LYDIAN STONE FLINT. 53 
There is a kind of sand which is naturally mixed 
with day, and has the name of Founder's Sand, from 
its being chiefly employed in the formation of moulds 
to cast metals in. At Neuilly, in France, there is a 
bed of perfectly transparent and crystalline sand. 
Each grain, when examined with a magnifying glass, 
is seen to consist of a perfect six-sided prism, termi- 
nated by two six-sided pyramids. 
The uses of the different kinds of Sandstone will be 
enumerated in the account of the rocks ('267, 268). 
89. LYDIAN STONE is a kind of flinty-slate, of greyish 
or velvet-black colour, not quite so hard as flint, opaque, and 
about twice and a half as heavy as water. 
It is usually massive, and, internally, has a glimmering 
appearance. 
This mineral occurs in beds in primitive clay-slate 
(257) ; and is found in Bohemia and Saxony, and also 
in the Pentland hills near Edinburgh. It was first 
noticed in Lydia, whence it derived its name. 
It is sometimes used as a touchstone to ascertain the 
purity of gold and silver. This was its use among the 
ancients. The metal to be examined is drawn along 
the stone so as to leave a mark, and its purity is judged 
by the colour of the metallic streak. A good touch- 
stone should be harder than the metals, or metallic 
compounds to be examined ; if softer, the powder of 
the stone mixes with the trace of the metal and ob- 
scures it. A certain degree of roughness on the sur- 
face of the best stone is also requisite, that the metal to 
be tried may leave a trace or streak sufficiently distinct. 
It must not, however, be too rough, otherwise the par- 
ticles of the metal will be hid amongst its inequalities, 
and no distinct trace will be formed. The touch-stone 
should also be of black colour, as this tint shows the 
colour of the streak better than any other. 
90. FLINT is a peculiarly hard and compact kind of stone, 
generally of smoke-grey colour, passing into greyish white, 
