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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 also 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:

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(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 infringe 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 without interception, and to those by which it is absorbed, and to the degree of absorption. It is found to accompany the rays of light in the most constant and inseparable manner through whatever substance, and in whatever direction it takes its course, this is strikingly exemplified in one of Sir W. Herschel's experiments (Phil. Trans. 1800, No. 13, Exper. 11), in which the heating effect is shown to accompany the rays of light in all the alterations of its course through a Newtonian telescope with four lenses.

Speaking in general terms, within ordinary limits, and for light of the same colour, we may say, that the heating effect increases or decreases in proportion to the intensity of illuminat

ing effect. Prof. Leslie considers the proportion to be precise and undeviating.

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(12.) Whatever we suppose to be the state in which the heat exists when it thus inseparably accompanies the luminous rays, it is evident that there must be some peculiar circumstance in the mode of its union which makes its effects sensible only under some particular circumstances; and under others endows it with properties which heat in its simple radiant state does not possess. It evidently exists in a state essentially different from that of simple radiant heat; and we may in general say, that it is never developed or rendered sensible except under such circumstances as produce at the same time some modification or change in the light itself.

Upon considering all these well established facts, I think, instead of using such terms as "calorific rays," and "luminous rays," it is much more conformable to facts, and involves no hypothetical ideas, to describe the phenomena by the terms rays of light," and the "heating power or property" of those rays.

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III. (18.) Thus far my remarks have been confined to the nature of the heating power of the sun when its rays are in that state in which they naturally are, as coming directly from that luminary. In the next place we have to inquire whether by any modification which these rays may be made to undergo by artificial means, we can attain to any further elucidation of the nature of this heating power.

(14.) It has been shown by the experiments of M. Berard (Biot, Traité de Phys. vol. iv. p. 603, &c.), that when light undergoes polarization, the heating power participates in that effect.

It has also been shown by the same distinguished philosopher (see Biot as above), that simple radiant heat when unaccompanied by light is susceptible of being polarized also. In considering these results, we must be careful not to confound them together; because simple rays of heat are capable of displaying the effects of polarization, and the heating effect in the solar beam also obeys the same impulse, we must not hastily conclude that the same agent existing in the same form is, therefore, the common heating principle in both cases.

(15.) The heating power of the sun is well known to be capable of being collected in a focus along with the luminous rays, both by reflexion and refraction. By the former means simple radiant heat may also be concentrated: this circumstance again shows a similarity, but does not prove an identity in the agents or powers.

In one of Sir W. Herschel's experiments (Phil. Trans. 1800, No. 15, Exper. 23), a focus of heat different from that of light seems to be proved. This opinion it is not my design at present either to maintain or controvert. I have only to observe, that granting its truth, it must not be applied as an argument to

show that simple radiant heat exists distinctly in the solar beam; or in other words, that the sun's heat is produced by an assent identical with that which emanates from non-luminous hot bodies; it merely shows that the refractive and dispersive power which the lens exercises on light is capable of eliciting from it a certain agent or set of rays which produces the sensation of heat, but not that of illumination; and by no means shows that that agent exists in a separate form when the rays of light are in their direct natural condition.

If the focus of heat be the same as that of light, and the experiment of Sir W. Herschel (Phil. Trans. 1800, No. 15, Exper. 19, 20) be admitted, where the concentration of simple radiant heat by a lens appears to be proved, the same remark applies as that just made with respect to concentration by a mirror.

(16.) The principal modification which the sun's rays are made to undergo, and from which conclusions relative to the nature of their heating power have been drawn, is the analysis to which they may be subjected by the prism. Into any discus sion upon the controverted points respecting these experiments, I shall abstain from entering. We will suppose it granted that a set of invisible heating rays are separated beyond the visible red rays. The existence of such rays in a distinct state in the spectrum cannot be considered as any proof that they have that distinct existence in the natural state of the rays. It by no means proves that any such simple heating rays must have come directly from the sun, and have been transmitted through the prism, with merely a change in their direction.

From the experiments above given, we may, in reference to this point learn thus much: the direct rays are not accompanied by any separate heating rays which are either stopped by glass, or bear a relation to texture more than colour. It therefore becomes an important object to try whether in the prismatic beam these heating rays possess those characteristics or not. With respect to one of the characteristics, viz. transmissibility through glass, we have no ground to assume that the heating prismatic rays possess it from the circumstance of their passing through the prism, because, when the light impinges upon the prism, we know that it has not any such separate rays accompanying it; and of the nature or properties of the rays during their passage through the prism, we are altogether ignorant.

(17.) My object in making these remarks is merely to attain if possible some clear ideas on the subject in question; and to point out those parts of it which appear to me to want further elucidation; and to several of which I have attempted to direct experimental research. The subject must always remain perplexed and obscure so long as we dispute about such terms as "calorific rays," "luminous," or "non-luminous heat," &c. The only way of arriving at clearness of ideas, and thence being able to pursue the inquiry in a satisfactory manner, is to fix upon

some definite criteria by which the nature of heating agents may be distinguished and compared; and instead of framing theories to explain the union of heat and light in the sun's beams, to describe the nature of the phenomena in conformity to the criteria above pointed out, as afforded by certain experimental facts.

It becomes necessary to subject to an examination by those criteria the supposed exterior heating effect of the prismatic spectrum; as well as the analogous exterior heat of the cone of light formed by a lens, and probably several other phenomena, before we can obtain from them any further information respecting the nature of the heating power which is so inseparably associated with the sun's light.

The

The remarks hitherto made apply to the subject of solar light and heat; but they might be extended also to the investigation of the relations of light and heat from terrestrial sources. want of some fixed criteria of definition must upon consideration be felt equally in following up this part of the subject as in the former; and to endeavour to supply that want should be the first business of the experimenter.

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Some further observations will probably form the subject of a future communication.

ARTICLE II.,

Astronomical Observations, 1824.
By Col. Beaufoy, FRS.

Bushey Heath, near Stanmore.

Latitude 51° 37′ 44.3" North. Longitude West in time l′ 20·93′′.

March 28. Emersion of Jupiter's second

8h 17' 24"

Mean Time at Bushey.

8 18 45

Mean Time at Greenwich.

satellite..

March 31. Emersion of Jupiter's first

satellite.

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April 5. Occultations by the moon, Si

8 13 49
28 15 10
12 15 44
12 19 01
12 21
13 21 51

19

Mean Time at Bushey. Mean Time at Greenwich. Immersion of a small star. Im. Jupiter's 4th satellite. Immersion of a small star. Em. Jupiter's 1st limb. Em. Jupiter's 2d limb. Clouds prevented the Immersion of Jupiter being seen: at the Emersion, the limbs of Jupiter, and the Moon, were tremulous.

derial Time..

13 22 01

April 7. Emersion of Jupiter's first 10 09 15

satellite....

Mean Time at Bushey.

10 10

36

Mean Time at Greenwich.

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