must be extra-red also. The absorption by dry air of the heat emitted by a platinum spiral raised to incandescence by electricity was found to be insensible, while that by the ordinary undried air was 6 per cent. Substituting for the platinum spiral a hydrogen flame, the absorption by dry air still remained insensible, while that of the undried air rose to 20 per cent. of the entire radiation. The temperature of the hydrogen flame was as stated, 3259° C., that of the aqueous vapour of the air was 20° C. Suppose, then, the temperature of our aqueous vapour to rise from 20° C. to 3259° C., we must conclude that the augmentation of temperature is applied to an increase of amplitude, and not to the introduction of periods of quicker recurrence into the radiation.

The part played by aqueous vapour in the economy of Nature is far more wonderful than hitherto supposed. To nourish the vegetation of the earth, the actinic and luminous rays of the sun must penetrate our atmosphere, and to such rays aqueous vapour is eminently transparent. The violet and the extra-violet rays pass through it with freedom. To protect vegetation from destructive chills, the terrestrial rays must be checked in their transit towards stellar space, and this is accomplished by the aqueous vapour diffused through the air. This substance is the great moderator of the earth's temperature, bringing its extremes into proximity, and obviating contrasts between day and night which would render life insupportable. But we can advance beyond this general

statement now that we know the radiation from aqueous vapour is intercepted, in a special degree, by water, and reciprocally, the radiation from water by aqueous vapour; for it follows from this that the very act of nocturnal refrigeration which produces the condensation of aqueous vapour upon the surface of the earth-giving, as it were, a varnish of liquid water to that surface-imparts to terrestrial radiation that particular character which disqualifies it from passing through the earth's atmosphere and losing itself in space.

And here we come to a question in molecular physics which at the present moment occupies the attention of able and distinguished men. By allowing the violet and extra-violet rays of the spectrum to fall upon sulphate of quinine and other substances, Professor Stokes has changed the periods of those rays. Attempts have been made to produce a similar result at the other end of the spectrum-to convert the extra-red periods into periods competent to excite vision-but hitherto without success. Such a change of period the author believed occurs when a platinum wire is heated to whiteness by a hydrogen flame. In this common experiment there is an actual breaking-up of long periods into short ones-a true rendering of invisual periods visual. The change of refrangibility here effected differs from that of Professor Stokes, first, by its being in the opposite direction, that is from lower to higher; and secondly, in the circumstance that the platinum is heated by the collision of the molecules of aqueous vapour, and before their heat has assumed the radiant form.

But it cannot be doubted that the same effect would be produced by radiant heat of the same periods, provided the motion of the ether could be rendered sufficiently intense. The effect, in principle, is the same whether we consider the platinum wire to be struck by a particle of aqueous vapour oscillating at a certain rate, or by a particle of ether oscillating at the same rate.

By plunging a platinum wire into a hydrogen flame we cause it to glow, and thus introduce shorter periods into the radiation. These, as already stated, are in discord with water; hence we should infer that the transmission through water will be more copious when the wire is in the flame than when it is absent. Experiment proves this conclusion to be true. Water, from being opaque, opens a passage to 6 per cent. of the radiations from the flame and spiral. A thin plate of colourless glass, moreover, transmitted 58 per cent. of the radiation from the hydrogen flame; but when the flame and spiral were employed 78 per cent. of the heat was transmitted. For an alcohol flame Knoblauch and Melloni found glass to be less transparent than for the same flame with platinum spiral immersed in it; but Melloni afterwards showed that this result was not general, that black glass and black mica were decidedly more diathermic to the radiation from the pure flame. The reason of this is now obvious. Black mica and black glass owe their blackness to the carbon diffused through them. The carbon, as proved by Melloni, is in some measure transparent to the extra-red rays, and the author had in fact succeeded in transmitting between 40 and 50 per cent. of the radiation from a hydrogen flame through a layer of carbon sufficient to intercept the light of the most brilliant flames. The products of combustion of the alcohol flame are carbonic acid and aqueous vapour, the heat of which is almost wholly extra-red. For this radiation the carbon is in a considerable degree transparent, while for the radiation from the platinum spiral it is in a great measure opaque. By the introduction of the platinum wire, therefore, the transparency of the pure glass and the opacity of its carbon were simultaneously augmented; but the augmentation of opacity exceeded that of transparency, and a difference in favour of opacity remained.

No more striking or instructive illustration of the influence of coincidence could be adduced than that furnished by the radiation from a carbonic oxide flame. Here the product of combustion is carbonic acid; and on the radiation from this flame even the ordinary carbonic acid of the atmosphere exerts a powerful effect. A quantity of the gas, only onethirtieth of an atmosphere in density, contained in a polished brass tube four feet long, intercepted 50 per cent. of the radiation from the carbonic oxide flame. For the heat emitted by solid sources, olefiant gas is an incomparably more powerful absorber than carbonic acid; in fact, for such heat the latter substance, with one exception, is the most feeble absorber to be found among the compound gases. For the radiation from the hydrogen flame, moreover, olefiant gas possesses twice the absorbent power of

carbonic acid; but for the radiation from the carbonic oxide flame at common tension of one inch of mercury, while carbonic acid absorbs 50 per cent., olefiant gas absorbs only 24. Thus we establish the coincidence of period between carbonic acid at a temperature over 3000° C., the periods of oscillation of both the incandescent and the cold gas belonging to the extra-red portion of the spectrum.

It will be seen from the foregoing remarks and experiments how impossible it is to examine the effect of temperature on the transmission of heat, if different sources of heat be employed. Throughout such an examination the same oscillating atoms ought to be retained. The heating of a platinum spiral by an electric current enables us to do this while varying the temperature between the widest possible limits. Their comparative opacity to the extra-red rays shows the general accord of the oscillating periods of our series of vapours with those of the extra-red undulations; hence, by gradually heating a platinum wire from darkness up to whiteness, we gradually augment the discord between it and our vapours, and must therefore augment the transparency of the latter. Experiment entirely confirms this conclusion. Formic ether, for example, absorbs 45 per cent. of the radiation from a platinum spiral heated to barely visible redness; 32 per cent. of the radiation from the same spiral at a red heat; 26 per cent. of the radiation from a white-hot spiral, and only 21 per cent. when the spiral is brought near its point of fusion. Remarkable cases of inversion as to transparency occurred in these experiments. For barely visible redness formic ether is more opaque than sulphuric; for a bright red heat both are equally transparent, while for a white heat, and still more for a nearly fusing temperature, sulphuric ether is more opaque than formic. This result gives us a clear view of the relationship of the two substances to the luminiferous ether. As we introduce waves of shorter period, the sulphuric augments most rapidly in opacity; that is to say, its accord with the shorter waves is greater than that of the formic. Hence we may infer that the molecules of formic ether oscillate as a whole more slowly than those of sulphuric ether.

When the source of heat was a Leslie's cube filled with boiling water and coated with lampblack, the opacity of formic ether in comparison with sulphuric was very decided; with this source also the position of chloroform, as regards iodide of methyl, was inverted. For a white-hot spiral, the absorption of chloroform vapour being 10 per cent., that of iodide of methyl is 16; with the blackened cube as source, the absorption by chloroform is 22 per cent., while that by the iodide of methyl is only 19. This inversion is not the result of temperature merely; for when a platinum wire heated to the temperature of boiling water was employed as a source, the iodide was the most powerful absorbent. Numberless experiments, indeed, prove that from heated lampblack an emission takes place which synchronizes in an especial manner with chloroform. This may be thus illustrated. For the Leslie's cube coated with lampblack, the absorption by chloroform

is more than three times that by bisulphide of carbon; for the radiation from the most luminous portion of a gas flame the absorption by chloroform is also considerably in excess of that by bisulphide of carbon; while for the flame of a Bunsen's burner, from which the incandescent carbon particles are removed by the free admixture of air, the absorption by bisulphide of carbon is nearly twice that by chloroform; the removal of the incandescent carbon particles more than doubled in this instance the relative transparency of the chloroform. Testing, moreover, the radiation from various parts of the same flame, it was found that for the blue base of the flame the bisulphide was the most opaque, while for all other portions of the flame the chloroform

was most opaque. For the radiation from a very small gas flame, consisting. of a blue base and a small white top, the bisulphide was also most opaque, and its opacity very decidedly exceeded that of the chloroform when the flame of bisulphide of carbon was employed as a source. Comparing the radiation from a Leslie's cube coated with isinglass with that from a similar cube coated with lampblack, at a common temperature of 100° C., it was found that out of eleven vapours all but one absorbed the radiation from the isinglass most powerfully; the single exception was chloroform. It may be remarked that whenever, through a change of source, the position of a vapour as an absorber of radiant heat was altered, the position of the liquid from which the vapour was derived was changed in the same manner.

It is still a point of difference between eminent investigators as to whether radiant heat up to a temperature of 100° C. is monochromatic or not. Some affirm this, others deny it. A long series of experiments enables the author to state that probably no two substances at a temperature of 100° C. emit heat of the same quality. The heat emitted by isinglass, for example, is different from that emitted by lampblack, and the heat emitted by cloth or paper differs from both. It is also a subject of discussion whether rocksalt is equally diathermic to all kinds of calorific rays,-the differences affirmed to exist by one investigator being ascribed by others to differences of incidence from the various sources employed. MM. De la Provostaye and Desains maintain the former view, Melloni and M. Knoblauch maintain the latter. The question was examined by the author without changing anything but the temperature of the source. Its size, distance, and surroundings remained the same, and the experiments proved that rock-salt shared in some degree the defect of all other substances; it is not perfectly diathermic, and it is more opaque to the radiation from a barely visible spiral than to that from a white-hot one.

The author devotes a section of his memoir to the relation of radiation to conduction. Defining radiation, internal as well as external, as the communication of motion from the vibrating molecules to the ether, he arrives by theoretic reasoning at the conclusion that the best radiators ought to prove the worst conductors. A broad consideration of the subject shows at once the general harmony of the conclusion with observed facts. Organic substances are all excellent radiators; they are also extremely bad

conductors. The moment we pass from the metals to their compounds, we pass from a series of good conductors to bad ones, and from bad radiators to good ones. Water, among liquids, is probably the worst conductor; it is the best radiator. Silver, among solids, is the best conductor; it is the worst radiator. In the excellent researches of MM. De la Provostaye and Desains the author finds a striking illustration of what he regards as a d natural law-that those molecules which transfer the greatest amount of motion to the ether, or, in other words, radiate most powerfully, are the least competent to communicate motion to each other, or, in other words, to conduct with facility.

II. "Remarks on Sun Spots." By BALFOUR STEWART, M.A., F.R.S., Superintendent of the Kew Observatory. Received March 8, 1864.

In the volume on Sun Spots which Carrington has recently published, we are furnished with a curve denoting the relative frequency of these phenomena from 1760 to the present time. This curve exhibits a maximum corresponding to 1788.6. Again, in Dalton's Meteorology' we have a list of auroræ observed at Kendal and Keswick from May 1786 to May 1793. The observations at Kendal were made by Dalton himself, and those at Keswick by Crosthwaite. This list givesFor the year 1787.... 27 auroræ,

1788.... 53
1789....45

د,

For the year 1790.... 36 aurora;

showing a maximum about the middle, or near the end of 1788. This corresponds very nearly with 1788-6, which we have seen is one of Carrington's dates of maximum sun spots.

The following observation is unconnected with the aurora borealis. In examining the sun pictures taken with the Kew Heliograph under the superintendence of Mr. De la Rue, it appears to be a nearly universal law that the faculæ belonging to a spot appear to the left of that spot, the motion due to the sun's rotation being across the picture from left to right.

These pictures comprise a few taken in 1858, more in 1859, a few in 1861, and many more in 1862 and 1863, and they have been carefully examined by Mr. Beckley, of Kew Observatory, and myself. The following Table expresses the result obtained :