proper kind, are propagated through the ethereal media, and pass into our eyes, and reach and agitate the nerves of the retina, they produce in us the sensation of light, in a manner more or less analogous to that in which the vibrations of the air affect our auditory nerves in producing sound.

5. That as, in sound, the frequency of the aerial vibrations, or number of excursions of each molecule of air, determines the note, so, in light, the frequency of the vibrations made on our nerves, in a given time, by the ethereal molecule, determines the color of the light and that, as the extent of the vibrations of air determines the loudness of the sound, so the extent of the vibrations of the ethereal molecule determines the intensity of light.

:

To understand how waves are produced, by the vibrations of the particles of an elastic medium, we have only to study the waves produced upon the surface of a pond of water when rain is falling; it will be found that the particles of water driven down by a single drop of rain force the adjacent ones upwards, the air above, in consequence of its being more elastic, yielding sooner to the pressure exerted by the displaced particles on those surrounding the point of disturbance. Thus a wave is raised round this point by the momentum of the falling drop, and, as soon as such momentum is spent, the effect of gravity draws the raised particles down to the common surface of the liquid; the momentum, however, which they have acquired in this descent, carries them below the level, or point of rest; and, in descending below this point, they cause, among the adjacent particles of still water, a motion similar to that by which they were themselves actuated by the falling drop. The second wave thus produced will, in its descent, cause a third; and thus a series of waves, decreasing in height as they increase in breath, will be produced in rings, vibrating up and down, as is easily proved by placing a cork upon a smooth surface of water and producing a series of waves, when the cork will produce the same vibrations as the particles of water upon which it floats. Such vibrations present the appearance of waves,

as the motion is communicated from one particle to those adjacent, as is represented by Fig. 1, which we may suppose to be a section of the surface of water at the time of the appearance of the waves, the dots being the separate particles.

m

Dr. Young, in applying the undulatory theory of light to the explanation of the various phenomena, while considering the results of Sir D. Brewster's researches on the laws of double refraction, first proposed the hypothesis of transversal vibrations, which has since been shewn to be a necessary consequence of dynamical principles, and is most important to the explanation of our subject. These vibrations he illustrated by the propagation of undulations along a stretched cord, agitated at one end, which, supposing a person to hold in his hand, and, by moving first quickly up and down, a wave will be produced, which will run along the cord to the other end; and then, by a similar movement, but from the right side to the left, another wave will be produced, which will run along the cord as the former; but the vibrations or undulations of each will be in planes at right angles to each other and independent of each other, one being in a perpendicular plane and the other in a horizontal plane; so that, according to this theory, Fig. 2 may be supposed to represent a a ray of ordinary or unpolarized light. I have chosen this representation of a single ray of ordinary light, which is a drawing of one of Mr. Woodward's beautifully simple card models, as it conveys at once to the mind a distinct notion of the planes in which the vibrations take place. A beam of light we may therefore conceive to consist of a succession of systems of waves following each other with immense rapidity, and comprising an immense number of rays, the vibrations of which are performed in every possible plane.

ELYDORIC PAINTING.

that can be procured,) is to be applied on them with a pallet knife. To this layer, when dry enough to admit of scraping, more is to be added, if ne

cessary.

As it is of very great importance for the preservation of this kind of painting, that the layers be free from oil, that they may better imbibe the colors laid on them, it is necessary that their surface be made very smooth, and that it be very dry and hard.

The artist is next to procure a circle of copper, about two inches diameter and one-fourth of an inch in height, extremely thin, and painted black on the inside. This circle, is to contain the water on the surface of the picture.

Water distilled from rain or snow is preferable to any other; ordinary water, on account of the salts which it contains, being pernicious to this mode of painting.

The colors are to be finely levigated between two oriental agates, carefully preserved from dust, and mixed with oil of poppies, or any other cold siccative oil, which should be as limpid as water.

All the colors being ground, are to be placed in small parcels on a piece of glass, and covered with distilled water.

The materials being thus prepared, the subject to be painted is to be faintly traced with a black-lead pencil on one of the pieces of cloth above-mentioned. The tints are then to be formed on the pallet from the little heaps under the water; and the pallet placed, as usual, in the left hand. The picture is to be held between the thumb and forefinger, supported by the middle finger, and the necessary pencils, the third and little fingers. The hands rest on the back of a chair to give a full liberty of bringing the work nearer to, or removing it far from the eye.

After having made the rough draught with the colors still fresh, the circle of copper, which is to surround the picture, is to be fitted exactly to the surface. Distilled water is then poured within this circle till it rises to the height of one-eighth of an inch, and the eye is held perpendicular over the object. The third finger of the right hand, while painting, should rest on the internal right angle of the picture. The work is then to be re-touched, the artist adding color and softening as he finds requisite. As soon as the oil swims on the top the water is poured off, and the picture carefully covered with a watch-glass, and dried in a box by a gentle heat. When dry enough, it is to be scraped nearly smooth with a knife, the artist repeating the former method till he is satisfied with his work.

It is at this period that the advantage of this new method particularly shows itself for the purpose of finishing; as the water poured on the picture discovers every fault of the pencil, and gives the power of correcting and perfecting it with certainty.

When the work is finished it is put under a fine glass, from which the external air is excluded, and then it is dried by means of a gentle heat.

ELECTRICITY OF STEAM.
To the Editor.

SIR.-I take the opportunity of mentioning, in
reference to the subject of the electricity of steam,
a circumstance which occurred to myself some time
ago. I was making an experiment, in which I was
collecting, in a gas-holder, the gas and steam which
were thrown off from two retorts containing re-
spectively tar and water. I was very much sur-
prised to observe, shortly after the commencement
of my experiment, the discharge of at least two
elective sparks from the top of the glass tube which
is used for ascertaining the quantity of gas by the
quantity of water contained in the vessel. My
attention at the moment being otherwise occupied,
I
gave little heed to the circumstance, thinking it
probable that this manifestation of electricity was
owing to some peculiar chemical action resulting
from the union of the gases. The accidental dis-
covery of electrical phenomena in steam generated
under high pressure, inclines me to think otherwise
now. You will be much the best judge whether
the publication of this circumstance will assist,
though ever so feebly, in the elucidation of the
subject.

Orford Place, Chelsea.

S. M. TAYLOR.

ON MANAGING BALLOONS. To the Editor.

SIR. A suggestion having occurred to me to render balloons more manageable, I beg to submit it to your scientific readers. It consists in having a fire in the car, and a tube (open at the top) from the fire or stove to pass throngh the balloon, (from the bottom through the top,) but not so as to touch the gas, the hole through which the tube passes being intended to be closed up with cloth, so that if the tube were removed, it would have no aperture for the gas to escape, as the cloth tube would be left in the cavity. This tube must have a valve in it near the bottom which can shut or open the passage at pleasure; and it is by means of hot steam or heat passed through it that the balloon is to be caused to ascend and descend. When wished, then, for it to ascend, the valve must be caused to open the passage, when the heat passing through the balloon will dilate the gas, and the balloon will consequently rise. For descent, then, the valve must be closed. But the balloon must not be nearly full, otherwise it could not expand, but must burst. No ballast will, it is presumed, be required, and probably some progression might also thereby be obtained, by employing a sort of nearly horizontal sail and inclining it one way or the other, as the balloon ascends; and the resistance of the air would then, it appears, cause the ballooon to progress. Suppose, then, the aeronaut wished to go eastward, he must let a current of hot air pass through the balloon, and at the same time incline the east end of his sail upward, and then close the passage so as to let the gas cool (or, if any method should occur, he may send in some cold air,) and as the balloon would thereby descend, he must turn the east end of the sail downward, and the balloon would go in the contrary way.

But the car ought to be fire-proof. It does not seem impossible to produce the same effect without fire by means of an apparatus in the car to condense and exhaust the gas; but then the tube must, instead of passing through the balloon, open into the balloon itself, and this method would be very laborious.

But with respect to ballast, I am obliged to confess my ignorance why any is ever required, as it seems that the only reason why it is adopted is because too much gas is put in the balloon, which, if not full, must expand as it rises into a more rarefied atmosphere, as, the more the outside air be rarefied, the more so will be the gas within; and, if the balloon be full before it rises, it cannot expand, but may burst.

The machine lately exhibited at the Polytechnic Institution of a cross sail applied to a balloon, seems to be the same as was invented by me, and also by another person, signed "Volitor," several years ago, and, which, on being turned round, rose in the air. (See the Philosophical Magazine for 1814, and the Mechanics' Journal, &c.)

Oval, Kensington.

L. GOMPERTZ.

To form Figures in Relief on an Egg.-Design on the shell any figure or ornament you please, with melted tallow or any other fat oily substance; then immerse the egg in very strong vinegar, and let it remain till the acid has corroded that part of the shell which is not covered with the greasy matter: those parts will then appear in relief, exactly as you have drawn them.

MEDICAL RECEIPTS.

SNUFFS.

THE base of all snuffs is, or ought to be, ground tobacco, the part of the plant used being the stalks of the plant. The following are general directions on the manufacture of it :

Perfumed Snuffs.-The tobacco being 'ground and sifted, as the scented snuffs do not require much pungency, it may be mixed with mahogany sawdust, or that from rose-wood, or else the pure tobacco powder may be soaked in water, (the water being serviceable to steep cabbage and lettuce leaves in to make “penny Havannahs ;") then pressed nearly dry, then dried, afterwards moistened with rose-water, or orange flower water; and finally sufficiently dried for sale a little oil of tartar being put into all the moist snuffs to keep them damp at all times. French snuff is scented with the root of calamus aromaticus.

Macouba Snuff owes its flavor, and perhaps its sedative powers, to the previous fermentation of the tobacco leaves by moistening them with cane juice, and probably producing in this way an acetate of the narcotic principle of the tobacco, if such exist, as there is reason to believe there does.

Lundyfoot's Snuff, which has a burnt odour, somewhat like malt, is prepared by partially torrifying the materials, and is imitated by moistening any light colored snuff with empyreumatic oil.

Eye Snuff-Triturate and mix together, in a marble mortar, 5 grains of sulphate of mercury with 2 scruples of liquorice root, powdered. Grimstone and others disguise this by various ingredients. It is a powerful errhine in producing a discharge from the nose, when snuffed up in small pinches, but it is not safe.

Cephalic Snuff-The following are various receipts recommended for this snuff:-Mix together equal parts of the leaves of asarabacca, marjorum, and of the lily of the valley, dried leaves of asarabacca 1 ounce, flowers of lavender 2 drams, dried leaves of asarabacca 3 ounces, leaves of the dock and flowers of lavender each 1 ounce, sage leaves, rosemary, lilies of the valley and the tops of sweet marjorum, of each 1 ounce, with a dram each of asarabacca root, lavender flowers and nutmeg; pound and sift it very finely.

Yellow Snuff.-Yellow ochre the size of an egg, add chalk to lower the color, grind with half an ounce of oil of almonds till fine, then add water by degrees, and two spoonsful of the mucilage of gum tragacanth till you have about a quart, mix this with purified snuff and dry it; then sift it through a very fine sieve to remove the color which does not adhere to the snuff, and moisten it afterwards with any scented water that may be desirable.

Tabac (Snuff) Perfumè aux Fleurs.-Put orange flowers, jasmine, common or musk roses, or tuberoses with the snuff for a day and a night, and sift them out; repeat this as often as necessary. Snuff does not heat with the flowers. Snuffs are also scented with musk, civette, ambergris, the essence of bergamot, rose-water, elder-flower water, &c.

KINDS OF CAMPHOR

Rough Camphor, Laurel Camphor, Camphora.— Obtained from the roots and shoots of the Laurus camphora, L. cinnamomum, and Capura curundu,

by distillation with water. Comes from China and Japan.

Dryobalanus Camphor.-Very little of this camphor comes to Europe, it being carried to China, where it sells for about thirty times the price of their own laurel camphor. Obtained by merely splitting the Dryobalanus camphora; the heart of this tree containing camphor mixed with oil (of camphor. It comes from Sumatra and Borneo.

Refined Camphor, obtained by sublimation with one-sixteenth its weight of lime in a very gentle heat. Camphor is stimulant, narcotic, and diaphoretic, in doses of 5 grains to 1 scruple, in pills or & bolus: too large a dose occasions vomiting and convulsions; its effects are counteracted by opium; it may be suspended in liquids, by means of mucilage, yolk of egg, or almonds. Camphor is put into boxes to keep insects from them, and is used in fireworks; it renders copal soluble in some essential oils.

Liquid Camphor, Oil of Camphor, Camphora liquida, Oleum camphora. From Dryobalanus camphora, by piercing the tree.

Rosemary Camphor.-Obtained from the oil by a careful redistillation, without addition, of onethird of the oil; the residuum affords crystals of camphor; on separating which, and re-distilling the remaining oil two or three times, the whole of the camphor, amounting to 1 oz. from 10 of the oil, may be obtained.

Sweet Marjorum Camphor.-Obtained in like manner, about 1 oz. from 10 of the oil; not volatile; when set on fire, it soon goes out.-Sage Camphor, 1 oz. from 8.

Lavender Camphor, 1 oz. from 4, or even less, of oil.

Thyme Camphor.-Crystals cubical, does not form a liquid solution either with nitric or sulphuric acid; is precipitated from nitric acid in a glutinous mass.

Soap Camphor.-Obtained from soaps, by solu. tion in water, adding muriatic acid, collecting the curd, washing it with boiling water, and pressing it to separate the liquid elaine. It is a mixture of stearic and margaric acids; used to make candles, which are very white, as neat as wax candles, and give a brighter light, but do not last so long.

Turpentine Camphor.-Obtained from spirit of turpentine, by passing muriatic gas through it, by which means it will yield about its own weight of a kind of artificial camphor, not dissolved by dilute nitric acid, and when dissolved by strong nitric acid not separated by the addition of water.

Citron Camphor.-Obtained from the white rectified oil of citrons, exposed to muriatic acid gas it absolves 286 times its bulk, or nearly half its weight, and yields about 9-10ths of camphor.

It will be seen immediately that these two last are muriates; the first, or that from turpentine, is the muriate of camphogen, or artificial camphor; camphogen being the supposed base of turpentine, while the citron camphor is the muriate of citrene; citrene being the new name of the oil of lemons.

Common Camphor is called the protoxyde of camphogen, and consists of carbon 10 parts, hydrogen 8 parts, and oxygen 1 part; by distilling it with nitric acid it absorbs 4 parts more of oxygen, and becomes camphoric acid.