670 Spectra of Metallic Vapours. in the red and orange, and one in the blue, but of greater wave length than the calcium blue. Phosphorus gives a number of green lines only. Hydrogen gas a red, a blue, and an indigo-blue band. The last spectrum in the fig. (235) is interesting as that of one of four rare metals which have been discovered by spectrum analysis, and as having a rather curious history. It was discovered by Professor Bunsen in 1860. When examining by the spectroscope the flame given off by a mixture of salts left from the evaporation of a large quantity of mineral water, he remarked some very bright lines which he was sure did not belong to either the soda or potash of the mineral water. After evaporating the enormous quantity of forty-four tons of this mineral water, he succeeded in separating the new metals, though from this large quantity of water, he had only 200 grains of the mixed metals. To the one he gave the name of Rubidium, from its ruby or red lines, which are, however, nearer the heat end of the spectrum than the potash lines. To the other, and the one indicated in the figure, he gave the name of Casium, from two very characteristic blue lines, while it gives also orange lines, and no red lines at all. A third metal, discovered in 1861 by Crookes, in the same way, is known as Thallium, from thallus (Lat.), a green bud. Its spectrum consists of a single very distinct green line. The fourth metal, discovered by a similar process, is known as Indium, from its spectrum being composed simply of two lines in the indigo. It was discovered (1864) in some zinc ores by the two German professors, Reich and Richter, of Friburg. (See Roscoe's 'Spectrum Analysis.") 905. To summarize the general results of such spectroscopic examination of incandescent metals, we may say that each metal has its own peculiar set of luminous rays or lines which it gives forth, and that it appears from a most careful examination of these, that there is no overlapping of the lines of one metal with those of another. Copper gives a set of green bands; zinc a set of bright blue and red bands; while brass gives both the copper and zinc lines, so that its composition, if not otherwise known, might in this way be unerringly deduced. The composition of atmospheric air is revealed by spectroscopic examination of it when a small quantity of it is rendered incandescent by the electric discharge within what is known as a Geiss. ler's tube. The oxygen lines, the nitrogen lines, and the hydrogen Reversal of Lines in the Spectrum. 671 lines of the water vapour, always present in greater or less quantity, are all distinctly recognisable by the experienced spectroscopist. 906. The question at once suggests itself—What mean those remarkable dark bands in the solar spectrum, if they correspond exactly in situation with the bright lines peculiar to particular metals, as in many instances they do? Does the dark D line mean that there is no sodium in the composition of the sun? The answer to the question was first given by the German professor Kirchhoff, and involves a new theory of the sun's constitution. Kirchhoff's theory is founded on the experimental fact that vapours and bodies in general absorb, and consequently fail to transmit, the very luminous rays which they emit when in a state of incandescence. Thus, if the vapour of sodium come between the slit of the prism and a flame impregnated with sodium vapour, and giving forth the characteristic D lines, the ethereal rays of the burning sodium are absorbed by the sodium vapour, and the band is completely cut off. The annexed figure (fig. 236) will serve as an illustration. In the spectrum, A A, the lines, C C, are such as would be produced by the incandescent vapour of sodium; while in the spectrum, B B, they are represented by the dark lines, D D. They are reversed by absorption on being transmitted through another portion of vapour. A B CC B DD Fig. 236. The same remarkable phenomena appear with other incandescent metals and their vapours. Thus, too, if sodium vapour be interposed between the flame of a candle, it cuts out the D line and gives us what is known as the reversed sodium spectrum; and, if we interpose other incandescent vapours, they will each cut out from the continuous spectrum their own peculiar lines. Here, then, we have the basis of Kirchhoff's theory, that the internal mass of the sun has an intensely white-hot surface, which emits white light-that is, light waves of all degrees of refrangibility; but outside of this light sphere or photosphere, as it is now usually termed, is an envelope or atmosphere of less hot, but still glowing, gases and vapours, which is called the chromosphere; it is composed of the vapours of the different metals, which may exist in a molten state in the photosphere, and exercises the same sifting or absorptive action on the rays issuing from within, as the artificial vapours mentioned above. 672 Constitution of the Sun and Stars. 907. From the foregoing statements, then, we should infer that sodium, hydrogen, calcium, strontium, iron, and the very rare nietal titanium, whose bright lines coincide exactly with dark lines of the solar spectrum, must all be present in the solar vaporous envelope.* The truth of this hypothesis is upheld by the fact that a spectroscopic examination of the reddish protuberances which appear under various shapes in total eclipses of the sun, displays bright lines in positions corresponding to the C (near), D, F, and near G, dark lines of Fraunhofer. This amounts almost to a demonstration of the existence of a gaseous envelope, composed to a great extent, if not principally, of hydrogen, the characteristic dark lines of which are C, F, and one in front of G. (See fig. 235.) The application of this method has extended the bounds of chemical analysis to the planetary and stellar spaces. It can be asserted, with complete moral certainty, that the atmospheres of certain stars contain many of the metals we are acquainted with— as, for instance, hydrogen, sodium, calcium, magnesium, iron, bismuth, antimony and mercury. The colours of the stars are due to their special absorption of certain portions of the spectrum, doubtless by their gaseous envelopes. The light of the moon shows the very same lines as the solar spectrum, showing that there is no additional absorbing atmosphere round the surface of the moon. On the other hand, the spectrum of the planet Jupiter shows unmistakable traces of additional absorption, due, no doubt, to the existence of an atmosphere. Chemical properties of light. Light, like heat and electricity, has in some instances a combining, and in other instances, 908. Thus when a mixture of chlorine and hydrogen is exposed to the direct rays of the sun or any intense light, as the oxyhydrogen or lime light, a sudden combination takes place with explosion. Under diffused daylight, the gases equally combine to form the same compound, but more slowly and gradually. When kept in the dark at the same temperature they do not combine. Bunsen and Roscoe have ingeniously made the rate of combination a measure of the intensity of light for photometrical purposes, and they have thus * Account has of course to be taken of the dark lines due to absorption by our atmosphere. The A and B lines of Fraunhofer are now regarded a air-lines. Chemical Properties of Light. 673 been able to make numerous comparisons on the relative intensities of artificial lights and the light of the sun. It is a rema kable fact that the combining power resides in those rays of the spectrum which are near to the more refrangible colours, the violet and the blue. When exposed to the yellow, orange, or red rays the gases show no tendency to combine. Under the influence of light, chlorine decomposes water, combines with the hydrogen, and sets free oxygen. In accordance with what has been above stated, this decomposing power is chiefly manifested by the most refrangible rays of the spectrum, blue and violet. In orange or red coloured glass no chemical action takes place. The chemical rays are therefore independent of those of light and heat. They extend beyond the most refrangible rays, and their existence may be there made evident by means of uranium glass. They may be sifted and practically separated from light by causing them to pass through a layer of deep orange (amber) or red coloured glass. These facts are brought daily into practice in the art of photography. We have in this art an interesting illustration of the chemical power of light, or rather of certain rays which are associated with it. Freshly prccipitated chloride of silver exposed to light, or to any of the coloured rays of the spectrum as low as the yellow, undergoes a remarkable chemical change. It blackens, and is reduced to the state of subchloride, or even of metallic silver, in which state it is rendered quite insoluble in certain liquids, by which the fresh chloride is readily dissolved. The white chloride kept in absolute darkness undergoes no change. On these reactions depends the production of photographic drawings on paper. The chemical operation of light is here indicated by an evident change of colour and properties. When other compounds of silver, such as the iodide and bromide, are used in thin layers on glass or mia, there is no visible change after exposure to light, but on pouring over the exposed surface, either immediately or after the lapse of many hours (provided the films are kept in the dark), a perfect image is produced in which all the parts which have received light are darkened, while those which were in shade remain unchanged. A solvent separates the unchanged bromide, and a per. manent impression is left in which light and shade are reversed. The plate, which in this state is called a negative, is subsequently employed for printing any number of positive impressions with the light and shade in their proper relations. 674 Chemical Effects of Light. 909. By the aid of these chemical changes and certain ingenious arrangements, light is made to record the amount of electricity in the atmosphere, the duration and force of the wind, and the relative degrees of light and darkness, with numerous other matters of scientific interest. Even the shape of a cloud, the waves of the sea in motion, and the discharge of a projectile from a gun, have been thus accurately delineated. The chemistry of light is perhaps most strongly seen in the power which it imparts to vegetation, of fixing the carbon in the vegetable structure and liberating the oxygen from the carbonic acid of the atmosphere. A noxious ingredient is thus removed and its place is supplied by a gas, without which no animal could live. In the absence of light, the green parts of vegetables cease to eliminate oxygen. The source of fuel has been already described in the section on Heat, and it is here referred to as an illustration of the influence of light. Light has been supposed to exert an influence on the volatility of camphor, as this substance is generally found deposited in crystals on the sides of bottles containing it, which were exposed to light. It has, however, been clearly proved that this is an effect of heat radiation and cooling, taking place more readily from the surfaces on which the camphor is deposited. A remarkable effect of light is shown in reference to certain crystals. Santonine in a pure state is in colourless crystals. By exposure to light these crystals acquire a brilliant yellow colour. Some crystals decompose light into its complementary colours. The crystals of platino-cyanide of magnesium are of a ruby red in one aspect and an emerald green in another. These are truly dichroic. Other crystals present the shades of yellow and violet. There are other remarkable properties possessed by light, such as the imparting electric conductivity to Selenium, as described by Dr. Siemens. These, however, require no special notice in this place. The section on optics would not be complete in the absence of any notice of the modern wave-theory, as contrasted with the old view of light being an emanation from the sun and other selfluminous bodies. Many of the phenomena of light admit of explanation on either view, and to some, the emanation theory, which has been generally adopted throughout this section, may appear more simple and intelligible for the purposes of description. Light was formerly regarded as an exceedingly minute material emanation from the sun or other luminous body, |