BOILING OF LIQUIDS IN GLASS VESSELS. 
331 
cessation of ebullition takes place, the temperature will rise 
and will sometimes reach 220^. This will be followed by 
a sudden and violent evolution of steam, constituting the 
phenomenon of bumping, by which the accumulated heat 
is disengaged, and the temperature of the liquid reduced 
again to its usual boiling point. The violence of these ex- 
plosions will, after some time, become so great as to cause 
the projection of a considerable part of the liquid, at once 
out of the flask, endangering in no slight degree the safety 
of the operator. This may be taken as a forcible illustra- 
tion of the phenomena which accompany the bumping of 
certain liquids when boiled in glass vessels; but the effects 
are seldom so marked as in this case. 
Several attempts have been made to explain these phe- 
nomena, but without much success. They have been as- 
cribed to a modification of the forces oi cohesion and adhe- 
sion in the liquid, caused by the expulsion of atmospheric 
air during the process of ebullition ; Donne having shown 
that some liquids acquire greatly increased cohesive and 
adhesive force when air is wholly excluded fro«m them. It 
may, indeed, be readily admitted that the immediate cause 
of the bumping of liquids is some modification of the forces 
of cohesion and adhesion ; but that this condition is induced 
simply by the expulsion of air, is a position that will not be 
so readily assented to. 
The effect appears to depend, partly on the condition of 
the liquid in which it occurs, and partly on that of the 
vessel containing the liquid. 
It was found by Marcet, and I have repeatedly verified 
the results, that if pure water be put into a glass flask, 
which has previously had oil of vitriol heated in it, the 
boiling-point of the water will sometimes rise as high as 
220° ; while, on the other hand, if the inner surface of the 
flask be coated with a thin film o{ shellac, the boiling-point 
of pure water heated in it will be sensibly below 212°. In 
