52 Guy, on Crystals of Arsemous Acid. 



octahedron, well-formed triangular prisms, terminated at 

 either end by triangular facettes, also twin-crystals or mdcles, 

 also equilateral triangles resting on half the adjoining 

 triangle as a base. I will not take up your time further by 

 specifying all the forms which at first puzzled and perplexed 

 me. Suffice it to say that, in the full consciousness that I 

 did not understand the things I saw, I determined to turn 

 for awhile from nature on the small scale to art on the large. 

 I procured octahedra of wood, and not being satisfied ynih. 

 them, prevailed on Messrs. Powell, of Whitefriars, to make 

 me the crystals of glass now before you. By studying these 

 large models, placing them in all sorts of positions, and viewing 

 them from different points and in different lights, I was pre- 

 pared to understand most of the appearances under the 

 microscope. The broader shadows of the transparent glass 

 crystals were reproduced in the small crystals of arsenious acid, 

 and the several postures which I caused the large crystals to 

 assume were recognisable under the microscope. I found 

 that the sublimed crystals adhered to the flat surface of 

 glass by their solid angles, by their edges, and by their 

 faces, as well as in positions less easily described. I also 

 inferred that the dark squares were crystals (octahedra) 

 adhering to the glass by their solid angles, in which position, 

 as my glass model taught me, the play of lights and shadows 

 was such as to occasion confusion and possible darkness. 

 This suspicion, which was strengthened somewhat when I 

 examined the sublimates by reflected light, became certainty 

 under the binocular microscope. Under that admirable 

 instrument, with reflected light, there are no dark masses, 

 and no obscure forms. The meaning of the dark oblong 

 forms and of the dark lines which I at first identified with 

 the acicular or prismatic cryistals of authors did not occur 

 to me till later in my inquiries. 



I have mentioned the frequent occurrence of the three-sided 

 prism with bevelled extremities. I do not mean the figure 

 sometimes described as a lengthened octahedron, but a figure 

 having the deceptive appearance of a triangular prism. Was 

 this a distinct crystalline form, or might it not be some aspect 

 of the octahedron ? It obviously could not be brought about 

 by any attitude of the whole crystal ; but my wooden model, 

 supplied by Professor Tennant, is cut in half by a plane parallel 

 to, and equidistant from, two of its faces, and these two equal 

 halves of the-crystal are made to rotate on each other, so 

 as to show the twin-crystal, or mdcle. Here, then, without 

 supposing any new form of crystal, there was new material 

 for speculation. I had seen the twin-crystal, or mdcle, in 



