ON THE EOILINGr OF MIXED LIQUIDS. 
233 
more volatile, it requires a certain heat, from which the 
vapors of the mixture take a temperature below the boiling 
point of the more volatile liquid; that is to say, the tempera- 
ture at which the sum of the tensions of the vapors of the two 
liquids is equal to the pressure of the atmosphere. Gay 
Lussac asserts that the upper portion may have a lower tem- 
perature; but if we consider the equalization of temperature 
which should take place at the end of some time, by the ascent 
of the inferior portion, or by those parts which are carried up 
by the bubbles to the surface, we can comprehend that the 
surface should not have a temperature lower than the boiling 
point of the more volatile liquid. In my experiments, the 
temperature of the upper portions of the mixture was a little 
higher than the boiling point of the more volatile liquid, 
when there was a covering of three inches of water upon a very 
small quantity of carburet of sulphur. 
When we use a still, the vapors of the two liquids pass 
over at a constant temperature into the refrigerent, and are 
condensed; but when there does not remain sufficient of the 
more volatile liquid to pass in drops, the tension of the vapors 
diminish, and is no longer sufficient to overcome the pressure 
of the atmosphere. The vapors do not now pass into the re- 
frigerent, and distillation ceases, although some of the vapors 
of the more volatile liquid still remain. 
To know the temperature of the vapor of two liquids which 
do not upon each other, it is desirable, knowing the boiling 
point of the two liquids, to find an expression of the tem- 
perature at which the sum of the elasticity of the vapors of 
the two liquids should equal the pressure of the atmosphere. 
For this it is necessary to know the law according to 
which the elasticity of the vapor of different liquids 
change with the temperature ; but this law is hardly 
known for water. In the meanwhile we may suppose 
that, for temperature near the boiling point, the elasticity 
of vapors of different liquids are equal, for equal distances of 
temperature to the boiling point. The temperature at which 
carburet of sulphur boils, under the mean pressure of the 
