298 Dr Gosse an the Baths of St Filippo^ and the formation 
this position they receive an equable and continued dash of wa- 
ter, in form of an artificial rain, during which, the deposition 
takes place, and the water rapidly escapes,— conditions all of 
which are necessary to obtain a solid and semitransparent cast 
like marble. A reduction of temperature in the chamber is al- 
so required to favour the deposition, but a current of cold air is 
not admitted, both because it would prevent the equable distri- 
bution of the water, and render the cast itself brittle. The 
chamber,, therefore, has openings only in its upper part, to per- 
mit the rapid escape of the contained vapours. The moulds are 
thus left exposed to the action of the water, from ten or twelve 
days to two or three months, according to the size and thick- 
ness, or inequalities of the cast. A cast of 2 or 3 inches 
diameter, requires a thickness of at least 2 or 3 lines ; one of 
6 inches, 4 lines ; of 1 foot, 8 lines. The casts are easily de- 
tached from the moulds, their edges clipped, and their front 
surface rubbed first with a subtile powder and water, and then 
polished with a paste composed of soap and calcined sheep 
bones. They acquire thus a smooth shining appearance, and 
their semitransparency is improved, while the back surface or 
reverse remains dull and rouoh. 
By an ingenious variation of the process, the artist is able to 
form casts of differently coloured marbles, presenting thus a 
white figure in relief em a darker ground, and vice versa. This 
is done by covering the plain parts of the mould with a thin 
paste, so that the deposition shall be made only into the sunk 
parts. When these parts are filled, the paste is detached, and 
the whole exposed to a water previously coloured, by dissolving 
a colouring substance in the last of the three pits. In this 
manner, while the raised parts have the ordinary colour, those 
of the ground exhibit that of the colouring matter employed. 
Metallic colours are preferred to vegetable, and oxides of iron, 
in particular, give their appropriate colours, without impairing 
the solidify of the stone. 
Lastly, Impressions of engravings may be transferred to 
stones thus artificially prepared. The subject is first engraved 
on a metallic plate, and a little printer'^s ink passed over, so as 
to colour hghtly the furrows. The plate is exposed to the 
action of the water in the manner already described, and 
when the cast is removed, the engraving is represented by black 
