FOR THE PRODUCTION OF VAPOURS. 239 
lower the boiling-point of the liquid. This lowering, and conse- 
quently the temperature of the boiling-point, varies not merely 
according to the different nature of the vessel and its purity, but 
even every particle of dust that falls into the liquid alters the 
relation of adhesion, and consequently the boiling-point. But 
if the liquid be protected against dust, and the bodies present 
be preserved from every change, the boiling-point, nevertheless, 
varies constantly ; for as soon as the temperature is sufficiently 
high to overcome the cohesion at any one place and form vapour, 
this expands even in the liquid, and from its assuming here the 
expansion and temperature corresponding to the pressure, it 
cools the liquid. If this boil at a temperature several degrees 
higher than that of the escaping vapour, the expansion of these 
latter will be considerable, and the cooling of the liquid produced 
each time correspond to this expansion, upon which a consider- 
able time elapses until its temperature is sufficiently high to 
overcome the cohesion. It is on this account that a sensitive 
thermometer constantly changes in such a boiling liquid, and 
the evolution of the vapour is always connected with a kind of 
thumping. If, on the contrary, the boiling-point has been ren- 
dered by the presence of pulverulent substances or wires but 
slightly higher than the temperature of the escaping vapour, a 
very inconsiderable expansion of the vapour occurs, and while 
the boiling-point remains nearly constant, every sudden boiling 
up and thumping is avoided. 
There does not exist an older physical experiment, nor one 
more frequently repeated than that of boiling water; but never- 
theless, what occurs in the process was not sufficiently known, 
and even now much still remains unexplained. 
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