the Athenæum, Manchester, that there is no locality in the world where a knowledge of chemical laws is of so much practical advantage as in this metropolis of manufactures, it was with peculiar pleasure that I lately heard of a most interesting application of those laws, in a newly-discovered process for burning smoke and economising fuel, by Mr., by which those two important objects are effected in a manner far superior to any thing of the kind that has ever been seen before. The particular chemical laws thus applied are those relating to the diffusion of the gases, and other elastic fluids, said to be first discovered by Dr. Priestley; but the scientific principles of which were certainly first described by Dr. Dalton, in a paper read before the Manchester Philosophical Society, in October, 1801, and applied by him to the explanation of various atmospherical phenomena, as is well known.

The principal feature of Mr. - -'s plan is the compelling all or most of the carburetted hydrogen, and other combustible gases, which escape inflammation when first generated from the coal in the furnace, to return again and pass through the fire, where they are converted into flame under the boiler. The process is effected by placing a small rotary fan in connexion with that part of the flue through which the smoke is passing off to the chimney after having left the boiler. The fan being made to revolve with considerable velocity, exhausts the smoke from the smoke flue, and at the same time propels it through an additional return flue, leading to an enclosed ash-pit, from whence it is, by the joint exhausting and propelling action of the fan, forced to pass through the firegrate, where complete combustion is effected, and the products of this second combustion again pass under and around the boiler as before, and hence up the chimney, in the state of vapour, carbonic acid gas, and azote, which are perfectly colourless.

Your chemical readers will be well aware, that this circulatory process, which, for want of a better word, I call double combustion of the fuel, cannot go on without a continual supply of atmospheric air, which is admitted at a small aperture, situated between the fan and the place where the latter communicates with the smoke flue; and it is in the particular adaptation of this aperture, with respect to the fan and to the chimney, that the peculiar philosophical principle which distinguishes this invention is brought to bear with so much effect.

It was demonstrated by Dr. Dalton, in the paper before referred to, that different gases act as vacua to each other, and it has recently been shown by Professor Graham, that the different gases have a tendency to

diffuse into each other, with different degrees of rapidity, which bears a certain relation to their specific gravity; and hence it shows that, by availing ourselves of this tendency in mixed gases, a sort of mechanical separation of the various gases may be effected. Now, it is precisely this separation that is effected by Mr. -'s process-the unconsumed carburetted hydrogen and carbonic oxide are returned under the fire-grate, mixed with a fresh supply of oxygen; while, at the same time, nearly the whole of the nitrogen and heavy carbonic acid gases are allowed to pass off below to the chimney.

From a consideration of this theory, the intelligent manufacturer will perceive, that, so far as the saving of fuel is derived from the burning of the smoke, it is not to effect that purpose alone that the fan or blower is required-the latter has a much more important duty to perform than that of merely forcing the smoke through the fire, and that is, the blowing of the fire. The effect of a strong draught generally, is not to burn the smoke, as usually understood, but rather to create such an intense heat, that a more complete combustion of the fuel is effected, and consequently, less smoke is produced.

Leaving out of view, for the present, the much disputed point, as to the absolute amount of heat that it is possible to save by burning the whole of the smoke in any case (although all who have seen Mr. -'s plan in operation, at his own works, are compelled to acknowledge that it entirely burns the whole of the smoke-therefore, that saving is effected, whatever its amount may be); it may be stated, that, in regard to returning the smoke to the ash-pit, which is an essential point in this invention, it does not appear that the use of the fan is absolutely indispensable; for the lighter gases, which constitute the most valuable part of the smoke, have, according to Dr. Dalton's law of the "Diffusion of the Gases," before referred to, a natural tendency to leave the carbonic acid gas, and rush into the ash-pit as they would into a vacuum, at the same time, from their buoyancy, carrying the light carbonaceous matters with them; but the atmospheric air, which is essential to supply oxygen to support the combustion of those gases, not having this diffusive tendency to so great an extent, requires to be supplied by artificial means, and for that purpose a very small fan is necessary. The whole of the atmospheric air that is found sufficient to supply oxygen to the furnace of a twenty-seven-horse boiler is admitted to the fan through an aperture of only 10 inches by 4, and certainly does not require more than a quarter of a horse power to propel it; because the smoke which passes through at

the same time, both from its levity and the natural tendency to diffusion, before mentioned, rather assists the propulsion of the fan than otherwise. By running the fan at a sufficient speed, of course any degree of draught can be obtained that may be thought necessary, even to the burning of the anthracite, or stone coal, or any common coal of inferior quality. Indeed, a considerable speed is necessary; for, as Sir Humphrey Davy long ago proved (and on which the theory of his safety lamp is founded), carburetted hydrogen, or coal gas, is the least combustible of any of the inflammable gases, and consequently requires a strong draught to effect its complete combustion. And here the question will no doubt occur to many, as to what is the probable effect of this necessarily strong draught, or "blast," against the boiler bottom. This is an important question; but it fortunately admits of being easily answered by any chemist who duly considers the rationale of the process, as given above; and the practical man, who will take nothing but facts, will find, upon inspection, that boilers have been working some months on this plan, with fires constantly under them, apparently capable of giving a welding heat to iron, without sustaining the slightest injury; and, moreover, he will find, that the underside of the gratebars, upon which such intense combustion has been going on, is never found at a higher temperature than the metal of the burner of an ordinary gas light.

The ash-pit is, under the circumstances, converted into a reservoir of mixed combustible gases and common air, in such proportions as enables the latter to yield sufficient oxygen for combining with the former in the production of carbonic acid and water (or rather steam), and thereby eliciting the maximum quantity of heat from a given quantity of fuel, besides sufficient oxygen to support the less perfect combustion of the raw coal, when first thrown upon the firegrate. Moreover, as these changes and combinations are being continually and rapidly effected, and although they are only the well known phenomena of ordinary combustion, yet they are, in this case, carried on in an atmosphere (so to speak) surcharged with a certain proportion of nitrogen and steam, which, being neither supporters nor combustibles, but being propelled by the fan in uniform mixture with other elastic fluids that possess these properties in an eminent degree, there can be no doubt that, in this process of double combustion, those two incombustible substances effect the very important purpose of diluting and modifying the oxidating property of the blast." In fact, the peculiar mixture of elastic fluids

39

thus effected, produces what is, I believe, called by some mineralogists, and others conversant with the use of the blowpipe, the "deoxidising flame." At any rate, a similar result is characteristic of Mr. -'s pro

cess,

in contradistinction to the ordinary process of blowing the fire with crude air; which latter method, whenever resorted to, has always effected the rapid oxidation of the grate bars, and the destruction of the furnace; but, according to Mr.'s plan, where the smoke itself is actually made in part the medium for blowing the fire, the draught produces a totally different result; while the manner of effecting it is as complete and simple as it is unique in its application to steam-engine furnaces.

In conclusion, allow me to observe, that, in the above attempt to illustrate what I conceive to be one of the greatest discoveries of modern times, I am in no way anxious to be considered as giving the only true explanation of the scientific principles concerned in it. My intention has been principally to state facts for the consideration of the more experienced chemist; and if, in attempting to theorise upon the subject, I have overstepped the legitimate province of the practical experimentalist, I have to plead, that I have been urgently requested so to do by several manufacturers, whose interests are likely to be most extensively affected by the invention; and I may add, that the subject is not the less interesting, as it affects the health and comfort of the population of all large towns.

R. ARMSTRONG, C. E., Manchester.

MR. MALLET'S PROCESSES FOR THE PROTECTION OF IRON FROM OXIDATION AND CORROSION, AND FOR THE PREVENTION OF THE FOULING OF

SHIPS.

The discovery of an effectual means of protecting iron, copper, and other metallic surfaces, from the injurious effects of exposure to atmospheric and aqueous influences, had long been an object of earnest, but nearly unavailing pursuit, as well among men of science as among mere practicians, when the successful application of iron to the building of ships gave suddenly a new impetus and great increase of importance to the inquiry. Sir Humphrey Davy had found. out how to save copper sheathing from corrosion, by means of zinc protectors; but subsequent experience showed that, in proportion as the copper was thus

electro-chemically preserved, it was rendered more liable to be fouled by the adhesion of animal and vegetable substances an evil scarcely inferior in magnitude to that of the destruction of the copper itself; and farther than the point so reached by Davy, science had not advanced, when the first iron ship was launched into the deep. Much was at one time said of certain patented processes of zincing, by which it was alleged iron could be so thoroughly coated, as not to leave a speck unexposed for air or water to act upon-and much was hoped from them; but one after another they all proved decided failures. In the best zinced sheets of iron produced by these processes, there were always found a number of spots which had been left bare, by the collection of rust on which, the protective power of the zinc, in respect to the remainder of the iron, was almost entirely neutralized. Of "anticorrosive" and "anti-barnacle" paints and varnishes there had been also an abundance, both before and since the days of Davy, but not one which could be said to have survived the test of practice, or which was not, more or less, of an empirical character.

So matters stood-that is to say, about the time of iron first coming into extensive use for the construction of ships -when the British Association were induced to take up the question, as one of the most practically important of the day, and to devote a portion of their funds to the institution of a series of experiments in relation to it, under the direction of Mr. Robert Mallet, of Dublin, a gentleman eminently fitted, by practical habits and experience, as well as by scientific knowledge, to do justice to the task intrusted to him. The details and results of these experiments are related in two reports made by Mr. Mallet to the Association, and published in their Transactions and though they go little farther than to show the defects of existing processes, (that of zincing more particularly,) they must be allowed to have accomplished a most valuable service, in having cleared the subject from the vast mass of false science and erroneous practice by which it had become encumbered.

Mr. Mallet, following out the course of investigation thus auspiciously commenced, has since happily mastered all the difficulties of the case, and devised a

series of remedial processes with so much of science, and therefore of sufficient reason in them, as to leave no doubt on our minds of their perfect efficiency. To indicate briefly Mr. Mallet's discoveries, they may be said to consist, first, in a method of zincing iron so perfectly, that not a spot of the iron is, or can be, left unprotected; second, in a method of protecting iron and other metals by means of palladium, (at a moderate cost.) which renders them as incorrodible by air and moisture as palladium itself (palladiumizing, it may be called, with as much propriety as we say, zincing, or gilding, or soldering;) and, third, in a new paint, to which, from its life-destroying properties, Mr. Mallet has given the name of zoofagous paint, by the application of which to vessels, whether of wood or iron, or with whatever material they may be sheathed, fouling is rendered impossible. The following details of these processes, which we have great pleasure in being the first to lay before the public, we extract from Mr. Mallet's specification, which has been just enrolled.*

1. The Zincing Process.

Supposing the articles about to be zinced, are plates and ribs of iron, intended to be employed in the construction of an iron vessel, they are first carefully cleaned from all adhering oxide. With this view they are immersed edgewise in a suitable vessel of wood, pottery, stone or lead, containing dilute sulphuric acid of the specific gravity of about 1.300 at 60° of temperature, or dilute hydrochloric acid of the specific gravity of about 1060 at 60° of temperature, formed by diluting these acids respectively as they are usually found in commerce with rather more than an equal bulk of water. As it is of importance that the scales of oxide should be detached as rapidly as possible, the diluted acid should be warmed; and this may be conveniently effected by means of a steam jacket round the vessel, or by blowing steam into the acid; the acid vessel, or 64 Cleansing Bath," as it may be termed, should be so constructed for operations on a great scale, that the lower portion of the acid, and the scales which are precipitated, can be occasionally withdrawn to prevent waste of acid, or the cleansing process from being inconveniently protracted. The iron must be wholly, not partially immersed, and the bubbles of gas formed on its surface must be free to ascend in the fluid and escape. As

• Enrolment Office, January 7, 1842,

soon as the scales of oxide have become detached or loosened, the articles are to be removed from the "cleansing bath," thrown into or washed with cold water, and struck or hammered to shake off and detach the scales. In the case of flat boiler plates they may be advantageously passed backwards and forwards, through the machine known to boiler makers as "a mangle." The surfaces of the iron are then to be thoroughly scoured, by hand or by any suitable machinery, with sand or emery, or with pieces of grit stone, while exposed to a small running stream of water, until they appear quite clean and of a bright metallic lustre. The articles are now, before being allowed to dry, to be plunged into a "preparing bath," consisting of the following mixture: A saturated cold solution of chloride of zinc is made by dissolving zinc or its oxide in hydrochloric acid; to this is added an equal bulk of a saturated cold solution of sal ammoniac; and to the mixed solutions as much more sal ammoniac in the solid state is added, as they will dissolve. Or, these solutions may be made and mixed hot, and the solid sal ammoniac then added, but the addition of some cold water will then be requisite to dissolve the whole of the salts so formed. The bath may also be formed of sulphate of zinc and sulphate of ammonia, or acetate of zinc and acetate of ammonia, or of any other soluble salt of zinc and ammonia or salt of manganese and ammonia. The nitrates of zinc and ammonia are the least advantageous, and it is stated that none answer the purpose so well as the chloride of zinc and sal ammoniac first before directed to be used. No free acid should be present in these solutions. As soon as the

surfaces of the immersed articles appear covered all over with minute bubbles of gas they are then in a fit state for combining with the metallic alloy with which they are next directed to be coated; but they may be allowed to remain in the preparing bath for any convenient length of time without injury or prejudice to the subsequent processes. The metallic alloy last referred to is prepared in the following manner: A quantity of zinc is melted in a suitable vessel (one formed of pottery or stone is found to answer best), and when it is in a state of fusion, mercury or quicksilver is added, in the proportion of 202 parts of mercury to 1292 parts of zinc (both by weight) being in the proportion of one atom of mercury to forty atoms of zinc, both upon the hydrogen scale. The two metals are well stirred or mixed together with a rod of dry wood or of iron coated with clay; and when this has been done there is added one or the other of the metals known to che

mists and others as potassium and sodium (the metallic bases, of which the well known alkalies potash and soda, are oxides) in the

41

proportion of a pound or thereabouts of potassium or sodium to every ton weight of the alloy of zinc and mercury, or in some cases less will suffice; either potassium or sodium will answer the purpose, but Mr. Mallet prefers the latter, as more easily obtained and more manageable. Whether it is potassium or sodium which is used, it is removed from the naphtha, or other fluid in which it is customary to keep these metals, in order to preserve them from oxidation, in small portions of not more than half an ounce at a time, and by means of a small inverted cup of wood, formed on the end of a stick, thrust rapidly below the surface of the alloy of zinc and mercury, so as to avoid any waste or combustion of the alkaline metal. A triple alloy is thus formed of zinc, mercury, and sodium or potassium, which, having been again stirred and mixed with the rod of dry wood, or of iron coated with clay, is now ready for covering or coating the prepared iron. The combination of these metals is facilitated, and their oxidation on the surface retarded, by pouring upon their fluid surface some of the liquor of the preparing bath, or strewing upon it some of the salts dissolved in that liquor in a dry state.

The plates or ribs of iron are now to be taken up out of the preparing bath, permitted to drain for a few seconds, and while still wet with the liquor of the preparing bath, immersed in the triple alloy in a state of fusion. As soon as they have acquired the temperature of the bath of alloy, they are to be withdrawn from the metallic bath edgewise or endwise, when they will be found covered with a perfectly uniform and coherent coat or surface of the alloy. The affinity of this alloy for iron is, however, so intense, and the peculiar circumstances of surface as induced upon the iron presented to it by the preparing bath are such, that care is requisite lest by too long an immersion the plates are not partially or wholly dissolved. Indeed where the articles to be covered are small, or their parts minute, such as wire or nails or small chain, it is necessary before immersing them to permit the triple alloy to dissolve or combine with some wrought iron, in order that its affinity for iron may be partially satisfied and thus diminished. At the proper fusing temperature of this alloy, which is about 680° Fahr. it will dissolve a plate of wrought iron of an eighth of an inch thick in a few seconds. No sputtering is produced by the immersion of the iron wet from the preparing bath into the alloy; but care is to be taken that there are no hollow places or cavities in the articles immersed which the alloy cannot wholly fill; lest in such case steam may be generated below the surface of the metal, and a dangerous explosion be thereby occasioned. It is

stated to be desirable that the melting vessels should be as deep and expose as small a surface as the nature of the articles to be immersed will allow. At the moment of immersion of the articles, the surface of the alloy is to be cleansed of all dross or oxide by a wooden skimmer. As soon as the iron plates or ribs are withdrawn from the alloy or "Metallic Bath," they are to be plunged into cold water and well washed therein. The surface of the iron is now in a condition permanently to resist corrosion and oxidation in air, or in salt or fresh water.

All the foregoing operations are best performed upon the plates or ribs after they have been bent and fitted to their places, and the plates have been riveted together into large pieces of eight to ten feet square or more. When again put "into frame," or placed in their respective positions in the ship's hull, they are directed to be united by rivets countersunk from the outside, and consequently headed inside the vessel. The countersunk heads of these rivets are to be also coated with the triple alloy in the manner before described, and tongs of iron are to be provided, having a very large mass of metal in their jaws, between which a hollow seat, of the shape and size of the countersunk rivet head, is to be formed to receive it. An alloyed rivet being seized by a pair of such tongs may have its point heated to a riveting or welding heat without injuring the coat of alloy upon its countersunk head; for the heat is carried off from the latter so fast by the contact of the large mass of iron in the jaws of the tongs, which are to be cooled occasionally, as to prevent the head of the rivet becoming hot during the heating of the point in a common smith's fire.

The hull of the iron vessel, being thus completed, and wholly covered with the alloy, is then to receive a coat of varnish all over, of either of the compositions about to be described. If possible, this varnish should be laid on with a spatula or thin flexible blade of horn, or some such material, as a brush produces minute air bubbles, which leaves spaces uncovered on the drying of the varnish. The varnish will dry, or get hard and coherent, at ordinary temperatures; but when practicable, it is desirable to expose it for some hours to a temperature of about 300 Fahrenheit, which gives it greater adhesion and durability. The iron surfaces may be warmed in successive portions by heat radiated from "chauffers" or open fires of coke, or by any other convenient means. The varnish may be either of a composition, which Mr. Mallet terms No. 1, or of another, which he terms No. 2. The composition, No. 1, is formed as follows:Take 50 lbs. of foreign asphaltum, melt and

:

boil it in an iron vessel, for three or four hours; add gradually 16 lbs. of red lead and litharge ground together to a fine powder in equal proportions, with 10 imperial gallons of drying linseed oil, and bring all nearly to a boiling temperature. Melt in a separate vessel 8 lbs. of gum anime (which need not be of the clearest or best quality); add to it two imperial gallons of drying linseed oil, boiling, and 12 lbs. of caoutchouc softened, or partially dissolved by coal tar naphtha (as practised by the makers of water-proof cloths). Mix the whole together in the former vessel, and boil gently until, on taking some of the varnish between two spatulas, it is found tough and ropy. When this "body" is quite cold it may be thinned down, with from 30 to 35 gallons imperial of turpentine, or of coal naphtha, which will make it ready for use. Mr. Mallet states this to be the best varnish he is acquainted with for this purpose. It is not acted on when dry and hard, by any moderately diluted acid or caustic alkali; it does not by long immersion combine with water, and so form a white, and partially soluble hydrate, as all merely resinous varnishes and all oil paints do; it is, moreover, so elastic, that a plate covered with it may be bent for several times without its peeling off. And, lastly, it adheres so fast, that nothing but a sharp edged instrument will scratch it off the surface of iron. The composition No. 2 is of a cheaper sort, but not quite so good. Common coal or gas tar is to be boiled in an iron cauldron, at so high a temperature, that the smoke from it is of a yellow dun colour; or the tar is to be caused to flow through red-hot iron tubes. The boiling passage through the tubes is to be continued until the residue is a solid asphaltum, breaking with a pitchy fracture. It is essential that the boiling should be carried on at this high temperature, as the permanence of the varnish in water depends upon the tar having been submitted to the temperature at which naphthaline is formed, by the decomposition or breaking up of the original constitution of the tar. Take 56 lbs. of this coal tar asphaltum; melt it in an iron vessel: add 10 imperial gallons of drying linseed oil, ground with 25 lbs. of red lead and litharge, in equal proportions; add to the whole, when well mixed, and after boiling together for two or three hours, 15 lbs. of caoutchouc, softened or partially dissolved by coal naphtha, as before described; and when cold, mix with 20 to 30 gallons of turpentine, or coal naphtha, which will make the varnish ready for use.

2. The Palladiumizing Process. The articles to be protected are to be first cleansed in the same way as in the case of zincing, namely, by means of the double