324 Intelligonce and Miscellaneous Ai'ticles. 



by means of an iron and a copper wire, the magnetic needle is not 

 deviated, whatever may be the temperature. But it is very different 

 if the copper rod be inclosed in a tube of hard glass, and the 

 temperature be brought to the point of fusion of the latter. If a 

 multiplier and a tangent galvanometer be placed in the circuit, it is 

 seen that long before the glass has become red-hot, the needle of the 

 multij)lier deviates ; by continuing the heat up to the melting-point, 

 the intensity of the current constantly increases, attains a maximum, 

 and remains constant. Long before this it is necessary to take away 

 the multiplier and only make use of the tangent galvanometer ; this 

 current is directed from the iron to the copper, across the charcoal 

 and glass ; that is to say, the iron during its oxidation disengages 

 negative electricity, and the copper, of which the surface remains 

 clear and bright, evolves positive electricity. Thus the copper, 

 although exposed to a high temperature, remains intact, as is also 

 the case when, in contact with zinc or iron, it is immersed in an 

 oxidizing liquid. It is consequently indebted to its electro-negative 

 state for its preservation at an elevated temperature. The current 

 remains constant as long as the temperature undergoes no sensible 

 variation and the iron does not become covered with a thick layer 

 of oxide ; but should the tube partially fuse and the copper touch 

 the iron, all the signs of electricity disappear. This fact proves that 

 th5 current is not thermo-electric. 



The disengagement of electricity, under these circumstances, is 

 due therefore both to calorific and chemical causes. Whilst the 

 iron is becoming oxidized it acquires a negative electricity, the sur- 

 rounding air taking the positive electricity, which is transmitted to 

 the copper by the medium of the red-hot coals and glass with which 

 the gases are in contact. 



Second experiment. — To ascertain the relation existing between the 

 current produced by the pyro-electric couple and that arising from 

 a Bunsen's couple of equal conductibility, I placed in the same 

 circuit a pyro-electric couple, and another with nitric acid, of which 

 the elements were of the same dimensions. These two couples 

 were placed successively so that the currents passed in the same 

 and in contrary directions. 



Representing the intensity of the current furnished by the Bunsen's 

 couple by x, and that of the pyro-electric current byy, it was found 

 with the tangent magnetoscope that 



x-\-y= sin 1 7°=29237, x-y= sin 10°=17365 ; 

 whence x=23301, y=5936, and consequently x : y : : 3*9 : 1. 

 Under the circumstances of my experiment the intensity of the pyro- 

 electric current was therefore only a fourth of that of the nitric acid 

 battery. 



We have already seen that sufficient heat for the complete fusion 

 of the glass must be avoided, as the iron and copper then soon 

 touching, all the signs of electricity would gradually disappear ; 

 but there is another cause which diminishes the intensity of the 

 current, the oxidation of the points of junction of the metallic wires 

 and the rods of iron and coj)per, when they are very near the hot 

 furnace ; this inconvenience is got rid of by employing long rods. 



