tery see what heat we get (fig. 48). Is not that beautiful?—it is a complete bridge of power. There is metallic connection all the way round in this arrangement, and where I have inserted the platinum, which offers some resistance to the passage of the force, you see what an amount of heat is evolved,—this is the heat which the zinc would give if burnt in oxygen, but as it is being burnt in the voltaic battery it is giving it out at this spot. I will now shorten this wire for the sake of showing you that the shorter the obstructing wire is, the more and more intense is the heat, until at last our platinum is fused and falls down, breaking off the circuit.
Here is another instance. I will take a piece of the metal silver, and place it on charcoal connected with one end of the battery, and lower the other charcoal pole on to it. See how brilliantly it burns (fig. 49). Here is a piece of iron on the charcoal, see what a combustion is going on; and we might go on in this way burning almost everything we place between the poles. Now I want to show you that this power is still chemical affinity—that if we call the power which is evolved at this point heat, or electricity, or any other name referring to its
