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coloured by sodium vapour in front of the slit. I then saw the dark lines D change into bright ones. flame of a Bunsen's lamp threw the bright sodium lines upon the solar spectrum with unexpected brilliancy. In order to find out the extent to which the intensity of the solar spectrum could be increased without impairing the distinctness of the sodium lines, I allowed the full sunlight to shine through the sodium flame, and to my astonishment I saw that the dark lines D appeared with an extraordinary degree of clearness.

"I then exchanged the sunlight for the Drummond's or oxyhydrogen lime-light, which, like that of all incandescent solid or liquid bodies, gives a spectrum containing no dark lines.

"When this light was allowed to fall through a suitable flame coloured by common salt, dark lines were seen in the spectrum in the position of the sodium lines.

"The same phenomenon was observed if, instead of the incandescent lime, a platinum wire was used, which being heated in a flame was brought to a temperature near its melting point by passing an electric current through it. The phenomenon in question is easily explained upon the supposition that the sodium flame absorbs rays of the same degree of refrangibility as those it emits, whilst it is perfectly transparent for all other rays."

Kirchhoff had in fact, as far as he had gone, produced artificial sunlight, because he had obtained the two double dark lines in his continuous spectrum. I will try to show the formation of the dark lines of the sodium: for this purpose we will again employ our electric lamp, and I will throw the continuous spectrum of the carbon points on to the screen, and then I will bring into the

lower carbon, which is shaped like a cup, a small quantity of metallic sodium; and we shall thus see that the vapour of the sodium has the power of absorbing the particular kind of light which it emits, and that in place of the bright sodium line we shall have a dark line. There you observe the dark sodium line. As a further illustration I have here a diagram (Fig. 53) representing what is seen when we look at the spectrum of burning sodium with an instrument such as that which Kirchhoff used. At the bottom (No. 2) we have a drawing of the ordinary sodium spectrum, giving us these

No1

No 2

FIG. 53.

bright double lines on a dark background, and above (No. 1) we see a drawing of the spectrum of burning sodium. Instead of two bright yellow lines, we here find we have two intensely black lines upon a bright continuous spectrum, the “D” light having been absorbed by the sodium vapour. The difference between the intensities of the lights on each side of these lines and in that particular part where the lines fall is so great as to give an actual shadow, which we see as a black line. There is a wellknown experiment by which we cast a shadow with a luminous object, such as a candle flame: so here, although these black lines are not wholly devoid of light, yet the

light is so much less intense than in the surrounding parts, that they appear black to us.

I can illustrate this to you in another way. Here (Fig. 54) we have a large sheet of non-luminous gas flame (bb) burning under a tall chimney (c), and the flame I can colour by sodium. In front of this I am going to ignite a flame of hydrogen (a), and I will also place in the hydro

a

FIG. 54.

gen flame some sodium compound; so that we shall have two sodium flames burning, one in front of the other. I want you to notice that the yellow rays passing from this large flame at the back through the hydrogen flame tinged with soda will be absorbed, and that the outer rim of this hydrogen flame will appear dark; in fact, it will look just as if the hydrogen flame was smoky,—as though

we had a smoky candle burning in front of the large flame. There is no carbon in this flame to produce a smoky appearance. We shall have nothing but pure hydrogen burning. We will light our hydrogen here, but we must first make

a

our large soda flame.

FIG. 55.

This we do by burning a little sodium, the fumes of which I waft into the flame. Now you see the large flame is turned yellow, and you will notice that in front we get a smoky flame. It is now very

distinct. If, instead of sodium, I next place some lithium in the flame, no black rim will appear. We shall get the red colour of the lithium flame, but it will not give us any black shadow, because it has no power of absorbing the yellow light. Hence we conclude that the smoky appearance was really caused by the absorption of the yellow “D” light by the sodium vapour in a state of incandescence.

Here is another most ingenious apparatus lately sent me by my friend Professor Bunsen, for exhibiting a constant black sodium flame absorbing the rays of the same degree of refrangibility as it emits. The little cap of yellow flame (d) which floats from the first burner in front of the larger yellow soda flame (g) absorbs the "D" D" rays, and in consequence we have the peculiar phenomenon of a constantly burning black sodium flame (Fig. 55).

I can also show you in a third way the fact that sodium vapour is opaque to the light which it gives off. I have prepared a tube containing some sodium which I can convert into vapour. By heating the tube as I am doing, it will become filled with sodium vapour, and you will see that it is perfectly colourless and transparent when we look at it with the white sunlight; but when we look at it with the yellow sodium light it will appear to be opaque. We shall then see that the tube containing the sodium vapour throws a dark shadow on the screen. [The lights were turned down, and the screen was illuminated with a yellow sodium flame.] Now the tube looks black; we cannot see through it; it throws a dark shadow. [Light was again admitted.] Now, by the daylight, it is colourless. This shows us,

then, very distinctly, that the sodium vapour is opaque for the rays which itself can emit.

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