of substances are present in the flame in the state of glowing gas.

By a simultaneous comparison of the lines in the Bessemer spectrum with those of well-known substances I was able in the year 1863 to detect the following substances in the Bessemer flame: sodium, potassium, lithium, iron, carbon, hydrogen, and nitrogen. At a certain stage of the operation I found that all at once the lines supposed to be due to carbon disappeared, and we got a continuous spectrum. The workman by experience has learned that this is the moment at which the air must be shut off; but it is only by means of the spectroscope that this point can be exactly determined.

The following table gives you a good idea of the changes seen to take place in the flame, (1) as seen by the unaided eye, (2) as seen by the aid of the spectro

No. 2, Fig. 42, represents the general appearance of the Bessemer spectrum towards the close of the "blow," drawn according to the plan proposed by Bunsen (see page 67). The striking analogy between the flame spectrum and that of carbon (No. 1, Fig. 42) renders it at first sight probable that the principal lines of the Bessemer spectrum are due to carbon in some form; and this conclusion is strengthened when we find that the lines. disappear at the moment when, by chemical analysis, we can show that the carbon has been burnt out. Still it has been stated that the lines are due to the presence of manganese, although they are brightly seen in the working of certain iron ores (such as the Ulverstone

beds) which contain scarcely any of this metal. And, singular and at present unaccountable as it may appear, it has lately been shown beyond doubt by Dr. Marshall Watts, that the bright lines in the Bessemer flame are in reality not identical with carbon lines, as we had long believed, but with those of oxide of manganese. Fig. 42a on the opposite page shows this singular fact clearly: No. 1 gives the iron spectrum; No. 2 that of the Bessemer flame; No. 3 that of oxide of manganese; and No. 4 the spectrum of the spiegel flame. Here again we must be content accurately to record ascertained facts, leaving the explanation for a future time. When the spiegeleisen is brought into the converter, a very

bright flame issues from the mouth of the vessel, and this flame exhibits a spectrum (No. 4, fig. 42a) which really contains the same lines as that of the Bessemer flame, although the general appearance of the spectrum is completely changed by the alteration of the relative brightness of the lines.

Those who are practically engaged in working this process would like spectrum analysis to do a great deal more; they would like to be told whether there is any sulphur, phosphorus, or silicon in their steel: questions which unfortunately at present spectrum analysis cannot answer, for this very good reason, that these substances do not appear at all as gases in the flame, but that they either remain unvolatilized in the molten metal, or swim on its surface in the slag of the ore; and consequently the lines of these bodies are not seen in the spectrum of the flame.

The next point to which I would direct your attention is one of a slightly different kind. We find that certain substances-not only gases, but liquids, and even solid bodies-exert at the ordinary temperature of the air a selective absorption power upon white light when it passes through them. In the following lecture I shall have occasion to show you, in various ways, the absorptive effect which glowing sodium vapour exerts upon the particular kind of yellow light which sodium itself gives off; but I would now consider some cases of selective absorption occurring at the ordinary temperature, and just indicate to you a most interesting and important branch of this subject which has been, to a certain extent, worked out, but in which a rich harvest of investigation still remains open. I refer to the absorption spectra obtained by the examination of various

coloured gases and liquids, especially of blood and other animal fluids. In the first place, then, it has long been known that certain bodies have at the ordinary temperature the power of selectively absorbing certain particular kinds of light. In Fig. 43 we have a representation of the selective absorption exhibited by two coloured gases: No. 1 shows the dark bands seen when white light passes through the violet vapours of iodine, whilst No. 2 gives the bands first observed by Brewster in the red fumes of nitrogen tetroxide, which have since been shown by Kundt to be identical with those which the liquid tetroxide

gives. Some coloured gases, such as chlorine, do not give any dark absorption bands; whilst, on the other hand, certain colourless gases, such as air and aqueous vapour, exert a remarkable power of selective absorption, and exhibit spectra filled with dark lines. I shall return to this subject of the atmospheric lines in a subsequent lecture. Perhaps the most striking instance of the formation of these absorption lines in the case of liquids is the one which I will show you of this colourless solution of a salt of the rare metal didymium. All the didymium salts possess the power of absorbing

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