1817.) on the Intensity and Nature of Electricity. 361 
wool. In this case the rod, being heated some degrees, and plunged 
into cold mercury, always becomes negative. The same rod being 
cold, and plunged into mercury heated a few degrees, becomes, on 
the other hand, positive. The hot rod, after having been negative 
in cold mercury, is still susceptible of passing into the positive state 
when heated to a higher temperature. 
There are, then, two positive states: the one takes place when 
the power of the glass is inferior to that of the mercury, and the 
other when the power of the glass enjoys too great a superiority 
over that of the mercury. AAs this last positive state is preceded, as 
well as the first, by a momentary equilibrium of forces, it can only 
be the etfect of a new weakening of the power of the glass produced 
by the immersion, by its too great expansion compared to that of 
the mercury, and by the subsequent revulsion of the last. The 
positive state, then, is the partition of the most feeble power, or 
of that which becomes so by the result of the pressure. 
5. When the hot rod is decidedly negative in the mercury, if we 
keep it a long time in that degree of heat, it passes to the positive 
state as soon as its power begins to get weak, and it is found almost 
inexcitable when it has cooled. 
Seventh Fact.—Mechanical pressures are capable of producing 
the same changes in electricity. 
1. If we merely touch the surface of mercury with the convex 
end of a large stick of sealing-wax, polished glass, or sulphur, we 
generally draw it back positively electric. If we strike the surface 
of the mercury slightly with it, we render it inexcitable: if we 
strike with greater force, we make it negative. This experiment 
may be repeated as often as we please. 
2. When the powers are well developed, a rod of glass comes 
equally negative out of mercury put into a very flat vessel as out of 
mercury contained in a conical vessel. When the power of the 
glass begins first to get weak in winter, the rod comes positive out 
of the shallow vessel, and negative out of the deep one. On the 
contrary, when the power of the glass begins first to get weak in 
summer, it comes negative out of the shallow, and positive out of 
the conical vessel. In the same circumstances we obtain the same 
results with two rods of glass of unequal thickness, and plunged 
each to the same depth in the same conical vessel. 
3. If we press a rod of glass over its whole length, so as neither 
to communicate heat nor moisture, and then plunge it into mercury, 
it becomes electric in it even when naturally inexcitable. If it is 
naturally positive it becomes negative, and it becomes more strongly 
negative than before when it is naturally negative. 
If, instead of pressing the rod, we merely wrap round it a dry 
linen cloth in different folds, and while we hold the cloth round it 
draw out the rod gently as from a case, it becomes positive in the 
mercury after two or three such frictions, when it is aaturally nega~ 
tive: and if it is naturally positive, it becomes inexcitable. 
4, When a large rod is decidedly negative in the mercury, if we 
