and transparency? How is it affected by gas-light, the lime-light, the electric light, &c. &c. A multitude of such questions are suggested at the first glance on the subject.

Some queer applications may be made of this pigment. If all the house fronts of a southern city were painted with it, what a relief to the daylight glare would be afforded, while at night the streets would gain all the light due to reflection from their white walls. Ladies might wear bright and gay canary-coloured dresses in-doors, which would become sober and unobtrusive black when out-of-doors. This would especially suit the Turkish ladies, whose indoor or hareem costume is coquettish in the extreme, but which they modestly cover with the clumsiest of possible cloaks when they go abroad, or otherwise appear in public.

A

THE ORIGIN OF THE SALT OF THE OCEAN.

READER writes from Manchester in reference to my note on "The Origin of the Salt of the Sea" (May 1881), and wishes to know "How it is that the rivers, being the medium by which the ocean receives its salinity, are not themselves salt?" This would be a very difficult question to answer if its main postulate were sound; but such is not the case. As many others besides the writer may very naturally believe that what we call "fresh water" is not salt water, some explanation is desirable.

All river water is mild salt water, so mild that we cannot detect it by the sense of taste, but its salinity is at once distinguishable by means of suitable chemical tests. Rivers vary in the nature as well as the degree of their salinity. A mountain stream fed by the rainwater that has only flowed over the surface of siliceous rocks is nearly pure or "soft"; the water of a streamlet fed by springs that have squeezed through porous soils contains whatever the water may have been able to dissolve on its way.

But the average composition of the saline matters dissolved in river waters differs materially from that of the ocean not only in quantity but also in kind. At first sight my theory seems to demand that the salts should be the same excepting in degree of concentration, if the saltness of the ocean is due to the continuous contributions of saline matters by the rivers which have remained and accumulated, while only pure water has been distilled away by the

sun.

'This would be the case if all these salts were equally soluble; but such is far from being the fact, as may be shown by the simple experiment of boiling down sea-water and observing the result. At

first, as the water is driven off, a chalky semi-crystalline deposit will be observed. This is sulphate of lime-the salt which, when calcined, forms plaster of Paris. Sea-water contains about 100 grains of this to the gallon, nearly as much as it can dissolve; therefore precipitation occurs on reducing but slightly the quantity of solvent.

Continue the boiling until the bulk of the sea-water is reduced to about one-third, when a further deposition will commence. This will consist of the chloride of sodium, of which the sea-water contains above a lb. avoirdupois to the gallon. These will be followed by sulphate of magnesia (Epsom salts), of which sea-water contains about 330 grains to the gallon. Then will come down the chloride of potassium, 50 grains to the gallon. By still further evaporation of the small quantity of the residual bitter brine, the precipitation of all the above-named will continue, with the addition at last of a small quantity of bromides, only about 20 grains to the gallon.

Now, it is evident from this that we cannot go on adding more and more of the sulphate of lime, seeing that as regards this salt seawater is practically a saturated solution. But this is not all. If a more soluble salt of lime were brought into the sea, its lime would combine with the sulphuric acid there combined with magnesia, or soda, or potash, and would, in obedience to a curious chemical law, leave these bases to combine in preference with that one which would form an insoluble combination.

Thus the total quantity of lime in sea-water is limited by the solubility of sulphate of lime, and this amounts to only one part in about 400 of water. I put this in italics because it appears to me a very important law, having considerable influence on the limitations of oceanic life, and it is a law that I have never seen worked out and enunciated as above.

IN

THE ORIGIN OF Oyster-Shells, Corals, etc.

the previous note I have referred to the salinity of river-water, usually described as its "hardness," and to the saltness of the sea as due to the continuous contributions of saline matter by rivers, and also to the general fact that the salinity of river-water differs in composition as well as in concentration from that of the

ocean.

The chief source of the hardness of river-water is lime, and this exists most abundantly in the form of carbonate of lime, dissolved in excess of carbonic acid. Carbonate of lime, ie., the material of chalk and limestone rocks generally, is nearly insoluble in pure

water or rain-water, but if the water becomes by any means supplied with carbonic acid, it acquires the power of dissolving a small quan, tity of any ordinary limestone with which it may come in contact.

To illustrate this, buy a pennyworth of lime-water from a druggist; put some in a wine-glass, and then, by means of a quill or piece of glass tube, blow through it. The carbonic acid from the lungs will first convert the soluble lime into insoluble carbonate, the precipitation of which will render the water milky. Then continue blowing for some time. The turbidity will reach its maximum by the complete conversion of all the lime into carbonate, after which the carbonic acid becomes free, having no more lime with which to combine. It will now begin its next operation, that of dissolving the carbonate it had previously precipitated, and if the experiment is continued long enough, and the original lime-water was not too strong, the whole of the carbonate may thus be dissolved and the solution again rendered clear.

All vegetable matter in the course of decay forms carbonic acid; thus all rain-water that falls upon old grass, dead leaves, peat, or other vegetable matter not vigorously growing, picks up this solvent of limestone, and all rain-water that has travelled far over anything but bare rocks is ready for the work of dissolving limestone rocks, and does so, as may be proved by testing the waters of any river that, during any part of its course, has flowed through a limestone region; and there are very few rivers of any magnitude that have not.

Thus the quantity of dissolved limestone constantly flowing into the sea is enormous ; but, as I have shown in the previous note, the quantity that sea-water is capable of holding is limited by the very small solubility of sulphate of lime. What becomes of the excess?

This question is easily answered by dredging the sea bottom, or by a walk along the sea coast, or even by a visit to Billingsgate or any other fish market. The oysters, the mussels, the cockles, &c., must get the lime for their shells from the sea-water; so also must the associated millions of coral animals that build up islands and fringe great continents with the remains of their calcareous encasements.

The multitude of other creatures that clothe their soft bodies in stony armour all derive material from the same source, and all of them are engaged in the recompensing work of producing new limestones by depositing their remains on the sea bottom, which, when upheaved or otherwise laid bare, will form the dry-land lime-rocks of the future, which, in their turn, will be washed and liquefied by the

same water that has again and again been evaporated from the ocean. surface and recondensed as rain.

THE ANGLO-AMERICAN ARCTIC EXPEDITION.

‘OMMANDER CHEYNE has gone to America to seek the

modest equipment that his own countrymen are unable to supply. He proposes now that his expedition shall be "AngloAmerican." I have been asked to join an Arctic Council, to cooperate on this side, and have refused on anti-patriotic grounds. As a member of the former Arctic Committee, I was so much disgusted with the parsimony of our millionaires and the anti-geographical conduct of the Savile Row Mutual Admiration Society, that I heartily wish that in this matter our American grandchildren may "lick the Britishers quite complete." It will do us much good. My views, expressed in the Gentleman's Magazine of July 1880, remain unchanged except in the direction of confirmation and development. I still believe that an enthusiastic, practically trained, sturdy Arctic veteran, who has endured hardship both at home and abroad, whose craving eagerness to reach the Pole amounts to a positive monomania, who lives for this object alone, and is ready to die for it, who will work at it purely for the work's sake-will be the right man in the right place when at the head of a modestly but efficiently equipped polar expedition, especially if Lieutenant Schwatka is his second in command.

They will not require luxurious saloons, nor many cases of champagne; they will care but little for amateur theatricals; they will follow the naval traditions of the old British "sea-dogs" rather than those of our modern naval lap-dogs, and will not turn back after a first struggle with the cruel Arctic ice, even though they should suppose it to be "paleocrystic."

SCIEN

MR. WALTER POWELL.

CIENTIFIC aërostation has lost its most promising expert by the untimely death of Walter Powell. He was not a mere sensational ballooner, nor one of those dreamers who imagine they can invent flying machines, or steer balloons against the wind by mysterious electrical devices or by mechanical paddles, fan wheels, or rudders.

He perfectly understood that a balloon is at the mercy of atmospheric currents and must drift with them, but nevertheless he regarded

it as a most promising instrument for geographical research. I had a long conference with him on the subject in August last, when he told me that the main objects of the ascents he had already made, and should be making for some little time forward, were the acquisition of practical skill, and of further knowledge of atmospheric currents; after which he should make a dash at the Atlantic with the intent of crossing to America.

On my part, I repeated with further argument what I have already urged on page 113 of the Gentleman's Magazine for July 1880, viz., the primary necessity of systematic experimental investigation of the rate of exosmosis (oozing out) of the gas from balloons made of different materials and variously varnished.

Professor Graham demonstrated that this molecular permeation of gases and liquids through membranes mechanically air-tight, depends upon the adhesive affinities of particular solids for other particular fluids, and these affinities vary immensely, their variations depending on chemical differences rather than upon mechanical impermeability. My project to attach captive balloons of small size to the roof of the Polytechnic Institution, holding them by a steelyard that should indicate the pull due to their ascending power, and the rate of its decline according to the composition of the membrane, was heartily approved by Mr. Powell, and, had the Polytechnic survived, would have been carried out, as it would have served the double purpose of scientific investigation and of sensational advertisement for the outside public.

If the aëronaut were quite clear on this point-could calculate accurately how long his balloon would float-he might venture with deliberate calculation on journeys that without such knowledge are mere exploits of blind daring.

The varnishes at present used are all permeable by hydrogen gas and hydro-carbon coal gas, as might be expected, à priori, from the fact that they are themselves solid hydro-carbons, soluble in other liquid or gaseous hydro-carbons. Nothing, as far as I can learn, has yet been done with siliceous or boracic varnishes, which are theoretically impermeable by hydrogen and its carbon compounds; but whether they are practically so under ballooning conditions, and can be made sufficiently pliable and continuous, are questions only to be solved by practical experiments of the kind above named. Now that the best man for making these experiments is gone, somebody else should undertake them. Unfortunately, they must of necessity be rather expensive.

W. MATTIEU WILLIAMS.