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seconds to fall through an interval of temperature which As the proportion of carbon increases in steel, the first hitherto and subsequently only occupies about 6 seconds. break in cooling travels more and more to the right, and Turn to the diagram, and see what actually bappens when gradually becomes confounded with the second break, the iron contains carbon in the proportion required to which, in steel containing much carbon, is of long duraconstitute it mild steel (shown by thin continuous line, tion, lasting as much as 76 seconds in the case of steel Fig. 7); there is not one, but there are two such breaks in containing 1-25 per cent. of carbon (thick continuous the cooling, and both breaks occur at a different tempera- line, (Fig. 7). ture from that at which the break in pure iron occurred. [In the experiments shown to the audience the spot of

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2000* 150 1100 1050 1000 950* 900*850*800*750*700* 650" 600 550 500° 450 400 350

TEMPERATURE

Fig. 7.–The curves in this diagram show how the rate of movement of the spot of light varies with different samples of steel. The stoppage of

the movement of the spot of light of course indicates the evolution of heat from the cooling mass of steel, F (Fig. 5). light moved slowly and uniformly along a screen ten feet' Biron ; cool such red-hot pure iron, whether quickly or in length. It halted for a few seconds as the temperature slowly, and it becomes soft ; it passes to the a soft modiof the cooling mass of steel fell to about 850°C., and fication-there is nothing to prevent its doing so. It when the metal was at dull redness, the spot of light appears, however, that if carbon is present, and the metal remained stationary for 68 seconds, and then resumed its be rapidly cooled, the following result is obtained : a course.]

certain proportion of the molecules are retained in the Now, it may be urged, evidently the presence of carbon form in which they existed at a high temperature-the has an influence on the cooling of steel when left to itself: hard form, the B modification—and hard steel is the result. may it not affect molecular behaviour during the rapid cooling which is essential to the operation of hardening ? We

IRON know that the carbon, during rapid cooling, passes from the state in which it is combined with the iron into a state Q. OR SOFT

BOR HARD in which it is dissolved in the iron; we also know that,

IRON during slow cooling, this dissolved carbon can re-enter

WHEN IRON COOLS IRON into combination with the iron so as to assume the form

DOWN FROM BRIGHT in which it occurs in soft steel. Osmond claims that this

REDNESS TO 855°C. second arrestation in the fall of the thermometer corre

IT CHANGES TO CL IRON sponds to the recalescence of Barrett, and is caused by the re-heating of the wire by the heat evolved when carbon leaves its state of solution and truly combines with the iron.

PURE IRON AT TEMPERATURES

IRON AT HIGH TEMPERATURES If it is hoped to harden steel, it must be rapidly cooled BELOW 855°C. AND IRON

OR, IF CERTAIN OTHER before the temperature has fallen to a definite point, not CONTAINING CERTAIN OTHER

ELEMENTS BE PRESENT , lower than 650°, or the presence of carbon will be un ELEMENTS IF COOLED SLOWLY.

AFTER BEING RAPIDLY COOLED. availing. But what does the first break in the curves

(OSMOND)

(OSMOND) mean? You will see that a break occurs in electro

Fig. 8. type iron which is free from carbon (thin dotted line, Fig. 7); it must then indicate some molecular The main facts of the case may, perhaps, be made clearer change in iron itself, accompanied with evolution of by the aid of this diagram (Fig. 8) which shows the relation heat - a change with which carbon has nothing what between a and B iron. This molecular change from B ever to do, for no carbon is present; and Osmond iron to a iron during the slow cooling of a mass of iron or argues thus :— There are two kinds of iron, the atoms of steel is, according to Osmond's theory, indicated by the which are respectively arranged in the molecules so as to first break in the curve, representing the slow cooling of constitute hard and soft iron, quite apart from the iron, as is proved by the fact that it occurs alone in electropresence or absence of carbon. In red-hot iron the mass iron. A second break, usually one of much longer duramay be soft but the molecules are hard—let us call this . tion, marks the point at which carbon itself changes from

*he dissolved or hardening carbon to the combined ! shown by Spring, even at the ordinary temperature, while. carbide-carbon. It follows that, if steel be quickly cooled i in the case of steel, it must take place far below incipient after the change from B to a has taken place but before fluidity-indeed, at a comparatively low temperature, as is the carbon has altered its state--that is, before the change shown by the following experiment on the welding of steel. indicated by the second break in the curve has been Every smith knows how difficult it is to weld highly reached--then the iron should be soft, but the carbon, carburized hard tool-steel, but if the ends of a newly hardening carbon ; and as such, the action of a solvent fractured /-inch square steel rod, a (Fig. 10), are placed should show that it cannot be released from iron in the black carbide form. This proves to be the case, and affords strong incidental proof of the correctness of the view that two modifications of iron can exist.

It will be seen, therefore, that, although the presence of carbon is essential to the hardening of steel, the change in the mode of existence of the carbon is less important than has hitherto been supposed. The a modification of iron may be converted into the B

Fu, 10. form by stress applied to the metal at temperatures below a dull red heat, provided the stress produces permanent together and covered with platinum foil, b, so as to exclude deformation of the iron, but the consideration of this the air, and if the junction is heated in the flame of a question would demand á lecture to itself

. I am anxious Bunsen burner, c, the metal will weld, without pressure, to show you an experiment which will help to illustrate so firmly that it is difficult to break it with the fingers, the existence of molecular change in iron. Here is a long bar of steel containing much carbon. although the steel has not attained a red-heat.

The question now arises, What is the effect of the In such a variety of steel, the molecular change of the iron presence of other metals in steel, of which much has been itself, and the change in the relations between the carbon heard recently? (1) Manganese. Osmond has shown that and the iron, would occur at nearly the same moment. It this metal enables steel to harden very energetically, as is is now being heated to redness, but if you will look at

well known. If much of it be present, 12 to 20 per cent, in this diagram (Fig. 9), you will be prepared for what I want iron, no break whatever is observed in the curve which re

presents slow cooling (see line marked "manganese steel" b

Fig. 7). That is, the iron never shows such a change as that which occurs in other cooling masses of iron. Then you will say such a material should be hard however it is cooled. So it is. There is one other important point of evidence as to molecular change connected with the addition of manganese to submit to you Red-hot iron

is not magnetic. Hopkinson has shown that the temFIG. 9. --The bar of steel, a, inch in section and 18 inches long, heated to

perature of recalescence is that at which iron ceases to be bright-redness and firmly fixed in a vice or other support at b. A weight of about 2 pounds is rapidly hung on to the free end, and a light

magnetic. It may be urged that B iron cannot therefore pointer, s, is added to magnify the motion ot the bar. It rema.ns per be magnetized. Steel containing much manganese cannot fectly rigid for a per.od varying from 3 to 40 seconds, and then, when the bar has co sled down to very dull redness, it suddenly bends, the

be magnetized, and it is therefore fair to assume that the pointer falling from 6 to 8 inches to the position c*.

iron present is in the B form Hadheld has given you to see in the actual experiment. One end of the red- i metallurgists wonderful alloys of iron and manganese in not bar a will be firmly fixed at b, a weight not sufficient to

proportions varying from 7 to 20 per cent. of manganese. bend it is slung to the free end, which is lengthened by the attracts the sphere of iron, but if nothing is changed,

This core of iron round which a current is passing, addition of a reed, c, to magnify any motion that may take place. Now remember that as the bar will be red-hot it except by replacing the core of iron with a core of ought to be at its softest, you would think, when it is freshly Hadfield's steel, it is impossible to make a magnet of it. withdrawn from the furnace and if the weight was ever to

[Experiment shown.] have power to bend it, it would be then ; but, in spite of the concludes that, "no magnetizing force to which the

Prof. Ewing, who has specially worked on this subject. rapidity with which such a thin bar cools down in the air metal is likely to be subjected in any of its practical and becomes rigid, points of molecular weakness come applications would produce more than the most miniwhen the iron changes from $ to a, and the carbon passes tesimal degree of magnetization” in this material. Trom hardening carbon to carbide-carbon ; at that moment, at a temperature much below that at which it is withdrawn

It has been seen that quantities of manganese abuve 7 from the furnace, the bar will begin to bend, as is shown per cent. appear to prevent the passage of iron into the by the dotted lines d', c'. It has been found experimentally a form. In smaller quantities manganese seems merely that this bend occurs at the point at which, according to to retard the conversion, and to bring the two loops of Osmond's theory, molecular change takes place. Mr.

the diagram nearer together. Coffin takes advantage of this fact to straighten distorted other elements on steel. I will only add that tungsten

Time will not permit me to deal with the effect of steel axles.2

There is a sentence in the address which has just been possesses the same property as manganese, but in a delivered before Section G, by Mr. Anderson, which has

more marked degree. Chromium has exactly the redirect reference to molecular change in iron. He says :- verse effect, as it enables the change of hard s iron

to a soft iron to take place at a higher temperature “When, by the agency of heat, molecular motion is raised to than would otherwise be the case, and this may explain a pitch at which incipient fluidity is obtained, the particles of the extreme hardness of chromium steels when hardened iwo pieces brought into contact will interpenetrate or diffuse in the same way as ordinary steels. into each other, the two pieces will unite into a homogeneous There are a few considerations relative to the actual whole, and we can thus grasp the fall meaning of the operation working of steel with which I can deal but briefly, notwithknown as welding.'”

standing their industrial importance. The points d and It is, however, possible to obtain evidence of inter- b, adopted in the celebrated memoir of Chernoff to which change of molecular motion, as has been so abundantly

" Trans American Society Mechanical Engineers, ix., 1888, s. 155 1 * Études Métallurgiques,” par Osmond, p. 6 (Paris : Dunod, 1888.) * Prcc. Roy. Soc, xlv., 1889, PP. 318, 445, and 457. 2 Trans. American Soc. Civil Engineers, xvi., 1887, p. 324.

3 Proc. Inst. Civil Engineers, xciu. l'art m., 1888.

I have referred already, change in position with the appears to break up this crystalline structure in a manner degree of carburization of the metal. It is useless to analogous to mechanical working. If the mass of metal attempt to harden steel by rapid cooling if it has fallen in is very large, such as a propeller shaft, or tube of a large temperature below the point in the red) a, and this is the gun, the change in the relations between the carbon and point of “recalescence" at which the carbon combines the iron, or true “hardening ” produced by such oil with the iron to form carbide-carbon : it is called V by treatment is only effected superficially—that is, the Brinell

. In highly carburized steel, it corresponds exactly hardened layer does not penetrate to any considerable with the point at which Osmond considers that iron, in depth, but the innermost parts are cooled more quickly cooling slowly, passes from the B to the a modification. than they otherwise would have been, and the developNow with regard to the point b of Chernoff. If steel ment of the crystals, which would have assumed serious be heated to a temperature above a, but below b, it proportions during slow cooling, is arrested. It depends remains fine-grained however slowly it is cooled. If the on the size of the quenched mass, whether the tenacity of steel be heated above b, and cooled, it assumes a crystal- the metal is or is not increased, but its power of being line granular structure whatever the rate of cooling may elongated is considerably augmented. This prevention be. The size of the crystals, however, increases with the of crystallization I believe to be the great merit of oil temperature to which the steel has been raised.

quenching, which, as regards large masses of metal, is Now the crystalline structure, which is unfavourable to certainly not a true hardening process. the steel from the point of view of its industrial use, may There has been much divergence of view as to the be broken up by the mechanical work of forging the hot relative advantages of work on the metal, and of oil

hardening, but I believe it will be possible to reconcile these views, if the facts I have so briefly stated be considered.

The effect of annealing remains to be dealt with. In a

very complicated steel casting, the cast metal probably ELONGATION STRENGTH PER SQUARE

contains much of its carbon as hardening carbon, and the CARBON PER CENT

mass which has necessarily been poured into the mould 00 01 02 03 04 05 06 07 08 09 10 11 12 at a high temperature is crystalline. The effect of an

nealing is to permit the carbon to pass from the "hardening” to the "carbide" form, and, incidentally, to break up the crystalline stucture, and to enable it to become minutely crystalline. The result is that the annealed casting is far stronger and more extensible than the original casting. The carbide-carbon is probably interspersed in the iron in fine crystalline plates, and not in a finely divided state. It would obviously be impossible to "work”-that is, to hammer-complicated castings, and the extreme importance of obtaining a fine crystalline structure by annealing, with the strength which results from such a structure, has been abundantly demonstrated by Mr. J. W. Spencer, whose name is so well known to you all in Newcastle.

The effect of annealing and tempering is in fact very complicated, and I can only again express my wish that it were possible to do justice to the long series of researches which Barus and Strouhal have conducted in recent years. They consider that, annealing is demonstrably accompanied by chemical change, even at temperatures slightly above the mean atmospheric temperature, and that the "molecular configuration of glass-hard steel is always in a state of incipient change, ... a part of which change must be of a permanent kind.” Barus says " that during the small interval of time within which appreciable annealing occurs, a glass-hard steel rod suddenly heated to 300° is almost a viscous fluid."i Barus considers that glass-hard steel is constantly being spontaneously "tempered” at the ordinary temperature which, he says, “acting on freshly quenched (that is hardened] steel for a period of years, produces a diminution of hardness about equal to that of 100° C., acting for a period of hours."

The nature of the molecular change is well indicated in

the long series of researches which led them to conclude FIG. 11 shows the way in which the tenacity of steel containing varying that in steel "there is a limited interchange of atoms

amounts of carbon is increased by oil hardening,' while at the same time the elongation rapidly diminishes.

between molecules under stress, which must be a property

common to solids, if, according to Maxwell's conception. mass; and the investigations of Abel, of Maitland, and of solids are made up of configurations in all degrees of Noble, have shown how important “work” on the metal is.

molecular stability." When small masses of hot steel are quenched in oil, they change in the relations between the carbon and the iron

Barus and Strouhal attach but little importance to the are hardened just as they would be if water were used as during the tempering and annealing of hard steel. They a cooling Auid. With large masses, the effect of quench- consider that in hardening steel the strain once applied ing in oil is different. Such cooling of large hot masses to steel is locked up in the metal in virtue of its

* This was well shown in Prof. Akerman's celebrated paper on " Hardening Iron and Steel," Journ. Iron and Steel Institute. 1879. Part ii. p. 101.

Phil. Mag., xxvi., 1888, p. 209.

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viscosity”; tempering is the release of this molecular when steel faces are used for the armour plates, the strain by heat.

material contains Toto lo per cent of carbon, Highly carburized steels harden very energetically by It has been pointed out that the crews of the fleet 2 very slight modifications in thermal treatment, and it will Spithead numbered no less than 21,107 men. This it has be evident that a very hard material is unsuitable for been shown is “ a remarkable figure, considering the great industrial use if the conditions of its employment are such economy in men which prevails in a modern navy as com as to render it desirable that the material should stretch. pared with the navy of Nelson's day. A hundred year To turn to very “mild” steel which does not harden, it ago the normal requirements of a fleet were one man to is certain that, although wrought iron passes almost a little over four tons, but now, thanks to the part played insensibly into steel, there can be no question that not by steel and hydraulic power, we require but one man to merely the structural but the molecular aggregation of every seventeen tons. Thus it may roughly be said that ar even 'steel containing only to per cent. of carbon is aggregate of 20,000 men at the present day correspond profoundly different from that of wrought iron. Formerly, to an aggregate of 80,000 men in the days of Nelson, as Sir F. Bramwell pointed out in a lecture delivered at The latest type of battle-ship weighs, fully equipped, abou: the Royal Institution in 1877," by the year 1830 ... from 10,000 tons, there being about 3400 tons of steel in the small beginnings in Staffordshire and at Birkenhead hull, apart from her armour, which, with its backing, wil: sprang a wonderful wrought-iron navy, but steel was a weigh a further 2800 tons. luxury : it was made in small portions sold at high prices, From the use of steel in the Royal Navy and in the as much as a shilling or eighteenpence a pound. It was mercantile marine, let us pass on to its most notable use employed for swords, cutlery, and tools, needles and other in construction. If the President of the French Republic purposes where the quantity used was but trifling, and was justified in appealing, in a recent speech, to the Eiffe. where the importance of the superior material was such Tower as “a monument of audacity and science," what as to justify the large expenditure incurred. It was felt are we to say of the Forth Bridge, the wonders of which in those days that steel was worth paying for because it will be described by Mr. Baker on Saturday? By hus was trusted ; indeed its trustworthiness had passed into kindness I am able to place in the position in the troph a proverb "_"as true as steel.”

justified by the carbon it contains, a plate from the Forth The class of steel which was formerly employed, as I Bridge, which fell from a height of some 350 feet, and have just indicated, for weapons and tools belonged to being of excellent quality, doubled itself on the rock the highly carburized, readily-hardening class. It was below. A single span of the Forth Bridge is nearly as the "mild steel" containing but little carbon which was long as two Eiffel Towers turned horizontally and tied destined to replace wrought iron, and when attempts were together in the middle, and the whole forms a complicated made to effect the general substitution of steel for iron, steel structure weighing 15,000 tons, erected without the fears as to its character and trustworthiness unfortu- possibility of any intermediate support, the lace-like fabric nately soon arose, so that from about the year 1860 of the bridge soaring as high as the top of St. Paul's until 1877 steel was viewed with suspicion. We can now The steel of which the compression members of the explain this. Doubts as to the fidelity of steel, even when structure are composed contains to per cent. of carbon it was obtained free from entangled cinder, arose from and 10% per cent. of manganese. The parts subjected to ignorance of the fact that, on either side of a com- extension do not contain more than 100 per cent of paratively narrow thermal boundary, the iron in steel can carbon." practically exist in two distinct modifications. The steel Time will not permit me to pass the members of each was true enough, but from the point of view of the special class in review. I can only refer to very few. Steel for duties to be intrusted to it, its fidelity depended on which the manufacture of pens contains about 16 per cent of modification of iron had to be called to the front. carbon, and 16 to 18 tons of steel are every week let Artificers attempted to forge steel after it had cooled loose on an unoffending world in the shape of steel down below the point a of Chernoff, at which recal- pens. escence occurs, and they often attempted to work highly Steel rails contain from ito ito per cent. of carbon, carburized steel at temperatures which were not sufficiently and, in this class, slight variations in the amount of carlow.

bon are of vital importance. An eminent authority, Mr. Steels may be classified from the point of view of their Sandberg, tells us that in certain climates a variation of industrial use according to the amount of carbon they bo per cent in the amount of carbon may be very serious contain, and I have attempted to arrange in this trophy The great benefit which has accrued to the country from certain typical articles, grouped under certain definite the substitution of more durable steel rails for the old percentages of carbon ranging from io to i per cent. wrought-iron ones may be gathered from the figures [This was a trophy 18 feet square, with various typical which Mr. Webb, of Crewe, has given me, which show articles of steel arranged in order according to the that "the quantity of steel. removed from the rails amount of carbon they contained. I am greatly indebted throughout the London and North-Western system by to Mr. J. W. Spencer, of Newcastle, who kindly lent me wear and oxidation is about 15 cwt, an hour, or 18 tons the fine series of specimens of which the "trophy” is a day." built up.] Each class merges into the other, but the Gún-steel contains i tot per cent. of carbon, and it members at either end of the series vary very greatly. may contain o per cent. of manganese. It is in relation It would be impossible to make a razor which would cut to gun-steel that oil-hardening becomes very important. from boiler plate ; and conversely, a boiler made of razor The oil-tank of the St. Chamond Works (on the Loire steel would possibly fracture at once if it were super-is 72 feet deep, and contains 44,000 gallons of oil, which heated and subjected to any sudden pressure of steam is kept in circulation by rotary pumps, to prevent the oil Speaking generally, if the steel contains, in addition to being unduly heated locally when the heated mass of carbon, io per cent. of manganese, each class of steel, as steel is plunged into it. at present arranged, would have to be shifted a class Now with regard to projectiles. To quote some recent backwards towards the left of the trophy.

remarks of Lord Armstrong, "the heaviest shot used in At the present day, instead of steel being manufactured the Victory was 68 pounds, while in the Victoria it will and used in small quantities, about 4,000,000 tons are be 1800 pounds; and, while the broadside-fire from the annually employed in this country. Let us see how it is

1 Address by Mr. Baker, Section G, British Association Report, 15 used. A steel fleet, the finest fleet in the world, has recently assembled at Spithead. The material of which

* Times, August 19, 1889.

3 Journal of the Iron and Steel Institute, 1888, ii. p. 94. it was made contained 10 to to per cent. of carbon, and 4 Times, August 3, 1889.

p. 1182.

Victory consumed only 325 pounds of powder, that from of the film has been the subject of much controversy. the Victoria will consume 3000 pounds. The most for- Barus points out that “the oxygen molecule does not inidable projectiles belong to the highly carburized class penetrate deeper than a few thousand times its own of steel. Shells contain 0-8 to 0 94 per cent. of carbon, dimensions, and that it probably passes through the film and, in addition, some of these have o'94 to 2 per cent by a process allied to liquid diffusion. The permeable of chromium. The firm of Holtzer shows, in the Paris depth increases rapidly with the temperature, until at an Exhibition, a shell which pierced a steel plate 10 inches incipient red heat the film is sufficiently thick to be thick, and was found, nearly 8co yards from the plate, britile and liable to rupture, whereupon the present pheentire and without flaw, its point alone being slightly dis- nomenon ceases, or is repeated in irregular succession. torted. Compound armour-plate with steel face, which face contains o-8 per cent. of carbon, is, however, more Looking back over all the facts we have dealt with, it difficult to pierce than a simple plate of steel.

will be evident that two sets of considerations are of (A prominent feature in the "trophy," among the class special importance: (1) those which belong to the relaof highly carburized steels which contain over it per tions of carbon and iron, and (2) those which contemcent of carbon, was a fine suspended wire 180 of an plate molecular change in the iron itself. The first of inch diameter, of remarkable strength, supporting a weight these has been deliberately subordinated to the second, of 2 cwt., or a load of nearly 160 tons to the square although it would have been possible to have written inch. The strength of the same steel undrawn, would much in support of the view that carburized iron is an not exceed 50 tons to the square inch. A similar wire alloy of carbon and iron, and to have traced with Guthrie manufactured by the steel company of Firminy attracted the analogies which alloys, in cooling, present to cooling much attention in the Paris Exhibition by supporting a masses of igneous rocks, such as granite, which, as the shell weighing 18co lbs., or a load of 158 tons per square temperature of the mass falls, throws off “atomically inch.]

definite "2 bodies, leaving behind a fluid mass of indefi. Lastly, I will refer to the highly carburized steel used nite composition, from which the quartz and feldspar for the manufacture of dies. Such a steel should contain solidify before the mica. This view has been developed 08 to i per cent. of carbon, and no manganese. It is with much ability in relation to carburized iron by Prof. usual to water-harden and temper them to a straw colour, Howe, of Boston, who even suggests mineralogical and a really good die will strike 40,000 coins of average names, such as “cementite," "perlite," and “ferrite,” for dimensions without being fractured or deformed; but I the various associations of carbon and iron. am safe in saying that if the steel contained to per cent. I am far from wishing to ignore the interest presented foo much carbon, it would not strike 100 pieces without by such analogies, but I believe that the possibility of molecracking, and if it contained % per cent. too little carbon, cular change in the iron itself, which results in its passage it would probably be hopelessly distorted, and its engraved into a distinctive form of iron, is at present the more imsurface destroyed, in the attempt to strike a single coin. portant subject for consideration, not merely in relation

The above examples will be sufficient to show how to iron, but as regards the wider question of allotropy in diverse are the properties which carbon confers upon iron, metals generally. but as Faraday said, in 1822, “It is not improbable that Many facts noted in spectroscopic work will have, as there may be other bodies besides charcoal capable of Lockyer has shown, indicated the high probability that giving to iron the properties of steel.” The strange thing is the molecular structure of a metal like iron is gradually that we do not know with any certainty whether, in the simplified as higher temperatures are employed. These absence of carbon, other elements do play the part of various simplifications may be regarded as allotropic that metalloid, in enabling iron to be hardened by rapid modifications. cooling. Take the case of chromium, for instance: The question of molecular change in solid metals chromium-carbon steels can, as is well known, be ener- urgently demands continued and rigorous investigation. getically hardened, but Busek? has recently asserted that Every chemist knows how much his science has the addition of chromium to iron in the absence of car- gained, and what important discoveries have been made bon does not enable the iron to be hardened by rapid in it, by the recognition of the fact that the elements cooling. So far as I can see, it is only by employing the act on each other in accordance with the great law of electrical method of Pepys that a decision can be arrived Mendeleeff which states that the properties of the elements at as to the hardening properties of elements other than are periodic functions of their atomic weights. I firmly carbon.

believe that it will be shown that the relation between A few words must be devoted to the consideration of the small quantities of elements and the masses in which they colours which, as I said (see ante, p. 11), direct the artist are hidden is not at variance with the same law. I have in tempering or reducing the hardness of steel to any deter- elsewhere tried to show that this may be true, by examinate standard. The technical treatises usually give- mining the effect of small quantities of impurity on the not always accurately, as Reiser has shown-a scale of tenacity of gold. temperature ranging from 220° to 330°, at which various In the case of iron, it is difficult to say what property tints appear, passing from very pale yellow to brown yellow, of the metal will be most affected by the added matter. purples, and blues, to blue tinged with green, and finally to Possibly the direct connection with the periodic law will grey. Barus and Strouhal' point out that it is possible be traced by the effect of a given element in retarding or that the colour of the oxide film may afford an indication promoting the passage of ordinary iron to an allotropic of the temper of steel of far greater critical sensitiveness state ; but " the future of steel ” will depend on the care than has hitherto been supposed. It is, however, at with which we investigate the nature of the influence present uncertain how far time, temperature, and colour exerted by various elements on iron, and on the thermal are correlated, but the question is being investigated by treatment to which it may most suitably be subjected. Mr. Turner, formerly one of my own students at the Is it not strange that so many researches should have School of Mines,

been devoted to the relations between carbon, hydrogen, That the colours produced are really due to oxidation and oxygen in organic compounds, so few to the relations was shown by Sir Humphry Davy in 1813, but the nature of iron and carbon, and hardly any to iron in association

with other elements ? I think that the reason for the comStahl und Eisen, ix. 1889, p. 728. * ** Das Hårten des Stahles," P. 78 (Leipzig, 1881). See also Löwenherz, parative neglect of metals as subjects of research arises estackrift for listrumentenkunde, ix, 1889, p. 322. Bull. Ú S.Ceo Survey, No. 27, 1886, p. 51.

+ Bull. U.S. Geo. Survey, No. 35. 1886, p. 51. * Sir Humphry Davey, Thomson's Ann. Phil., i., 1813, p. 131 ; quoted

2 Phil. Mag., June 1884, p. 462. by Tarner, Proc. Phil. Soc., Birmingham, vi., 1889, part 2.

3 Phil. Trans. Roy. Soc., clxxit., 1888, p. 339.

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