is degeneration which lops the worn and aged stems, prunes the weakly foliage, trims the budding growths, and so directs and moulds the outlines of the organic whole. It is to evolution and progress that the world of life largely owes its forward march. But hardly less is the debt of gratitude due by the living hosts to degenerative change and retrogression which, though stern and ofttimes cruel in their ways, nevertheless mark wisely and well the pathways of life, and prevent the useless and weak from cumbering the ground.

ANDREW WILSON.

495

SCIENCE NOTES.

IN

SPHERICAL DUST.

N the course of a rather famous controversy between Pasteur and Pouchet, carried on not merely by means of words but by actual research, Pouchet discovered that some of the microscopic egg-like particles found in atmospheric dust remained unaltered in size and shape when kept for some time at a red heat in a platinum crucible, and, therefore, argued that they are mineral particles, not organic structures.

Dr. Phipson confirms this, and finds that ordinary fine dust con. tains not only small angular particles of sand, but also rounded, eggshaped or globular siliceous particles. He heated micrococci and diatoms (microscopic siliceous vegetables that walk about in water), and found that they do not retain their forms after such treatment, but that the remains of fossil species of these creatures resist the action of heat and retain their forms, and he concludes that the minute siliceous bodies found in the atmosphere are also fossil; that they are micrococci of another age.

This appears to me a rather far-fetched explanation, and I venture to offer the following, which is much simpler, and, I think, more probable :

We know that when flints, or fragments of rock of any size or hardness, are swept along by water, and thus shaken up together, they become rounded, as we see them on the sea-beach or bottom of a running stream. Siliceous dust particles are but pebbles of smaller size, that are more or less stirred and rolled together as they are swept along by the wind. As the same particles settle on the surface of the ground, and are blown up again and again, and backwards and forwards, there must be among them many that have existed as dust for years or even ages, and thus have become as completely rounded as the pebbles of our chalk-flint shingle.

A patent was secured in America about twelve years ago for pulverising fuel by agitating ordinary coal-dross, or "slack," in a suitable chamber, whereby it became reduced to a fine powder by

the collision of particles with each other. The result was the production of a fine dust composed of spherical or nearly spherical grains. If coal dust can be thus shaped in a few minutes, sand dust may be in a few years.

AB

INVERTED PHOTOGRAPHS OF THE SUN.

BOUT seventeen years ago Mr. Nasmyth announced that "the bright surface of the sun consists of separate, insulated, individual objects or things, all nearly or exactly of one definite size and shape, more like that of a willow-leaf than anything else." I here quote Sir John Herschel's description,' who proceeds to state that these" are evidently the immediate sources of the solar light and heat," and that "we cannot refuse to look upon them as organisms of some peculiar and amazing kind; and though it would be too daring to speak of such organisation as partaking of the nature of life, yet we do know that vital action is competent to develop both heat, light, and electricity." In Herschel's "Outlines of Astronomy" these willow-leaves are pictured most definitely as geometrically regular and uniform bodies, "crossing one another in all directions, like what are called spills in the game of spillikins."

This extraordinary discovery was presently questioned, and these still more extraordinary speculations were refuted by other observations; and the appearances upon which they were based were described as resembling" rice-grains," " crystals," "flocculi," "granulations," 97 66 straws," things," "bits of white thread," "cumuli of cotton wool," "clouds," "excessively minute fragments of porcelain," "untidy circular masses," things twice as long as broad," "three times as long as broad," " ridges," " waves," "hill knolls," &c. &c.

Here was disagreement among doctors, with a vengeance, not on any abstract conclusion, or profound metaphysical quibble, but on what was visible to different pairs of eyes aided by telescopes. Such conflicting testimonies demand the intervention of some unimpeachable witness. This has been obtained by making the sun draw his own portrait.

The difficulty of doing this has been considerable, on account of the excessive glare of light dazzling the salts of silver as it dazzles our eyes, and producing, under ordinary exposure, a blurred round blotch, such as we see when we dare to blink at the noonday sun. The indomitable Janssen, who perilled his life by escaping in a balloon from besieged Paris, and then travelled half-way round the Familiar Lectures on Scienti fic Subjects, p. 83.

1

globe in order to see an eclipse of the sun, which, after all, the cruel clouds concealed just at the critical moment, has with characteristic perseverance obtained his revenge, and compelled the sun to give his own evidence, in the form of magnificent pictures, that have settled the willow-leaves and many other solar phantasies. When I saw these at the Royal Astronomical Society last year, I could not refrain from a small chuckle, seeing that they strikingly confirm my theoretical conclusion, published eleven years ago, that these mysterious mottlings are tongues of veritable flame, bending over in various directions according to the direction of the rushing cyclones, and other solar storms, that eternally rage in the solar atmosphere above them.

In order to obtain these magnificent pictures, M. Janssen has had to devise special apparatus, that shall limit the exposure to measurable, but almost inconceivably small, moments of time, such as the ten thousandth part of a second, or even shorter periods.

In the course of these struggles with the sun he has made a very curious photographic discovery.

Most of my readers are doubtless aware that in the ordinary course of photography a "negative" picture is first obtained, and a “positive” printed from it. This is a necessary consequence of the primary fact upon which photography is based, viz. that when certain salts of silver are exposed to light, they become darkened in proportion to the degree of such exposure. Hence in the direct pictures the lights are represented by shade and the shade by lights. A picture from this negative reverses the ights and shades again, and thus gives the resemblance to nature.

Such is the ordinary experience of photographers; but M. Janssen finds that if the exposure of his sun pictures is continued beyond the time for obtaining the best result, the negative image gradually becomes fainter and indistinct, then nearly disappears. By still further exposure a new picture comes into existence, a positive picture with the spots black, and the face of the sun bright, and the minor details shown as when viewed in a telescope. By further exposure after this a second neutral stage is reached.

He has further applied this principle to terrestrial photography, and, after exposures of from one to three hours, obtained direct positive pictures of landscapes. A picture of the Park of Meudon thus produced shows the sun as a white spot on a dark sky. From these positives he obtains other positives by similar protracted exposure, and negatives from negatives.

The interest of these discoveries is purely philosophical, no practical photographic advantages being, at present, obtainable. VOL. CCL. NO. 1804.

K K

GASES SINGING THEIR OWN DENSITIES.

THE accurate weighing of gases for the determination of their specific gravities is a delicate and difficult operation, in spite of all the devices that have been invented. A new and curious one has lately been proposed by Herr Goldschmidt, who extorts from the gas a vocal declaration of its own density, by simply filling a tube, first with air, and then with the gas or vapour to be tested, striking the tube when thus filled, and recording the note emitted.

Having obtained this record, and stated the number of vibrations due to each note, all that remains to be done, according to Herr Goldschmidt, is to divide the number of vibrations of the air by the number of the gas, and the square of this quotient expresses the specific gravity of the gas, that of air being unity.

I may add, that the number of vibrations due to all the notes of the diatonic and chromatic scales has already been determined and tabulated; for the determination of the intermediate tones, I presume that a siren, or instrument producing the same tone as the gas by successive impulses that can be counted, would be required.

SINC

ARSENICAL WALL PAPERS.

INCE the printing of my February notes on the fattening of pigs, &c., by arsenic, I have met with the record of a striking example of its action on human beings. Some years ago Kopp, the celebrated German chemist, was engaged in producing those splendid coal-tar dyes which are obtained by boiling aniline with dry arsenic acid. He found that he gained 22 lbs. in ten weeks, without detriment to his general health, and that he lost this additional weight when he left off these researches. The same effect is observed on the workmen engaged in the works where rosaniline colours are made in wholesale quantities.

A well-known Birmingham firm, at whose works arsenical ores, &c., were smelted, and a great variety of very odorous gases poured forth (sulphuretted hydrogen being a speciality), was continually subject to actions for nuisance, on the ground that they were poisoning their neighbours. Their principal witness on these trials was a man living on the works (a watchman, if I remember rightly), who when he entered upon his duties there was a lean and hungry Cassius, but after some years of service became a Falstaff.

I have long held very heretical opinions on the subject of poisoning by arsenical wall-papers, even going so far as to believe that, if they have any effect at all, it is beneficial, on account of the powerful