102 RAPHIDES FORM. [BOOK i. 



strained to assume this form, as being fitted to fill up certain 

 spaces better than one of the oblique or rectangular shape. 



This being the observed figure, it is not difficult to account 

 for the black margins observed in certain crystals, which may 

 be occasioned by a rectangular prism resting obliquely, or 

 by an oblique or triangular prism, as well as by the hexa- 

 gonal; for, as- the sides of either are not in the plane of 

 direction of transmitted light, the rays are reflected, and that 

 portion which reflects the light appears dark. 



Besides these methods of determining the form of acicular 

 raphides, if a little alcohol be added to water containing 

 them, and examined immediately after by the microscope, 

 the crystals can be often seen to roll over and over, and 

 some of them will, whilst revolving, present alternately a 

 pointed and then a flat extremity, just as if a wedge were 

 revolving on its axis ; showing that such crystals are four- 

 sided, but two of the sides only attenuated to form the 

 pointed appearance, the other two suffering no diminution of 

 their breadth at any point. 



That the four-sided prism is the ultimate figure of these 

 minute crystals is rendered more probable by the occurrence 

 of rhombohedral and rhombic prisms without pyramids, of 

 the same composition, in the same plant, but of much greater 

 widths ; and there can be no doubt that these latter bodies 

 and the acicular are two modifications of crystal of the same 

 substance. The most decided proof of their being four-sided 

 is obtained by pressing lightly on the piece of glass which 

 covers them, whilst examined under the microscope, when 

 those which appear six-sided instantly appear four-sided, 

 owing to the square crystal resting obliquely : this can be 

 seen in the minute crystals of Squill, or in the large square 

 ones of Quillaja saponaria. 



The rounded masses, which may be termed Conglomerate 

 Raphides in opposition to the acicular variety, seldom present 

 more than the pyramid of each little crystal composing them ; 

 but in a few cases, where an opportunity is afforded of 

 examining the prism, it can be seen to be rectangular and 

 terminated by a four-sided pyramid. 



Classes of Plants in ivhich they are found. No division 



