486 Royal Society. 
electricity. The author gives, in conclusion, an analytical inves- 
tigation of Mr. Airy’s general formula. 
2. “Variation of the Magnetic Needle as observed at Washing- 
ton City, D. C., from 35 30™ July 24th to 35 July 25th, 1840, in- 
clusive (Gottingen mean time),” by Lieut. Gillies, of the United 
States Service. Communicated by Samuel Hunter Christie, Esq., 
Sec. R.S. 
Feb. 2.—A paper was read, entitled “Experimental Researches in 
Electricity :” Eighteenth Series; by Michael Faraday, Esq., D.C.L., 
F.R.S. Section 25. On the Electricity evolved by the Friction of 
Water and Steam against other bodies. 
The object of the experiments related in this paper, is to trace 
the source of the electricity which accompanies the issue of steam 
of high pressure from the vessels in which it is contained. By 
means of a suitable apparatus, which the author describes and de- 
lineates, he found that electricity is never excited by the passage of 
pure steam, and is manifested only when water is at the same time 
present ; and hence he concludes that it is altogether the effect of 
the friction of globules of water against the sides of the opening, 
or against the substances opposed to its passage, as the water is 
rapidly moved onwards by the current of steam. Accordingly it 
was found to be increased in quantity by increasing the pressure and 
impelling force of the steam. The immediate effect of this friction 
was, in all cases, to render the steam or water positive, and the 
solids, of whatever nature they might be, negative. In certain cir- 
cumstances, however, as when a wire is placed in the current of 
steam at some distance from the orifice whence it has issued, the 
solid exhibits the positive electricity already acquired by the steam, 
and of which it is then merely the recipient and the conductor. In 
like manner, the results may be greatly modified by the shape, 
the nature, and the temperature of the passages through which the 
steam is forced. Heat, by preventing the condensation of the steam 
into water, likewise prevents the evolution of electricity, which 
again speedily appears by cooling the passages so as to restore the 
water which is necessary for the production of that effect. The 
phenomenon of the evolution of electricity in these circumstances 
is dependent also on the quality of the fluid in motion, more espe- 
cially in relation to its conducting power. Water will not excite 
electricity unless it be pure; the addition to it of any soluble salt 
or acid, even in minute quantity, is sufficient to destroy this pro- 
perty. The addition of oil of turpentine, on the other hand, occa- 
sions the development of electricity of an opposite kind to that 
which is excited by water ; and this the author explains by the par- 
ticles or minute globules of the water having each received a coat- 
ing of oil in the form of a thin film, so that the friction takes place 
only between that external film and the solids, along the surface of 
which the globules are carried. A similar, but a more permanent 
effect is produced by the presence of olive oil, which is not, like 
oil of turpentine, subject to rapid dissipation. 
Similar results were obtained when a stream of compressed air 
