If we compare the numbers thus obtained by experiment with those calculated by Clausius on the theory of hollow vesicles impeding the passage of the rays, we shall notice a most remarkable difference between the experimental and calculated numbers.

Ratio of Chemical Intensities of direct Sunlight to diffused Light.

Thus, whilst the theory requires that at an elevation of 20° the relation of diffuse light to sunlight was as 100 to 491, the experiments at Heidelberg showed a relation of 100 to 35, those of Kew of 100 to 30, at Cheetham Hill of 100 to 19, and at Owens College of 100 to 10, whilst the differences at higher altitudes

becomes still greater. The Heidelberg observations were made on the summit of the Königstuhl, at an elevation of nearly 2000 feet above the sea level, and therefore at a position beneath which a very considerable portion of the densest air was situated; when the sun attained an altitude of 40°, the direct sun's rays exert the same amount of chemical action as the diffused light of a cloudless sky. At Kew Observatory, this point of equality is not nearly reached when the altitude of the sun is 42°. At Pará, under the equator, this difference between the chemical intensity of direct and diffuse sunlight becomes even more striking, for with an altitude of 77° the ratio of direct to diffuse is less than 0.5; that is, if 100 rays come from the diffused daylight only 50 come from the direct sunlight. This is certainly a very remarkable result. We thus see that the high tropical light curves are mainly caused, not by the increase of the chemically active rays in the direct sunlight, but by the enormous increase in the chemical activity of the diffuse light. It must, however, be borne in mind, that in the Pará observations the sky was not cloudless, and much light is reflected from the heavy cumuli; it is, nevertheless, remarkable, that under a tropical sun at an altitude of 80°, the diffuse daylight should exert a chemical action twice as great as the direct sunlight.

That the relation between the chemically active constituents of sunlight, direct and diffused, is quite different from the relation of the visible rays, can be easily ascertained. In some of the experiments made at Cheetham Hill, the shadow of a small disc was thrown on a horizontal surface of white paper, and careful estimations made of the relative brightness of the shaded and unshaded portions of the surface. A comparison of these results with those obtained at the same time for the chemical rays showed that when the sun's mean altitude was 25° 16', the mean ratio of the chemical intensities of direct and diffuse light was 0.23 (or for 100 of diffuse light there was 23 of direct sunlight), whilst the ratio of the visible intensities was 4.0 (or for 100 of diffuse light there was 400 of direct sunlight). This shows that the action of the atmosphere was 174 times greater on the chemical than on the visible rays. Again, at Owens College, with a mean altitude of 12° 3', the ratio of chemical intensity was 0.053, that of the visible intensity being 14; or the action of the atmosphere was 264 times as great upon the chemical as upon the visible rays.

How can we seek to explain this unexpected result-that the sun shining brightly, and casting a dark shadow, should at a height of 20° be capable of producing a chemical action of only th of that produced by the diffuse light from the whole of a cloudless sky?

The explanation may be rendered plain by an experiment. Let us take a very slightly milky liquid-such as water containing th grain of suspended sulphur in the gallon. So slight is the opalescence that we can scarcely detect it. Nevertheless, this minute trace of most finely-divided sulphur is sufficient to cut off the chemically active rays; the bright flash of carbonic disulphide in nitric oxide cannot explode the bulb when the opalescent solution is placed between it and the

bulb; but the bulb explodes instantly when the light is allowed to pass through pure water.

We

We have here an exact imitation of the condition of the atmosphere as regards the chemically active rays. We see that light of a high degree of refrangibility cannot pass through the water containing the finely-divided sulphur; it is reflected back again by the particles of sulphur. So, too, the atmosphere is filled with particles which reflect the blue rays and transmit the red. What the exact nature of these particles may be, it is hard to say. We know, however, that the air is always filled with minute solid bodies. We see that in the sporules which are constantly present and cause fermentation and putrefactive decomposition. We see it also in the fact that particles of soda can always be detected in the atmosphere by spectrum-analysis. notice these particles as motes dancing in the sunbeam, or in those grander paths of light which sometimes shoot up into the sky from a setting sun. The phenomenon may, perhaps, be caused by that finelydivided extra-terrestrial meteoric dust, which is, according to many physicists, constantly falling through the atmosphere to the earth's surface. These solid particles in the air may produce the above effects, and certainly do produce them; but we must remember that small particles of water are also able to transmit only red rays, and that, as Forbes has shown, the glorious ruddy tints of the setting sun are doubtless partly caused by aqueous vapour.

If the white beam of the electric lamp be passed through a tube 3 feet long, fitted with glass plates at each end, and filled with a scarcely visibly opalescent liquid, all the blue, green, and yellow rays will be completely cut off, and the immerging beam of light is deep red. Here indeed we have an artificial sunset. The finely divided sulphur reflects blue light and transmits red. If the visible light is diminished to onethird by means of opalescent sulphur, the chemically active rays are altogether cut off. The variation in the amount of this finely-divided matter, whether solid or liquid, in the air, will naturally produce variation in the tints of sunrise and sunset, and the presence at sunset of more aqueous vapour on the point of being condensed than at sunrise will explain the greater depth of colour in the setting than in the rising sun; the tints of dawn being, according to Mr. Brayley, those of evening in the reverse order.

In opal glass we have perhaps a still better illustration of the action of the atmosphere upon the chemically active rays. The opalescence of the glass is caused by the presence of very minute particles of bone-ash (calcium phosphate), or of arsenic trioxide, which are disseminated throughout the mass. By reflected light this glass appears white or blueish-white, by transmitted light it appears orange. If we place a bright source of white light behind the glass, we see that the direct rays are red, whilst the general diffused light reflected from the particles of the finely-divided matter in the glass is blueish-white.

That the size of the particles between which the light passes modifies the character of the transmitted ray scarcely admits of doubt.

This is most clearly exemplified in the beautiful phenomena of blue and ruby gold investigated by Mr. Faraday. Gold in thin plates reflects yellow, and transmits green light; but when suspended in a very fine state of division in water, it transmits blue, purple, or ruby light, according to the state of division in which it is precipitated.

The blue, purple, and ruby solutions all contain metallic gold in suspension, as Mr. Faraday has most conclusively shown, and yet they transmit totally different rays.

Hence we may fairly suppose that the varying size of the reflecting particles may aid in producing the widely differing sunset tints, from deep ruby-red to yellow and even blue; for we are not without several well-authenticated cases in which the sun has been seen to be blue. Thus, in the year 1831, a blue sun was noticed over a great part of Europe, as also in America.

We have seen that the light transmitted by finely-divided sulphur is red-it is, however, singular that blue sulphur can be formed. If we add ferric chloride to solution of sulphuretted hydrogen, we get a transient but very splendid purple tint; and we may ask ourselves whether this can be due to the size of the particle. If we heat sulphuretted hydrogen water up to 200° C. the gas decomposes, sulphur being deposited, and the solution attains a deep blue colour. Can this possibly be due to the minute division, almost approaching solution, which the sulphur attains? We find that on cooling the colour disappears, sulphur is deposited, and the liquid becomes milky. If we dissolve sulphur in sulphuric trioxide (anhydrous sulphuric acid), no chemical action that we know of occurs, and we get a magnificent deep-blue colour. Can this again be due to the minute division of the sulphur, thus permitting the blue rays alone to pass?

Finally, it is interesting to learn that both the analogues of sulphur, selenium and tellurium, yield magnificently coloured liquids when acted upon by sulphuric trioxide. Selenium in this state yields a deep olivegreen solution, and tellurium a magnificent ruby-red colour.

Can these colours likewise be caused by the reflection or absorption of one kind of light and the preferential transmission of another kind by finely-divided particles?

The ruby-red gold liquid and ruby-red gold glass are both as transparent, and the one is apparently as truly a liquid as the red solution of tellurium. Yet we know that finely suspended metallic gold is the cause of this red tint. Are we acting contrary to analogy in supposing that the colour of this red liquid is caused by the particles of finelydivided tellurium, or that of these blue and green liquids by the particles of sulphur and selenium?

The speaker felt that he was here entering upon debateable ground, that, namely, of the cause of the colour of natural bodies; it was with much diffidence that he brought forward these examples of coloured solutions, and he did so only because they forced themselves on to his notice in the consideration of the plainer and now somewhat better understood phenomenon of the Opalescence of the Atmosphere.

[H. E. R.]

GENERAL MONTHLY MEETING,

Monday, June 4, 1866.

WILLIAM POLE, Esq. M.A. F.R.S. in the Chair.

Edward Beanes, Esq. C.E. F.C.S.
Robert C. L. Bevan, Esq.

were elected Members of the Royal Institution.

John Hogg, M.D.

was admitted a Member of the Royal Institution.

The Special Thanks of the Members were returned to Sir Henry Holland, Bart. the President, for his Eighth Annual Donation of £40 to "the Donation Fund for the Promotion of Experimental Researches" (see page 151).

The PRESENTS received since the last Meeting were laid on the table, and the thanks of the Members returned for the same, viz. :

FROM

Asiatic Society, Royal-Journal, Vol. II. Part 1. 8vo. 1866.
Astronomical Society, Royal-Monthly Notices, 1865-6. No. 6. 8vo.

British Architects' Institute, Royal-Sessional Papers, 1865-6. Part III. No. 1. 4to. Editors-Artizan for May, 1866. 4to.

Mechanics' Magazine for May, 1866.
Pharmaceutical Journal for May, 1866.

Practical Mechanics' Journal for May, 1866. 4to.
Franklin Institute-Journal, Nos. 480, 481, 442, 483.

8vo. 1866.
8vo. 1866.

Geographical Society, Royal-Proceedings, Vol. X. No. 3.
Geological Society-Quarterly Journal, No. 86. 8vo. 1866.
Horticultural Society, Royal-Proceedings, 1866. No. 4. 8vo.

Jones, H. Bence, M.D. F.R.S. Hon. Sec, R.I.-Third Report of the Cattle Plague
Commissioners.

fol. 1866.

Leeds Literary and Philosophical Society-Annual Report, 1864-5. 8vo.

Catalogue of the Library. 8vo.

1866.

Macpherson, John, M.D. M.R.I. (the Author)-Cholera in its Home. 8vo. 1866. Mechanical Engineers' Institution, Birmingham-Proceedings, August, 1865. Part 3.

8vo.

Meteorological Society-Proceedings, No. 24. 8vo. 1866.

Photographic Society-Journal, No. 169. 8vo. 1866.

Royal Society of London-Proceedings, Nos. 82, 83. 8vo. 1866.

Sidney, Rev. Edwin, M.A.-Constantinus, R.: Lexicon Græco-Latinum. fol. 1592. United Service Institution, Royal-Journal, Appendix to Vol. IX. 8vo. 1865.