THE ROYAL ARTILLERY INSTITUTION. 
277 
The fact that combustible, and especially inflammable solid sub¬ 
stances, if of sufficiently low specific gravity, and reduced to a 
sufficiently fine state of division to allow of their becoming and 
remaining for a time suspended in air, may furnish mixtures with 
the latter which partake of explosive character, scarcely needs to be 
pointed out. The ignition of a particle of silch a substance, surrounded 
by atmospheric oxygen, will, under these conditions, at once communi¬ 
cate to others immediately adjacent to it; and if the particles of 
suspended solid matter be sufficiently numerous and finely divided, the 
ignition will spread throughout the mixture with a rapidity approaching 
that of a mixture of inflammable vapour and air, the development of 
gaseous products and heat being sufficiently rapid and considerable to 
produce explosive effects, which may even be of violent character, their 
violence being regulated by the nature and inflammability of the solid 
substance, the proportion and state of division in which it is dis¬ 
tributed through the air, the quantity of the mixture, and the extent 
of its confinement. 
Explosions of an accidental nature produced in this way are believed 
to have occurred in connection with operations in the chemical laboratory ; 
but it was scarcely to be expected that the first clearly authenticated 
cases of any importance should have arisen out of the apparently harm¬ 
less operation of grinding corn. 
That a mixture of very fine flour and air will ignite with a flash when 
light is applied to it, and produce in a very mild form the species of 
explosion observed on applying a light to lycopodium suspended in air, 
is not very difficult of demonstration ; but it is not easy to realise the 
possibility of the production of violent explosive effects by the ignition 
of such a mixture, even upon a very large scale, though the vapidity of 
its ignition be accidentally favoured by the warmth of the atmosphere. 
Cotton mills have been known to be rapidly fired by the ignition of 
cotton particles suspended in the air ; but, compared with flour, cotton 
is very combustible. Flour, when absolutely dry, would contain only 
about half its weight of carbon, and about 6 per cent, of hydrogen; the 
remainder consisting of nitrogen and mineral substances—constituents 
which, by absorbing heat instead of contributing to its development, 
must tend to reduce the rapid combustibility of the substance. Yet the 
possibility of very serious calamities arising out of the accidental ignition 
of a mixture of flour-dust and air has been but too conclusively demon¬ 
strated. 
On the 9th July, 1872, the inhabitants of Glasgow were startled by 
an explosion which was heard to a considerable distance, and occurred in 
some very extensive steam flour-mills, of which the front and back walls 
were blown out, while the interior was reduced to ruins, and speedily 
enveloped in flame, destroying the remainder of the extensive buildings. 
Several persons were killed, and a number of others were severely burned, 
or injured by the fall of masonry. That the explosion was not occasioned 
by the steam-boilers employed as motive power in the mill was soon 
made clear ; and by the evidence of persons employed in the mill at the 
time of the explosion, its origin was conclusively traced to the striking 
of fire by a pair of millstones, through the stopping of the “ feedor 
supply of grain to them, and the consequent friction of their bare sur- 
