56 
THE PRINCIPAL FLOOR. 
exhibited. The series commences with examples of the 
incrustation of one mineral by another, forming pseudomorphs 
by simple mechanical investment. By far the larger number of 
pseudomorphs, however, are due to chemical alteration ; the 
action consisting”, as here illustrated, either in the loss of a con- 
stituent ( e.g ., native copper in form of cuprite, by removal of 
oxygen) or in the gain of a constituent {e.g., malachite after 
cuprite, by addition of carbonic acid), or in the exchange of one 
constituent for another {e.g., pyrites converted into limonite, by 
loss of sulphur, and gain of oxygen and water). Finally, the 
change may be so profound that the secondary mineral is 
entirely distinct in composition from the one which it replaces, 
as seen in such a case as that of the replacement of calcite 
(carbonate of lime) by quartz (silica). A few examples of the 
mineralisation of organic structures form a link between this 
case and the objects in the adjacent one, No. 34. 
Volcanic Products. Case No . 39. 
This Table Case contains a collection of minerals and lavas, 
chiefly from Vesuvius and Etna, which were formerly exhibited 
with the rock-specimens in the Upper Gallery, but were removed 
some time ago, in consequence of lack of space. Each specimen 
is furnished with a label, recording the name of the substance 
and its locality. 
Precious Metals, Aluminium, &c. Case No. 35. 
A rather miscellaneous assemblage of metallurgical specimens 
is exhibited for convenience in this case. 
The series of precious metals includes not only specimens 
bearing on the production and applications of gold and silver, 
but also samples of such metals as platinum and palladium. 
Platinum was discovered in 1735, by a Spanish traveller, who 
obtained it from certain river- deposits in New Granada. It was 
afterwards discovered, also in alluvial detritus, in several other 
localities, especially, in the Ural Mountains. After Wollaston, in 
the early part of the century, had suggested a method of treating 
the metal, so as to render it useful in the arts, the Russians 
turned their attention to platinum as a metal of economic 
importance, and soon afterwards issued platinum coins. The 
metal is shown in the finely-divided form of platinum black, in the 
cellular condition of spongy platinum, in massive form as fused 
by the oxy hydrogen blow-pipe, and in the state of foil aud 
wire. 
Aluminium was obtained in the condition of a powder, by 
Wohler, as far back as 1827, but it was not until Henri Sainte- 
Claire Deville produced it, in 1854, in the form of bars, that the 
metal acquired any industrial importance. Aluminium may be 
readily obtained from Cryolite — a mineral found in Greenland, 
