Chemistry and Physics. 65 



SCIENTIFIC INTELLIGENCE. 



I. Chemistry and Physics. 



1. On the Influence of Foreign substances on the Form, the 

 Size and the Purity of Crystals separating from a solution. — In 

 continuation of his investigations on isomorphism, Retgkrs has 

 studied the influence, in a solution from which crystals are separat- 

 ing, of the presence of foreign substances upon the form, the 

 size and tbe purity of these crystals. It is well known for exam- 

 ple that sodium chloride, which crystallizes in cubes from solution 

 in pure water, separates in octahedrons if urea be present. Lead 

 nitrate which separates from an aqueous solution in white porce- 

 lain-like crystals, appears in perfectly clear, transparent crystals 

 if nitric acid be added to the solution. Ammonium chloride 

 which is deposited from solution in water in insignificant grains 

 and skeleton-like crystals, separates in crystals a centimeter long 

 if the solution contain some ferric chloride. If a cubo-octahe- 

 dron of sodium chloride be placed in a concentrated aqueous solu- 

 tion of salt, the author observes that the cubic faces grow faster 

 than the octahedral, and so produce finally a cube; while a simi- 

 lar cubo octahedron placed in a solution containing urea, becomes 

 finally an octahedron by the more rapid growth of the octahedral 

 faces. The cause of this difference lies evidently in a contact 

 difference in the two cases between the crystal-faces and the 

 liquid ; a change in fact in the capillary attraction. Adhesion 

 depends upon the nature of the liquid as well as upon that of the 

 solid, the same liquid acting very differently upon different solids. 

 Moreover the cubic and the octahedral faces of the cubo-octahe- 

 dron are physically quite different ; this difference being often 

 evident in their difference of luster. Sodium chloride gives cubes 

 in pure water, or in water containing ferric chloride or a lead hal- 

 ide; but it yields octahedrons in water containing urea or chro- 

 mium chloride. The potassium halides give cubes in all these 

 cases, except where the solution contains a lead halide, when the 

 crystals are octahedrons. Potassium chloride however, which 

 generally crystallizes in cubes, gives cubo-octahedrons if the solu- 

 tion contains urea. Ammonium chloride and bromide, which 

 separate in trapezohedrons from an aqueous solution, crystallize 

 in cubes if the solution contains urea or chromium chloride ; while 

 if it contains ferric chloride, the ammonium chloride separates in 

 cubes and the bromide in trapezohedrons ; the reverse being the 

 case when the solution contains a lead halide. Ammonium iodide 

 crystallizes in cubes from aqueous solutions and from those con- 

 taining urea, while the crystals are octahedrons if the solution 

 contains chromium or ferric chloride or a lead halide. As to the 

 size of crystals, the author concludes that every crystal has a 

 maximum limit, beyond which there is no further growth. This 



Am. Joue. Sci. — Third Series, Vol. XLV, No. 265. — January, 1893. 

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