This metal has a yellowish-white colour and a feeble lustre. It is brittle and infusible, and takes its name from the Greek word kroma, signifying colour; because its compounds constitute some of the finest mineral colours. There are two compounds of chromium and oxygen. The first is an oxide of a dark-green colour, and is composed of 28 parts, or 1 equivalent of the metal, and 12, or 14 of oxygen. The second combination forms an acid called chromic acid, and is composed of 28 parts, or 1 equivalent of the metal, and of 24, or 3 equivalents of oxygen. r Chromic acid is a crystalline compound, of a dark ruby-red colour, and unites with the oxides of metals and with ammonia, forming salts called chromates. GEOMETRICAL QUESTION. To the Editor of the Mechanic and Chemist, Sib,—Although I am anxious at all times to be corrected when I am "mistaken," yet I cannot admit that I am mistaken in what I have written respecting my pedes trian question. I have submitted the question to friends, on whom I can rely, and they cannot conceive the possibility of a doubt as to my being correct. It is perfectly true, as stated in your notice to correspondents in No. 85, that 1. /, &c, are equal spaces, and P p, &c, are equal spaces; but it does not follow that p I, &c, are parallel to p L, nor will they all point to the north. I suspect, from the erroneous reasoning, that the writer of the notices to correspondents and "L. L.", are identical. However, to put the matter beyond the doubt of a child, if you place yourself in one corner of a large room, and place your boy in another corner on the same side, and he move towards the corner opposite to you, do you mean to contend, that if you move continually towards the boy (who is always progressing and changing his place), you will move in a $ might line $ Or, which amounts to the same thing, can you for a moment contend*, that if you walk direct from the corner in which you are placed, to the opposite corner, you will be continually moving towards the boy, although he is only travel ling towards such last corner? Either of these positions is preposterous, and yet these are the positions which you and "L. L." contend for I It might as well be said (and the cases are parallel), that because it planet is retained in its orbit by centripetal and centrifugal forces (each of which, taken alone, is a rectilinear force), it will not move in a curve, but in a straight line! This, we know, is not the case; and it is equally clear that the pedestrian must move in a curve, and not in a straight line. Your and "L. L.V error consists in assuming a point where the two parties are to meet, from the rates of their travelling, and is, in fact, nothing more than an answer to the following question, scarcely fit for the consideration of a schoolboy of ten years of age :—" If A is distant from London eight miles, and B is distant from London ^8' + C* miles, and A travels four miles an hour, and B five, how long will they be before they reach London?" Ana. "Two hours." Of course, this is all that can be made of" L. L's" solution. The question to which he gives the answer is similar to the following :—" A and B are fifty miles asunder, directly east and west, and are each travelling in a straight line to a place directly north of B, so situated, that if B travels three miles an hour, and A four miles an hour, they will both reach it at the same time. How far distant is the place from A and B respectively?'' But, in such a case, it can never be said, as in my question, that A is travelling towards B, for he is continually travelling direct to a place in advance of B, and to which B is also travelling. In fact, the matter is too clear to waste farther words about it. But, after all, I should like to see a correct and concise solution to my question. W. N. (We repeat that "W. N." is utterly mistaken, and a correct and concise solution to his question has already been given. Our answers to correspondents are not, as he surmises, supplied by correspondents ; and we know no more of "L. L.", than that he favoured us with an accurate, though somewhat too brief solution of the above problem. "W. N." appeals from our decision; he has a right to do so; and if he can persuade " L. L." and the rest of our mathematical readers, that two lines, near together, and each pointing to the north, are not to be considered as parallel, then, indeed, the question would assume a new aspect; but, unless this be conceded, p c is evidently a straight line. "W. N." appears to forget, that the light was to be continually due north of the traveller.—Ed.] THE CHEMIST. ON ALKALIES. fContinued from page 285.J Brucia (Vegeto).—This alkali received its name from Mr. Bruce, the Abyssinian traveller, who first made known the tree, the false augustura, from the bark of which the alkaline substance is obtained. The bark is powdered and digested in ether, which frees it from oily matter; it is then treated with alcohol. This solution is then evaporated, which leaves impure brucia. To purify it, it must again be dissolved in water, and saturated with oxalic acid, then evaporated to dryness; the colouring matter is then removed by alcohol, and leaves pure oxalate of brucia, which is decomposed by lime; and the brucia may be taken up in a free state by boiling alcohol, from which it may be obtained by slow evaporation in the form of crystals. In this state it is a true hydrate, which, by fusion, loses a considerable quantity of water. Brucia is intensely bitter, is sparingly soluble in water, fuses at a temperature near 212" Fab., and, on cooling, looks like wax. Exposed to moist air, it swells, and becomes pulverulent. It combines with acids, forming neutral salts. Mr. Liebeg's analysis of brucia is as follows :— Carbon 70.88 Nitrogen 5.07 Oxygen 17.39 Hydrogen .... 6.C6 100.09 Cinchonia (Vegeto).—To obtain this alkali, the alcoholic extract of the grey bark of cinchonia * is to be entirely dissolved in boiling water, strongly impregnated with hydrochloric acid; calcined magnesia is then added in excess, to fix the colouring matter; it is allowed to cool,filtered, and themagnesian precipitate washed with cold water. The precipitate having been dried on a stove, is to be repeatedly treated with alcohol, in order to dissolve all the bitter matter. The alcoholic liquors are now mixed, and the cinchonia crystallizes on cooling. It is white, and soluble in 700 parts of cold water. When dissolved in alcohol, and still more in an acid, its taste is very bitter. It unites with acids, forming salts, more or less soluble. When it is heated in a glass tube, it evolves ammonia. Some years • Peruvian bark. back, MM. Pelletier and Dumas analyzed cinchonia, which afforded them Carbon 78.G7 Hydrogen 7.06 Oxygen 6.16 Nitrogen 9.11 100.00 Codeia (Vegeto) was discovered by Robiquet in 1832. In treating a solution of opium with muriate of lime, muriate of morphia is precipitated in combination with another substance, which is codeia. From this precipitate the latter is separated by ammonia; the major part of the morphia is precipitated. The supernatant solution of codeia, with some non-precipitated morphia, is concentrated, until the chlorohydrate of ammonia begins to crytallize, and with it the double salt of morphia and codeia. The crystals are to be dissolved in water, and filtered through animal charcoal; a slight excess of caustic potassia is then added, by which means the codeia alone is precipitated. It is then taken up by alcohol or ether, which yields it pure by evaporation. Codeia is insoluble in alkaline solutions; it combines with acids, forming neutral salts, which are decomposed by tannin. According to M. Robiquet, it is composed of Carbon 71-339 Nitrogen 5.353 Hydrogen .... 7-585 Oxygen 15.723 100.000 G. W. S. Piesse. IMPROVED CHEMIST'S MORTAR. To the Editor of the Mechanic and Chemist. Sib,—It is well known, that in pulverizing drugs, a considerable portion is lost, besides the offensive odour that arises, from some—aloes, for instance. I think the following simple plan would effectually remedy the evil:—'lake a piece of fine cloth, make it in the form of a bag, but open at both ends; one end should be made wide enough to slipover the top of the iron mortar, round which should be fastened a thin band of India-rubber, rather smaller than the top of the mortar, so as to slip over it; the other end of the bag should have a small ring of the rubber fastened to it, Bo as to slip over the pestle; so that when one end is slipped over the top of the mortar, and the other over the pestle, not a particle of the powder can escape; and it does not in the least prevent the action of the pestle, or cause a loss of time, as it can be taken off in a moment. I have made one myself, and have used it for several years, and the expense is only trifling. I remain yours, &c, J. Thomson. INSTITUTIONS. LECTUBES DURING THE WEEK. Lmdtm Mechanics' Institution, 29, Southampton Buildings, Chancery Lane. Wednesday, April 15, W. Rider, Esq., on Engraving. Friday, April 17, Institution closed. At half-past eight precisely. ■ - Literary and Scientific Institution, 6 and 7, Great Smith Street—Wednesday, April 15, Quarterly General Meeting. At eight o'clock. Poplar Institution, East India Road.—Tuesday, April 14, T. Archer, Esq., on Igneous Geology. At eight o'clock precisely. QUERIES. I. What composition is that which will change any coloured hair a jet black? 2. What will make the hair curl? 3 How to make the white daring soap, such as is commonly sold in the shops?" A. H. D. Coventry. [The first question has been so often asked and answered in the " Mechanic," that we are compelled to decline saying anything more about it; we. nevertheless, leave it open for any of our correspondent* who can describe a method superior to the nitrate of silver. We do not think that " a hair-dresser" would be benefitted by speculating in soap-boiling; but he might find *one advantage in adding a little sweet oil to lather ; this is, in the language of old writers of receipts, " proved."—Eo.j How to make the writing-fluid, that, at the urn* of using, is of a greenish blue, but soon turns a deep black? J. M. H. How to stain white whalebone blue or green? J. H. How to make yeast different from that which is procured at the brewers? W. T. How to obtain fluoric acid? T. Wear. Can any of your correspondents inform me *hat to put with glass to make it melt easier P--. (Borax.) How to make bird-lime? What to pot in bread seals to prevent their cracking? I bare found putting them in a cold place does not effect the object W. V—K. The best method of staining leather, or, at least, the ingredients I should use, to produce a *kr blue and a gross green? A. Z. ANSWERS TO QUERIES. To wake Blue Ink—Prussian blue, | oz.; oxalic acid, \ oz.; water, 16 oz.. The above its colour, and is not affected by acids, though, of course, it will not stand a strong alkali. J. M. H. "V. Knox." "1. Why is the apparent brightness of a star increased by the use of a telescope, while that of a planet is not?" This, I conceive, arises from the fact of the planet, when viewed through a telescope, being magnified in size, while a star is only magnified in lustre; it not appearing through a telescope to be increased in size, but to still remain a mere point [The intensity of the light emitted by a star, is estimated at about 800,000 times greater than that of a planet. If a star does not appear to the naked eye so bright as when magnified by a telescope, it is because our organs of vision cannot appreciate the impression of so extremely small an image.—Ed.] "2. Why are some stars visible only at particular seasons of the year?" Because, by the apparent motion of the sun in the ecliptic, all stars (except those circumpolar stars, which are always above the horizon) rise and set nearly four minutes earlier every day. Therefore those stars (except as above mentioned) which are now visible in the night, will, in six months hence, be above the horizon in the day time, and will, consequently, be then invisible, because of the sun's light, and vice versa. "3. How the apparent place of a star, at any time, is calculated from its mean place at the beginning of the year?" A full answer to this would occupy too much space. In the first place, we must find the mean place at the given time by the annual variations in right ascension and declination. To this the proper corrections must be made for precession, aberration, and nutation, and we shall have the true place of the star. The altitude of the star for the given time and place may then be found, and that increased by the refraction in altitude, will be the star's apparent place. The correction for refraction, however, is not necessary in occultations and near approaches of a star to the moon, &c, as both sides are then affected by it equally. "V. Knox" will find the necessary tables for the above corrections, in "Galbraith's Mathematical and Astronomical Tables," which is a very useful work. W. N. "S. C." "There was an occultation of the star a Lconis by the moon; the time for Greenwich was 7h. 10 m. immersion, and 8 h. 21 m. emersion; would there be any difference, and what the difference, between the time at Greenwich, aud in fi0° 36' north latitude, and 2° 27' west longitude?" It is impossible, from the question as stated, to give a correct answer. In consequence of the large parallax of the moon, her apparent places, as seen in different latitudes and longitudes at any given moment, will always be different. The only correct way of solving "S. C's" question is, to compute the times of the immersion and emeision for the given place; which is a work occupying considerable time and space. (See any work on practical astronomy or spherical trigonometry, or Galbraith's book above referred to ) The difference in actual time asked by your correspondent is trifling; but, as the motion of the moon in the zodiac is fro n west to east, the more westerly the place, the earlier the occultation lakes places. However, your correspondent must recollect, that to obtain the time on his meridian, it is necessary to subtract from the Greenwich time, 9 m. 48 s., answering to the difference of longitudes. That is, that the occultation, as observed at Greenwich, commences at 7 n. 0 in. 12 s. Weymouth time, and ends at 8 h. 11m. 12 s. "Weymouth time. Being an amateur astronomer myself, 1 shall be happy at any time cither to give (so far as is in my power) or to receive any information or assistance in astronomy, either through this work, or by letter in a private way. W. N. To Stuff Birds.—The principal beauty of studied birds, consists in their being well shot; for the large species, ball-shot from a rifle should be used, and, for smaller ones, dust-shot. As soon as one is killed, a little wool should be laid upon the bleeding orifice, the feathers laid in order, and the head wrapped up in tow; it should then be packed in hay, and then quickly conveyed home. You then lay it upon a clean cloth, and part the feathers of the breast and abdomen . then divide the skin, taking care not to soil the feathers from the breast to the rent; or they may be opened under the wing: and those who have beautiful breasts, as the divers, &c, may be opened on the back. Separate the skin from the muscles and cellular tissue, by means of the finger or a blunt instrument. Push up the thighs, and deprive them from the flesh, and break the bone about its middle; draw the skin over the body, and remove it also from the wings to the second joint; treat them as you have done the thighs; then turn the skin over the head, and remove the occipital part of the skull, so that you may be able to scoop and wash out the brain. Remove the eyes, dissect away as much flesh as possible from the skull, and, when you have finished shining it, rub the skin over with chalk, to remove adipose matter; wash it clean with a sponge and warm water, then cover it all over with either the following solution, powder, or soap:— Solution.—Muriate of mercury, loz.; alcohol, 8oz. Powder.—Muriate of mercury, | oz.; burnt alum, \ oz.; tanner's baik, 31b.; camphor, 4 oz. Soap.—Camphor, 5 oz.; arscnious acid, 2 lb.; white soap, 21b.; subcarbonate of potass, 12 oz; powdered lime, 4 oz. To keep the Hand* from Chapping.—Tincture of Peruvian balsam, 2 oz.; tincture of benzoin, 1 oz.; tincture of aloes, loz.; hydrochloric acid, 10 drops. Manipulator. The title of the work to which "R. H. S." alludes, is "The Cabinet-maker's Guide," in which he will find the information he requires. There are two works under this title; the first of which was published anonymously, and printed by or for Knight and Lacy, Paternoster Row; the second, which is little more than a reprint of the other, except the article on upholstery, is edited by J. Stokes, and published by Dean and Muuday, Threadnecdle Street; however, I think the first will suit the purpose of " R. H. S." the best. B. T. TO CORRESPONDENTS. The Penny Postage. — Complaints having reached us of the difficulty of procuring our work in some of the country villages and other places, our correspondents are reminded, that the" Mechanic And Chemist" is forwarded through the post ttoo days prior to the date of publication, upon the following terms; vis.:— Subscription for one year' {inrludititj postage and supplementary Ao*., to be paid in advance) t Half-year A Crown Quarter of a year Haifa Crown. All letters to be prepaid, and addressed to D. A. DotWNBY, City Frets, 1, Long Lane, Aide'sgate Street. SI am man n, Hamburgh.—Has addressed to us a number of queries concerning certain reservoirs of water, but lias omitted to explain what and where they are. I. Q. G. in our next. F. M.—Once more, the oxide of iron is the substance used for razor-straps, and as it may be obtained of various qualities, more or less mordantt it is well adapted to various similar purposes. B. Tongue.—M. Daguerre announced some time ago, that he had succeeded in substituting silvered copper for the plated metal originally employed. We believe it is customary for country postoffices, to charge a halfpenny, for the conveyance of prepaid letters to isolated houses; but all such questions will probably be settled, when the measure is brought finally before the House at the end of the present sessions. Junius.—Proposition 1 is correct; tie 2nd. and 3rd. are untrue. I. C.—The various nostrums, which are sold to make hair grow on bald places, are of no utility whatever, except to tfte sellers. It is generally supposed that uneasiness of mind is unfavourable to the preservation of the hair. The fol. lowing recipe may be serviceable in some cases: take a pint of the finest old port, and, after dinner, slightly moisten the part therewith, and drink ttie remainder. If about fail/-a dost n patients afflicted with the same complaint, assemble together daily, and persevere in the proposed remedy, they will soon experience andproclaim its efficacy. W. TV receipt is incorrect, and might, if through any ot\ rsiyht it had gone to press, have caused a great deal of mischief. S. S.—If lie will enclose the model, and write upon it "confidential" it will only be examined by on* person. Eheathm.—Puge 258, for " p/,M wsAp'p''. London: Printed at " The Citt. Press," I, Long Lane, Aldersgate, by D. A. Doddkky (to whom all communications for the Editor must be addressed, postage paid); published every Saturday, Im Ci. Hlhgkr, Holywell Srreet, Strand; and may h«- h,irf M all HoofcaeMerr and Newtmen in Town and rmnitry, |