Natural History.— Mineralogy. 191 
carried away by the percolating water, and leave the marble, 
for the first time, of a pure snow-white colour*. Whole rocks 
appear to be changed in this way by a chemical process. In il- 
lustration of this, it is remarked, that the rock of the old quar- 
ry named Di Poggio Silvestro, was formerly useless, but du- 
ring the lapse of time has become pure white marble : and fur- 
ther, that the different kinds of Carrara marble change in the 
course of time, and generally become purer. 
24. On the Formation of Rock-Crystal. — Spallanzani re- 
marks, that the numerous beautiful rock-crystals in the cavities 
of the Carrara marble, continue still to form, and from a pure 
acid fluid. Ripetti, in his tract 6 4 Sopra TAlpe Apuana ei 
Marmi di Carrara, 1811,” adduces some new observations in 
favour of this opinion, and tells us, that, on opening a drusy 
cavity, there was found 1 \ lb. of the above fluid, and among 
the solid crystals, a soft mass the size of the fist, which, on ex- 
posure to the air, hardened into a substance having the charac- 
ters of calcedony. According to Daubuisson and Beudant, the 
opal of Hungary is sometimes found in a soft state. 
25. New Analysis of Heliotrope. — According to Brandes and 
Firnhaber it contains, Silica, 96.25 ; Oxide of Iron, 1.25 ; Alu- 
mina, 0.83 ; Water, 1.05, = 99.58. 
26. Colouring of Marble.— The ancients were acquainted 
with a mode of communicating various lively and very durable 
colours to marble, hitherto unknown to us. Lately Ripetti, in 
his tract published at Florence in 1821, entitled “ Sopra l’Alpe 
Apuana ei Marmi di Carrara,” throws some light on this sub- 
ject. Among the different sorts of marble of Carrara, that of 
the quarry Di Betogli is particularly fine granular, and pure 
snow-white, but at the same time uncommonly changeable. 
When exposed to the air for some months, it loses a portion of 
its water and its carbonic acid, becomes brittle, and specifically 
lighter, and in two years suffered a loss of 7 per cent. The 
workmen, on account of its property of easily falling in pieces, 
name it burnt marble (M. concotto or Salone). This evil is 
cured in part by rubbing it with Euphorbia characias, L. ; but 
it is principally employed to colour the marble, which absorbs the 
* The workmen say, II marmo si purga. 
