1817.] on the Intensity and Nature of Electricity. 357 
electric with wool and the rod alone without power, it becomes, on 
the contrary, positive when it is cooled. In these two cases it is 
found inexcitable when it returns to the temperature of the air. 
The nascent electricity of the rod, then, is negative in the mer- 
cury when the latter is naturally without power, and positive when 
it still preserves power. 
4. The electricity produced by friction with wool is equally sus- 
ceptible of changing its nature when we cool the substance rubbed. 
{ have allowed a large rod of glass to cool in mercury to 14° at a 
time when it was strongly positive upon rubbing it with wool. After 
taking it out, I rabbed it from time to time on the sleeve of my coat, 
as it recovered its temperature. At first it was inexcitable; some 
time after it was pretty strongly negative ; at the end of some mi- 
nutes it became inexcitable ; finally it showed itself positive, and 
continued so. Sulphur and wax treated in the same way appeared, 
on the contrary, positive, then inexcitable, and then negative, as 
usual. When the temperature of the external air is 23° or 21°, we 
may obtain the same results by exposing these substances to the 
cold of the atmosphere. 
From this fact it results that, if we progressively weaken the 
power of glass, while we preserve that of mercury at a constant 
degree of force, the electricity of the rod becomes successively 
positive, negative, and positive, according to its degree of relative 
weakness. 
Fourth Fact.—Artificial heat produces on the electric power 
similar effects as artificial cold does. 
1. If we gradually heat a glass rod when it is naturally very elec- 
tric, and at each degree of heat acquired we plunge it into mercury 
at the common temperature of the atmosphere, it becomes the more 
electric the more its temperature is elevated above that of the mer- 
cury. When we allow it to cool, its electricity again diminishes, 
and returns gradually to its original state. 
The case is different when we heat the mercury, and allow the 
rod to remain at the temperature of the atmosphere. The rod 
comes out of the mercury so much the more feebly electrified the 
higher the temperature of the mercury is elevated above its own. 
It must be understood, however, that this effect takes place only 
during the two or three first times that the rod is plunged into the 
mercury; for if we continue the process, the rod becomes more and 
more electric in proportion as it becomes hot. Its electricity, how- 
ever, is much weaker than that which it acquires when it is hot, 
and the mercury cold. 
It seems surprising that hot glass becomes so electric in cold mer- 
cury, and cold glass so little in hot mercury. But we must consider 
that if, in the conflict of the two opposite forces, the one is natu- 
rally superior to the other, the effect produced ought to increase 
when the superior force is increased, and diminish, on the contrary, 
when the inferior force is augmented. 
2. When glass and mercury are naturally inexcitable the one by 
