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the more compact forms, where infiltrated solution?, forming part 

 of the material, existed at the moment of aggregation. 



Dj". Hayes made some remarks on the formation of macle 

 crystals, the true theory of which he stated that he had many 

 vears 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 example 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 ordinary crystal- 

 lization. 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 tliey 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, 

 forming a regular, solid crystal of a hexagonal form. 



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 pi'ocess of construction ; 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, representing 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 inte- 

 rior of the crystal has 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 supposi- 



