denominated thallium, from the beautiful green ray which characterizes its spectrum.

(77) The position of the bright lines in the spectra of the gases is not affected by the temperature of incandescence, but their brightness is; and it happens thus, that lines which are invisible at the lower temperature, become visible when the temperature is much raised; and this process continues until, at extreme temperatures, the spectrum becomes continuous. Thus lithium, whose spectrum consists of a single red line at ordinary temperatures, exhibits an orange and a blue band when the temperature is highly raised.

(78) The rays of light emitted by an incandescent gas are those which it absorbs at lower temperatures. This property, which has been denominated the reversal of the spectra of the gases, was first discovered in the spectrum of sodium vapour. When a little common salt is introduced into the wick of a spirit-lamp, the spectrum exhibits the bright yellow double line which is characteristic of sodium; and it is found that the vapour of sodium, at the ordinary temperature, stops or absorbs precisely the same ray, when it is interposed in the path of the continuous light emanating from an oil lamp, the light of which gives with the prism a continuous spectrum. A more exact process is to introduce a small piece of metallic sodium into a glass tube closed at both ends, and from which the air has been expelled by introducing hydrogen. On heating the tube by means of a spirit-lamp, the metal is vaporized. By such means Bunsen and Kirchoff found that the vapours of the alkaline metals, lithium, potassium, strontium, and barium, when interposed in the light of a continuous spectrum, absorb precisely the same rays which they emit when self-luminous.

(79) The general law, of which the foregoing is a par

ticular case, was enunciated by Kirchoff in 1860. It may be thus expressed:-The relation between the power of emitting, and the power of absorbing any given ray, is the same for all bodies at the same temperature. This law applies to the calorific, and to the chemical rays, as well as to those of light.

The relation between the emissive and the absorptive powers of any gas is determined by the difference of temperature of the gas in the two cases. The greater that difference, the more marked is the phenomenon of reversal.

The mole

The law above stated, and which had already been demonstrated, in the case of heat, to be a necessary consequence of the conditions of equilibrium of temperature, has been shown by Professor Stokes to be a necessary consequence of the laws of propagation of vibratory motion. cules of an incandescent gas vibrate in a definite manner, producing rays of definite refrangibility. Hence the ether within a gas will be set in vibration by these same rays, which will thus communicate to it their vis viva, and be absorbed. On the other hand, the waves whose time of vibration is different, will not set the ether in motion, and will be transmitted through it without loss.

(80) We have already adverted to the similarity of the absorptive spectra of the gases, to the phenomena of the solar spectrum; and we are at once led to the conclusion, that the light of the Sun is that of an incandescent solid, surrounded by an atmosphere of vapours, which absorb certain definite rays. We have now only to compare, by the spectroscope, the position of any of these rays, with those of the bright lines in the spectra of the metallic vapours, and if we find them to be identical, we may conclude with certainty that that vapour exists in the solar atmosphere, and therefore the metal itself in the body of the Sun. This comparison has

been made by Kirchoff, and he has thereby ascertained that sodium, bismuth, magnesium, chrome, nickel and iron, all exist in the Sun. The metals, gold, silver, mercury, tin, lead, aluminium and silicum, on the other hand, do not exist in the Sun, their specific rays being wanting in the solar spectrum.

(81) It has hitherto been assumed that, when a ray of light is incident upon the surface of a transparent medium, the intromitted portion pursues, in all cases, a single determinate direction. This is, however, very far from the fact. In many,—indeed in most cases, the refracted ray is divided into two distinct pencils, each of which pursues a separate course, determined by a distinct law.

This property is called double refraction. It was first discovered by Erasmus Bartholinus, in the well-known mineral called Iceland spar. After a long series of observations, he found that one of the rays within the crystal followed the known law of refraction, while the other was bent according to a new and extraordinary law not hitherto noticed. An account of these experiments was published at Copenhagen, in the year 1669, under the title, "Experimenta Crystalli Islandici dis-diaclastici, quibus mira et insolita refractio detegitur."

A few years after the date of this publication, the subject was taken up by Huygens. This distinguished philosopher had already unfolded the theory which supposes light to consist in the undulations of an ethereal fluid; and from that theory had derived, in the most lucid and elegant manner, the laws of ordinary refraction (37). He was, therefore, naturally anxious to examine whether the new properties of light, discovered by Bartholinus, could be reconciled to the same theory; and, in his desire to assimilate the two classes

of phenomena, he was happily led to assign the true law of extraordinary refraction. The important researches of Huygens on this subject are contained in the fifth chapter of his "Traitè de la Lumiere."

(82) The property of double refraction is possessed by all crystallized minerals, excepting those belonging to the tessular system, i. e. those whose fundamental form is the cube. It belongs likewise to all animal and vegetable substances, in which there is a regular arrangement of parts; and, in fine, to all bodies whatever, whose parts are in a state of unequal compression or dilatation. The separation of the two refracted pencils is in some cases considerable, and the course of each easily ascertained by observation; but it is generally too minute to be directly observed, and its existence is only proved by the appearance of certain phenomena which are known to arise from the mutual action of two pencils. In Iceland spar, the substance in which the property was first discovered, the separation of the pencils is very striking: and, as this mineral is found in considerable masses, and in a state of great purity and transparency, it is well fitted for the exhibition of the phenomena.

A

B

(83) Carbonate of lime, of which Iceland spar is a variety, crystallizes in more than 300 different forms, all of which may be reduced by cleavage to the rhombohedron, which is accordingly the primitive form. The angles of the bounding parallelograms, CAB and ABD, in the rhombohedron of Iceland spar, are 101° 55' and 78° 5'. Two of the solid angles, at A and O, are contained by thre obtuse angles; while the remaining four are bounded by one obtuse and two acute angles. The line AO, joining the summits of the obtuse solid angles, is called the axis of the rhom

C