be accidentally detached, a partial explosion may occur in
consequence.

307. _Q._--Does the contact of water with heated metal occasion an
instantaneous generation of steam?

_A._--It is found that a sudden disengagement of steam does not immediately
follow the contact of water with the hot metal, for water thrown upon red
hot iron is not immediately converted into steam, but assumes the
spheroidal form and rolls about in globules over the surface. These
globules, however high the temperature of the metal may be on which they
are placed, never rise above the temperature of 205 deg., and give off but very
little steam; but if the temperature of the metal be lowered, the water
ceases to retain the spheroidal form, and comes into intimate contact with
the metal, whereby a rapid disengagement of steam takes place. If water be
poured into a very hot copper flask, the flask may be corked up, as there
will be scarce any steam produced so long as the high temperature is
maintained; but so soon as the temperature is suffered to fall below 350 deg.
or 400 deg., the spheroidal condition being no longer maintainable, steam is
generated with rapidity, and the cork will be projected from the mouth of
the flask with great force.

308. _Q._--What precautions can be taken to prevent boiler explosions?

_A._--One useful precaution against the explosion of boilers from too great
an internal pressure, consists in the application of a steam gauge to each
boiler, which will make the existence of any undue pressure in any of the
boilers immediately visible; and every boiler should have a safety valve of
its own, the passage leading to which should have no connection with the
passage leading to any of the stop valves used to cut off the connection
between the boilers; so that the action of the safety valve may be made
independent of the action of the stop valve. In some cases stop valves have
jammed, or have been carried from their seats into the mouth of the pipe
communicating between them, and the action of the safety valves should be
rendered independent of all such accidents. Safety valves, themselves,
sometimes stick fast from corrosion, from the spindles becoming bent, from
a distortion of the boiler top with a high pressure, in consequence of
which the spindles become jammed in the guides, and from various other
causes which it would be tedious to enumerate; but the inaction of the
safety valves is at once indicated by the steam gau