28 DR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XVIII.) 
that surface where the film is believed to exist, and such a globule, though almost 
entirely water, may well act as an oil globule, and by its friction render the wood, 
&c. positive, itself becoming negative. 
2126. That water which is rendered ineffective by a little saline or acid matter 
should still be able to show the effect of the film of oil (2121.) attached to it, is perfectly 
consistent with this view. So also is the still more striking fact that alkalized water 
(2092.) having no power of itself should deeply injure the power of olive oil or resin, 
and hardly touch that of oil of turpentine (2121.), for the olive oil or resin would no 
longer form a film over it but dissolve in it, on the contrary the oil of turpentine 
would form its film. 
2127. That resin should produce a strong effect and sulphur not is also satisfactory, 
for I find resin in boiling hot water melts, and has the same effect on the balance 
(2124.) as oil, though more slowly ; but sulphur has not this power, its point of fusion 
being too high. 
2128. It is very probable that when wood, glass, or even metal is rubbed by these 
oily currents, the oil maybe considered as rubbing not merely against wood, &c., but 
water also, the water being now on the side of the thing rubbed. Under the circum- 
stances water has much more attraction for the wood rubbed than oil has, for in the 
steam-current, canvas, wood, &c. which has been well soaked in oil for a long time 
are quickly dispossessed of it, and found saturated with water. In such case the 
effect would still be to increase the positive state of the substance rubbed, and the 
negative state of the issuing stream. 
2129. Having carried the experiments thus far with steam, and having been led 
to consider the steam as ineffectual by itself, and merely the mechanical agent by 
which the rubbing particles were driven onwards, I proceeded to experiment with 
compressed air*. For this purpose I used a strong copper box of the capacity of 
forty-six cubic inches, having two stop- cocks, by one of which the air was always 
forced in, and the other retained for the exit aperture. The box was very carefully 
cleaned out by caustic potash. Extreme care was taken (and required) to remove 
and avoid oil, wax, or resin about the exit apertures. The air was forced into it 
by a condensing syringe, and in certain cases when I required dry air, four or five 
ounces of cylinder potassa fusa were put into the box, and the condensed air left in 
contact with the substance ten or fifteen minutes. The average quantity of air which 
issued and was used in each blast was 150 cubic inches. It was very difficult to de- 
prive this air of the smell of oil which it acquired in being pumped through the con- 
densing syringe. 
2130. I will speak first of undried common air: when such compressed air was 
* Mr. Armstrong has also employed air in much larger quantities. Philosophical Magazine, 1841 , vol. xviii. 
pp. 133, 328, 
