Dr. Frankland has answered this question, and I think satisfactorily. When coal is imperfectly burned, as it is whenever smoke is visible, a certain quantity of coal tar is formed and thrown into the air in vaporous condition. This coal-tar vapour condenses on cold surfaces, especially on the surface of water, as may be seen by examin ing some of the tanks in which the London water companies compel their helpless victims to hoard a stagnant reserve.

Dr. Frankland examined the effect of a film of coal tar spread upon the surface of a shallow vessel of water, and found that it acts as a cover or shield, greatly retarding, or almost preventing, the evaporation that would otherwise take place. Here, then, we have an explanation of “dry fog." The particles of water floating in the smcky atmosphere of a coal-burning city are coated with an oily fim, which, although inconceivably thin, constitutes a varnish, retards the evaporation, and renders possible the maintenance of their liquid state under conditions of atmospheric dryness that would dissipate them into invisible vapour in the country.

That such a fog should irritate the eyes, tickle the nose, and inflame the throat, is not surprising, when we consider how the varnished water-particles must strike their moist membranes, and there adhere and deposit their coal-tar varnish upon such sensitive surfaces.

The dark colour of the "London peculiar" is also explained. The country fog or mountain mist is white, being composed of particles of pure water; the London fog and the Manchester or Sheffield or Birmingham fogs have the delicate brown tint of water in which coal tar has been diffused.

Such a fog has but little power of self-dissipation, like that so Seautify displayed by the morning mists that rush up the mountain sides and vanish as they rise. It is only removed by a sweeping breeze which blows it beyond the valley in which the reeking town is bered. Hence our London fogs only display their full hideousness dening a dead calm

I'

A NOVELTY IN LEATHER.

I is now well and practically known that gelatine undergoes some curious changes when subjected to the action of chromic gerd, or of a salt containing much of this acid, such as bichromate of potash Quite a multitude of patented processes for converting photomaps into some sort of printing plates depend primarily on this Letion. The Woodburytype, the Autotype, Photolithography, Photo

aphy, Photozincography, &c. &c. &c., are carried out by genausy turning to practical account the action of chromic acid

In the early days of photography, Mungo Ponto discovered that paper which had been dipped in a solution of bichromate of potash and dried in the dark became, like the chloride and other salts of silver, sensitive to the action of light; and further investigation proved that this property belonged, not to the body of the paper itself, but to the size varnished over its surface. This chromatised gelatinous size became insoluble in proportion to its exposure to the light, so that when the paper was washed, the variable removal of the variably soluble gelatine left a faint picture, if the prepared paper had been duly exposed in a camera.

This faint shadowy suggestion skilfully followed up, led to the production of more decided pictures on a surface of glass covered more thickly with gelatine, and treated with chromate of potash, or chrome alum.

Such a thick film washed away in different degrees produced a printing surface, exaggerated very conveniently by the fact that the insolubility was accompanied by a swelling or thickening of the gelatine film. It would be a long story to tell how carbon dust was sprinkled over this variable glue, to which the dust variably adhered; how the gelatine picture was actually stamped into metal by the "natureprinting process;" how gelatine picture-films were skinned off from one surface and transferred to another; how their varying adhesion to water and repulsion of oil was made available for inking them like lithographic stones; how zinc plates are etched from these gelatine pictures; how lithographic stones have the picture film laid upon them; how such films are tortured until they submit to receive an electro deposit of copper upon them, which brings the picture standing up in bold metallic relief to be backed with type metal and wood, and then printed in a common press with ordinary type. By walking down New Oxford Street anybody may see some of these results in the Autotype Gallery there, and thousands of book and newspaper illustrations, supposed by the uninitiated to be wood engravings, are produced by one or another of these processes, each of which is a triumphant example of the union of science and art.

But this is not what I intended to write about when I began this note. Its proper subject is leather.

Take a solution of gelatine-clear soup will do, and smell it. Take some tincture of galls or infusion of oak bark, and smell that. Then mix them together and smell the mixture. A new odour will have become created, a very familiar odour, suggestive of St. Crispin; the smell of a practical shoemaker's shop, the smell of leather. If both solutions were clear as they should be, another change is observable, the mixture becomes turbid with a turbidity due to flocculent particles;

this is tanno-gelatine or the essence of leather. It is gelatine rendered tough and insoluble.

The skin of animals is mainly composed of gelatine, and the process of tanning consists in converting the soft and soluble gelatinous integument into tough and insoluble tanno-gelatine.

The reader will now perceive the drift of this long preamble, which is simply that chromic acid is about to be substituted for oak bark, catechu, sumach, divi-divi, valonia, and the other sources of tannic acid. So much having already been done by photographers with chromic acid and gelatine, and during so many years, it is only surprising that chromising-(if I may coin a word)—as a substitute for tanning should not have been invented long ago. In spite of the old adage, something that is very like leather may be produced by steeping prepared hides in a solution of bichromate of potash instead of ordinary tan liquor.

The inventors and their representatives of course claim many advantages over ordinary tanning, one of these being rapidity of action, less than half the time being required for the leathering of the gelatine. We have yet to learn what is the quality of the new product. The records of the patent office include a very long list of processes for shortening the tedious process of ordinary tanning, such as sewing up the skins as bags and forcing the liquor through them; the application of the principle of exosmosis by exposing one side to a dense solution and the other to a weak one; pricking the skin with small holes, &c. &c. &c.; but, with the exception of the latter (Snyder's process), the usual effect of rapidity is to produce harshness or brittleness, and this whether the hastening means be mechanical or chemical; thus the best leather is still that which is slowly tanned by old-fashioned simple immersion in unsophisticated tan liquor made from oak bark only.

We shall see what the chrome leather proves to be; this can best be done by wearing a pair of boots made from it. Engineering tells us that the new leather is being made "in fourteen tanneries in Germany, and is being introduced into Russia, Belgium, France, and Italy."

PRACTICAL SCIENCE IN FRANCE.

HE French Association, framed on the model of our British

THE French

Association, has had a successful meeting. Nature tells us that "the most attractive excursion was undoubtedly to the caves where champagne is manufactured by the old process," and that "a demonstration of the principles of the operation was given in the

caves of Pommery, where Madame Pommery kindly permitted the visitors to make practical test of the quality of her celebrated produce." I have not seen the report in which is embodied the results of this practical testing, which, if properly written, should begin and end with "Hip, hip, hurrah!" If not, the French savans are not yet on a level with the red lions of our British Association.

ΜΥ

A PERFORATED MOUNTAIN.

Y old friend Torghatten was lately introduced to the British Association at Swansea by Professor W. J. Sollas. It is an insular granitic rock, one of the thousands of such islands that fringe the cost of Scandinavia. It is situated a few miles south of the Arctic circle, and composed of stratified granite or "gneiss." When seen at a distance from the south, it is remarkably like a round-topped broadrimmed hat. It is 824 feet high, and pierced with a very curious natural tunnel 530 feet long (Professor Sollas says 600, but this is wrong). This tunnel is 250 feet high at its western entrance, 66 feet high at its eastern entrance, and about 200 feet high in the middle. The floor slopes downwards from east to west, being 470 feet above the sea-level on the east side, and 400 feet on the west. As the passenger packet passes on the east side, the daylight is seen. fairly through the mountain.

Professor Sollas attributes its origin to mechanical disintegration aided by joints. When I first visited this region in 1856, but little was known of this remarkable perforated mountain, beyond what could be seen in passing. I then ventured to suggest an explanation of its origin, which the accurate measurements subsequently made by Norwegian surveyors help to confirm. Torghatten stands out a short distance from the mainland of Norway, and to the west of it, of course. Every valley opening up on this coast is more or less terraced, and these terraces indicate a former submergence of this part of Scandinavia in varying degrees, the maximum reaching about 600 feet. By eye measurement at the time, I estimated the height of this tunnel at about 600 feet, and thus concluded that once upon a time the waves dashed against that part of the rock and battered out this tunnel as one of those ordinary sea-caves that abound on every rocky coast where the material of the rock varies in hardness or friability. I was not then aware of the difference between the height of the east and west opening, only having seen it from the east. The fact that the west side of the tunnel, which is exposed to the open sea, is about four times as high as the east mouth, confirms my theory, seeing that all the well-known sea caverns of this kind on our own and other coasts have similar

proportions in relation to their sea and inland extremities. The down slope of the floor corresponds in like manner, the west side being 70 feet lower than the east.

Besides this, the maximum height of the cavern corresponds remarkably with the height of the highest terraces, being 650 feet against their 600. The additional 50 feet is accounted for by the height of roof above sea-level, and the subsequent falling of the roof, as shown by blocks now lying on the floor. Such a cavern, started at the time of maximum submersion, would have its floor lowered as the land rose above the sea when they formed the lower terraces that abound in the valleys.

The "joints" described by Professor Sollas undoubtedly exist, and mechanical disintegration has taken place since the original excavation of the tunnel. This is proved by the blocks that have fallen from the roof and now cumber the floor, just as the boulders lie on the floor of a cavern under Dunluce Castle, which only differs in being now at the sea-level. On a subsequent visit nineteen years later, I observed several abortive attempts at similar caverns on the rocks of the neighbourhood, that is, hollows which overhang on the face of the cliffs, where joints and the mechanical disintegration described by Sollas were exhibited. But mere mechanical disintegration, and consequent falling of rock, cannot excavate a long tunnel. Horizontal traction, as well as vertical fall, is required. The material separated by the joints must be carried away from one end to the other-530 feet, in this case; or, at least, from the middle to each end-265 feet in each direction. The only agents we know capable of doing this with granite rock or pseudo-stratified gneiss are the sea-waves or a torrent river.

Such caverns abound inland in limestone, but these are due to the solvent action of water containing carbonic acid. It has no such action on gneissic or other similar metamorphic rocks, while every seacoast formed of such rocks exhibits more or less of such perforation by the waves. St. Katherine's Rock, at Tenby, is an insular mass perforated by a tunnel closely resembling Torghatten; the cliffs of Mohir on the Irish coast, and the whole face of the serpentine formation of Cornwall about the Lizard, abound with such tunnels, arches, sea-caverns, &c., all visibly done by the waves hammering out the softer portions of the rock. But an ancient sea-cavern upraised some 600 feet above the present sea-level is a rare phenomenon, and nobody need wonder that it is the subject of strange legends, such as one that I have narrated in "Through Norway with a Knapsack."

W. MATTIEU WILLIAMS.