The generation of this light and of this heat merits a moment’s attention.  Before you is an instrument—­a small voltaic battery—­in which zinc is immersed in a suitable liquid.  An attractive force is at this moment exerted between the metal and the oxygen of the liquid; actual combination, however, being in the first instance avoided.  Uniting the two ends of the battery by a thick wire, the attraction is satisfied, the oxygen unites with the metal, zinc is consumed, and heat, as usual, is the result of the combustion.  A power which, for want of a better name, we call an electric current, passes at the same time through the wire.

Cutting the thick wire in two, let the severed ends be united by a thin one.  It glows with a white heat.  Whence comes that heat?  The question is well worthy of an answer.  Suppose in the first instance, when the thick wire is employed, that we permit the action to continue until 100 grains of zinc are consumed, the amount of heat generated in the battery would be capable of accurate numerical expression.  Let the action then continue, with the thin wire glowing, until 100 grains of zinc are consumed.  Will the amount of heat generated in the battery be the same as before?  No; it will be less by the precise amount generated in the thin wire outside the battery.  In fact, by adding the internal heat to the external, we obtain for the combustion of 100 grains of zinc a total which never varies.  We have here a beautiful example of that law of constancy as regards natural energies, the establishment of which is the greatest achievement of modern science.  By this arrangement, then, we are able to burn our zinc at one place, and to exhibit the effects of its combustion at another.  In New York, for example, we may have our grate and fuel; but the heat and light of our fire may be made to appear at San Francisco.

[Illustration:  Fig. 1.]

Removing the thin wire and attaching to the severed ends of the thick one two rods of coke we obtain, on bringing the rods together (as in fig. 1), a small star of light.  Now, the light to be employed in our lectures is a simple exaggeration of this star.  Instead of being produced by ten cells, it is produced by fifty.  Placed in a suitable camera, provided with a suitable lens, this powerful source will give us all the light necessary for our experiments.

And here, in passing, I am reminded of the common delusion that the works of Nature, the human eye included, are theoretically perfect.  The eye has grown for ages towards perfection; but ages of perfecting may be still before it.  Looking at the dazzling light from our large battery, I see a luminous globe, but entirely fail to see the shape of the coke-points whence the light issues.  The cause may be thus made clear:  On the screen before you is projected an image of the carbon points, the whole of the glass lens in front of the camera being employed to form the image.  It is not sharp, but surrounded