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analysing them again they were found to contain from ·002 to ·005 % of hydrogen. He points out that, on account of the low atomic weight of hydrogen, these percentages should be multiplied by, say, thirty to make them comparable with the volumes of phosphorus or of sulphur. Baedecker ('Deut. Ing.,' 1887, vol. xxxii. p. 187) confirms these views about the action of acids.

Heyn (Stahl und Eisen,' 1900, vol. xx. p. 780) states that by heating and cooling steel in an atmosphere of hydrogen it becomes brittle.

The study of this subject is as yet in its infancy, and is certainly beset with many difficulties. It may, therefore, be of interest to mention where experiments on occluded gases can be found :-L. Troost and P. Hautefeuille, ‘Comp. Rend.,'1875, vol. lxxx. p. 788, and An. Chim. Ph.,' 1876, 5th ser. vol. vii. p. 155; M. Reynard, “Ing. Civ.,' 1877, pp. 91 and 210; A. H. Allen, .I. and S. I., 1879, p. 480, and 1880, p. 181; F. C. G. Müller, ‘Deut. Ch. G.,' vol. xii. p. 11; ibid. • Glaser's An.,' 1880, vol. vii. p. 138; ibid. “Stahl und Eisen,' 1882, vol. ii. p. 531; N. Zyronski, Soc. I. Min.,'1884, p. 101 ; H. M. Howe, ' Eng. Min. J.,' vol. xlv. p. 236. Nitrogen, Harbert and Tynam, I. and S. I.,' 1896, vol. ii. p. 161.

It would be wrong to compare occlusion by metals with the absorbing power of fluids-for instance, with water, which takes up very much larger quantities of carbonic acid when cold than when warm. On the contrary, metals behave in a most erratic manner, and usually absorb certain gases while in a molten state, which they give off again when cooling. Thus molten silver and copper absorb oxygen, and expel it with much force as they harden. Therefore it can hardly be expected that the annealing process will drive out all the gases occluded by iron or steel, even if carried out in a vacuum, although a certain proportion would disappear. The gas given off by steel in the soaking pits is said to be hydrogen. If this gas should be found to be the cause of brittleness, it may one day be necessary to dry the gases used in melting-furnaces.

Burnt Iron.--It has been attempted to explain the mystery of burnt iron as being due to the presence of a large percentage of occluded oxygen ; but iron can unquestionably also be burnt' in a vacuum and in gases containing no oxygen. This has led to the view that the occluded gases in the iron or steel-chiefly hydrogen and nitrogen-leave the metal and form innumerable cavities, thereby destroying its continuity and making it rotten (burnt).

W. M. Williams (Chem. S.,' 1870, vol. xxiv. p. 790) found oxide of iron in burnt iron.

H. Caron, Comp. Rend.,' 1872, vol. lxxiv. p. 662. Iron could be burnt in air, hydrogen or nitrogen gas.

Professor Ledebur, 'Jahrb. B. H.,'1883, p. 19. Phosphoric iron is more easily burnt than pure qualities. He believes burning not to be due to oxygen, and mentions dead iron. See also ‘I. and S. I.,' 1888, vol. i. p. 386; 1897, vol. i. p. 234 ; 1898, vol. i. p.

145. In spite of this diversity of opinion there can be no doubt that iron and steel do get burnt if exposed to excessive heat, and are then both red- and coldshort; and also that in re-heating furnaces, and even in annealing furnaces, burning can very easily be brought about if the fire grates are left partly uncovered and pure but heated air is allowed to impinge on the plates.

Recalescence.--It might be that burning is simply due to an excessively high temperature, for iron perhaps and steel certainly exhibit strange phenomena when heated. One of these has been called recalescence, to which attention was first drawn by Mr. Barrett, who showed that if a piece of hard steel is heated to redness and then allowed to cool slowly it will, when almost black (in a dark room), suddenly reglow or recalesce, and then get dark again.

The subject has been very exhaustively investigated, and seems to be closely related to the specific heat of iron, which shows strange irregularities at certain temperatures, occasionally changing from a positive to a negative quantity, i.e. the temperature rises while heat is being abstracted.

Barrett, ‘Phil Mag.,' 1873, vol. xlvi. p. 472.
G. Forbes, ‘R. Soc. Edinb.,' 1873-4, vol. viii. p. 363.

M. Pionchon, ‘Comp. Rend.,' 1886, vol. cii. p. 1451. Specific heats of iron up to 1,000° C.; breaks at 660° C. and 723° C.

F. Osmond, ‘Comp. Rend.,' 1887, vol. ciii. pp. 743, 1135. Recalescence of mild steel between 670° and 690° C., 735° and 775° C., 820° and 860° C., and other periods for harder steels.

Brinell, ‘I. and S. I.,' 1886, p. 365.

A. Ledebur, · Stahl und Eisen, 1887, vol. vii. p. 447. Recalescence of four qualities of steel and two of iron (analysis). There is a difference between cooling and heating the samples.

H. Tomlinson (Phys. S.,' vol. xix. p. 107) notices seven points of recalescence.

H. F. Newall, ' Phil. Mag.,' 1888, vol. xxv. p. 510.

F. Osmond, ‘I. and S. I.,' 1890, i. p. 38. Recalescence of fifteen qualities of iron and steel. Pure (electrolytic) iron possesses the recalescent properties, but ferro-manganese does not.

Temnikoff, Gorni J.,' 1887, p. 308.

T. Andrews (C. E.,' 1888, vol. xciv. p. 192) gives temperature readings down to 0° F., which show two distinct breaks similar to those noticed by F. Osmond and others at high temperatures,

Sir W. Roberts-Austen deals very fully with this subject in his reports to the Alloys Research Committee of the Institute of Mechanical Engineers.

The above experiments show that the recalescent property is possessed by pure iron, and that chemical admixtures modify it somewhat. It is worthy of note that a large percentage of manganese removes the property, and also makes steel very ductile at a blue heat, which is a temperature corresponding with one of the recalescent periods.

Other metals have not been experimented upon in this line, but the task might be a profitable one, particularly as regards copper, which is exceedingly brittle at a dull red heat.

A careful study of the above might also throw light on the question of annealing and tempering (or quenching); for it is strange that copper, brass, and manganese steel should be hardened by annealing and softened by quenching, while steel behaves exactly in the reverse way. It is evident that the chemical composition, particularly the carbon, plays an important part, and it even seems as if absolutely pure iron hardens if annealed.

Annealing. It is well known that the annealed iron wire has an initial hardness, which disappears on straining it. Basic (Siemens) steel, made of pure pig and low in carbon, usually shows a slight increase in its ultimate tenacity after annealing, while tempering lowers its elastic limit. The reverse is the case with acid Siemens steel, which generally contains more carbon and less manganese. Internal strains are never entirely removed by annealing (p. 24). Tool steel can be thoroughly annealed by heating it to redness and cooling slowly till it is just dark, and then quenching. Mild steel gives better bending tests if quenched at a cherry-red heat than if quenched at a very dull red heat. The literature on the subject is chiefly restricted to the question of removing the injury done by punching, and to the influence of quenching on the comparatively hard steel used for guns.

The well-known optic glass works, Schott Bros., Jena, have adopted a novel method of annealing. They argued that, however slowly a glass might be cooled, the outside would always be hotter than the inside until uniformly cold, and then the inside would be in tension. They therefore cool their glass masses step by step, taking care to maintain their annealing furnaces at constant temperatures for long periods at a time, and then cooling rather rapidly to the next step. By this means the glass has repeatedly passed through a period in which it was of an equal temperature throughout, and can now be produced so as to be without strains, which is proved by its optical properties.

According to practical experience and also theoretical investigations by J. A. Brinell (* Stahl und Eisen,' vol. v. p. 611, also abstract 'I. and S. I.,'1886, vol. i. p. 365), annealing can produce both bad and good results. Steel or iron which is kept at a dull red heat for a long time becomes rotten; if kept at a bright red heat for a long time, it partakes of the nature of burnt material. The best results are obtained by heating to a good red heat for a short time and letting the object cool, rapidly at first and then slowly. See also ‘I. and S. I.,' 1882, vol. i. p. 209; 1882, vol. ii. p. 532; 1898, vol. ii. p. 137, and w. Rudeloff, · Mitt. Berlin,' 1891, p. 109.

Effects of Quenching Red-hot Steel.–For thin plates it seems advantageous to remove the sheared edges before tempering the samples, while for thick plates the reverse appears to be true. Quenching in oil gives more toughness, while quenching in water, and particularly in mercury, increases the hardness. Quenching at a duil red heat thoroughly anneals tool steel. Quenching in molten lead or in boiling water produces nearly the same effect as annealing. This should not be done to any structures exposed to corrosive influences, as the minute particles of lead which adhere to the iron produce very severe local corrosion.

Galvanised Steel.-Injury, possibly due to chemical action or to the absorption of vapours, is produced if iron or steel articles are galvanised at too high a temperature. In order to guard against this danger, the fires for heating the zinc baths should always be placed round their sides; this allows the impure zinc (containing iron) to

fall to the bottom, where it floats on a layer of molten lead. If mixed with the other zinc, the temperature of the bath would have to be raised too high in order to keep it fluid.

Another class of influences-chiefly mechanical treatments—have to be noticed, as they affect the quality of iron and steel.

The Influence of Hammering or Cogging the Cast Ingots, or of Rolling them direct.-In England the practice is either to hammer or cog the ingots, while in Germany this is not always done, which is made possible by the different chemical compositions. Hammered ingots show a better surface when rolled, and are said to be a little denser. The question of waste is intimately bound up with the above, and there can be no doubt that there is less scrap from plates whose ingots were cogged or hammered than if no preliminary shaping had taken place. From a boiler-maker's point of view a large amount of scrap is

advantage, for the edges are sometimes overheated, and are never of exactly the same quality as the rest of the plate. As regards the final quality of the material several important investigations have been carried out, but being in each case confined to steel of one company, the deductions are not conclusive as regards other makers.

J. Riley, ‘I. and S. I., 1887, p. 121. Influence of rolling, hammering, and annealing. This is a very exhaustive paper, and the conclusion to be drawn from it is that an excessive amount of hammering and rolling is not necessary. From one and the same charge thick plates are both weaker and more ductile than thin ones, which have received more work.

W. Parker ('I. and S. I.,' 1887, p. 134) mentions experiments to show that tenacity and elongation increase with rolling.

H. Allen, ‘C. E.,' 1888, vol. xciv. p. 240. Rolled wire and drawn wire are neither much stronger nor more ductile than the billet unless tested in an unannealed condition.

Dr. Kirkaldy gives a few experiments.

W. H. Greenwood, C. E., 1889, vol. xcviii. p. 83. Fluid compressed steel. The benefit derived from this preliminary compression does not seem to dissipate during the subsequent manipulations.

The immediate effects of hammering, &c., are investigated in the following papers :

H. Tresca, ‘M. E.,' 1878, p. 315.

Ibid., 'Comp. Rend.,'1883, vol. xcvii. pp. 222, 515, 928. Localisation of heat developed by a hammer blow.

M. Lau, ‘Comp. Rend.,' 1882, vol. xciv. p. 952.

M. Osmond, Comp. Rend.,' 1885, vol. c. p. 1228. Motive power for rolling steel is 2 times as much as for iron. Dr. J. Kollmann, Ver. Gew.,' 1880, 2nd ser. vol. lix.

p.

6. Local Heating.--Serious injury is sometimes done to a steel plate by drawing out one of its corners, even though this be done at a red heat. Flanging, whether done by hand or presses, has also led to fail

It has never yet been reasonably demonstrated that this is due, as some contend, to the local heating alone, or, as others believe, to the straining produced by the unequal expansion of the locally heated parts; other influences, such as the chemical composition of the material and the processes through which it has passed, are probably important factors.

ures.

The following list contains published cases of cracked plates :

Sir N. Barnaby, ‘I. and S. I., 1879, p. 242. List of forty-three steel failures at Chatham in six months.

W. Denny, N. A.,' 1880, vol. xxi. p. 185. List of steel failures in his own yard.

W. Parker, ‘N. A.,' 1881, vol. xxii. p. 12. Cracked plates of the steam yacht · Livadia.'

A. C. Kirk, ‘N. A.,' 1882, vol. xxiii. pp. 131 and 137. Cracked flanged plate.

J. F. Barnaby, 'Enging.,' 1883, April 20; D. S. Smart, 'C. E.,' 1884, vol. lxxx. p. 102. Sketches of cracked plates.

W. Parker, ‘N. A.,' 1885, vol. xxvi. p. 253. Failures of thick steel plates.

H. Goodall,' C. E.,'1888, vol. xcii. p. 10. Sketches of some cracked plates. (See also p. 256.)

Of course this list contains only a very small fraction of the number of cracked plates, for as long as it is not settled whether these failures are due to the material, or to the mode of working the plates, it will be the custom, as now, for the steel-makers to replace the failed plates.

Blue Heat.—Perhaps some, though certainly not all, such failures are due to working the steel at a blue heat. This is another mysterious phenomenon connected with iron and steel. The salient features are that steel will bend without fracture at temperatures ranging from 0° F. to 450° F. (see Dr. J. Kollmann, p. 149), and again above 550° F., but it is quite rotten between these limits; and, further, a piece of steel which has been bent or hammered at this particular temperature, but without breaking, acquired and retained a permanent excessive brittleness (see p. 260). This brittleness can be removed by annealing, but time alone (8 years) does not restore the quality. Experiments on the subject, as well as failures, which are attributed to working steel at a blue heat, will be found in the following papers

M. Valton, ‘Berg-H.-Z.,'1877, vol. xxxi. p. 25; D. Adamson, ‘I. and S. I.,' 1878, p. 402; W. Denny, ‘N. A., 1880, vol. xxi. p. 185; C. E. Stromeyer, C. E.,'1886, vol. Ixxxiv. p. 114; ibid., 1888, vol. xcii. p. 89; W. Parker, ‘I. and S. I.,' 1887, p. 136; G. B. Craig, ‘N. A.,' 1888, vol. xxix. p. 113; Rudeloff, Mitt. Berlin,' 1889, p. 97; Board of Trade Report, August 31, 1886 ; 'Am. R. M. M. A.,' 1892; Prof. A. Ledebur, Glaser's An.,' 1886, vol. xviii.p. 205; A. Martens, * Deut. Ing.,' 1892, p. 172.

Besides these cases a few illustrations will be found on p. 256, and it is also possible that some of the failures noted in the following list may be due to the same cause, or perhaps to the

Influence of Time.-- There are indications that exposure or time alone can change a tough material into a brittle one in the same way that elastic (amorphous) sulphur slowly changes into the hard and brittle condition. It is also said that the nature of pure tin, as well as of nickel steel, is permanently changed when exposed to cold. Phosphoric steel seems to possess this quality.

List of Spontaneous Failures.—Z. Colburn, 1860, p. 32. Old boiler stays said to be brittle.

Professor Thurston, ‘I. and S. I.,'1875, p. 342. Old rails had grown brittle, and improved on re-rolling.

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