MMiscellanies. 385 
dergoing a sudden evaporation, they experience a slow one, and that 
instead of spreading themselves, as is the case at a common tempera- 
ture, they suddenly assume a spherical form like drops of mercury 
on glass. It is also known, that when the metal, in cooling, has 
reached a temperature below that of a dull red heat, the drops of 
water become flattened on their surface, and undergo a sudden evap- 
oration, accompanied by a brisk ebullition. ‘The same phenomena 
have been observed to attend the heating of a considerable quantity 
of water. It has been remarked that, by pouring water, drop by 
drop, into a platina crucible raised to a white heat, the crucible might 
be filled almost completely, and preserved for a long time in this 
condition, without undergoing any very perceptible evaporation ; but 
that when the crucible was removed from the fire, and permitted to 
cool down to a dull red heat, the water suddenly enters into a violent 
ebullition and is rapidly converted into vapor. 
These facts were explained on the supposition, that at a red heat 
the water is not in contact with the sides of the vessel, and that the 
radiant heat, which alone penetrates it, passes through it almost en- 
tirely without heating it, so that the feeble elevation of temperature, 
which results from the slight proportion of heat left in its passage, is 
more than compensated by the evaporation which takes place at the 
surface of the liquid. Not satisfied with this explanation, M. Le- 
chevalier undertook a series of experiments to ascertain the true 
theory of these phenomena. _ He constructed a cylindrical boiler of 
copper, six inches in length, and one inch in diameter, whose sides were 
_two lines in thickness, and pierced at one of its extremities by a round 
orifice, two lines in diameter. After having filled it with water, he 
closed it with a plug-of wood, which was encased by a strap of iron 
in order to secure it in its place. After having left it forty eight hours 
with the orifice downwards, in order that the plug might swell so 
as completely to occupy the opening, he subjected the boiler toa 
red heat by means of a forge. He then withdrew the plug; but no 
vapor escaped, although the boiler contained a certain quantity of 
water. It is necessary to unplug the boiler while it is still red hot, 
‘and to do it without loss of time; for when it has fallen to a tempe- 
rature below redness, the liquid it contains is rapidly converted into 
steam accompanied with detonation, and giving at the same time a 
recoiling motion to the boiler. In an experiment where the cooling 
was suffered to take place upon the floor of the forge, the explosion 
