APPENDIX. 
549 
some difference of opinion exists between Raspail and Mohl’s de- 
scription ; 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, vanish- 
ing 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. 
“ Raspail’s reason for considering the acicular crystals to be hex- 
agonal 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 ex- 
tends the whole length of the crystal : from this he reasons that its 
figure is six-sided, the lateral planes reflecting the light which im- 
pinges upon them, and consequently are seen as darkened margins, 
whilst the surfaces which are superior and inferior, being in a po- 
sition favourable for the transmission of the rays, are transparent. 
This is the argument brought forward in Raspail’s 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 could not 
present a similar aspect, which is the case, as will be shown here- 
after. 
“ 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 sur- 
mounted by a pyramid. Mohl’s idea of the shape certainly ap- 
proximates the truth more than Raspaifs ; but it can be shown, 
though the acicular 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 tour 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,^o o 
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 
N N 3 
