The heat would seem to destroy the lightreflecting power of the dust by breaking up the larger motes into smaller ones, and by carbonizing, or in some way changing their color, and thus makes them less light-reflecting." Mr. Aitken's experiments in dust are carried out to prove that without dust "fogging" is impossible, and that it is only necessary to purify the air from the one to have nothing of the other: indeed fog is but another form of atmospheric dust, and hence we must look upon our breath' as seen on a cold morning as evidence of the dusty state of the air; and every puff of steam as it escapes into the atmosphere will remind us still more powerfully of the same disagreeable fact." Fog, mist, cloud, or rain are but the visible forms of atmospheric dust saturated with water; and when air thoroughly vaporized is affected by a fall in temperature the necessary result is condensation from its invisible to its visible form, the outcome being one of these four closely allied phenomena. Of the first three the mist-form is the coarsest in texture, the particles composing the fog being so fine as scarcely to fall through the air, while the cloud-form occupies the medium; the rain itself being any of these while falling, whether in enveloping mist or heavy downpour.

In a

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of combustion from a clear part and from a smoky part of a fire" will be "found to be about equally foggy," that, indeed, under all conditions combustion is bad as a fog-producer; "bad, whether the combustion be perfect, as in a Bunsen flame and a clear fire, or imperfect, as in a smoky flame and smoky fire."' He then proceeds to state how hopeless is the expectation that the adoption of fires with perfect combustion, the gas fires, for instance, now so widely advocated, would minimize or diminish the fogs so universally dreaded. Having previously shown that the nuclei of fog and cloud particles are not formed by the motes or visible dust seen in the air, owing to the destructibility of these by combustion, Mr. Aitken demonstrates they must be composed of a much finer and quite invisible form of dust, the same, however unobserved, that is a ceaseless agent and ever present in enormous quantities in our atmosphere." This, owing to natural laws, exists altogether apart from artificial fog or dustproducing agencies, and is, therefore, beyond the sphere of human intervention. At the same time we learn that one of the most active, if not the most active fog-producer, is sulphur; and when we call to mind the vast quantities of this material daily consumed, not only in this or that town, but almost throughout the kingdom, it will hardly be a matter of wonder that, circumstances being otherwise favorable, yellow fog should be increasingly dense and frequent. The following comparative figures adduced by Mr. Aitken will enable the reader to realize this fact more clearly :-Sulphur, it is well known, exists in coal, and is, therefore, burned along therewith, the quantity of burned sulphur escaping from our domestic, and manufacturing, and engine chimneys being accordingly very great the average amount of this substance in coal has been determined as somewhat more than 1.2 per cent, but taking it at 1 per cent, and putting the amount of coal annually consumed in the London district at a little over 7,400,000 tons, the result would be that in London fires there would be an annual consumption of 74,000 tons of sulphur, which in an average day is at the rate of about 200 tons, in a winter day the amount being greater still. As one

The results of Mr. Aitken's long series of tests and experiments is, that, while the smoke of our towns intensifies fogs, they exist quite apart from any such admixture, and, indeed, must always do so under given circumstances, such as, for instance, our climate. word, Mr. Aitken considers man's influence over fogs-nil. This, however, is not so disheartening as it at first appears, for the fogs he refers to are those natural phenomena only so called in the country, town-bred people denominating them as white mists; hurtful no doubt in certain ways, but quite free from the injurious artificially introduced poisons characteristic of the fogs in manufacturing towns, receiving as the latter do evil and infectious gases as well as volumes of smoke and soot. But showing clearly as he does that combustion of any kind produces fog particles, relief from the evil by improved smoke appliances seems as far off as ever; for he proves realizably that " products

is decidedly not without a relieving as pect. It is, of course, well known that during fogs the air is clarified by no currents, but owing to its stagnation becomes a receptacle for every foul and noxious gas and deadly germ that floats in the impure air above the confined and suffocating streets; and to such an extent is this the case that, were it not for the fogs, it is possible that pestilent and deadly dangers would hover about every household, and probably sweep away thousands where now the yellow fog, with its train of asthmatic and chest complaints, claims its fifties. The reason of this is the "powerful antiseptic properties of the sulphurous acid formed by the burning sulphur," the fog becoming, therefore, a huge disinfectant,

always, indeed, disagreeable, and at times offensive, but nevertheless possessing purifying properties possibly preventive of terrible and universal evils.

If this be so, as there seems ample reason to believe, what we have to hope for is some practical and inexpensive invention that will enable the householder, as well as the manufacturer, to perform his share in minimizing as far as possible what is fast becoming an almost intolerable excess of smoky combustion, at the same time recognizing both the probable highly beneficial antiseptic properties of what we may call bearable fogs, and the fact of the inevitable existence of these as long as our physical environment is what it is.-Good Words.

TWENTY years ago, to have one's likeness taken was a trying ordeal. The patient to be operated on was placed in as strained an attitude as the ingenuity of the photographer could devise; his head fixed in something resembling a vice; he was cautioned not to wink for a length of time which seemed to depend on the state of the photographer's temper; and then in the course of a few weeks he received pictures of a staring idiot supposed to be himself. All who were at all proud of their personal appearance-all women and most men-were disgusted with the art. Now all is changed; the operation is generally over in a second or two; freckles, pimples, and cross-eyes are improved away, and everybody is surprised how comely he is. This rapid progress in the art of photography is to some extent due to improvements in lenses and various mechanical appliances, but more especially to the discovery that the salts of silver in combination with gelatine yield a far more sensitive plate than could ever be obtained by the old collodion process.

Within the last two years some remarkable photographs have been taken which show the wonderful perfection to which the art has attained. Likenesses of restless children, crying or laughing, are now so common as hardly to need mention; even the act of kissing, transitory as it

is, is sufficiently prolonged to enable a photograph to be taken, the momentary rest, when lips meet lips, are enough for the artist's purpose. But movements far more rapid than the act of kissing (which, after all, is often not so very transitory) are now seized by photography. Athletes performing in mid-air, birds flying, the course of projectiles, waves breaking on the coast, have all been photographed with a definition and clearness that leaves little to be desired. Photos of the Irish mail, rushing along at the rate of forty-five miles an hour, show the outlines perfectly defined; while the spokes of the engine-wheels are plainly delineated, proving the operation to have been so rapid that the wheels had not time to move any appreciable distance. Perhaps, however, the most. remarkable photographs of moving objects are those obtained by Mr. Muybridge of horses running and jumping; in these, positions of the limbs are shown which are far too transitory for the human eye to detect; what the eye sees in watching a horse running is an average of the successive positions assumed by the horse's legs; photography alone can give an accurate idea of their position at any definite point of time. The attitudes shown in photogaphs seem at first sight to be absurd, and certainly differ very much from representations

by engravers and painters; photographs show the real positions at certain moments of time, while painters depict, and rightly too, the apparent positions.

To the astronomer the art is invaluable, and some of the most remarkable discoveries in astronomy have been made by its aid. Large photos of the sun are taken every day it is visible at Greenwich and elsewhere, and thus a permanent record of the exact size and shape of every sunspot is obtained; these, when compared with electrical and other meteorological conditions, will help to settle the question whether and in what way the sunspots affect the weather. To such a perfection has the manufacture of gelatino-bromide of silver at tained, that M. Janssen, of Paris, photographs the sun in less than one twothousandth of a second. Again, the solar corona, as to the nature of which such varied speculations have been rife, is only visible during the very few minutes that a total eclipse of the sun lasts, and the observations that can be made in so short a time are necessarily very imperfect. Recently, however, Dr. Huggins has succeeded in photographing the corona without the intervention of an eclipse. The corona is especially rich in violet rays; now, the eye is less sensitive to small variations in the violet rays than it is to the other colors of the spectrum, whereas the violet is just what photography deals with most effectively. By cutting off the other rays, Dr. Huggins has succeeded in photographing the corona by means of its own violet light, and that, too, at a time when hitherto observations have been impossible. When his method is perfected, astronomers will be able, with the help of the camera, to study the corona and solar protuberances at their leisure.

The recent transit of Venus has afforded a fine opportunity for calculating the distance of the sun, and it is expected that, with the assistance of the hundreds of photographs obtained, the distance of the sun from the earth will be calculated to within 300,000 miles. The numerous comets, too, have not been allowed to pass without leaving their images behind, which show their shapes and positions far more perfectly than has hitherto been possible. But perhaps the most remarkable achievements are the

photographs of spectra of stars and nebulæ. Not long ago it was hardly possible to photograph stars of the fourth or fifth magnitude, and even the brighter nebulæ shone with far too faint light to enable photographs to be taken. But, recently, not only have the fainter nebulæ and stars, as low as those of the fourteenth magnitude which are only visible through most powerful telescopes, been photographed, but their light, even when dispersed by the prism, has still been strong enough to leave its impress on the sensitive plate. Dr. Huggins and Professor H. Draper have each succeeded in photographing spectra of nebulæ and stars of the twelfth magnitude, and thus determining some of the elements contained in worlds so distant from us that their light, travelling 186,000 miles per second, has taken thousands of years to reach us. Such photographs are especially useful, because they show the faintest lines in the spectra which have hitherto escaped the most practised eye.

Hardly less remarkable are some of the discoveries of Captain Abney, the prince of photographers, in his experiments on the infra-red of the spectrum ; he has recently shown that between the earth and the sun and quite outside our atmosphere, there exists accumulations of benzine and alcoholic derivatives.

Alcohol in temperance drinks, alcohol in rain water, alcohol in space, alcohol everywhere.

Again, in meteorology the art of photography will prove to be of immense use. A regular system of photographing the clouds by means of a specially made cloud-camera, which acts automatically, has just been commenced. The form and disposition of. clouds have always been regarded as an index to the weather, and weather records compared with cloud-photographs will doubtless afford valuable information and assistance in weather prognostications.

To the geographer and ordnance surveyor the camera will soon be regarded as an indispensable part of their outfit. The tedious operations of making sketches of a district will be obviated, and perfect pictures with hardly a chance of error will easily be obtained.

To the medical man too, and the chemist, photography is found to be a valuable assistant. At the Glasgow Medical

School the successive stages of surgical operations, sections of tumors and diseased structures, and in fact any remarkable forms of disease, are photographed, and the prints shown to medical students and distributed among the profession to assist in the diagnosis of rare forms of disease. Dr. Lennox Brown and Mr. Cadett have recently got some wonderful photos of the interior of the larynx. By an adjustment of mirrors in the mouth and the electric light to illuminate the throat, they obtained per fect pictures of the various positions of the laryngeal muscles during the act of singing; and we may expect that such photos will be found of great value, not only in the teaching of classes of medical students, but as aids to the study of the mechanism of the voice. Further, Dr. Koch has recently got some remarkable photographs of bacteria and bacilli by the aid of the camera and microscope; and here, again, such pictures may be made of incalculable value in disseminating a knowledge of these minute but most formidable enemies of mankind.

In medical jurisprudence, when it is stated that the crystals formed by the one-thousandth of a grain of arsenic have been successfully photographed, it will easily be seen that, in cases of poisoning, photography may prove a very valuable assistant in the detection of crime. A novel use of the art is now being made in the Municipal Laboratory of Chemistry at Paris; photographs of chocolate, tea, coffee, pepper, milk, cheese, etc., as seen through the microscope, are taken and distributed; and, by comparing samples of such articles with photos of the pure article, an easy method is afforded even to non-professionals of detecting adulteration.

escape the eye in microscopic examinations. A large photo, six inches in length, of a small fly's tongue measuring about one-seventieth of an inch, shows the hairs and various markings with remarkable clearness. A simple calculation shows this photograph to cover an area 176,coo times as large as the original object. Again, views of the internal structure of wood show conclusively whether the wood is weak or strong; in strong wood the concentric rings appear close in texture, while the radial plates are numerous, broad, and thick. It has even been suggested that such photos might be used as trade advertisements. The internal structure of metals, too, has been examined by the joint aid of the camera and microscope; laminæ of the metals are reduced to extreme tenuity by the action of acids, and when sufficiently translucent are photographed through the microscope; gold and silver are said to have a fibrous structure, while tin is granular.

Photography is utilized by the microscopist in other directions. Accurate views have been secured of the most minute objects, just as they appear under the most powerful microscope. Photos of minute diatoms, polycystina, infusoria in motion, bacilli, and trichini have recently been obtained by the writer of this article under a power of 1000 diameters. The cilia of animalcula, blood corpuscles, the microscopic structure of bone and tissue are shown most distinctly, and details are seen easily which often

Till recently, no one would ever have dreamed of applying photography to acoustics; but it is now possible to photograph sound, or, speaking more accurately, sound-vibrations; and Professor Boltzmann is now announced as the discoverer of what at first might well be regarded with incredulity. The soundvibrations are communicated to a thin platinum plate, and the movements of the plate, after being magnified by a solar microscope, are reflected on to a screen, and photographed by rapidly drawing a sensitive plate across the image. Every letter when pronounced gives a separate and distinct impression, the vowels showing regular undulatory vibrations, while the consonants give curves and lines of very varied forms. The uses of an arrangement like this may be innumerable. We can almost imagine that when the process is perfected, eavesdroppers and spies will have a very easy time, and need to run no risks in order to obtain secret information; a small instrument secretly placed in a room, and acting automatically, may copy down every word spoken; nay, it is far more chimerical to expect that photography may one day take the place of shorthand reporters.

But besides all the varied ways in which photography has been utilized in science,