We have already seen in our Histmy of the Science, that Laplace, Lavoisicr, and Volta discovered that elec tricity was generated during the evaporation of carious fluids.
In their experiments they made use of two different kinds of apparatus. In both of thcse, the bodies that were evaporated were insulated by means of stands of glass coated with sealing wax. When the disengage ment or the absorption of the electric matter was be lieved to be instantaneous, the body was made to com municate directly with the electrometer by means of a chain or a thread of archil ; but when the electric mat ter was supposed to be disengaged successively, and continued for a certain time, they made use of Volta's condenser, which has the property of accumulating small quantities of electric matter.
Having put iron filings into a vessel with a large aperture, they poured upon them sulphuric acid weak ened with three parts of water. After a brisk efferves cence, and the rapid disengagement of inflammable air, the condenser was so powerfully electrified, that it af forded a lively spark, and they found from the electro meter that the electricity was negative.
When sulphuric acid was, in like manner, thrown in to some vessels that contained powdered chalk, fixed air was rapidly disengaged, and the condenser and electro meter indicated that the electricity was negative. It was, however, less than in the preceding experiment, and produced no sensible spark.
When nitrous gas was produced, they obtained a si milar result ; but in order to increase the electricity, they used six vessels at once, each containing iron fil ings, and they poured over diem nitrous acid diluted with about two parts of water. The effervescence and the production of air were extremely rapid, and nega tive electricity was distinctly produced ; but as the cir cumstances under which the experiment was made, were not favourable, the electricity was very weak. Three small chafing-dishes tilled with burning charcoal were next insulated, and made to communicate with Volta's condenser. The negative electricity which was generated was so sensible, that by augmenting the quan tity of charcoal they could easily have obtained from it a spark.
The next experiments were made upon the evapora tion of water, produced by throwing it upon three insulated furnaces of hammered iron, which communi cated with the electrometer. In three successive expe riments the electricity was distinctly perceptible. In the first, however, it appeared to be negative, but in the other two it was decidedly positive. In order to ac count for this result, they conjectured that the cooling produced by evaporation had increased the signs of po sitive electricity more than the evaporation had diminish ed them. Volta assisted at these experiments.
The same subject was more deeply investigated by Saussure, who has given a full account of his experi ments in his Voyage dans les Allies. Volta having ob served that the electricity produced by evaporation was always negative, Saussure attempted to repeat the ex periment by throwing a mass of red-hot iron into a small quantity of water in a coffee-pot with a large mouth, and suspended by silk strings. He found, however, that the electricity thus produced was negative. This re sult being quite opposite to that of Volta, he varied the experiment in the following manner. He insulated a small chafing-dish by silk cords, and placed upon it the coffee-pot, with a small quantity of water. One elec trometer was connected with the chafing-dish, and ano ther with the coffee-pot, and when the water had conti nued for a few minutes to boil strongly, both the elec trometers exhibited signs of negative electricity, as in the experiments of Volta. He found also that the eva poration produced by the effervescence of iron in sul phuric acid, and by the effervescence of chalk in the same acid, likewise gave negative electricity.
In order to account for the positive electricity gene. rated in the first experiment, M. Saussure conjectured that the intensity of the heat to which the water was ex posed by the contact of a body brought to a white heat, was the cause of the electricity produced by the evapo ration, and that a combination was then formed, by which a new quantity of the fluid was developed. The quan tity of electricity was so great in the experiment, that the balls of the electrometer were separated to the greatest possible distance. In order to verify this con jecture, that the electricity was produced in some way or other by the combustion of the water or the iron, Saus sure tried to procure positive electricity, by moderating the heat of the iron. Into a large insulated iron cruci ble, five inches high, four in diameter, and six lines thick, and made red-hot, he threw small quantities of water successively, till the crucible had the degree of heat sufficient to make water boil. The electricity pro duced at each projection of the water was carefully ob served, and destroyed. The electricity which was ex hibited was always positive. It was very strong at the first projection, and gradually diminished to the twelfth, when it was scarcely perceptible. In the repetition of this experiment, Saussure found that when a small quan tity of water was thrown into the crucible the moment it was taken from the fire, and while it had a pale red colour approaching to white, he never could obtain any indication of electricity.