ON THE BOILING OF MIXED LIQUIDS. 237 
is equal to the atmospheric pressure. Hence the temperature 
of the boiling point continually changes, and will not be con- 
stant, except in the case where the quantities of the gas 
which escape by the boiling hold the same proportion as the 
liquids which remain behind. 
This takes place, for example, in concentrating alcohol by 
distillation. When we arrive at a certain proportion of alcohol 
and of water, the quantities of the vapors of the two liquids are in 
the same proportion as the quantities of the remaining liquids; 
then whatsoever passes by distillation has the same compo- 
sition as that which remains; no further concentration of the 
alcohol takes place. If we add to the mixture some substance 
very attractive of water, such as potassa or the chloride of 
calcium, we diminish by this the tension of the vapor of the 
water; and the temperature remaining the same, the vapors 
of the mixture consist almost all of the vapors of the alcohol, 
which alone passes over in the distillation. 
In the experiments upon the boiling of two liquids which 
do not mix, there often takes place strong explosions, when 
the less volatile liquid occupies the upper part. These ex- 
plosions are so violent when we boil the volatile oils with water, 
as to render it necessary to stop the operation to avoid the frac- 
ture of the vessel. If we introduce the bulb of a thermometer 
into the inferior liquid, but as near as possible to the surface of 
junction of the two liquids, the thermometer will be seen to 
rise immediately before the explosion. The liquids are then 
perfectly tranquil, they do not boil, there is no distillation, 
and the superior forms a continuous mass upon the top of the 
inferior liquid. In this state the thermometer often takes a 
temperature of 3°, 5°, or even 10° C. above the boiling point 
of the mixture. Suddenly an explosion takes place, the upper 
mass is broken, a great quantity of vapor is disengaged, and 
the thermometer falls to the temperature of ebullition of the 
mixture, at which it remains as long as the vapors of the in- 
ferior liquid can freely escape through the superior. 
It thus appears, that the coherence of the particles of the 
upper liquid may be so great as to hinder the formation of 
vol. iv. — no. in. 31 
