I. Knowing the maximum tensions of pure water and pure alcohol, can we
calculate directly the tensions of the vapors of any mixture whatever of
alcohol and water?
Yes, we can calculate this tension by a general formula, provided we
take into account the affinity of water for alcohol, which increases the
value of the total latent heat of evaporation of the liquid. The results
of the calculation are fully confirmed by experience. We thus establish
the following laws:
a. For any temperature whatever, the maximum tension of the vapors of a
mixture of water and alcohol is always comprised between that of pure
water and that of pure alcohol.
b. The tension of the vapors of a mixture of water and alcohol
approaches the tension of alcohol so much the nearer in proportion as
the proof is higher; and, reciprocally, if water is in excess, the
tension of the vapors approaches the tension of the vapors of water.
c. The curves of the maximum tensions of vapors formed by all mixtures
of alcohol and water are represented by the same general formula, one
factor only of which is a function of the richness of the alcoholic
solution.
It results, then, from these laws that we may determine with the
greatest exactness the richness of a solution containing alcohol and
water, if we know the tension of the vapors that it gives off at a
certain temperature.
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