100 BAPHIDES FORM. [BOOK i. 



Besides these two usual kinds, there are other forms, but 

 of more rare occurrence, some of which are observed of regular 

 crystalline figure ; as the rhombohedron in some cells of Calla 

 sethiopica, and bark of Cascarilla ; octohedron, according to 

 Meyen, in the stem of Tradescantia virginica ; the rectangular 

 prism in Quillaja saponaria; and oblique prisms, which occur 

 with acicular crystals, in Scilla maritima : but still there are 

 a few varieties which present an irregular crystalline figure, 

 some of which can be observed also in Tradescantia virginica, 

 and in the inner layers of the bark of the Lime tree, where 

 they seem very thin and pointed at the extremities, appearing 

 like slices cut longitudinally from the middle of a square 

 prism, which may be imagined to possess a four-sided pyramid 

 at each end. 



Form. With respect to the form of the acicular Raphides, 

 some difference of opinion exists between Raspail and MohPs 

 description; the former asserting that they are six-sided 

 prisms, terminated at each end by a pyramid with the same 

 base ; whilst the latter describes them as right-angled four- 

 sided prisms, vanishing into points. It is a difficult matter 

 to decide between these two opinions, if an entire crystal be 

 the subject of examination ; for, even if magnified 1000 times, 

 the figure is not clearly defined : but, by having recourse to 

 some delicate manipulation, the proper shape can be then 

 ascertained, when it can be shown that neither of the two 

 opinions is correct in all points. 



RaspaiPs reason for considering the acicular crystals to 

 be hexagonal prisms arises from the appearance they present 

 with transmitted light, when some (but not all) are seen to 

 exhibit two dark margins and a streak of light between them, 

 which extends the whole length of the crystal : from this he 

 reasons that its figure is six-sided, the lateral planes reflect- 

 ing the light which impinges upon them, and consequently 

 are seen as darkened margins, whilst the surfaces which are 

 superior and inferior, being in a position favourable for the 

 transmission of the rays, are transparent. This is the argu- 

 ment brought forward in KaspaiPs Organic Chemistry, in 

 favour of the hexagonal figure, but there are no attempts to 

 prove if any other form or position of an acicular crystal 



