke suggested 

 that crystallization merely exhibited the vari- 

 ous regular forms resulting from the combina- 

 tion of small globular particles ; and this 

 opinion has been adopted by many writers. 

 There is, however, a difficulty in conceiving 

 how globular particles should with undeviat- 

 ing exactness constantly assume the same 

 primitive figure, in (for instance) the crystal- 

 lization of common salt. The simple form of 

 these crystals is always a cube ; now we can- 

 not comprehend the reason why this should be 

 the case on the supposition that they are 



merely an accumulation of globular particles ; 

 for these particles can unite only by mutual- 

 attraction, and might, for ought we perceive to 

 the contrary, assemble in any other primitive 

 form as readily as that of the cube. A mare 

 recent theory supposes the particles to have 

 either the figure of the primitive crystal or 

 that of a cube, a solid angle, or a paral- 

 lelopipedon ; and we can pretty well discover 

 in every instance, on this hypothesis, how the 

 primitive form is in variably determined. Much 

 light has been thrown upon this subject by the 

 microscope, which brings immediately under 

 the eye of the observer the whole process of 

 crystallization, from (in several cases) the 

 primitive form, to the most intricate combina- 

 tion which it ultimately develops. A more 

 beautiful sight cannot be conceived than that 

 which is presented when any saline solution 

 is suffering gradual evaporation under the mi- 

 croscope, and the crystals begin to shoot and 

 extend themselves over the field of the instru- 

 ment. It is not exaggeration to say that it 

 partially draws aside the veil that is cast over 

 the first creation of material forms. Adopting 

 the hypothesis, that primitive chaos was a 

 fluid holding in solution the particles of all 

 solid matter, we may imagine the small drop 

 of water on the stage of the microscope to be- 

 that chaos, from which we see glaciers and 

 rocks rising up with a rapidity, precision, and 

 beauty truly astonishing. 



In Plate 35 the figs. 44 and 45 exhibit 

 the compound crystals of common salt. These 

 have been selected for illustration in conse- 

 quence of their showing the process of super- 

 position in the most simple manner. The 

 figures closely resemble each other ; but 

 in fig. 44 the various laminae are not so 

 easily recognized as in fig. 45. The crystal 

 commences with the formation of the nucleus, 

 which in this case is a very minute cube, com- 

 posed of cubic particles ; a layer of four 

 similar cubes immediately begin to form under 

 this nucleus, and raise it up, or propel it for- 

 ward ; a third layer of nine similar cubes then 

 form under the layer of four ; and thus the 

 crystal goes on increasing, each successive 

 layer taking an additional cube into the side. 

 Now as the cubic particles are small beyond 

 our comprehension, and since the aggregation 

 will frequently be affected by the superposi- 

 tion of lamina?, the thickness of these particles 

 only, it -is easy to conceive the reason why, 

 even in microscopic crystals, the sides frequently 

 present to our weak vision, and its limited 

 aids, a perfectly polished surface free from 

 all irregularities. In fig. 44, the laminas 

 are not perceptible towards the apex of the 

 crystal ; whilst in fig. 45 the layers are dis- 

 tinctly recognised: these varieties are the 

 result of mere accidental circumstances, in the 



