1817.] on the Intensity and Nature of Electricity. 359 
ginning. If we now bring the mercury into a room where the 
temperature is 46° or 50°, and then plunge the rod into it, we are 
much surprised to find it inexcitable. On exposing the mercury 
again to the open air, it speedily recovers its power, and loses it 
again when brought into the room. 
If we cool a glass rod to 32°, or lower, at a time when it is 
naturally very electric, and then bring it suddenly to the fire to raise 
its temperature to 86° or 140°, we find it, when cooled again, in- 
excitable in mercury; or if it is still electric, its electricity has 
changed its nature. This shows that its power has no longer the 
same ratio of force to that of mercury. This diminution does not 
take place when we allow it to return of itself to the temperature of 
the air. In that case it is more electric than before the cooling. 
All that I have said of glass in this fifth fact and in the fourth is 
common to sealing-wax, sulphur, silk, and wool, some trifling 
differences excepted. 
Thus we may say that, if the first effect of heat is to increase the 
tension of the electric power, the second and subsequent effect is to 
repress the expansive force by bringing it nearer the centre of acti- 
vity of the attraction ; and this effect is produced with so much the 
more facility the rarer the fluid is. : 
Heat, then, acts on the electric power in the way of attraction 
by opposing its expansion; and cold in the way of an expansive 
force by favouring its developement. 
Sixth Fact.—These different impressions of heat do not merely 
modify the tension of the electric power; they change, likewise, 
the nature of the electricity. 
1. If we gradually heat the rod to 212°, when it is strongly 
negative in mercury, and at each degree of elevation plunge it into 
mercury at the temperature of the atmosphere, it always comes out 
more and more negative. If we gradually heat the mercury in its 
turn, and plunge the rod (at the temperature of the air) into it at 
every 10° of elevation, it comes out, on the contrary, inexcitable 
at 176°, and positive when the temperature rises to 212°. If we 
allow it to get hot in the mercury, it becomes again negative, 
2. When the rod heated to 212° is strongly negative in the cold 
mercury, if we continue to heat it, its negative state diminishes by 
little and little. At 410°, or about that temperature, it becomes 
inexcitable in winter. Beyond that degree it comes out positive, 
beginning at the bottom of the rod. If we raise the temperature 
still higher, the rod becomes inexcitable, and does not recover its 
electric state again. In summer, after having become positive, it 
passes a second time to the negative state; nor is it necessary to 
heat it so much as in winter. 
When the cold rod comes out positive from mercury at 212° at 
the first immersion, if we continue to heat the mercury, and always 
keep the rod at the temperature of the air, it becomes inexcitable, 
and contipues so till the mercury is heated to the boiling point. 
Mercury, then, heated to 317° has not sufficient force to make the 
