STRUCTURE.] RAPHIDES FORM. 101 



could not present a similar aspect, which is the case, as will 

 be shown hereafter. 



As respects the summit being a six-sided pyramid, its 

 existence does not seem discoverable, for the crystal from 

 about its centre gradually vanishes to a point, having no 

 angular interruption, such as is observed in the large crystals 

 in the root of Iris florentina or the wood of Quillaja, where a 

 regular right-angled prism is surmounted by a pyramid. 

 MohFs idea of the shape certainly approximates the truth 

 more than RaspaiPs; but it can be shown, though the aci- 

 cular crystals are four- sided, they are not always right-angled 

 prisms, as he asserts. To witness these facts, the crystals 

 must be obtained, by lengthened maceration, free of cellular 

 tissue, and then crushed into fragments, when many will 

 present an obliquely transverse fracture which exhibits four 

 sides, some having the angles right angles, and others acute 

 and obtuse angles; in fact, the transverse section of such 

 would resemble frequently a rhombus. To examine more 

 satisfactorily the fractured ends of these minute crystals, 

 which scarcely measure the To ^ 00 part of an inch, it is most 

 convenient to place their fragments in a watch glass, with a 

 small portion of Canada balsam, and to heat the whole till 

 the balsam has acquired a viscid consistence, then let it be 

 removed from the source of heat, and be stirred whilst the 

 mass is cooling till sufficiently hard, when the broken Ra- 

 phides will be sustained in it in all directions ; and fre- 

 quently a few can be observed erect, which cannot fail to 

 give a true outline of their real form, which by this method 

 appears to be a four-sided prism, the angles being frequently 

 oblique, and in other instances to put on the rectangular 

 condition. Occasionally some of the crystals are observed to 

 exhibit a triangular section of the isosceles shape, which 

 seems to be an anomaly : but, to account for this, it is to be 

 remembered that it is not opposed to the laws of crystallisa- 

 tion for certain bodies to crystallise in halves, consequently 

 the half of a four- sided prism, taken from one angle to the 

 opposite one, would be triangular ; and it is conceived that 

 when a crystal exhibits this triangular section it has belonged 

 to the compact bundle of crystals in which it has been con- 



