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Mr. T. C. Porter. 



[Nov. 9, 



This comparatively little known salt is formed by dissolving in 

 50 c.c. of water, say, 20 grammes of crystallised ferric ammonium 

 sulphate (iron alum), and adding drop by drop, with constant shaking, 

 2 - 33 c.c. of ammonia solution, sp. gr. 0*88. The mixture must be left 

 to evaporate spontaneously, when it deposits hexagons of the very 

 interesting pleo-chroic salt; generally, some quantity of another basic 

 iron salt is also formed. 



Marignac's salt, which is strongly magnetic, is also formed when a 

 solution of iron alum evaporates spontaneously above a certain tem- 

 perature, and the orientation has been, as a rule, more noticeable when 

 the crystals have been deposited from an iron alum solution than 

 when deposited from the solution first mentioned, probably because 

 the rate of deposition from the alum has been very much slower. 

 Figs. 1 and 2 are photographs of the salt being deposited from a 

 solution of ammonium iron alum by spontaneous evaporation on glass 

 slides : fig. 1 in a powerful magnetic field, fig. 2 under conditions as 

 like as possible to those of fig. 1, but under the earth's magnetism 

 only. Numerous trials have satisfied the writer that other conditions 

 being the same, crystals of the alum and of Marignac's salt form in 

 the powerful magnetic field as readily as they do in the weak 

 one. The position of the magnet poles is not indicated on the 

 photographs, but it will be seen that, in many obvious cases, 

 where the hexagons rest on one of their prismatic sides, the 

 principal axes of the hexagonal prisms are parallel (to the magnetic 

 lines of force). Many of the crystals, however, if not most of them, 

 are resting in their most stable position, viz., on one of the hexagonal 

 faces. This is noteworthy, because if a crystal of the salt which has 

 grown to a large size (and away from the magnet) be suspended by a 

 thread of unspun silk attached to the centre of one of its prismatic 

 faces by a minute morsel of wax, between the blunt poles of the 

 electro-magnet, it sets itself in exactly the same position as the 

 orientated crystals in the photograph, viz., with its principal axis 

 parallel to the lines of force. When the same crystal is suspended 

 from the centre of one of its hexagonal faces, it sets itself with the 

 planes of an opposite pair of the prismatic faces perpendicular to the 

 lines of force, so that if in figs. 1 and 3 the visible hexagons are 

 orientated, they should be disposed with a pair of opposite sides 

 perpendicular to the lines of force, and therefore parallel to the longest 

 sides of the rectangles which are the observer's view of the hexagonal 

 crystals resting on one of their prismatic sides. There are certainly 

 signs of some of the hexagons being thus orientated, but since there 

 are notable exceptions, and, moreover, since there are six positions 

 favourable to an orientation hypothesis in one complete revolution of 

 the crystal in its own plane, one cannot say definitely whether they 

 are orientated or not. 



