353 On the Influence of Temperature, &c. [May, 
the other, if we heat only the rod, frequently the increase of a 
single degree of heat is sufficient to produce electricity. Frequently 
2°, sometimes 4°, and sometimes 8°, are required according to the 
degree of inexcitability. But when we heat the mercury only, we 
must raise it 4°, 8°, 16°, or 32°, higher than the rod before elec- 
tricity is excited by a single immersion. 
The electric power of mercury to act on that of glass requires, 
therefore, four times more force than is necessary for that of glass 
to act on mercury. Hence the electric power of glass is to that of 
mercury :: 2: 1, 
Fifth Fact.—1. Ihave said that on heating the rod its electricity 
augments progressively. This is the case only to 212°. Beyond 
that degree the electricity of the rod in the cold mercury diminishes 
more and more, and disappears entirely at about 410°. When it is 
inexcitable at this degree of heat, if we allow it to cool, it does 
not fail to become electric, and to recover nearly its original degree 
of intensity. 
Whatever be the natural tension of the electric power, the cold 
rod comes out equally inexcitable from the two first immersions into 
mercury of the temperature 212° or 257°. I say from the first two 
immersions, for it appears electric at the following immersions as 
soon as it becomes hot. 
We must observe that when the hot rod is inexcitable in cold 
mercury at 531°, it is still electric in mercury of the temperature 
of 140°; and that when the cold rod is inexcitable in mercury of 
the temperature 257°, a rod heated to 302° does not cease to be 
electric in it. The inexcitability, therefore, which takes place in 
these two circumstances is not the result of the destruction of one 
of the two powers by the heat, but of a momentary equilibrium 
between the two forces which act against each other ; and this equi- 
librium is equally produced here either by increasing the weaker 
power, or by giving to the strongest an excessive increase of force. 
2. Heat does not always act as a force increasing the tension. In 
certain circumstances it has the property of weakening it. 
During the time of strong tension, if we heat in winter a glass 
rod se as to make it red hot, and if, after keeping it in that heat for 
some time, we allow it to cool down to the temperature of the air, 
it is no longer excitable in mercury, or it is soonly very weakly. In 
summer we may obtain the same result by heating only to 410°. It 
is not possible in winter to weaken in this manner the power of 
mercury, because we cannot raise its temperature higher than 347°. 
But in summer it is only requisite to heat it for an instant to 167° to 
find it less excitable when cold than it was before. 
There is, however, a method of weakening the power by a very 
small degree of heat even during a time of strong tension in winter. 
If we expose for several hours mercury to the cold of the atmos- 
phere when the air is at 32°, and a cold wind blowing; and if we 
plunge into it now and then a glass rod, it always comes out electric. 
But its electricity is much weaker towards the end than at the be- 
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