N ON -METALLIC MINERALS. 
101 
Steel is commonly hardened by being plunged, when red hot, 
into water ; and it is afterwards tempered by being re-heated 
until the surface assumes a tinge varying from a light straw 
colour to a deep blue. An interesting series of polished 
specimens of steel at the different colours for tempering will be 
found in this Case. The tints appear to be due to extremely 
thin oxidised films. 
Attention should also be called to a series illustrating the 
production of steel and malleable iron from fusion of pig-iron 
and haematite by Olay's process. There will also be found here 
several examples of Musket's tool steel , and numerous fragments 
of steel shot and armour plates. In connexion with steel 
manufacture it should be remarked that somes fine examples 
of Bessemer steel will be found in the lower compartment of 
Pedestal-case No. 13. A sample of Whitworth steel, cast under 
pressure to ensure soundness in the ingot, and a specimen of 
Siemens' steel will also be found here. 
The metallurgical series, though far from complete, has out- 
grown the six table -cases to which it was formerly restricted, 
and it has been found necessary to devote several cases in the 
Central Area to this department of the collection. It is, however, 
needless to allude further to these cases, since they have already 
received brief notice at pp. 61, 63. 
HOBSESHOE CASE. 
“ Non-Metallic ” Minerals and their Applications. 
Formerly the collection of non-metallic minerals was confined 
to a large glass case, of horseshoe form, running round a 
similarly-shaped space in the centre of the room, which admits 
light to the Lecture Theatre situated below. Many years ago, 
however, it was found necessary to extend this case by adding 
others, which are supported on the cornice of the balustrade 
surrounding the opening from the Principal Floor to the Lower 
Hall. The case is divided into 28 compartments or separate 
sections, each equivalent to an independent case. The 28 cases 
form a closed series, somewhat O-shaped ; but though the col- 
lection has thus lost its horseshoe form, its original name is 
still conveniently retained. 
In the Horseshoe Case the visitor will find all the gems and 
ornamental stones, the spars, and earthy minerals. These are 
grouped together under the general term of te non-metallic,” in 
conformity with popular usage and practical convenience ; but 
visitors will bear in mind that whilst many of the species here 
exhibited, such as the diamond, are strictly destitute of metal, 
by far the larger number contain some metallic element, prin- 
cipally, however, the rarer metals of low density, such as potas- 
sium and aluminium, and but rarely the common heavy metals 
of commerce. 
