the hydrogen disengaged in bubbles from a metallic wire plunged into the mercury is this actual radiant hydrogen,cocons veyed and collected on its surface from all parts of the mercury. That, 4thly, the friction of the hydrogen so radiated produces the electricity, and not the electricity the hydrogen. And, lastly, that the gyration of the fragments themselves is a consequence of the re-action of the hydrogen they dart out during their oxidation by the water.

1

29 59. All these phænomena, however, are much better accounted for on the principles of this Lecture, from a knowledge of the properties conferred on mercury by alloying it with potassium but, first, it is necessary to premise, that the mere contact of a metal capable of amalgamating, even for an instant, communicates its peculiar properties, almost in the moment of contact, to the whole mass. The experiments in Art. 33, abundantly prove this; and it may be readily shown also by the following. Let a quantity of mercury be placed in a vessel of muriatic acid; no action takes place; but if touched with a zinc wire it presently becomes covered with bubbles, copiously disengaged from every part of the surface.

60. In the circumstances of M. Serrulas's experiments, it is therefore obvious that his mercury must have been always sensibly impregnated with potassium and the supernatant liquid, a solution of potash; and that it was so, is proved by the effects of the electric current, which agree precisely with those I have stated, as being always produced in such circumstances (Artis cles 18, 28); but the cause assigned to these effects by Mr. S. viz. the electro-positive energy of the pellicle, is proved not to be the real one by the simple fact, that the violence of the motion is ways proportional to the cleanliness of the surface, and is greatest when there is no pellicle at all; besides which the pellicle here consisted of metallic bismuth, a substance incapable b of producing any such effect as shown in Art. 38. zónush brosine 61. The gyration of the fragments is produced as follows: ab strong Voltaic excitement takes place at the point of contact of b two metals so different as mercury and potassium. The mercury 13 becomes strongly positive, and the floating fragments negative. The circuit is completed by the alkaline liquid; and the mercury, being alloyed with a portion of potassium, and being itself the positive pole of the combination, we have here the case of p Art. 21; and the result, as stated by M. Serrulas, is precisely as in that experiment, the currents radiating from the point of immersion. These once produced, drive before them the frag ment in which they originate, in the direction in which it exposes the greatest surface to their action.

62. The attraction of the pellicle to a metallic rod plungedar into the the mercury is also a direct consequence of the alloy of potassium present in the mercury, as is also the disengagement:q VIDIG JRAT

of gas from the wire. It is, in fact, precisely the experiment described in Art. 25, and has nothing whatever to do either with the floating fragments, or with any hydrogen they may be discharging at the time, farther than that their contact serves to furnish potassium to the mercury.

63. It is needless, therefore, to push this examination further, as all the phænomena observed by Mr. S. are only particular cases of those I have described. With regard to the radiant hydrogen producing currents by its impulse, I would ask how it happens that currents are producea (when the positive pole is placed in contact), while a thick and tough coat of oxide covers the whole surface; and, one would think, must effectually defend it from the action of the hydrogen. Yet we have seen, in Art. 18, that the currents continue their course under this crust; and it will hardly be contended, that the hydrogen finds a passage between the oxide and the metal.

Nitric Ether. By Mr. Whipple.

(To the Editors of the Annals of Philosophy.)

GENTLEMEN,

Elaboratory, London, Sept. 17, 1824.

SINCE to give my opinion on the directions given by Dr. Ure in his Chemical Dictionary, for conducting a retort distillation of nitric ether, would be yλavna eis Abnvas nouice, I shall feel obliged if favoured with an insertion of the following question : Whether the directions for the management of a glass retort in forming nitric ether, be correctly stated in Dr. Ure's Chemical › Dictionary?

I remain, Gentlemen, your most obedient servant,
G. WHIPPLE.

ARTICLE XIII.

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

Bushey Heath, near Stanmore.

Latitude 51° 37' 44.3" North. Longitude West in time 1' 20.93".

Occultations of stars by the moon.

Sept. 4. Immersion of a small star.
Sept. 15. Immersion of a small star.

19 10 540 Siderial Time. 3 28 46-2)

Remarks on Solar Light and Heat. By B. Powell, MA. FRS.

(46.) I concluded a former portion of these inquiries with some remarks on the small development of a heating effect which is observable exterior to the cone of light formed by a lens. The investigation of its nature appearing to me a topic of considerable interest as bearing upon many other parts of the science of light and heat, I have been led to try several experiments upon the subject. The results of these trials in which the circumstances and conditions of the case have been varied in several different ways, I here propose to give in a tabular form, and to introduce them by a few remarks on the nature and object of the experiments, as well as on the sort of conclusion, which can safely be deduced from them.

(47.) The existence of the effect in question being admitted, two suppositions obviously present themselves as to its nature. It may be attributed to certain rays of light refracted to a position beyond the principal body of rays which go to form the focus. These may be rendered invisible or nearly so from the proximity of other rays of infinitely superior intensity. And thus the phenomenon may be nothing more than simply the ordinary heating effect of these rays displayed on the black bulb of the instrument. Again, it may be supposed (which seems to have been the idea of the first observer of this, and kindred phenomena), that it is owing to some sort of radiant heat. From his analogical view of the subject, it would have followed that these were rays of a peculiar kind different from those of terrestrial heat, and exhibited as existing in a separate state from the rest of the solar rays by the refractive powers of the lens ; rays in fact of a sort of intermediate character between light and common heat. If, however, because we cannot see the rays producing this effect we should think it necessary to infer that they must be rays of simple heat, or if we had any better experimental reasons for such a conclusion, still it would not, I conceive, be at all necessary to suppose any thing peculiar in their nature, or that they were really an emanation entirely sui generis; for there would still be no proof whatever that they had passed through the thick glass of the lens in the form of rays of simple heat. It is obvious that we may, in all respects, as well suppose them to have originated, or have been separated in some way from the deflected rays of light after their passage through the lens; and it would seem that such a supposition is the more incumbent on us, when we admit what has, I conceive,

been above sufficiently proved, the non-existence of any rays of heat (at least in a free and separate state) in the solar beam.

(48.) It is obvious that by several different applications of the differential thermometer, we may examine the validity of such views, and decisively ascertain whether this heating power is of such a nature as to affect the bulb of black glass alone, or also to produce some effect on the plain one, which is equally absorp tive for simple heat. With this view several of the following experiments were tried. In Experiments, Nos. 1, 2, and 3, I observed the effect which would arise when each of the bulbs was respectively placed just without the rays, as compared with the indication when they were both equally exposed, by having the focus thrown between them (they being at nearly 0.75 inch distance), and when both away from the rays at an equal distance under the shadow of an opaque screen which surrounded the lens on all sides. In this disposition of things, it is evident that if the effect were due to simple radiant heat, the indication when the focus was between the bulbs, should not differ from that displayed when they were at a distance. Again the effect when the plain bulb was nearest should have been as much below the point at which the instrument stood when away, as the effect with the black bulb nearest was above it.

(49.) The first of these conditions took place only in Exp. 3; a difference is perceptible in 1, 2, 4, 5, 6, and 7.

4

The second is observed in some degree in Nos. 4, 6, 7, though not to the extent which the supposition would require; it is, however, completely shown in No. 12. An effect on the plain bulb is shown in Nos. 1, 2, and 6, as well as subsequently in No. 16. The effect also when the focus was between the bulbs is in general greater than on the black bulb when the other was at a greater distance.

Thus far then we can only conclude, that though simple heat may be in action, it is not the sole cause of the effect: light unquestionably contributes to it. The aperture of the lens was varied in the two sets of experiments. This does not seem to have at all altered the effect, yet it must have altered any effect which was immediately dependent on the convergence of the

rays.

From some other trials I was convinced that the light of the focus reflected in different ways from the inner surface of the glass case affected the results. Hence Exp. 6 and 7 were tried without the case; but the effects were still nearly the same relatively to each other, the total intensity of each being of course diminished.

(50.) In prosecuting the inquiry, my next idea was to present a surface absorptive for simple heat, but only covering the bulb in part in order that the increased radiation might not counterlet yd sbsui suo

balance the effect. This was done in Exp. 8, 9, and 10, upon the bulb before plain, by attaching to it a small piece of lightbrown silk. In these a decided action was produced on this bulb; whilst an equal one in the contrary direction was displayed by the smooth black surface; but this might be occasioned by the greater power of absorbing light now acquired by the bulb before transparent. In the next experiment, therefore (No. 11), attached a similar piece of silk to the black bulb; the effect, if merely due to light, ought by this means to have been considerably diminished; but it is evident on inspection that it was quite equal to that on the plain black surface. Some additional effect of simple heat must, therefore, have made up for the dimi nution which must have been occasioned by the decreased absorption of light.

b Exp. 12 was tried immediately after No. 11, and shows more decidedly that the alteration of the aperture makes no difference in the effect. It would seem then (since with a diminished aperture the total number of rays is less), to increase with the more accurate convergence, instead of being diminished, as it would be if owing to the straggling rays of light, or to less refrangible rays of heat.

2

(51.) According to the view of the phenomena before adverted to, an exterior and invisible heat accompanying the concentration of light by a lens was considered as analogous to the sepa ration of peculiar invisible heating rays beyond the red end of the prismatic spectrum. It would, however, admit of considerable question, whether the distance from the rays at which the former effect is perceptible is not much greater compared with space occupied by the coloured edges of the section of the cone, than would be at all proportional to the distance of the exterior heat in the other case beyond the visible boundary of the red rays. If, again, according to the experimental authority on which both facts were originally brought forward, the exterior effect in the one case is the maximum, ought there not to be some analogous result in the other? It might, perhaps, from the difference of circumstances, be impossible to ascertain this satisfactorily; but if only the smallest portion of the exterior red fringe of the cone be made to glance upon the surface of the bulb, the effect is enormous compared with the greatest indica tion while any sensible space intervenes between the bulb and the light. These considerations led me to the idea of comparing the exterior effect with a common simple lens, and a compound achromatic one of the same aperture. This would show whether it were owing to any thing connected with the dispersion or different refrangibility of the coloured rays; if so, the effect with an achromatic lens would be altogether or nearly imperceptible. The lenses I employed were a small achromatic one made by Dolland, of about one inch aperture, and 7·5 focal New Series, VOL. VIII.

U