We will place another reflector, e, at the far end, to receive and concentrate those rays, in the focus of which we will place a candle, F, with a little phosphorus on it, to show you that the rays of heat are passing through.
There you see the candle is lighted, thus proving that there is a strong current of radiant heat coming from the hot ball, through the tube to this end. And you see by this smoke that there is a current of air passing the other way.
Now, we want to know how much that air is heated in passing the whole length of this tube against that shower of radiant heat, or whether air absorbs radiant heat at all; but, before going to the other end, where the hot ball is, we will take two thermometers that have been lying here, side by side, both indicating a temperature of 69°. One of them, g, we will hang at this end, about opposite to the centre of our tube, which, I think, will give us a fair average of the entering air, first removing, however, the candle that has been lighted, and the reflector.
We will hang the other thermometer in the ascending tube, at the end near the heated ball. We have had two glasses, H, inserted here, so that we might observe what was going on within by the smoke from this taper. You see there is a strong current of air passing up the tube, all of which must come from the far end, flowing against the strong current of radiant heat going in the opposite direction. Now, leaving this thermometer to rise or fall according to the temperature of the air flowing through, we will go to the other end and examine another very interesting part of this experiment: it is the manner in which the radiant heat is received and appropriated by different substances.
Radiant heat is thrown from a hot body in every direction equally, but no two kinds of substances receive those rays of heat in the same manner, nor do they make the same use of them after they have received them.
Every substance receiving heat, however, must give a strict account of it. It must give out an equal amount of heat, or, what is taken as an equivalent, some action or power.
I have a sheet of ordinary tin, and as I hold this polished side behind this light, you see it throws a belt of light across the room; and as I put it in front of the end of our tube, and turn it so that the rays of heat will be reflected in your faces, I think some of you will be able to feel the reflected heat. The rays of heat are turned from their course, and thrown in a belt of light across the room, similar to the rays of light.
But you cannot give away and keep the same thing. This bright polished surface appropriates but a very small portion of the radiant heat. A thermometer hanging for some minutes against the back has scarcely risen one degree; but we have given the other side a coating of lamp black, with a little varnish, and by turning that side towards the pipe, the result will be quite different. By this coat of black varnish the whole character of the sheet of tin is changed. The black, however, has but little to do with it; if it were white, or red, or blue, the formation of the surface being similar in every respect, the result would be the same almost precisely.
Instead of acting merely as a guide-post, to change the direction only of the rays of heat, as before, it now becomes a receiving depot, absorbing nearly all the heat that comes to it. It must soon become filled, however. The thermometer hanging at the back has risen six degrees already, and is going up rapidly; it must soon begin to distribute its extra stores. But mark the different manner of distributing the heat. Instead of reflecting the whole all in one direction, as when received on the other side, it now radiates them equally in every direction.
Some solid substances allow the rays, both of heat and light, to pass directly through them without either reflecting or absorbing them. Other substances allow the rays of light to pass through them, but absorb much of the radiant heat, like clear glass.