tions were proprogated to great distances. If Sicily then is so often shocked, the powers which agitate it must exist in volcanoes that burn within its own bosom, and in the surrounding sea. Situated in the midst of such grand operations of nature, Sicily must be exposed to all the effects which such powerful causes are capable of producing. The chemical subterranean operations require that the earth should every where be traversed by vast cavities and canals, running in various directions; and the forces of the operations act on the different parts of these cavities. But it is natural to believe, and many facts in this memoir demonstrate the truth of it, that places in the vicinity of the three great volcanic outlets ordinarily feel the force with the greatest violence. In this respect the situation of Palermo is very advantageous; since it is distant from Ætna, and from Eolia and is near to Sciacca only, which is the least energetic. And this grand and respectable city would be less exposed to such grievous disasters, than all the other cities of Sicily, did its edifices possess that character, which they might easily be made to possess, which constitutes true solidity and resisting firmness.

ART. XXI.-Remarks on Solar Light and Heat. By BADEN POWELL, M. A. of Oriel College, Oxford. [Annals of Phil.]

SIB

[To the Editor of the Annals of Philosophy.]

Ix explanation of the design of the present communication, I conceive it necessary to observe in the first instance, that having been engaged in experiments on solar light and heat, I have laid accounts of some of them before the Royal Society (see reports of Royal Society, Annals, Feb. 1824): those accounts, however, being confined to the mere detail of the experiments, I wish through the medium of your journal to offer some remarks on the subject, of a more general nature, and which may be considered as forming a sort of introduction to such experimental researches.

If then in taking a brief review of the present state of our knowledge upon this subject, my remarks and statements should not be of a nature wholly new, my design as thus explained will be a sufficient excuse; and the more so as I could not proceed to the few experiments here given without such preliminary considerations.

I am, Sir, your obedient Servant,

B. POWELL.

I. (1.) Speaking according to our ordinary sensations, we are accustomed to say that the sun communicates both light and heat. Light is transmitted in a way which we term radiation. The heat from non-luminous hot bodies is transmitted to a distance in a way closely analogus; and to which the same name has been applied. In the first instance we might suppose that the sun sends out two separate emanations, one of light, and another distinct from it, and similar to that of radiant heat from a mass of hot water, and this perhaps, was the first view taken of the subject, though a confused idea of some very close and intimate connexion subsisting between the solar light and heat appears to have prevailed.

(2.) This subject, as might naturally be expected, attracted the early notice of experimenters. A very slight examination sufficed to show that the rays of solar heat (whatever their' nature might be) differed essentially in many properties from those of terrestrial heat, whether radiated from luminous or non-luminous bodies. Whether there existed a separate set of heating rays distinct from those of light, and at the same time differing in many respects from rays of terrestrial heat; or whether these differences depended on some unknown property of the rays of light, was a question which for a long time remained without any direct investigation, and on which even now, we have, perhaps, no very precise ideas.

Among the earliest experiments on the subject, if not actually the first, were those of Mr Boyle, on the different degrees of heat communicated by the sun to black, white, and red-coloured surfaces. These were extended and confirmed in the well-known investigations of Dr Franklin, &c.

"Mr Boyle caused a large block of black marble to be ground into the form of a spherical concave speculum, and found that the sun's rays reflected from it were far from being too powerful for his eyes, as would have been the case had it been of any other colour; and although its size was considerable, yet he could not set a piece of wood on fire with it; whereas a far less speculum of the same form, made out of a more reflecting substance, would presently have made it flame."--[Boyle on Colours, &c.]

Scheele conceived that the Sun's rays of light produced heat, not when in motion, but only when stopped by the interposition of solid bodies.-[Treatise on Air and Fire, &c.]

Mr Melville seems to have viewed the matter nearly in the same light, and to have conceived reflection at an opaque surface, the cause of excitation of heat from the sun's rays.[See Phil. Mag. June, 1815, a paper by Dr Evans.]

(3.) In later times the experiments of Prof. Leslie, Sir H. Davy, &c. have sufficiently established the property possessed by that emanation (whatever its nature may be, whether simple or compound) which is derived from the sun, of producing greater heat in bodies in proportion as their surfaces owing to darkness of colour have the capacity of absorbing rays of light. It has been equally well established by Prof. Leslie, Count Rumford, &c. that the heat emanating from a mass of non-luminous hot matter, has no such relation to the colour, though a very close one to the nature and texture of the surface.

(4.) The experiments of Sir E. Home [Phil. Trans. 1821, Part 1.] are particularly deserving of attention, as exhibiting what might at first sight be considered an exception to the above remarks; a greater effect being produced in some instances on a white than on a black surface. A more attentive examination, however, will show us that these experiments prove thus much. The heat occasioned by the rays of the sun when received directly, or when in some degree intercepted, as by thin white cloth, on the skin, is greater than that communicated by conduction to the same skin, through a black cloth in contact with it, which is itself, in the first instance, heated by absorbing the rays.

A white skin is scorched, and a Negro's skin is not, in ten minutes, by the direct rays of the sun; that is, as before, the outer coat of the skin allows some of the direct rays to pass through and affect the sentient substance beneath; whereas in the case of the black skin, the rays are absorbed by the black surface, and so affect the sentient parts only as heat of temperature.

II. (5.) As to the nature of this heating effect, the greatest difference of opinion has long prevailed among the most disfinguished philosophers; one party maintaining the totally distinct existence of light and radiant heat in the compound solar beam; the other contending for the absolute identity of the two: the same principle being merely displayed under two different modifications.-[See Sir W. Herschel, Phil. Trans. 1800. Part II.; Leslie on Heat, p. 162, Biot, Traité de Physique, vol. iv, p. 690, &c.]

Without entering upon an examination of the merits of either theory, we may proceed to remark, that the first object in the inductive examination of this subject is to ascertain distinctly what peculiar properties of this heating emanation we can fix upon by which its nature may be defined, and by

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the help of which we may be enabled to compare it with other heating emanations.

(6.) Among the most obvious properties of the solar rays, we perceive that before adverted to, viz. that they produce heat on bodies in proportion to the darkness of their colour, and not in regard to the absorptive qualities of the texture of their surfaces from the heat from non-luminous bodies. This relation is universal, and without any exceptions; it is consequently one which we can satisfactorily adopt as the foundation of a distinctive description.

(7.) We may from this advance to another test, which will afford an additional characteristic. It has been distinctly shown that all heating emanations from terrestrial bodies, whether luminous or not, are more or less stopped, or even in some cases totally intercepted, by the interposition of a glass screen. Similar experiments may easily be tried on the solar rays.

That little or no diminution of effect is produced on a blackened thermometer exposed to the sun, by the interposition of glass, has been shown by several experiments. As it is remarked by De la Roche [Biot, Traité de Phys. vol. iv. p. 611]. I have frequently observed the same thing, taking notice of the temperature of the glass, as will be subsequently seen. But there is another part of the question which still appears to me to want further examination. The sun's rays produce some heating effect on surfaces of a light colour. I have, therefore, tried whether also in this case, and when the texture of the surface was very absorptive for simple radiant heat, a glass screen has any power to diminish the effect.

Two thermometers were exposed together to the direct and screened rays, one having its bulb coated with indian ink; the other with a thin paste of chalk and water; the bulbs were free from contact. If there existed in the solar beam any rays of such a nature that they were affected by the texture rather than the colour of surfaces, and were not capable of passing through glass, they would be affected by a surface of chalk more than one of indian ink. If they formed only a small proportion of the whole, the diminution, when glass was interposed before the inked thermometer, might be so small as to be imperceptible; but with the whitened surface, it would be much more conspicuous.

(8.) The following are the results of two sets of experiments conducted on this principle:

(9.) These results exhibit a very close agreement in the ratio of the risings of the two thermometers, when exposed, and when screened, and this with glasses of different thickness, at different times, and with different absolute intensities of the sun's rays; as also when the colours of the bulbs were mutually changed. The mass of the bulb A was somewhat greater than B; the glass acquired no heat sufficient to interfere with the results; and the thermometer was always placed so that the bulbs were not near any object which might radiate heat. The temperature of the air affecting both surfaces equally would tend to diminish the ratio. To its variation, I conceive, the trifling difference in the ratios may fairly be ascribed.

(10.) Hence, I think, we are entitled to conclude, that there do not exist in the solar beam in its natural state any rays of the description just alluded to; but that the whole emanation consists of one sort of rays distinguished by the two characteristics of affecting substances with heat in proportion to the darkness of their colour and being wholly transmissible through glass without heating it; and that these same rays when they impinge on the eye are capable of producing the sensation of vision; and by the absorption of some, and the reflexion of others, of their constituent parts, at the surfaces of bodies, produce the phenomena of colours.

(11.) The heating effect maintains an intimate relation to light both in respect to the substances which it traverses