Art of making Gun-Flints. 87 
that the upper part of a flint is always of a better quali- 
ty than the lower. Flints are considered as imperfect or 
intractable when naturally deprived of any of the exter- 
nal characters before indicated, or when injured by long 
exposure to the air. Most of the masses are subject to 
have whitish opaque spots and a kind of knots where 
the material is harder than in the other part of the stone. 
When these accidents are too abundant, the stone is re- 
jected as useless. 
The physical characters are as follow Specific gra- 
vity of the white silex pyromachus from the banks of 
the Cher proved to be 26041 ; that of the blackish kind 
from the chalk isles of Laroche Guion was 25954. In 
this respect it does not differ essentially from the other 
varieties of silex, of which the specific gravities are 
usually between 26100 and 25900. Hardness a little 
superior to that of jasper, but inferior to that of agate 
and chalcedony. It is nearly the same as that of the 
other common flints. Brittleness. It is more brittle than 
most other siliceous stones. The light coloured is more 
so than the darker ; these last being rather more scintil- 
lant, and wear away the hammer more quickly. When 
two pieces of the silex pyromachus are strongly rubbed 
together, they emit the peculiar well known smell of si- 
liceous stones, but in this variety it is more strong than 
in any other. 
Chemical characters. Action of the air. 
The silex pyromachus, deprived of its natural coating, 
and exposed for a long time to the changes of the atmos- 
phere, acquires a second white friable coating, consist- 
ing of the silex reduced to powder $ and even its inter- 
nal part loses its greasy appearance and semi-transpa- 
rency, so as to become whitish. In this case the specific 
gravity, which was 2595% becomes 2575% consequently 
it has lost 200 parts of the weight it possessed at first. 
