red-hot, and which have been of course originally calculated to a factor of safety of six.

A most important question is the effect of temperatures, whether high or low, on the elasticity of the materialwhether iron will take a permanent set with greater facility at a high temperature? These data are really more important than those on the ultimate strength, as they would show the influence of temperature on the elastic limit. Here again is a vacancy in existing knowledge, which can scarcely be said to be filled up by the few experiments of the late M. Wertheim on very small wires. He found, however, that the elasticity of small steel and iron wire "increases from 15° C to 100°, but at 200° it is not merely less than at 100°, but sometimes even less than at the ordinary temperature."

ment for registering higher temperatures were in existence, that the powers of conduction are still less. Some of Mr. Peclet's experiments also seem to be vitiated by his disregard of Dr. Joule's discovery that water is heated by being mechanically stirred up. It is, however, certain that water can only moisten a metallic plate when at a lower temperature than 171° C. As soon as the water gets thus repelled, the heat radiated by the metal is reflected back from the surface of the liquid; the metal gets hotter and hotter, with a corresponding diminution of its conducting powers; its outside, exposed to the fire, would more or less oxidise, and with a similar result; and a like effect is produced on the inside-on the roughened surface of which incrustation would rapidly adhere, forming a calcareous coating, conducting with about sixteen times less power than iron.* All these tendencies are of a progressive character, leading to very high temperatures in the plate, even to a red-respect to wrought iron, which has scarcely received the heat. This tends to explain how rivet-heads close to the fire are soon burnt away by the friction of the current of heated gases on the red-hot metal; how thick fire-boxes are sooner burnt out than lighter ones, the process being often arrested at a certain thickness; how internal flues of thick plates so often give trouble; how externally fired boilers are most deteriorated at the corners from the junction of the three plates; and similar results well known to practical men. Another proof that thin plates conduct more heat than thick plates is afforded by some experiments lately made in Prussia, with two egg-end boilers, exactly similar in every respect, except that one was constructed of steel plate inch, while the other was of wrought-iron about inch thick. The steam generating power of the steel boiler was to that of iron as 127.49 to 100f-a result which can only be accounted for by the relative thickness of the plates. Thick plates are also more liable to blisters, one of which would considerably diminish the conducting power of the spot where it happened to form.

There is, however, another very important point with attention it deserves. As would appear from a number of phenomena, there seems to be a sort of thermal elastic limit with iron. When heated, and when its consequent dilatation of volume does not exceed that which corresponds to (perhaps) boiling point, it returns to its original dimensions. Beyond a certain temperature it does not contract again to its pristine volume, but takes a permanent dilatation in consequence, apparently, of its elastic limits having been exceeded. A number of observerst have determined the fact with cast iron, and though wrought iron has not been expressly investigated in this direction, there is no doubt that it exhibits a similar behaviour. Thus, a number of years ago, an Austrian engineer, named C. Kohn, remarked that a boiler about 12 metres long and 1.57 in diameter, with a thickness of plate of 0.011, permanently expanded, at a temperature corresponding to a steam pressure of 5 atmospheres, (153° C) by 0.07193, and did not, when cold, return to its original dimensions. The same thing has been noticed, by means of very accurate measurements, with other boilers. A number of experiments by Lt.-Col. H. Clerk, of Woolwich, on wrought iron cylinders and plates,§ bear distinct evidence to a dilatation of volume in wrought iron, when repeatedly heated and suddenly cooled. In experiment 7, for instance, "two flat pieces of wrought iron, each 12 inches long, 6 inches deep, and inch thick, were heated and cooled twenty times, one being immersed to half, and the other to two-thirds, its depth in water. That imends fully 3 inch; the other had similar indentations, but only to one-half the amount. They both turned up Mr. Fairbairng observed no effect on the strength of into the form of an arc," the convex side of which plate iron up to almost 400° F. At a "scarcely red" heat appeared in the portion heated and cooled. Unfortunately, the breaking weight of plates was reduced to 16-978 tons the specific gravities of the different portions were not from 21 tons at 60° F.; while at a "dull red "it was only tried by Colonel Clerk. A succession of trials of 13.621 tons. MM. Tréméry and P. Saint Brice,|| aided by the kind produced cracks in the metal, thus explainthe celebrated Cagniard Latour, found that at nominally the ing how boiler plates are cracked by imperfect circulation same temperature (rouge sombre), a bar of iron was reduced and by cold feed-water let in near the fire; and, in strength to one-sixth of its strength when cold. This the thicker the plate, the more permanent dilatation of is much greater diminution of strength than that found volume and consequent danger. Mr. Kirkaldy found by Mr. Fairbairn. Apart from other causes, this might that iron highly heated and suddenly cooled in water, easily be due to the fact that incandescent iron affords a is hardened," being injured, in fact, if not afterwards different tinge during a dull day to what it does in a clear hammered or rolled. This permanent dilatation of volume light. In fact, the great impediment to all these in- must be necessarily accompanied with a diminution of vestigations is the want of a thermometer for high specific gravity, thus affording another close analogy temperatures; but M. Tréméry's result is perhaps more between straining iron by loads in excess of the me conformable with daily experience. Mr. Fairbairn's data chanical elastic limits, and straining by heat. Lajerhelm would show that the ultimate strength of wrought iron is found long ago that the specific gravity of iron is direduced to about one-half; but M. Tréméry's result ex-minished by strains in excess of the limit of elasticity, plains the generally instantaneous collapse of flues when

While it is certain that boiler plates can assume very high temperatures, even up to red-heat, authorities differ as to the diminution of ultimate strength which is caused by heat, while its effect on the elasticity of the plate has been scarcely attended to. The experiments on the ultimate tenacity of iron at high temperatures, conducted by Baudrimont,‡ Seguin, and the Franklin Institute, can scarcely be looked upon as of much value, for they were made on a very small scale, and with no re-mersed one half contracted or became indented on the gard to the temporary and permanent elongations-or to the effect of heat on the elasticity and ductility.

and this result has been completely confirmed by Mr. Kirkaldy's numerous experiments. The smith calls iron "burnt" which has been rendered brittle in working through the often repeated applications of heat, or through

* Comptes Rendus, xix., 231.

† Percy's Metallurgy, vol. ii., p. 872.
Technologiste. 1850-51, p. 102.

? Proceedings of the Royal Society, March 5, 1863.
Poggendorf's Annalen, 2 s., vol. ii., p. 488.

too high a temperature. Iron rendered brittle by strains in excess of the limit of elasticity has been long popularly termed "crystallized." Both these states are accompanied with a dilatation of volume and attendant hardness and brittleness, and both seem to be referable to very similar causes. In fact, a very general belief exists that very ductile good iron, used in the form of a steam boiler, soon gets brittle. There are some applications of metal to a steam boiler peculiarly liable to be strained beyond the limits of elasticity; by mechanical force, by the mechanical force of expansion and contraction, and by dilatation of volume through heat all three acting simultaneously. Such is the case with fire-box stay-bolts. Accordingly, they are found to get very brittle when of wrought iron-which is a much less ductile metal than copper. Mr. Z. Colburn states that he has "frequently found these stays (where made of wrought iron) to be as brittle, after a few years' use, as coarse cast iron." He has broken them off from the sides of old fire-boxes, sometimes with a blow no harder than would be required to break a peach stone."*

3-THE CHEMICAL EFFECTS OF THE INCANDESCENT FUEL.

The remarkable diminution of elasticity and of tenacity caused by the combination of the red hot iron with sulphur; the absence of all elasticity and tenacity in the oxides of iron, show that, even if a flue do not at once collapse, or a shell explode, through getting red-hot, the boiler is more or less injured every time it gets overheated. A defective circulation, by permitting such a temperature as to drive the water off the plate, would soon lead to local injury. Particular spots in externally fired cylindrical boilers are sometimes, as is stated by Mr. L. Fletcher of Manchester, thus affected, and in an apparently mysterious way. A new boiler in which a heap of rags were accidentally forgotten, had the spot burnt out in a few days, doubtless through the resulting defective circulation and its consequences. The plates just above the fire of internal flues also suffer in this manner. It is perhaps possible that turned joints, secured by bolts, and allowing an occasional reversing, or rather rotating, of the ring, might, in some cases, be here of service. At any rate, universal experience proves that the thicker the plate the easier does it get red hot; and these chemical facts also point to the desirability of a minimum of thickness. In fact, the wearing away of the plates through these causes, if mechanically strong against pressure, often gets arrested at a certain thickness. In Germany and France, some of the best manufacturers still make the plates over the fire of, for instance, inside flues, slightly thicker than anywhere else; but the combined chemical and mechanical actions of the heated fuel cause most wear and tear in a thick plate, and thus justify American practice in this respect. In that country, fire-box plates of good charcoal iron are made only or of an inch thick, and, with stays four inches apart, give good results under nearly 150 lbs. steam pressure.

THE FEED-WATER.

The wear and tear of a boiler which occurs in the form

of corrosion, properly so-called, may be divided into two principal kinds :-(1.) Internal, and (2.) external. The progress of both is necessarily intensified by the mere effects of temperature; each, however, has its stronglymarked, distinct character-not merely as to position, but also as to origin and results.

Whatever physical changes may be induced in iron by the long continuance of a high temperature which is not succeeded by the application of the impact of the hammer of the pressure of the rolls, it is certain that long-contamed red heat leads to the loss of its metallic consistency. Its surface gets converted to a greater or less depth into forge scales, which, according to Berthier, consist of a crystallized compound of peroxide and protoxide of ion. The mechanical action of the gases-and especially of the free oxygen contained in every flame-forced at a high velocity by the draught past the more or less heated plates, would also aid these chemical combinations-upon 4. THE CHEMICAL AND PHYSICO-CHEMICAL EFFECTS OF the same principle as iron filings, thrown through a gas fame, burn in the air; and upon the same mechanical principle as the incandescent lime is worn away by the Lame of the oxyhydrogen blow-pipe. These actions would take place with any fuel, even with pure charcoal. But when mineral fuel, which mostly contains more or less iron pyrites, is used, there is much more danger to the plates, especially over the fire, in getting red hot, as the dames would then hold sulphurous acid, and often A steam boiler is in the position of a vessel into which volatilised sulphur. A familiar illustration of an action large volumes of water are continually forced; while the of this kind is afforded by the fact that a piece of red-heat applied, driving off all volatilizable matter, leaves beSot iron plate can be easily bored through by means hind a concentrated solution with a chemical character deof a stick of sulphur, the combination forming sulphide pendent on that of unvolatizable matters in the feed-water. of iron. Dr. Schafhaeut, of Munich, has given great The specific gravity of the substances found in the water attention to the changes in plates subjected to the naturally causes them to sink towards the bottom, at action of fire; twenty-five years ago he read a paper which part the solution is generally more concentrated, before the Institution of Civil Engineers, and more however much it may be stirred up by the ebullition. recently he has published an essay, both on this sub- Mr. J. R. Napier lately stated that a piece of zine "about ect, in a Munich periodical. He has brought forward a four feet long, by three inches broad, by three-sixteenths Lumber of facts, founded on chemical analyses of plates of thick, placed in a marine boiler for three weeks" to a exploded boilers, showing the danger, due to chemical depth of 18 inches in the water, showed a corrosion action alone, when the plates of a boiler become red hot. which rapidly decreased "up to the highest part, He notices that the iron of the inside of the plates, in getting which, in the steam, appeared to be little affected."+ red-hot, decomposes the water, and combines with the oxy- This accounts for the fact that all boilers, even those ingen thus freed. It also loses some of its carbon. The out- ternally fired, like locomotive boilers, have their plates side combines with the free oxygen and with any sulphurous most affected towards the bottom, and that internal acid in the flame. He states that iron made with pit corrosion always shows itself to a greater extent below the coal is much more affected than charcoal-made iron; water line. The bouilleur of the form of boiler known becoming laminated at the original joints in the pile out as the French boiler is also generally more affected than of which the plate has been rolled. It is possible that any other part. To resist this sort of slow action, it is portions of oxide are carried into these joints, and it is at clear that the more the bulk of metal the better, and it any rate certain that irou gives way easiest at these is for this reason that the bottom plates of most marine places. This points to the great value of really homo- boilers are made thicker, while these same plates in locomogeneous plates, such as those of cast steel, in which homo- tive boilers have to be often renewed. Any chemical or geneity has been obtained by the only known means of fusion. physico-chemical action of the kind is of course intensified by temperature, and this is one of the causes why externally fired boilers give way most a little in front of

Steam Boiler Explosions, 1860, p. 321

tTransactions of the Institution of Civil Engineers. Vol. 1840; p. 435.

Bairisches Kunst und Gewerbeblatt. June, 1863.

Pêclet, Traité de la Chaleur. Vol. ii., p. 73.

† Institution of Engineers in Scotland, Session 1864-5.

the furnace. But the plates above the water-line also get more or less corroded, and not merely with the usual character of rusting, but in that peculiar form known as pitting, which generally shows itself much more strongly inarked below the water-line.

The presence of a concentrated solution of an acid or alkaline character, kept at a high temperature for years in contact with iron plates, would be sufficient to account for much corrosion. But the internal corrosion of steam boilers has many features of such a mysterious character, that no accredited explanation of its attendant phenomena has yet been put forward. In the first place, plates thus attacked show a number of irregular holes like a pockmarked human face, or like the small craters seen on the moon's surface. (See Fig. 4.) The writer has Fig. 4.

(The internal surface of a plate of an old wrought iron boiler, showing, one-quarter of the full size, the ordinary appearance of pitting.) also sometimes observed two or three little irregular excavations like this in a plate otherwise showing a large surface quite intact. Sometimes the plate is most pitted round a projecting bolt; at others, one plate will be perfectly sound, while that rivetted to it will be almost eaten away, both having been the same time at work, and under, of course, apparently exactly similar conditions. With locomotive boilers this pitting has been ascribed to galvanic action between the brass tubes and the iron plates. But it is notoriously well-known to locomotive superintendents, that boilers with iron tubes are often worse pitted than those which have run the same distance with brass tubes. Besides, all iron boilers, with or without brass, whether used for stationary, locomotive, or marine, purposes, are subject to pitting.

plains how plates above the level of the water, especially in marine boilers, get very rapidly corroded in portions, while another part of perhaps the same plate is scarcely affected. The concentrated water in a marine boiler is known to be generally acid. "Of all the salts contained in sea water," says Faraday," the chloride of magnesium is that which acts most powerfully" on the plates. He shows that a cubic foot of sea-water contains 3.28 oz. of this salt; and, at the same time, points to the danger of voltaic action in a boiler through the contact of copper and iron. In a smaller degree the contact of cast with wrought iron, or between the different makes of wrought iron in the same plate, or between contiguous plates, acts in the same way. It is not improbable that some hydrochloric acid is present in the steam of marine boilers." Mr. J. C. Forstert has tested some of the condensed steam from the safety-valve casing, and from the cylinder-jacket of the Lancefield, and found both decidedly acid." With an exciting liquid in the condensed steam, it is thus explicable how the plates of marine boilers often get corroded in a most capricious manner; while, at the same time, the current of steam would create a certain amonnt of friction on the oxide, clearing it away to act on a fresh surface.

The crucial test of this explanation of pitting would be the observation of the absence of the phenomenon from plates of an electro-homogeneous character. This homogeneity could only be expected from fused metal, such as cast-steel. Accordingly, while the writer was in Vienna a short time ago, he was assured by Mr. Haswell, the manager of the Staatsbahn Locomotive Works, that some locomotives made of cast steel plates in 1859, for the Austrian Staatsbahn, had been working ever since without showing signs of pitting, though under similar conditions iron plates had severely suffered in this way. Pitting may thus be fairly defined as a form of corrosion localised to particular spots by voltaic action. It is also probably aggravated through the motion of the plate by mechanical action, and the expansions and contractions through alternations of temperature. All boilers are most pitted near the inlet for the feed water, and with inside cylinder locomotive boilers there is generally more pitting at the smoke-box end-no doubt caused by the more or less racking action on these plates. A state of corrosion at particular spots would probably be kept up to a greater intensity by the incrustation being mechanically thrown off. With a quicker voltaic action, caused by any unusual intensity of the exciting liquid, the sides of the cavities in the plates would be sharper and less rounded off; as in the case of the boiler fed with mineral water from ironstone workings, which exploded last year at Aberaman, South Wales. (See Fig. 5.)

The fact that pitting occurs in marine boilers when distilled water from surface condensers is used, does not affect this explanation. Water distilled in this way, from whatever cause, after repeated boiling, is stated to carry the salinometer even higher than sea water, thus proving that it is not pure.§ In the next, there is the absence of incrustation, which to some extent always protects the plates of boilers from the chemical action of its contents. In this way the mechanical buckling of the plates, directly and indirectly causing the furrows we have spoken of-by continually clearing particular lines of surface from incrustation and oxide, reduces these particular spots, with respect to corrosion, to the condition of the plates of a boiler fed with water which deposits no incrustation. Corrosion will also

Fifth Report of the Committee of the House of Commons concerning the Holyhead Roads, p. 194.

Institution of Engineers in Scotland, 1864-5. Introductory address by Mr. J. R. Napier.

An explanation which seems to meet all the circumstances of the case is the following:-Mr. Mallet, in a .report addressed to the British Association some years ago, showed that wrought iron and steel (blis er steel probably), "consist of two or more different chemical compounds, coherent and interlaced, of which one is electro negative to the other." In fact ordinary wrought iron, being also welded up from differently worked scrap, is far from being an electro-homogeneous body. In a boiler, the hot water, nearly to dryness, the salt and the water are decomposed, magWhen a solution of chloride of magnesium is evaporated more or less saturated with chemical compounds, is the ex-nesia and free hydrochloric acid being formed; or M g C1+ citing liquid, and the electro-positive portions of the plates H 0=Mg0+HCI. are thus quickly removed to a greater or less depth. This explanation meets most of the known circumstances with respect to pitting; it even, in a great measure, ex

Institution of Mechanical Engineers, 1863. Discussion on Mr. James Jack's paper "On the Effects of Surface Condensers on Steam Boilers."

control of the engine-driver; and (2) any overloading of this valve is made punishable by a fine of not more than £100, "in addition to any other liabilities" which may be incurred by such an act. The boilers of all vessels carrying passengers, before clearing out of port, are subjected to a careful inspection by an engineer-surveyor of the Board of Trade, who can require the boiler to be tested in the usual way to twice the working pressure; and, if he think fit, he can, as the result of such an examination, place the option before the shipowner of either lowering the working pressure or renewing the boiler. Armed with such powers, the government surveyor is also responsible for any explosion which may directly occur through wear and tear. When an explosion takes place on a passenger railway, one of the Board of Trade inspectors of railways examines the fragments and reports upon the accident to the government board, who communicate it to the railway board. The reports are then printed, in order to be presented to Parliament, and this is the extent to which the British government can interfere in these cases. As with other railway accidents, however, the Board of Trade inspector is examined as a witness in any action for damages against the railway company. All other boilers in the United Kingdom are worked without any government or municipal interference whatsoever. Within late years, however, private companies (the first of which was organised by Mr. Fairbairn, of Manchester) have been formed for the prevention of boiler explosions. In return for a small annual fee, or for a small annual insurance premium, the boilers of any subscriber or insurer are periodically inspected, and, if required, tested by skilled engineers. There can be no doubt that these companies have already prevented a great amount of loss and disaster.

lastly on the 25th of January, 1865, as many different regulations have been issued with respect to steam boilers of all kinds. Beginning by requiring that every boiler, even of wrought iron, should be submitted to a hydraulic test of five times the working pressure, this has been successively lowered down to a threefold pressure in 1843, and lastly to a twofold pressure, by the Imperial decree of this year. The previous law fixed the minimum thickness of the plates-a regulation which undoubtedly did much injury to boiler making in France. The old Prussian regulation of the 6th of May, 1838, also fixed the thicknesses of the plates, but did not require any hydraulic test. By the Regulativt of the 31st of August, 1861, this was completely altered. The construction of the boiler was left entirely in the hands of the maker; but stationary boilers had to withstand a threefold, and locomotive boilers a twofold, hydraulic pressure. In the same way as with the present French law, the test had to be repeated after any considerable repairs. On the 5th of March, 1863, a ministerial decree reduced the testing pressure for old locomotive boilers down to 14 of the working pressure; and another Circular Erlass, published on the 1st of December, 1864, reduced the test for all kinds of boilers down to twice the working load. There is now no material difference between the French and the Prussian regulations respecting boilers; and it may be expected that those continental states, such as Russia, Switzerland, and Spain, which have more or less copied the old French law of 1843, will also adopt the present alterations. There is also some talk about altering the present Austrian law, which determines the thickness of the plates, but only demands a double pressure test. The Belgian réglement § also requires double the working pressure for common boilers, but only 1 for tubular boilers. According to Article 31, the test must It may thus be said that there are three distinct plans for be annually applied to locomotive, portable, and marine the general management of steam boilers:-1. There is boilers, as also after all considerable repairs. There does the continental plan; 2. the free English and American not seem to be any general law in Italy, but in the special mode; 3. what may be termed the Manchester system. acts authorizing railway companies, similar requirements The continental mode consists in a strict supervision, to the French regulations are laid down, and govern- sometimes ruled by formulæ, of the original construction, ment commissioners see that they are carried out. Each and there its action may be said, for the most part, to end. of the smaller German states also has its law, more or less It does not, and cannot, without periodical inspections, take like that of France and Prussia. Mecklenburg-Strelitz||| into account the effects of wear and tear. It may even requires that common boilers be proved to three, and be doubted whether the old French law, for instance, did tubular boilers to twice the working pressure; to be re- not do more harm than good as regards construction. newed every fourth year, and every time that the The official formula, according to which were calculated boiler is repaired or altered; Saxony, that cylindrical the thicknesses of the plates, founded as it was upon the boilers be tried to twice the working pressure, and tubular assumptions that a cylindrical boiler formed an exact boilers to a pressure three atmospheres above it. Bavaria** circle, and that a plate, however thick, conducted the now requires double the working power pressure for new, same amount of heat to the water, was obviously incorrect. and one and a half for old boilers; while both Hanover What may be termed the ordinary English and American and Brunswick each have a somewhat similar regulation.†† plan throws the onus of proof of the negligence of the The French law, and indeed most of the others, require owner on those damaged by an explosion. This system is two safety valves; and many are extremely minute in subject, besides other difficulties, to all the objections that their directions with respect to glass gauges, steam gauges, exist against the trial of scientific questions by a jury, not and other fittings. In Great Britain there are no express composed of experts, and unaided by scientific witnesses. legislative enactments with respect to boilers beyo id those The continual occurrence of explosions in those cities and stated in two clauses of the Merchant Shipping Act, States of America in which boilers are used without any according to which (1) one safety-valve in every boiler of supervision by the authorities, and their undue occurrence a vessel carrying passengers shall be placed beyond the in England with boilers that are not subjected to systematic inspection, sufficiently prove that steam boilers cannot be worked at hap-hazard. On the other hand, the system of organised inspection by the English boiler companies, and the similar system according to which the passenger vessels are inspected by government officers. have given universal satisfaction. A proper estimate of the value of the Manchester and Board of Trade system, compared with the continental and with the laisser An-faire plans could only be well based on numerous statistics. Unfortunately, such do not appear to have been formed. It is stated, however, that in a average of 277 boilers, there were two explosions in the French department of the Haut-Rhin within ten years; and,

Décret concernant la Fabrication et l'Etablissement des
Machines à Vapeur. 25 Janvier, 1865.

Düsseldorfer Zeitung. 24 ste September, 1861.
Reichs Gesetz-Blatt fuer das Kaiserthum Oesterreich, 1854;

p. 229.

Ministère des Travaux Publics, Machines à Vapeur.Règlement. Donné à Laeken le 21 Avril, 1864.

Grossherzoglich Mecklenburg-Strelitzer Offizieller zeiger, No. 11, 1863. Gesetzliche Verordnungen, die Anlegung von Dampkesseln betreffend. Dresden, Meinhold und Sehne, 1865. **Regierungs-Blatt fuer das Koenigreich Bayern, 22 Februar,

1865.

+ Gesetz-Sammlung fuer das Koenigreich Hannover, 1863. Merchant Shipping Act, 27th June, 1854, Nos. 289 d 298.