The vapor pressure

Between 1792 and 1796, Volta studied the density and tension of saturated vapors, formulating fundamental laws of their behavior. His results, little known, anticipated those later attributed to Dalton.

Saturated vapors

After research on the thermal expansion of air and unsaturated water vapor, Volta devoted himself between 1792 and 1796 to research on the density and tension of saturated vapors and their dependence on temperature. These results, obtained when the scientist was particularly engaged in the debate with Galvani over the alleged animal electricity, were also not sufficiently publicized, and he was not given credit for some important findings subsequently obtained by other scientists.

Thermometer for studying air expansion (Volta Temple, Como)

Little-known results

In the postscript to a letter to Vassalli dated October 27, 1795, published in Br. Ann., Vol. XI, 1796, p. 127, and in Ant. Coll., Vol. III, p. 381, the results obtained up to that point were summarized. These same results were obtained by Dalton in 1801 and are now reported in texts under his name.

Volta's laws

VOLTA

The amount of vapor is the same in a space that is empty or filled with air, rarefied or dense, and depends solely on the degree of heat.
The pressure that [the steam] balances increases in geometric progression as the heat increases in arithmetic progression.

The comparison with Dalton

DALTON

The force of the steam […] is the same under any pressure of an other elastic fluid as it is in vacuo.
Upon examination of the number in the table […] there appears something like a geometrical progression in force of vapour.

Minutes of notes on saturated vapor pressures at different temperatures (Volta’s autograph, Cart. volt. H33_, Lombard Institute)

An underestimated contribution

As can be seen, they are identical; moreover, with regard to the second law, we observe, as Polvani has clearly pointed out, ‘that while Volta’s law concerns both vapour pressure and density, Dalton’s law concerns only pressure: furthermore, while the former comes from a theoretical interpretation of the increase in tension, and leads to separating the effect due to the increase in density from that due to the increase in elastic stress, Dalton’s law consists purely of a statement of experimental facts’.