PURIFICATION OF GAS.-JOHNSON'S PATENT. The complete separation of the impurities of coal gas, by a process at once economical and efficient, has long been a problem occupying the attention of the chemists of this and other countries. The impurities which gas contains after the tar and ammoniacal liquors have been separated by condensation, are themselves gaseous, or highly volatile, and consist of hydrosulphuric acid, carbonic acid, hydrosulphate of ammonia, carbonate of ammonia, cyanide of ammonium, and some minute quantities of other substances. The offensive odour and deleterious properties of the first of these substances pointed out the necessity for its separation at an early period of gas manufacture; and this was first completely effected by Mr. Clegg, in the apparatus which he erected at Stonyhurst College, Lancashire, in 1807-1808. He used cream of lime for this purpose, kept in agitation by mechanical means. This process has the effect of separating all the other substances enumerated, except the ammonia, which is rendered caustic, and escapes in the form of gas, taking with it (unless excess of lime has been used) considerable quantities of sulphur, masked by the ammonia, so as to be insensible to ordinary tests. With sufficient lime, however, nothing can more effectually separate the acid impurities than a properly arranged series of Clegg's purifiers; but they have the disadvantage of requiring considerable pressure to work them, and the continued application of motive power. A great improvement was effected by the discovery of Philips, that hydrate of lime, merely moistened, would produce the same effect as the lime water, and, in consequence, the dry has almost entirely displaced the wet lime purifier. The dry purifier requires no motive power, and the product is less bulky and more easily disposed of; but its chief advantage is the greatly diminished pressure required for working it. When we consider the loss resulting to the gas companies from leakage, from the reduction of the illuminating power of the gas by decomposition, caused by contact with the heated iron of the retorts, and from the rapid destruction of the retorts by the deposit of carbon, the product of that decompo sition, the vast improvement effected by Philips' invention can be readily appreciated; for all these sources of loss diminish rapidly with the diminution of pres sure. The lime purifiers continued for many years to be the only mode of purification, until it was found that the ammoniacal gas was a serious impurity, not only by diluting the illuminating gas, but by its energetic action on the fittings, meters, and the metallic ornaments of the apartments where gas was used. Mr. Lowe, the well-known gas engineer, suggested, and we believe used, diluted acid for separating the ammonia from gas; and an apparatus for the use of acid for this purpose was made the subject of a patent, a few years ago, by Mr. Croll. This process is now in use at the Horseferry-road station. Solution of sulphate of iron was used at Glasgow, and at some places in England; and by the veteran chemist, D'Arcet, at the Hôpital of St. Louis, in Paris. A patent was taken out subsequently for the muriate of the same base, and for the salts of manganese. These had previously been patented, and used at two stations in France by Mallet, but the disadvantages of the process had prevented its adoption in the principal works of that country, notwithstanding the influence of the patentee through his collaborateurs, the chemical professors, and a favourable Report of the Academy of Sciences. These disadvantages are the same as those of the wet-lime purifiers, viz., the great pressure required for working the apparatus, and the use of motive power; so that, with the exception of the metropolitan stations, where, from the restrictive operation of acts of parliament, the gas companies cannot use a stream of water, they have scarcely been at all adopted. At the great majority of the country stations the washing process is alone employed, although not sufficient for perfect purification; and notwithstanding the loss of illuminating power by the partial solution of the heayy hydro-carburets. A series of experiments has just been completed at the Horseferry-road, the principal station of the Chartered Gas Company, to test the merits of an entirely novel plan of separating the ammoniacal compounds. They were instituted by the spirited and liberal directors of that. company, under the care of Mr. Upward, the deputy superintendent. They have been, we believe, perfectly successful; fully confirming the statements of the patentee, Mr. Johnson, who asserted that, by its adoption, the whole of the ammoniacal compounds could be separated-the gross pressure of the establishment reduced by 40 to 50 per cent.and a saving of 30 per cent. of lime effected, without the slightest deterioration to the quality of the gas. Mr. Johnson's plan is based upon the discovery that, under a certain state of circumstances, certain salts will act upon the ammoniacal compounds while in the dry or solid state as efficiently as the same salts act in solution. It therefore stands in the same relation to other plans, as the dry to the wet-lime purifier. The state of circumstances alluded to, Mr. J. found to be, the presence of a sufficient quantity of the water of crystallization to bring the reacting atoms into contact. In the experiments at the Horseferry-road, the sulphate of iron was used. Its action upon the ammoniacal compounds of the gas is to form sulphate of ammonia, by the acid uniting to the volatile alkali, while the sulphur and cyanogen compoumds are absorbed by their union with the oxide of iron. In the acid plan previously used at this station, the ammonia alone was absorbed, leaving the hydro-sulphuric and hydrocyanic acids to be taken out by the lime. It is thus seen that, while the saving in lime is effected, a valuable product is formed; to separate which from the waste lime, several patents have been granted. In the course of the progress of the experiments it was discovered that the results were less successful as regards economy, when the thickness of the layer of material was increased beyond a certain point, as the weight of the salt compressed the lower layer, which thereby became more difficult of complete saturation. To obviate this, Mr. Upward, the conductor of the experiments, suggested the introduction of some neutral and lighter substance, as sawdust, or breeze, to separate mechanically the particles of the purifying material. The result was highly satisfactory, and he has, by the ingenious suggestion, added much to the value of the already valuable invention, which appears to give a complete solution to the difficult problem of perfect purification. RAILWAY SIGNALS. Sir,-I beg to offer the following suggestions through the medium of your pages: Let there be at least one "guard carriage" to each train, furnished on the top with a strong air-reservoir and safetyvalve, capable of supporting a pressure of about 20 lbs. per inch. This vessel can be supplied by a small condensing syringe, worked by the wheel or axle of the carriage, with a disconnecting apparatus to be applied when it is "blowing off," to economize the "wear and tear of the working barrel. On the top may be fixed the usual railway whistle, to be used by the guard in such cases as the following: 1st. Should the engine-driver neglect to sound his signal at the proper place, he might be reminded of his duty by one blast of the air-whistle. 2nd. Two quick blasts-quicken your speed. 3rd. Two slower blasts-slacken your speed. 4th. Three blasts If at rest-go on. If in motion-stop. 5th. Four blasts-look out - there is danger. 6th. Four slower here, &c., &c. be very careful The air-whistle should be shriller in tone than that for steam; and the disconnecting apparatus might be self-acting, so as to keep the reservoir charged, and yet not wear the working parts more than necessary. A few years ago, I suggested to a gentleman connected with railways, that the motion of the train should cause a bell to be rung at the station when it was about a mile distant; and that gates connected with each other, on each side of the railway, might close at the same time, and so cut off the communication with the common road. I was told that the "Act" did not allow of anything self-acting-there must be an attendant for that purpose. Shortly afterwards, however, gates constructed as I had proposed were introduced, but whether from PESCHEL'S ELEMENTS OF PHYSICS. the hint I had thrown out or not, I cannot tell. The ringing of a bell, or some other means of calling attention to a coming train, seems especially necessary now that "express trains are in so much use; particularly at those stations where they do not "pull up.” Hoping the above remarks may prove useful, I remain, Sir, yours, &c., Stockton upon Tees, Oct. 6. 1846. " THE LORD JOHN RUSSELL AND SIR Mr. Editor,-In the number of your Magazine, published on the 19th of last month, a correspondent who signs himself "Screw," gives account of the performance under steam of the schooner, Lord John Russell; and having been witness to the performance at the measured knot of her sister, the Sir Robert Peel, on the 30th ult., you will perhaps with a feeling of fair play towards both these candidates for public patronage, permit me also in your pages to state the particulars. Both schooners are of precisely the same dimensions, tonnage, and power, -are driven by engines of the same construction, and have screws in all particulars alike; and as both were built by the same talented constructor from the same design, the speed of the one may, under similar circumstances, be taken as that of the other. The difference shown between the two in this respect is due, as I conceive, to the more definite means so creditably taken in the case of the Sir Robert Peel, to ascertain the speed with precision. 355 time, have its effect in removing the many misapprehensions and groundless suspicions which the public mind has been induced to entertain with regard to the simple agent which produced it. Much honour is due to the company to whom these vessels belong, for the boldness and decision they have shown in adopting the use of the scrw, at a time when its opponents had so largely succeeded in bringing it into general disesteem. The merited success which they now meet with, while it must be gratifying both as a test of their soundness of judgment, and as affording encouragement to a principle which offers such large public advantages, will, no doubt, urge them still further to prosecute the course on which they have so well entered. And under the prudent, yet liberal management which at present directs their affairs, we may hope to see in the proprietors of these screw schooners the germ of a great and opulent company. The above trial of the Sir Robert .Peel was made with the vessel light; the screw being by measurement 20 ins., of its 92 ins. of diameter, out of water; and though the increased immersion of the ship when loaded forbids us to expect so high a result, yet in such case a much improved effort from the screw must certainly be looked for, not only from the increased surface with which it will then act, but because working thus much out of water, the solidity of the fluid is much impaired by the amount of air taken round by the instrument. This may account for the higher amount of slip exhibited on this trial as compared with that of the Lord John Russell, at the above speed; for with 43 revolutions of the engines, it amounts to 16.6. I remain, Sir, yours, &c., R. N. PESCHEL'S ELEMENTS OF PHYSICS.- Six runs were made over the measured knot in Long Reach, during the last quarter of the ebb and the first quarter of the flood tide, from 2 h. 30 m. to 4 h. 15 m. of the above day, and a very fair amount of minor obstructions and hindrances were met with; yet an accurate register of the time occupied by each run, carefully compared and reduced, gives a mean speed of 8.128 nautical, or 9-365 statute miles per hour: a speed which, ascertained by equal severity of tests as the effect of 40 horses power engines on a vessel of 320 tons, stands, I believe, unrivalled; and the publication of the fact must, in good gravings and Wood-cuts. Longman and Co. In noticing the first volume of this English translation of Peschel, (Mech. Mag., Vol. XLII., p. 329,) we questioned whether the time of the translator might not have been * Elements of Physics. By C. F. Peschel, Principal of the Royal Military College at Dresden. Translated from the German, with Notes, by E. West. Vols. II. and III. Foolscap 8vo, with En better occupied in compiling a text book from the many excellent works on natural philosophy in our own language; but bestowed our humble meed of praise, at the same time, on the production of the German professor, as being equal to any with which we were acquainted, whether home or foreign, for extent and variety of information, methodical arrangement, and apposite illustration. Some "drawbacks and imperfections" of a technical character we pointed out to the notice of the translator, and are glad to perceive that one of the most material has been rectified in the second and third volumes. We objected to "the confusion of English and foreign weights and measures that prevailed, to the great annoyance and inconvenience of the English reader." The translator has now judiciously reduced "all the measures to English standards." But though the path of the English reader has been so far well cleared, and the book now presents, as far as literals are concerned, quite a home-like appearance, we must own that, in perusing the additional volumes before us, we have been a great deal more forcibly struck with its foreign source than we were before. Formerly it was a mere affair of signs and symbols that every now and then annoyed us-the use of foreign for British weights and measures,—a stumbling block which, with the ready help of a Kelly or Waterton, could be got over in an instant; but now there is a more serious fault interwoven with the very substance of the workthe putting of foreign names and things where English ought to be, or the omission of English names where every English reader would expect to see them recorded with the highest distinction. Professor Peschel is either but imperfectly acquainted with the scientific literature of England, or very much blinded to its merits by continental and local prejudices; for certain it is, he has not, by any means, done it adequate justice. What could the translator (an Englishman) expect would be said in England of an account of the steam-engine in which no mention is made of either Worcester or Savery? Incredible, yet true! Following in the wake of Arago, Peschel regards Papin as being next to Hero, the person to whom the modern world is most indebted for the priceless and incalculable benefits of steam power, and passes over in utter silence men who contributed ten times more than either to its triumphant success. WATT, to be sure, has due honour paid to him, (HIS was a name which could not be so readily passed over,) but it is at the expense of all the many Englishmen who have followed in Watt's steps, and contributed so largely to carry out and improve his views and plans. In like manner, Peschel repeats the old story of "electrotyping having been simultaneously invented by Jacobi, of Petersburgh, and Spencer, of Liverpool," when it has been placed beyond all doubt, and is admit. ted by all impartial writers, (see Ure's Dictionary, Shaw's Manual, &c., &c.,) that the art was disclosed to the world before either, by the journeyman printer, Jordan, and (without boasting be it added) in the pages of the Mechanics' Magazine. We might multiply instances to the same purpose; but these may suffice to indicate the sort of foreign feeling that pervades the work in things essential, and which must qualify, to a considerable extent, its fitness as a test book for British youth. 66 Having referred to the section of the work which treats of the steam-engine, we must not leave it to be supposed that it is in respect of the history of the invention alone that it is faulty; for it has other blemishes, not a few. We are told ex. gr. (vol. II.,p. 236) that the principle on which it (the steamengine) acts, is this "-that "steam, at a temperature exceeding that of boiling water, possesses so great an expansive force, as to propel bodies against which it is directed,such bodies returning to their original position, when the steam, by being condensed, is deprived of this force;" whereas it is afterwards (p. 238) shown, as is the fact, that in an engine acting in the way stated, "only half the motion of the piston is used as an effective force, namely, its descent," (in consequence of the vacuum produced under it by the condensation of the steam); that is to say, that steam, to the expansive force of PESCHEL'S ELEMENTS OF PHYSICS. which Peschel ascribes everything, acts only when it is "deprived of this force"!* Again, speaking (p. 249) of the advantages of expansively-worked steam-engines, Peschel says: "These advantages are counterbalanced by this drawback, that they are suited only to produce rectilinear motion, as the stroke of the piston is not performed uniformly; and if their movements are to be equalized by means of a fly-wheel, it must necessarily be of such extreme dimensions as would render its use objectionable. Besides, these engines are more complicated in their construction, and consequently more expensive in the first instance, as well as more likely to get out of repair and difficult to mend. For these reasons they have been less generally adopted, and in England have almost entirely ceased to be used." What will our friend Mr. Hoseason say to this, who has been promoted in the naval service of his country, expressly for his successful endeavours to introduce the working of steam expansively into our man-of-war steamers? What the lords of the Admiralty, who have so laudably seconded his exertions by ordering every steamer under their controul to be provided with expansive gear? Peschel, living in the heart of Germany, may never have seen the sea, and may therefore be excused for some ignorance of what is going on upon it; but it is not so excusable in one of our own islanders, even though residing in the Boeotian region of Amersham, to assist in giving currency to such erroneous statements. We pass on to the more agreeable task of presenting our readers with some specimens of the better portions of the work,-those where the information afforded is at once correct and full, and brought down to the latest period: Expansion of Alcohol. The expansion of alcohol has also been investigated repeatedly, because of the use made of this fluid as a thermoscopic substance, for observations at a low temperature. Munke's experiments are the most recent, and the results he has obtained the most satisfactory. He makes the expansion of pure alcohol (0.806 at 32° Fah.) to be for every degree of heat above this temperature, the volume at 32° being reckoned 1; the expansion, however, at 43° is rather more than 357 tracts with the like regularity for every degree below the freezing point of water. Munke has further established the fact, that pure alcohol has the greater expansive and contractile force, and that this force diminishes in proportion to the quantity of water combined with it. He states the volume of pure alcohol at 122° Fah. to be = 1.056071; that of water at the same tem. perature to be=1.011570; and that of alcohol, not perfectly pure (i. e., whose specific gravity, at 54° Fah. is 0·808) to be 1.054394. Freezing Mixtures. = The application of heat from an external source is not always indispensable in order to liquefy solid bodies; many substances undergo this change of form if they merely come in contact with a liquid. Nearly all the crystallized salts possess this property. If a mixture be made of one of these salts with snow, or with some other liquid, the action of these substances on each other will convert one or both of them into liquids, one portion of their free caloric being consumed in effecting this change, as also part of the heat of the vessels containing the mixture. The consequence is, that the greater the degree of cold generated during the liquefaction of the mixture, the more quickly will the liquefaction be brought about, and the greater will be the quantity of heat taken up and rendered latent by the substances composing these freezing mixtures. The following experiments will serve to illustrate the principles laid down above: First Expt. Place a thermometer in a glass, into which throw some finely-pulverized sulphate of soda (Glauber's Salts) and some chlorate of aminonia, or nitrate of potassa (saltpetre)-Shake the mixture and the thermometer will indicate a considerable depression of the temperature. Second Expt. One part nitrate of ammonia and 1 of water at 50° mixed together produce a cold of 5°; or 5 parts of chlorate of ammonia, and 5 of nitrate of potassa, with 10 parts of water at 50°, lower the the temperature to 10°. These salts may be crystallized again, and they will be equally available for use. Third Expt. Five parts of hydrochloric acid poured on 8 parts of freshly-pulverized sulphate of soda reduced the temperature from 50° to 1o. Water placed in thin glass vessels or tubes, and set in the mixtures named in this and the preceding experiment, is very quickly converted into ice. "Fourth Expt. One part of snow or |