Part 3 (1/2)

Now, let us examine a little more minutely how this influence is exerted upon the _air_, which is the subject we are especially interested in at present.

Does it commence at the top, and heat it, layer by layer, until it reaches the bottom? Not at all; but it pa.s.ses through the whole forty-five miles of air, heating it very little, if any, and falls upon the solid substances at the earth's surface, heating them, which, in turn, heat the air by its individual particles coming into immediate contact with those solid hotter substances.

We will endeavor to ill.u.s.trate this in a crude way.

[Ill.u.s.tration: Fig. 7.]

Here we have a tin tube, _a_, fifteen feet long and ten inches in diameter, open at both ends; two feet from one end we introduce this ascending pipe, _b_, the upper end of which is merely inserted in a small flue, extending to the top of the building. The height of this flue is sufficient to make a current of air pa.s.s through this tube, as you will see by holding this smoking taper at the far end. We will now place a large heated ball, _c_, at this end, and outside of that we will place this reflector, _d_, pressing it quite close to the end of the tube, so that no air can enter here.

The rays of heat from this ball, or from any other warm body, are thrown like rays of light, in every direction equally; there would, therefore, be some of the rays thrown through this tube to the other end without any reflector, but the proportion that would reach the other end would, of course, be small.

We therefore collect those going the other way, and change their course, and then send them straight through the tube to the far end.

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 pa.s.sing 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 pa.s.sing the other way.

Now, we want to know how much that air is heated in pa.s.sing 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 gla.s.ses, 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 pa.s.sing 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 pa.s.s directly through them without either reflecting or absorbing them. Other substances allow the rays of light to pa.s.s through them, but absorb much of the radiant heat, like clear gla.s.s.

Rock salt is one of the best non-absorbents of radiant heat, allowing nearly the whole of the rays of heat to pa.s.s through un.o.bstructed.

We will now return to our experiment at the other end of the tube. I find there is something wrong here--the mercury in the thermometer has risen several degrees. I knew this was rather a crude arrangement for ill.u.s.trating this very beautiful and interesting part of our subject, but I hoped it would a.s.sist me a little in conveying to you the idea I desired to impress upon your minds. I find, however, that it is scarcely delicate enough to ill.u.s.trate perfectly what I wanted to show.

But this increased temperature is not owing to the effect of radiant heat on the air coming from the far end, for I find by the heat at the top of the pipe, between the heated ball and this ascending pipe, I, and by the current of heated air on the side next the ball, that there is a current of _circulating air_ that _has been heated_ by coming into immediate _contact_ with the hot ball.

I designed this smaller tube, _k_, to carry off the air thus heated, but it appears to be too small.

We ought to have had a piece of rock-salt to have closed the end of this tube, so that the radiant heat would have pa.s.sed through without allowing any _circulation_ of _heated air_, but I was unable to find such a piece. But Professor Tyndall, in his lectures before the Royal Inst.i.tute of Great Britain, gives the results of a large number of very accurate and beautiful experiments tried for the purpose of determining whether the forty-five miles of atmosphere surrounding the earth absorbed _any_ of the sun's rays, and if so, how much?

These experiments prove, in the most conclusive manner, that dry pure air is almost a perfect non-absorbent of radiant heat. Thus, were the air entirely dry and pure, the whole forty-five miles through which the sun's rays have to pa.s.s, would absorb a very small fraction thereof, so that in the length of our tube it would be but an exceedingly small fraction of one degree, that is, for pure dry air.

But is the air of this room pure and dry? Very far from it.

Professor Tyndall found that the moisture alone in the air of an ordinary room, absorbed from fifty to seventy times as much of the radiant heat as the air does. Air and the elementary gases--oxygen, hydrogen and nitrogen--have no power of absorbing radiant heat, but the compound gases have a very different effect; for instance, olifiant gas absorbs 7950 times as much as air; ammonia, 7260; sulphurous acid, 8800 times.

Perfumes, also, have a wonderful power of absorbing radiant heat.