862 On the Influence of Temperature, &c. [May, 
make a firm ligature on it with tape from its upper extremity to 
within 22 inches of its lower end, and in this state plunge it into 
mercury, the naked part of the rod comes out at first inexcitable, 
and some time after more or less weakly positive. On undoing the 
Jigature we find the rod instantly strongly negative in the mereury. 
So that when we apply pressure to a part only of the rod, the 
fluid of the part not pressed acquires expansion. 
5. However strong the tension of the power may be, if we press 
the rod with vigour, and for a long time, along its whole length, it 
at last loses its power. It is on this account that when we keep it 
plunged in mercury for a longer or shorter time, when it is not very 
strongly negative it becomes gradually inexcitable, then positive, 
and at last finally inexcitable. 
Eighth Fact.—Electricity likewise changes its nature under the 
influence of the principle of humidity. 
When the rod is decidedly negative in mercury during dry and 
cold weather, if we plunge it into water, and, after drying it with a 
linen cloth, plunge it again-into mercury, it comes out positive. If 
we moisten it again, spreading the water over every part of the 
surface with the finger, and, after drying it, plunge it into mer- 
cury, it comes out inexcitable. In proportion as the moisture eva- 
porates, either by leaving it in the air, or by moistening it with 
ether, and allowing it to evaporate, it becomes successively positive, 
inexcitable, and negative, as before the experiment. 
We must observe that in this case the power is only weakened 
when the moisture has not been applied to the whole surface, and 
that in the contrary case it is destroyed. 
it results in general from these facts that the different electric 
states of glass in mercury are the difierent effects of two powers 
whose forces are variable, and frequently change their ratio to each 
other. In the mutual pressure of these powers it is the one which 
is most powerful at the instant of their reaction which is always 
negative, and it is the weakest which is positive. We increase the 
electric intensity by weakening to a certain point the weakest power, 
or by increasing in the same way the strongest power; because in 
both cases we increase the ratio between the forces. On the other 
hand we diminish the electrical intensity by weakening the strongest 
power, or by increasing the weakest ; because in these two cases we 
diminish the ratio between the forces. The two powers are inex~ 
citable the one by the other when the two forces are to each other 
in a ratio of equality or in equilibrium. Finally, the electricity 
changes its nature when the ratio of the forces becomes inverse ; 
that is to say, when the weakest power becomes superior to the 
strongest. 
