236 MAGNUS ON THE FORCE REQUISITE 
higher temperature than corresponds to the expansive force of 
the steam at the temperature existing. 
When the liquid only evaporates it will not be possible to ob- 
serve whether it has a higher temperature than the liberated 
steam, because the evaporation proceeds solely from the surface, 
and the deeper portion of the fluid may possess quite a different 
temperature. Nor even at the surface will the fluid be much 
warmer than the vapour, because there, where it is in contact 
with the air, its parts are not held back by exactly as great a 
force as in the interior, where every particle is surrounded on all 
sides with particles of the same nature. This is the reason why 
the above-mentioned phzenomenon never occurred when a bub- 
ble of air, however small, was situated above the water. 
On the contrary, during ebullition, when the formation of 
steam proceeds from the bottom of the vessel, the liquid will 
always be warmer than the evolved steam. This is most evident 
in boiling saline solutions: in these the cohesion between water 
and salt is greater than between the parts of the water among 
one another, and consequently a higher temperature is required 
to overcome this cohesion ; moreover, the steam, as long as it is 
still in the saline solution, retains an expansive force correspond- 
ing to this high temperature, otherwise it would again become 
fluid by the attraction of the salt. This is already manifest from 
the fact, that on introducing a salt, for instance soda into the 
water contained in the vacuum of a barometer, its expansive 
force is immediately diminished*. But to prove this in a still 
more convincing manner, I introduced some water into the tube 
abd, which was used for the production of the steam, and ex- 
posed it to a temperature of 100°C. The steam which had thus 
originated possessed an expansive force equal to the pressure of 
the atmosphere ; but if I now conyeyed into the water a solution 
of common salt, the expansive force of the steam decreased im- 
mediately by several inches of mercury. 
In the same manner I passed the vapours of boiling water, 
which had a temperature of 100° C., into a solution of common 
salt, which was maintained at 100°C. on a water-bath. The 
result was that the solution of salt became heated to 107°C., 
although the heating body, viz. the aqueous vapours, were not 
hotter than 100° C.; but these were absorbed by the solution of 
salt and yielded their latent heat to this until they had attained 
* Biot, Zraité de Physique, i. p. 285. 
