6 Si 
sun is nothing else than its motion from a 
centre towards a circumference, for it does 
not seem capable of any proper expansion 
by a separation of its parts like any other 
fluid. Hence, when it begins to expand in 
this manner, the motion is propagated through 
it, with a velocity far exceeding that of any 
other fluid whatever. Thus, even when the 
quantify is excessively small, as when an 
electric spark is sent through a glass full of 
water or of oil, the expansion is so violent as 
to dissipate the glass into innumerable frag- 
ments, with great danger to the by-stan ders. 
In violent lightning, when the electric fluid 
is much concentrated, the strength of the 
explosion is proportionable to the quantity. 
Every one has heard of the prodigious effects 
of lightning when it happens to strike build- 
ings, trees, or even the most solid rocks ; 
and in some cases, where the quantity of elec- 
tricity is still greater than in any flash of 
lightning, we hear of still more tremendous 
consequences ensuing. Dr. Priestley gives an 
instance of a targe flre-ball rolling on the sur- 
face of the sea, which, after rising up to the 
top-mast of a ship of war, burst with such 
violence, that the explosion resembled the 
discharge of hundreds of cannons tired at 
once. Great damage was done by it; but 
there is not the least doubt that most of its 
force was spent on the air, or carried down 
to the sea by the mast and iron-work of the 
ship. Indeed, considering that in all cases a 
great part of the force of electric explosions is 
dissipated in this manner, it may justly lie 
doubted whether they can be measured by 
any method applicable to the mensuration of 
other forces. Even in artificial electricity 
the force is prodigiously great, insomuch that 
Dr. Van Marum calculated that of the great 
battery belonging to the machine in Teyler’s 
museum to lie upwards of 900 pounds. 
Whenever the electrical fluid acts like 
common lire, the force ot the explosions, 
though exceedingly great, is capable of men- 
suration, by comparing the distances to 
which the bodies are thrown with their 
weight. This is most evident in volcanoes, 
where the projections of the burning rocks 
and lava manifest the greatness of the power, 
at the same time that they afford a method 
of measuring it. By means of the fire 
which kindles the volcanoes, the aerial fluids 
are suddenly restored to their elastic state ; 
and not only so, but their natural elastici- 
ty is greatly augmented, so that the explo- 
sions take place with great violence. The 
case is the same with gunpowder. The 
reason of the extreme quickness of the ex- 
plosion of gunpowder is, that it takes fire so 
readily bv the intimate mixture and combus- 
tibility of all tire materials. In volcanoes tire 
explosions likewise follow one another very 
quickly, and are by no means inferior in 
strength to those of gunpowder; but here the 
quantity of vapour makes up for the compa- 
rative slowness with which it is affected by 
the heat. Thus, though we could not by any 
other means contrive to fire cannon in such 
quick succession as we can do with gun- 
powder, yet in the huge furnace of a volcano 
the elastic matter is supplied in such quan- 
tities, that the explosions are in a manner 
unremitting; and even in ordinary experi- 
ments the confinement of aerial vapours 
has often occasioned violent explosions in 
chemical vessels. Carbonic acid gas though 
heavier than most, of the other aerial fluids. 
EXPLOSION. 
yet by its expansion contributes equally tc 
the force of the explosion, as is evident, in 
that of pulvis fulminans. This is com- 
pounded of sulphur, saltpetre, and salt of 
tartar. The latter we know contains much 
carbonic acid gas ; and it is probable that the 
violence of the explosion is occasioned by 
this air; for it is said that the greater quan- 
tity of it the alkaline salt contains, the 
greater force does it explode with. 
Next in strength to the aerial vapours are 
, those of aqueous and other liquids. '1 he 
most remarkable effects ot these are ob- 
served In steam-engines ; but there is one 
particular case from which it lias been in- 
ferred that aqueous steam is vastly stronger 
than the flame of gunpowder. This is when 
water is thrown upon melted copper ; for here 
the explosion is so strong as almost to exceed 
imagination ; and the most terrible accidents 
have been known to happen trom such a 
slight cause as one of the workmen spitting 
in the furnace where copper was melting. 
Here, however, it is most probable that a 
decomposition of the water takes place, and 
on this supposition, the phenomenon is ea- 
sily solved. The water being thrown in sub- 
stance upon the melted copper, is decom- 
posed by the violent heat ; and one part of 
it adheres to the metal, thus converting it 
into a kind of calx, while the other is eon- 
verted into inflammable air, which expand- 
ing suddenly, throws the melted metal about 
with the greatest violence by means of its re- 
action. 
In order to understand the manner in 
which this is accomplished, we' must con- 
sider some of the principles of gunnery laid 
down by Mr. Itojffrts. One ot these is, 
that though the air, in cases of ordinary 
velocity, makes no great resistance, it is 
far otherwise where the velocity of the 
moving body becomes very great. In all 
cases of explosion also there is in the first 
instance a vacuum made by the exploding 
fluid; and consequently the weight ot the 
atmosphere is to be overcome, which amounts 
to about 15 pounds on every square inch of 
surface. Supposing the surface of the ex- 
ploding fluid then, on that ot melted copper, 
to contain an area of four square inches, it 
meets with a resistance of 60 pounds from 
the atmosphere, and consequently commu- 
nicates an equal pressure to the fluid metal.. 
Even this must of consequence throw it 
about, unless the same pressure was exactly 
diffused over every part of the surface. But 
much more must this effect be increased by 
the immense velocity with which the fluid 
moves, and by which the resistance of the at- 
mosphere is augmented in a prodigious de- 
gree. The elastic fluid generated is then 
confined not only by the fluid metal and 
sides of the furnace, but by the air itself, 
which cannot get out of the way, so that the 
whole resembles a cannon closed at the 
mouth, and filled with inflamed gunpowder. 
Iieuce not only the melted metal, but the 
furnace itself, and the adjacent walls of tire 
building, are hurried off as they would be 
by the firing of a great quantity of gun- 
powder in a small space, and which is well 
known to produce analogous effects. 
Dr. Black, in explaining the phenomenon 
in question, supposes that the mere heat of 
the metal applied to the aqueous steam pro- 
duces the explosion ; and in proof of this, 
alleges that copper imbibes a greater quan- 
tify of heat during fusion than any other mc~ 
tal. Aqueous steam, however, seems to be. 
too slow for producing such sudden and vio- 
lent effects. Explosions, it is true, will be 
occasioned by it, but then it must be confin- 
ed for a very considerable time, whereas the 
effects of water thrown upon melted copper 
are instantaneous. 
It must be observed, that in all cases 
where a very hot body is thrown upon a small 
quantity of water in substance, an explosion 
will follow; but here the water is confined, 
and suddenly rarefied into steam, which can- 
not get away without throwing off the body 
which confines it. Examples of this kind 
frequently occur where masons, or other me- 
chanics, are employed in fastening cramps 
of iron into stones ; where, if there happens to 
Be a little water in the hole into which the 
lead is poured, the latter will fly out in such 
a manner as sometimes to burn them severe- 
ly. Terrible accidents of this kind have 
sometimes happened in foundries, when large 
quantities of melted metal have been poured 
into wet moulds-. In these cases, the sudden 
expansion of the aqueous steam lias thrown 
out the metal with violence; and if any de- 
composition has taken place at the same 
time, so as to convert the aqueous into an 
aerial Vapour, the explosion must be still 
greater. 
To this last kind of explosion we must 
refer that which takes place on pouring cold 
water into boiling or burning oil or tallow. 
Here the case is much the same whether we 
pour the oil on the water, or the water on the 
oil. In the former case, the water which 
lies at the bottom is rarefied into steam, and 
explodes; in the latter, it sinks down through 
the oil by its superior specific gravity, and 
explodes as it passes along.. In either case, 
however, the quantity of aqueous fluid must 
be but small in proportion to that of the oil 
a very great quantity would put out the 
flame, or destroy the heat, in whatever way 
we applied it. 
Another kind of exp losion is that which 
takes place in solid substances, where we can 
scarcely suppose either aqueous or aerial va- 
pours to be concerned. The most remark- 
able of these are the volcanic bombs men- 
tioned by sir William Hamilton in the great 
eruption of Vesuvius in 1779'. They were 
large pieces of lava, which burst in pieces like 
bombs as they fell to the ground; but he does 
not inform us whether their bursting was at- 
tended with any great violence or.net. In- 
deed, amidst such scenes of horror, and the 
continual tremendous explosions of the vol- 
cano, smaller phenomena of this kind would 
probably be overlooked. 
The only other kind of explosion we have 
to take notice of, is that produced by hydro- 
gen and oxygen gases, when mixed together 
and set on lire. This differs from any of 
those hitherto considered, because in reality 
there is an absolute condensation, rather tinm- 
an expansion, throughout the whole of the- 
operation ; and could the airs be made to 
take fire throughout their whole substance 
absolutely at the same instant, there would be 
no explosion, but only a sudden production 
of heat. From this cause also is derived a 
very singular phenomenon, taken notice of 
by Dr. Priestley in his experiments on that 
subject. Having inclosed several quantities 
of these gases in a copper vessel, firing them 
