zation, we are presented with phenomena dependent, not upon structure, that is to say, the positions and mutual distances of the particles with regard to each other, but upon the nature of the ultimate particles themselves. This peculiar action of matter upon light may even afford the means of detecting varieties in the nature of substances which elude the more direct means of chemical examination, and M. Biôt has shown that mixtures of the sugar of the cane and of grapes may thus be recognised and estimated, which would utterly defy the powers of chemical analysis.

He has applied the property with success, as a test of the value of different samples of the expressed juice of beet-root in the manufacture of sugar, and has thus conferred even a commercial importance upon a discovery, which at first appeared to have as little relation as possible to the arts or conveniences of life.

He has also applied it to the determination of the existence and quantity of sugar in the urine of diabetic patients. The progressive influence of remedies upon this formidable disease may thus be determined from day to day by simple inspection, and the physician can direct his practice accordingly.

An apparatus which can exhibit all these phenomena with sufficient precision for purposes of illustration, and which can be constructed at a moderate expense, has been contrived by Professor Powell, and is described below with his kind permission (67).

§ 266. Another curious application of the properties of polarized light has been made to practical purposes; and such

horizontal wave is produced by the balls; when at D' D', an elliptical; and when at D" D", a circular wave. At fig. 2, an attempt has been made to represent the balls in a state of elliptical movement.

It is to the balls alone that the attention is to be directed, the mechanism by which their motions are produced being out of the question; and it must be remembered that their motions only approximatively represent to the eye the path which each particle is supposed to describe.

(67) The apparatus represented in the next page is fixed vertically upon any convenient support. The light from a flame or from the clouds is thrown into the required direction by a small plane mirror, s, into the polarizing mirror, P, inclined at the proper angle to the axis. It thence passes through a small aperture up the axis of the

examples furnish an admirable rebuke to those who affect in their ignorance to treat with disdain all such scientific researches as cannot at once, according to their narrow notions, be rendered useful.

It is well known that in dangerous navigations, a commander of a ship will place himself at the mast-head, for the purpose of more clearly seeing the rocks and shoals at the bottom of the sea: experience has taught him that from this lofty position they are more visible than from the deck. The reason of this is apparent from the common laws of the reflexion, and refraction of light: the greater the angle of reflexion, the greater the quantity of light which is turned aside, and the less that which penetrates. The refracted light from the bottom of the sea is not perceptible amidst the glare of that which is reflected to the lower position, for the eye cannot appreciate an addition of less thanth of the whole quantity which affects it: but at the smaller angle from the higher position, the reflected light is so diminished that the refracted portion makes its due impression.

Now this purpose is still more effectually attained by viewing the objects at the bottom of the sea through a polarizing tube; for at the proper polarizing angle, nearly the whole of the reflected light may be extinguished, and they become perfectly visible by their direct refracted light.

brass tube, T, in which is inclosed a common test-tube, from 6 to 24 inches in length, filled with the liquid under examination.

The analyzing part, A, consists of a graduated rim, w, for measuring the rotation of the index, v, attached to the tube containing R, a rhomb of Iceland spar, at least 1 inch or thick in its natural state, having a very small hole, H, at the bottom, through which the light is admitted. In the upper part, another tube slides, carrying a lens, L, which magnifies the separation of the images, and gives two sufficiently large well-defined circular images of H, in which all the changes of tint can be distinctly observed.

Supports are omitted in the figure, and means should be provided for measuring, as accurately as possible, the length of liquid traversed by the ray in the tube.

W

R

IX. RADIANT HEAT.

$ 267. RADIANT heat, as contradistinguished from light, is subject to all the laws of optical phenomena, but is variously affected and modified by different forms of matter, according to the intensity with which it is propagated or projected. The recent experiments and investigations of M. Melloni, have even rendered it highly probable that differences exist between obscure rays of heat which are analogous to the differences between the coloured rays of light; which although they cannot, like the latter, be made the objects of sight, may be established by certain physical properties and relations: just as different physical properties may be found, independent of their colour, in the different rays of luminous spectra.

Radiant heat like light, can pass through a few substances, but is arrested by the greatest number, and is subject to reflexion, absorption, secondary radiation, refraction, and polarization.

But what shall we say with regard to the nature of this extraordinary agent which we thus loosely designate as heat? The idea of a medium of communication between its source and the objects affected by it is as necessary as in the case of light; and the reasons which direct us to the hypothesis of undulations in an ethereal medium are as valid for one case as the other. The supposition of a second ether interpenetrating the luminiferous ether would be extravagant; but if we conceive the vibrations to take place in the same elastic medium, how do the calorific undulations differ from those which constitute light? In direction? in velocity? in intensity?

If, again, we admit the explanation with regard to radiant heat, how shall we apply the hypothesis to the different classes of phenomena which we have already examined? to the phenomena of specific heat? of latent heat? &c.

The boldest imagination could scarcely form a conception of undulations rendered latent without annihilation; laid up in store, as it were, and capable of being drawn forth at pleasure in full measure and intensity.

We cannot have a better exemplification of the true value and use of such hypotheses, and we ought never to lose sight of the great lesson which is conveyed by the instance before us, viz., that such conjectures, however happy they may be in their

applications, should never be dogmatically contended for as the true physical explanation of the phenomena, but only regarded as useful generalizations and temporary helps towards that completeness of knowledge which may not be within the reach of man in his present condition, but for the attainment of which he is permitted and encouraged to strive.

With a view to such useful and practical application, the hypothesis of emission is best adapted to the calorific phenomena which we have examined; according to which bodies are conceived to consist of particles, each of which gathers round it, by its attraction, a quantity of an imponderable highly elastic fluid, to which the name of caloric has been given; that the particles of the bodies attract each other, besides attracting the caloric, and that the particles of the caloric repel each other.

The phenomena of radiation are, on the other hand, best and most elegantly explained by the undulatory hypothesis of an ethereal medium; and to these let us now turn our attention.

$ 268. When the rays of unmixed light are absorbed, they escape the cognizance of our senses: but when the calorific rays are absorbed, they produce the sensation, and universal expansion, of heat. If a perfectly transparent colourless glass tube be filled with ether, and be placed in the rays of the sun, they will be freely transmitted without raising the temperature of the liquid: but if a piece of charcoal be immersed in the ether, the radiant heat will be arrested, and the temperature will speedily rise to the boiling-point.

It is impossible to examine and experiment upon the light of the sunbeam, without observing that it is accompanied in all its affections by calorific rays. We obtain foci of heat, as well as luminous foci, from concave metallic mirrors and convex glass lenses, according to the laws of reflexion and refraction. The laws, however, of this radiant force are best studied in its projection from terrestial bodies, whose temperature is under our command, and may be varied for the purposes of experiment.

§ 269. The intensity of radiant heat, like that of light, and for the same reason, decreases as the square of the distance from the source of the rays:-thus, if a thermometer protected from the influence of all disturbing causes, be observed to rise a certain number of degrees at one inch distance from a heated surface, it will indicate four times less heat at two inches;

nine times less at three inches; and sixteen times less at four inches.

§ 270. The primary law of reflexion at an angle equal to the angle of incidence, is also easily recognised by holding a bright metallic plate before a common fire; the rays of heat may then be turned in any direction according to this law, and tested by our sensation; when we see the image of the fire in the mirror we also feel its heat.

If we fix two concave metallic mirrors, at a distance apart of about ten or fifteen feet, with their axes in the same line and with their faces parallel and opposed to each other, upon placing a thermometer in the focus of one, it will be found sensible to the effects of a heated body placed in the focus of the other. It is easy to assure ourselves that the effect is owing to reflexion, and not to the direct radiation of the heated body, by removing the thermometer out of the focus, and approaching it towards the source of heat, when it will be found to fall; or the same return to its original state may be produced by placing a screen between the thermometer and its own mirror: when the reflected rays will be cut off, although the direct communication with the radiating body will still be open. In the same way when a red-hot iron ball is placed in one focus, a piece of paper will be scorched in the other, and gunpowder and phosphorus may be inflamed even when the temperature of the ball is below ignition (68).

The heated body placed in one of the foci of these conjugate mirrors throws off its rays in all directions, and those which strike upon the surface of the nearest mirror are reflected,

(68) The annexed figure shows the disposition of the apparatus. A and B are the two polished metallic mirrors. The hot iron ball is placed in the focus, c, of the mirror, A, towards which it radiates its

heat. The diverging rays are reflected in a parallel direction upon B by a second reflexion from which they are rendered convergent, and fall upon the thermometer, D, placed in the focus to receive them.