of this invisibility is rather a physiological than a physical question; nevertheless, the suitableness of this arrangement is at once apparent, for if any other law were to holdif, for instance, the eye were affected by each substance in proportion to the difference between its radiation and that due to the temperature of the human body—it is difficult to conceive how we could either enjoy the advantages of darkness, or experience that variety of shade and colour which is one of the chief pleasures of vision. It is also worthy of remark that by the present arrangement our safety is secured, for the eye is generally able to detect the presence of combustion when it occurs.

RADIATION OF A PARTICLE; RADIATION OF GASES.

238. Having discussed the radiation from heated surfaces, that from thin plates or particles comes next to be considered. Take, for instance, a glass plate at a low temperature: this will stop nearly all the rays corresponding to this temperature, and therefore it will behave very much like a lampblack surface. But at a high temperature (above redness, for instance) it will pass a great many of the rays of this temperature; and hence, its proportional absorption being less, its proportional radiation compared to lamp-black will also be less. The radiation of such a plate will not therefore increase with the temperature as fast as that from a lamp-black surface. Many other bodies beside glass possess the property of being more opaque to heat of low than to that of high temperature, and for all these the radiation will not increase with the temperature so fast as that from a black surface.

The thinnest plates of solid or liquid substances which we can obtain will not, however, afford us the means of studying the radiation from a particle; in order to do this. recourse must probably be had to a gas, each of whose

particles we may perhaps suppose acts for itself, and is not fettered by the neighbouring particles in the way in which it would be in the solid or liquid state.

239. In studying the radiation of gases we are led to some very peculiar laws.

1. In the first place, we may say that the general absorption and radiation of gases are often small, while on the other hand the selective absorption and radiation of many of them are very strong. The feeble radiation from heated air was observed by Melloni, and the feeble absorptive power of it (and of many other gases) for light is familiar to every one. Nevertheless, by aid of electricity we are enabled to heat a portion of any gas or vapour to a very high temperature, so as to obtain a visible spectrum from it, which we may then analyze by means of the spectroscope. Such spectra when obtained are always discontinuous, that is to say, they consist of a very intense radiation of certain disconnected spectral rays, while the intervening spaces are totally, or nearly, dark. It matters not what gas be subjected to analysis, the result obtained is of this nature in all cases. The spectra of all gases are thus characterised by a few bright lines on a dark background.

2. In the next place, as far as we know at present, the bright lines given out by any one gas have not been found to coincide in spectral position with those given out by any other gas. One or two coincidences of this kind have been suspected, but these have not been confirmed by results of a more searching analysis. Elaborate researches on the spectra of gases have been made by various philosophers.

3. In the third place, the spectra of gases probably remain of the same character, with certain limitations to be afterwards mentioned, throughout a very wide range of temperature. Thus we know that the vapour of metallic sodium, as soon as it has attained a yellow heat, will give out exclusively the

double line Ɗ in the yellow, and it will continue to radiate this kind of heat up to the highest temperature we can produce. The same law holds for other gases and vapours, only we must make the following exception. If, for instance, one of the bright lines given out by a heated vapour be in the blue of the spectrum, and if this vapour be capable of existing at a red heat, we must not expect that it will give out the blue line at this heat, nor until the temperature rises to such a degree that blue becomes one of the constituent rays of that temperature. When this is reached the blue line will be given out, and when once given out it will probably continue for all higher temperatures.

240. These laws of gaseous radiation have lately become of great practical importance. Let us recapitulate them.

In the first place, the spectrum of an ignited gas consists of a few bright lines of definite refrangibility.

Secondly, these lines are probably not the same for any two substances.

Thirdly, the lines peculiar to any substance remain the same throughout a great range of temperature.

If to these three laws we add the following chemical one, namely, that at a very high temperature most substances are decomposed, we shall soon readily perceive the great practical importance of this combination of facts.

For if we already know the spectra of the various chemical elements, and if we heat a specimen of any substance presented to us for analysis sufficiently to resolve it into its elements and to drive these into the state of vapour, then will the position of the bright lines of the spectrum of the flame obtained enable us to ascertain what elements were present in the substance, since each element will furnish its own peculiar lines, which are supposed to be known and recognizable. It was first remarked by Professor Swan that by means of the well-known and peculiar double line

D the presence of a salt of sodium may be detected in a most delicate manner; and Bunsen and Kirchhoff, who have done much more than any one else to introduce and perfect this method of analysis, remark that by means of the spectroscope the presence of less than 200.000.000 of a grain of sodium may probably be detected. Bunsen has also by this means discovered two new metals, namely casium and rubidium. Our countryman Crookes has discovered thallium, and Messrs. Reich and Richter indium, by the same means.

An apparent exception to the law that the nature of the spectrum of a gas remains constant throughout a great temperature range, ought to be remarked in the case of nitrogen, which changes the nature of its spectrum at a very high temperature. This is viewed by some as an indication that nitrogen is in reality a compound body, since the same change takes place in the spectra of some other gases which we know to be compound.

241. Before concluding this chapter we ought to allude to the beautiful discovery of Kirchhoff, by which it has been proved that substances with which we are here familiar exist also in the atmospheres of the sun and stars. It had been observed, first by Wollaston and after him by Fraunhofer, that the solar spectrum contains a number of dark lines, while it is in other respects a continuous spectrum, and the latter observer extended his remarks to the spectra of many of the fixed stars. The origin of these lines for a long time remained a mystery, nor was this mystery diminished when it was found by Fraunhofer that a bright band corresponding in refrangibility to the double dark line D of the solar spectrum was produced by the light of a flame containing sodium. Sir D. Brewster was the first who prepared the way for the solution of this problem, by shewing that analogous (not identical) lines might be artificially produced by inter

posing a jar of nitrous acid gas in the path of a ray of light. The inference naturally drawn from this experiment was, that the lines of the solar spectrum do not denote rays originally wanting in the light of the sun, but are due to the absorption of his light by some substance interposed between the source of light and the spectator. It was doubtful, however, whether this stoppage of light occurred in the atmosphere of the sun or in that of our earth, until the matter was finally settled by Kirchhoff, not however before the true explanation had been divined by Professor Stokes.

Kirchhoff found that a sodium flame which gives out on its own account the double line D absorbs a ray of the same refrangibility when it is given out by a body of a higher temperature than the sodium flame, thus producing a dark line D instead of a bright one, and he therefore conjectured that the dark line D in the light of our luminary was occasioned by the presence of the vapour of sodium in the solar atmosphere, and at a lower temperature than the source of light. This belief was strengthened by his finding that many of the dark solar lines correspond in position with the