The nature of the body, too, has no influence on the law of the dissipation. On the 28th June, the electricity decreased per minute when pith balls were used ; and the same result was obtained when the balls were inad,1 of copper or of sealing-wax.
The next object of Coulomb was to discover thc re lation which subsisted between the humidity of the air and the dissipation of electricity. Ile therefore drew up the following Table, in which the first column marks the day when the observations were made; the second the state of Saussure's hygrometer ; the third the quan tity of water dissolved in a cubic foot of air, Whell the thermometer is between and 16° of Ileaumur, ac cording to the experiments of Saussurc ;* and the fourth, the dissipation of electricity per minute.
If we now wish to determine the law between the dis sipation of the electricity and the quantity of water in tho atmosphere when the thermometer is between 15° and 16°, the temperature when the four experiments were made, let us call 711 the power which expresses that re lation; then comparing the experiment with tl:u three others, we shall ol.tain, Hence it follows, that the diminution of the repul sive force, or, what is the same, of the electric density, is proportional to the cube of the weight of the quantity of water dissolved in a given volume of air.
The preceding conclusion, however, cannot be consi dered as certain, since it depends upon several circum stances which have not been determined with sufficient exactness. Coulomb proposed to enclose the balls in dif ferent kinds of air, to communicate to the air different degrees of humidity, and then to determine the law of the dissipation of the electricity of the balls; but he found this investigation too arduous, for want of proper instru ments for measuring the purity and the humidity of the different gases which he employed.
Although the quantity of water dissolved in a given quantity of air increases with the temperature, yet this is not indicated by the hygrometer of Saussure. As the conducting power of the air, therefore, must increase with the temperature, the dissipation of electricity ought to be greater in warm weather than in cold weather, even when Saussure's hygrometer indicates the same degree of humidity. This conclusion Coulomb confirmed by experiment; but the law of the increase depending on the augmentation of temperature, he has not been able to ascertain. Saussure* has published a Table, showing
the correspondence between the degrees of his hygro meter, and the quantity of water which a cubic foot of air holds in solution at every degree of the thermome ter. But this Table is not given by its author as cor rect; and therefore Coulomb merely concludes, in gene ral, that, in proportion as the temperature increases, the dissipation of electricity is not so great as it ought to be, by calculating from Saussure's Table.
The dissipation of electricity seems also to be affect ed by some other causes, which it is not easy to disco ver. On different days, when the barometer, the ther mometer, and the hygrometer, indicated that the air was in the same state, the dissipation underwent considera ble changes. The only general observation which Cou lomb was enabled to make, was, that when there was a sudden change of weather, and when the hygrometer varied sensibly in a few hours from moist to dry, the loss of electricity, relative to its density, remained, du ring some time, greater than it ought to be, according to the degree of dryness indicated by the hygrometer, and vice versa, when the' hygrometer changes suddenly from dry to moist. If, in 12 or 15 hours, for example, the hygrometer passes from moist to dry eight or ten degrees, and that it afterwards remains stationary at that degree of dryness for several days, it will often happen that the electrical density will decrease the first day af ter this rise of the hygrometer, at the rate of A per mi nute. Some days afterwards, though the hygrometer is stationary, the electric density will decrease more than ioo per minute. Coulomb ascribes this variation to the aqueous vapours contracting an adhesion to the air, after having continued in it a certain time ; and he supposes that the hygrometer will thus only attract to itself the aqueous parts, which are free, and which adhere feebly to the air. The hygrometer, therefore, upon any sud den change of weather, will indicate only the quantity of aqueous parts in the air that are free, and not the abso lute quantity of aqueous vapours. This opinion is sup ported by the fact, that the electrical dissipation is al most always fixed at time end of some hours, relatively to the hygrometer, when there is a rapid change from dryness or humidity; and that it is only in calm weather that the contrary takes place.