FOR THE PRODUCTION OF VAPOURS. 237 
a temperature at which the expansive force of the aqueous va- 
pours sufficed to overcome the pressure of the atmosphere, and 
moreover, the cohesion between water and salt, that is the tem- 
perature at which the solution of salt boils. The solution gradu- 
ally became diluted, and at the same time its temperature de- 
creased ; if more salt was then added, it again rose to the boiling- 
point corresponding to the new degree of concentration. 
With pure water, or any other pure boiling liquid, it is quite 
different than with saline solutions. In them the particle to be 
converted into vapour must likewise possess so high a tempera- 
ture that the expansive force of the vapours be not merely suf- 
ficient to overcome the pressure, but likewise the cohesion. The 
vapour formed at this higher temperature being no longer 
attracted by the water present, expands even within the liquid 
corresponding to the pressure under which it is; consequently 
the temperature of boiling water can never be so high as that 
of a solution of salt; nevertheless, however, boiling water has 
constantly a higher temperature than the steam which escapes, 
as was recently demonstrated experimentally by M. Marcet*. 
That, howeve., it is generally but very little higher, is evident 
from the following :—When a liquid is boiled in a vessel the sides 
of which attract tt more powerfully than its parts attract each 
other, these latter will separate more readily from one another 
than from the sides of the vessel. For this reason the liquid in 
such vessels cannot assume any higher temperature than that at 
which the expansive force of the vapour suffices to overcome 
the pressure and the cohesion of the liquid. This temperature 
is the highest which the liquid can acquire under the pressure 
existing, and it would indicate this if it could be boiled in vessels 
formed as it were of the same liquid, or, as already stated, in 
vessels the sides of which retain it everywhere more powerfully 
than the parts attract each other. If, on the contrary, it is 
boiled in a vessel whose sides hold it back with less force than 
its homogeneous parts, only a less force will be requisite to sepa- 
rate it from these sides than from its homogeneous parts, and 
consequently the formation of steam will result in this case more 
easily. Consequently the boiling-point is so much the lower 
the weaker the attraction of the sides or any body presents for 
the liquid. This may, therefore, really be lowered by the sides 
of the vessel, but never raised, at least not above the tempera- 
* Poggendorff’s Annalen, vol. lvii. p. 218. 
VOL. IV. PART XIV. 8 
