tubulated glass retort, a (see engraving, fig. 1), with a tubulated receiver, A, passing into the bottle, c. Ihe requisite heat is obtained by the lamp, d, and the whole apparatus supported by the brass stands, with sliding rings, e e. But the manufacturer who prepares nitric acid upon the large scale, generally employs distillatory vessels of stone ware. Fig. 2 represents the arrangement of the distillatory apparatus employed at Apothecaries' Hall for the production of common aquafortis. It consists of an iron pot set in brick-work over a fire.place, an earthen head is luted upon it, communicating with two or more receivers of the same material, furnished with earthenware stopcocks, the last of which has a tube of safety dipping into a basin of water. The nitric acid of commerce, as obtained by the above processes, is always impure, and muriatic and sulphuric acids may usually be detected in it. The former may be separated by nitrate of silver, and the latter by a very dilute solution of nitrate of baryia. To obtain pure nitric acid, therefore, add to that of commerce a solution of nitrate of silver as long as it produces any white precipitate, and when this has subsided, pour off the clear liquor, and add in the same way the nitrate of baryta, then distil the acid, and it will pass over perfectly pure. Nitric acid is a colourless liquid, extremely sour and corrosive, and veryintense in its action upon the greater number of inflammable bodies. Its specific gravity, as usually obtained, fluctuates between 1.4 and 1.5. At 40° the concentrated acid congeals; when diluted with half its weight of water, it freezes at about 2° below 0°. Its salts are called nitrates.

Nitrotis acid may be obtained by mixing two volumes of nitric oxide with one volume of oxygen; much heat is evolved, and the gasses become condensed to onethird their original volume, and form nitrous acid or rather vapour, for it is condensed at 0°. This gas supports the combustion of a taper of phosphorus and of charcoal, but extinguishes sulphur. It is readily absorbed by water, forming a sour liquid. Its specific gravity is as 3.162 to 1.000. It forms salts termed nitrites.

Hydrochloric acid may be readily procured by acting upon common salt or sal ammoniac by sulphuric acid: the evolved gas must be collected over mercury. The salt should be put in fragments into a small tubulated retort, which may be onefourth filled with it; the sulphuric acid should barely cover the pieces of salt; the gas is instantly extricated, and when its evolution slackens, it may be quickened

by the gentle heat of a lamp. It is convenient to put a long strip of folded blotting paper into the neck of the retort, which absorbs any liquid which may chance to go over, and prevents its soiling the mercury. Hydrochloric acid gas is perfectly unrespirable; it extinguishes the flame of a taper. Although permanently gaseous at all common temperatures and pressures, Mr. Faraday liquified this gas by genarating it in a sealed tube, so as to expose it to a pressure of about forty atmospheres at 50°. It was colourless, and possessed a refractive power inferior to that of water. Hydrochloric acid gas has a strong attraction for water; when it escapes into the air it forms visible fumes, arising from its combination with aerial vapour. A piece of ice let up into the gas over mercury, immediately liquifies it and absorbs it; and if a tall jar of the gas be carefully transferred, with its mouth downwards, from the mercurial to the water-trough, the water instantly rushes in with violence and fills it. Litmus paper is powerfully reddened by this gas. Water takes up 480 times its bulk of muriatic acid gas, and has its specific gravity increased from 1 to 1.210. This may be shown by throwing up a few drops of water into a tall jar of the gas standing over mercury, the gas disappears, and the mercury fills the vessel. There is considerable elevation of temperature during the condensation of the gas. When the liquid acid is pure, it is perfectly colourless, but it generally has a yellow hue, arising from particles of cork or lute that have accidentally fallen into it, or sometimes from a little iron. The acid of commerce almost always contains iron and sulphuric acid, and sometimes nitric acid. The concentrated acid emits fumes ; when exposed to the air, it boils at a temperature of 1I2J, and gives off the gaseous acid; it freezes at GO". It is decomposed by the chloric, iodic, and brnmic acids, and several of the metallic oxides. It forms salts called hydrochlorates.

Hydrocyanic or Frussic ac':d, may be obtained by introducing cyanuret of mercury into a tubulated glass retort, and pour upon it rather less than its weight of muriatic acid (specific gravity 1.200). Adapt a horizontal tube to the beak of the retort about two feet long and half an inch in diameter; fill the first third of the tube next the retort with small pieces of white marble, the other two-thirds with fragments of fused chloride of calcium, adapt to its end a small receiver surrounded by a freezing mixture; on applying a gentle heat to the retort, hydrocyanic vapour will pass through the tube, and be

Chemical Powers of Light.—M. Edmond Becquerel Has recently communicated to the Aeadlmie des Sciences, some important investigations on the chemical powers of solar light, which will probably lead to new and valuable results. It has been long known that light has the power of variously affecting certain chemical compounds; sometimes causing combination between two elements, and in other cases effecting the decomposition of compound substances; and it has been found that w hen a pencil of light is decomposed by passing through a prism of glass, those rays which possess this power are differently refracted from the coloured rays, and hence the existence of peculiar rays, to which the name chemical rays is given, has been deduced. The chief difficulty in experiments on these rays has been, the slow nature of the actions caused, and the difficulty of appreciating them. M. Becquerel has overcome these sources of uncertainty, aud is enabled to study the chemical powers of light with ease, and measure the effects produced, with considerable accuracy. The manner in which this is done is very simple. Two liquids of different densities, but both conductors of electricity, and of such nature as to act chemically upon each other when exposed to the influence of solar light, are selected; and a portion of both is put into a cylindrical vessel blackened on the exterior. A plate of platinum is placed in the denser of the two fluids, and another similar plate is also immersed in the lighter liquid; these plates being then connected by means of platinum wires with the two terminations of a very delicate galvanometer, the apparatus is complete. If when thus arranged a ray of light is suffered to pass through the mass of fluid, it causes chemical action to take place at the surface of contact between the two liquids, and a current of electricity which this sets in circulation is immediately rendered evident by the galvanometer. As the angle of deflection of the galvanometer indicates the power of the electric current, and as this is in exact proportion to the chemical action which originates it, it is evident that this arrangement gives an accurate measure of the power of the chemical rays of light, at different times, from different sources, and under various circumstances. M. Becquerel details some experiments on the quantity of these chemical rays, which is intercepted when a ray of light is made to pass through screens of different substances, such as rock crystal, mica, and variouslycoloured glasses; and states that he is still engaged in experi men ting on the subject—Athenatum.

A Short Name.—The Dutch journals announce that King William has invested the Sultan of Djocjokata, with the dignity of commander of the order of the Lion of the Netherlands; the Sultan's name is H am an koebocwonosen opatiingalogongabgurrachmansaydinpnnotogoniodc, the Fifth. To make Oats doubly Nutritious.—Instead of grinding the oats, break them in a mill, and the some quantity will be doubly nutritious. Another method is, to boil the com and give it to the horses with the liquor in which it has been boiled; the result will be, that instead of six bushels in a crude state, three bushels so pre

pared will be found to answer, and to keep the animals in superior vigour and condition.

INSTITUTIONS.

LECTURES WOKING THE WEEK.

London Mechanics Institution, 29, Southampton, Buildings, Chancery-lane. Wednesday, Nov. 18, A. J. Mason, Esq., Sketches of the United States, Geographical, Political, and Moral. Friday, Nov. 15, B. R. Haydon, on the History of Art, from the earliest time. At half-past eight precisely.

Westminster Literary and Scientific Institution, 6 and 7, Great Smith-street. Thursday, Nov.

14, H. W. Woelrych, Esq., on Education. At half-past eight

St. Pancras Literary and Scientific Institution, Colosseum House, New-road. Friday, Nov.

15, Mr. Joshua Crockfom, on the Application of the Force of Gravity. At half-past eight.

QUERIES.

How to make blue ink that will not change its colour? V. T. Z.

What will turn brass to a steel or any dark colour, so that it will polish the same as any other metal? C—s C—s.

TO CORRESPONDENTS.

Alpha.—The experiments of M. Desprets concerning the transmission of heat in liquids, were minutely described in a recent number of the "Mechanic."

Gulphy wishes to know which are the best works on boxing, wrestling, and fencing; for the first, we should say that the best books are those which condemn the practice in the strongest terms of reprobation; as for fencing, it may be will enough between a butcher and a pig or a sheep, and perhaps necessary for some military men; but those who persuade youth that sticking or shooting one another is honourable, merit the abhorrence of all good and sensible men. Scratching and biting may be considered as honour amongst the grey rats of Montfaucon, or the toads and water lizards in a horse pond; but it is not so between men; a generous heart and a spotless mind, that is true honour; and let those who pretend that honour is derived from the mutilation of their fellow-creatures, be consistent, and bow down with reverence to the Tom Cat, who, according to the principles which they propound, must be more honourable than any two-legged gentlemen upon earth.

J. Mitchell is requested to oblige "Juvenile Entertainer " with his address, that gentleman having some chemical discovery to communicate.

W. P. and E. P. shall be answered in our next.

London: Printed at "the City Press," 1, Long Lane, Aldersgate, by D. A. Doudney (to whom all communications for the Editor must be addressed, postage paid); published every Saturday, by G. Berger, Holy well-street. Strand; and may be had of all Booksellers and Newsmen in Town and Country.

PENNY POSTAGE.

(See Kugraving, front page.) [ No latter to be carried which is not irritten on stamped paper.]

The paper to be stamped in the making, under the superintendence of the Excise, and a price set upon the said paper, sufficient to remunerate government for the carriage and delivery of the game, in the form of letters. The amount paid for stamping to be charged on the usual price of such paper by the manufacturer to the retailer, and by the retailer to the consumer. The object to be attained in this, being, that whilst it removes all cause for complaint of monopoly on the part of the manufacturer, brings the amount of the carriage of such letters through a more direct channel, than by the said amount being brought in by the deliverers of such letters; at the same time, saving a great sum of money, which would otherwise be expended for labour, and which would necessarily be incurred by the additional number of letters arising from the reduced postage. The deliverer, not having to receive postage for the letters he carries out for delivery, would, of course, be enabled to deliver a considerable number more. and that in less time.

Letters weighing no more than is allowed at the present time for single letters, to be carried as single letters; whilst those letters which are above the weight of R single letter, and not exceeding double such weight, to be charged one penny extra, either at the time of posting, or on the delivery of the said letters. Above this double weight, and up to a certain weight to be fixed by government, twopence extra charge, above which weight no letters should be carried.

The stamp for the paper, on which all letters to be sent by post must be written, I would propose to lie that of the royal arms; the impression of which to be made on the opposite side of the paper to that on which a letter is commonly commenced; tut that on folding the letter, the impression of the royal arms may be outside, and the address to be written over it.

Another proposition I would make, and which, I presume, would tend materially to the furtherance of the object now under consideration, is the manner of constructing the letter-boxes at the several receiving.houses under the operation of the penny post. The additional number of letters arising from the reduction of the postage, will not only require additional time for the stamping of such letters with the district mark, but will, in many instances, cause numerous mistakes and

even delay. To remedy this inconvenience, I propose the letter-boxes to be thus constructed.

Description of Letter-box.

I propose that there should be fixed inside the letter-box, close to the mouth, three rollers (see engraving, A B c); these rollers are for the purpose of stamping the letters as they are put inI D is a box containing ink; the roller, c, which is to be covered with cloth for imbibing the ink, revolving in the box, will ink the type or stamp on the roller, B ; so that, if the end of a letter be introduced between the rollers A and B, and the roller be set in motion, the letter must necessarily be drawn into the box, and be stamped at the same time: motion to be given to the roller by the pull at E; and after the letter has passed into the box, the spring, i, brings the rollers back into their former position, ready for another letter. Thus, every person stamps his own letters, which precludes all possibility of detention. Stamp I, is the district mark; stamp 2, the date and time. As the type of the date stamp will require frequent altering; figures may easily be furnished with screws for receiving them on the roller.

W. Scott.

INVENTION OF GUNPOWDER.

Next to the invention of printing, there is no other that so much arrests our attention as that of gunpowder, which, by introducing artillery, and a new method of fortifying, attacking, and defending cities, wrought a complete change in the whole art and tactics of war. 1 his invention comprises several discoveries, which it is necessary to distinguish from each other. 1. 'i he discovery of nitre, the principal ingredient in gunpowder, and the cause of its detonation. 2. The mixture of nitre with sulphur and charcoal, which, properly speaking, forms the invention of gunpowder. 3. The application of powder to fire-works. 4. Its employment as an agent or propelling power for throwing stones, bullets, or other heavy and combustible bodies. 5. Its employment in springing mines and destroying fortifications.

All these discoveries belong to different epochs. The knowledge of saltpetre or nitre, and its explosive properties, called detonation, is very ancient. Most probably it was brought to us from the east (India or China), where saltpetre is found in a natural state of preparation. It is not less probable that the nations of the east were acquainted with the compo-