108 ON THE SPICULA IN THE WOOD OF THE WELWITSCHIA. 
some approximation to the measure of the angles by means of a 
doubly refracting prism fitting on to the eye-piece of the miero- 
scope; the mean of several measures gave 106? nearly as the value 
of the obtuse angle (that of cale-spar being 105? 5). With regard 
to the prismatic-looking crystals occasionally seen, I found several 
which, examined by favourable light, presented the figure a, b. 
This form of rhomboid resembles that which was called by 
Haüy the “ inverse," a peculiarity of which is, that its plane 
angles measure the same as the dihedral angles of the primary 
rhomboid. 
The crystals of the so-called crystallized sandstone of Fontaine- 
bleau (which are carbonate of lime containing sand) are instances 
of this form. 
As it appears from what I have stated that these crystals con- 
sist of carbonate of lime, the question remains, What is it that 
protects them from the action of acids ? 
I have made some attempts, but very imperfect, to throw some 
light on this question. 
I found that alcohol and ether, even when heated, had not the 
power of removing the protecting substance. ; 
But I found that if, after digesting with ether, the spicula were 
boiled in solution of caustic soda and subsequently immersed in 
dilute hydrochloric acid, the crystals disappeared, and their places 
were occupied by amorphous patches. 
There is one objection that may perhaps be taken to the view I 
have adopted as to the nature of the crystals, which I may as well 
notice. It may be thought that in the plant the lime was united 
to some organic acid, say the oxalic. But I think it will be ad- 
mitted that, putting aside the agreement in form with carbonate 
of lime, the fact of the crystals being unaltered in form by burn- 
ing, and retaining the power of acting on polarized light, is fatal 
to such an hypothesis. 
I remain, 
Dear Doctor Hooker, 
Very truly yours, 
Pu. YORKE. 
Joseph D. Hooker, Esq., M.D., ác. dc. 
