Intelligence and Miscellaneous- Articles; 487- 



pyramids), a form scarcely if ever met with among the crystals pro- 

 duced by the above- described technical process. Analogous local 

 actions are observed on natural minerals ; so that in some cases an 

 expert mineralogist may infer the place of origin of a mineral sub- 

 stance from its crystalline form only. On the other hand, conclu- 

 sions as to the mode of formation of minerals founded on the results 

 of laboratory experiments must be drawn with a certain degree of 

 caution. In fact, the chemical forces, when acting on large quan- 

 tities of substances, as in manufacturing processes, frequently 

 produce results very different from those obtained by the chemist 

 operating with comparatively small portions ; and, still more, the 

 results of natural operations, gigantic in quantity as in energy, 

 and extending through immeasurable periods of time, may scarcely 

 be comparable to mere laboratory investigations made with limited 

 quantities in some few hours or days. 



When immersed in solutions of other salts, the crystals of the 

 bibasic sulphate in question show some curious phenomena. In a 

 solution of sulphate of ammonia a hexagonal plate was gradually 

 converted, by superposition on both of its larger planes, into a 

 lengthened hexagonal prism, easily cleavable at any point in a direc- 

 tion perpendicular to its longitudinal axis. Thin plates of it taken 

 from the newly added portion show the characteristic optical pro- 

 perties of the common prismatic sulphate of ammonia. This instance 

 of episomorphism between a rhombohedral and a prismatic salt, or, 

 in other words, of two substances belonging each to a different cry- 

 stallographical system and nevertheless subject to the crystallogra- 

 phical laws of isomorphism, is highly interesting. The angular 

 values of both (the rhombohedral and the prismatic combination) 

 being, in this special case, very near each other, the existence of the 

 fact here alluded to was to be decided by optical investigation. 

 Trifling as the difference of the forms here in question may be, its 

 existence is a fact not to be denied ; and therefore such a formation 

 as just described could not take place if the disposition of the mole- 

 cules, by whose regular aggregation such crystals are formed, did 

 not go on with mathematical exactitude. Observation shows devia- 

 tions from the strict regularity of lines and angles to be of no rare 

 occurrence in crystallogenetic processes; precise measurements of 

 substances considered to be isomorphous have shown them not to be 

 absolutely congruent ; so that isomorphism, as far as it is concerned 

 in this character, has only an approximate value. Two substances 

 different in angular value, even when combined into one and the 

 same crystal, cannot be considered as having totally lost their re- 

 spective individuality. Their last constituent parts, representing 

 the crystalline molecules of both salts (sulphates of potash and am- 

 monia), are in juxtaposition to each other, as if they were but one 

 homogeneous substance. Their superposition without preceding 

 mixture is a proof that molecules of not absolute identity may be 

 deposited on each other in the same way as analogous particles 

 would be. Both these sulphates could be considered as absolutely iso- 

 morphous in the cry stallographical sense, but for the optical phenomena 

 characteristic of two distinct and mutually independent systems. Iso- 

 morphism, however, presupposes chemical analogy ; now the potash 



