550 
APPENDIX. 
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 Raphides will be 
sustained in it in all directions ; and frequently a few can be 
observed erect, which cann;)t 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 crystallisation 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 constrained 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 oc- 
casioned by a rectangular prism resting obliquely, or by an oblique 
or triangular prism, as well as by the hexagonal ; 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 w^ater 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 at- 
tenuated 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 rhombo- 
hedral and rhombic prisms without pyramids,, of the same compo- 
sition, 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 
