light moved slowly and uniformly along a screen ten feet in length. It halted for a few seconds as the temperature of the cooling mass of steel fell to about 850 C., and when the metal was at dull redness, the spot of light remained stationary for 68 seconds, and then resumed its course.]

Now, it may be urged, evidently the presence of carbon has an influence on the cooling of steel when left to itself: may it not affect molecular behaviour during the rapid cooling which is essential to the operation of hardening? We know that the carbon, during rapid cooling, passes from the state in which it is combined with the iron into a state in which it is dissolved in the iron; we also know that, during slow cooling, this dissolved carbon can re-enter into combination with the iron so as to assume the form in which it occurs in soft steel. Osmond claims that this second arrestation in the fall of the thermometer corresponds 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.

If it is hoped to harden steel, it must be rapidly cooled before the temperature has fallen to a definite point, not lower than 650, or the presence of carbon will be unavailing. But what does the first break in the curves mean? You will see that a break occurs in electrotype iron which is free from carbon (thin dotted line, Fig. 7); it must then indicate some molecular change in iron itself, accompanied with evolution of heat a change with which carbon has nothing what ever to do, for no carbon is present; and Osmond argues thus:-There are two kinds of iron, the atoms of which are respectively arranged in the molecules so as to constitute hard and soft iron, quite apart from the presence or absence of carbon. In red-hot iron the mass may be soft but the molecules are hard-let us call this

The main facts of the case may, perhaps, be made clearer by the aid of this diagram (Fig. 8) which shows the relation between a and B iron. This molecular change from B iron to a iron during the slow cooling of a mass of iron or steel is, according to Osmond's theory, indicated by the first break in the curve, representing the slow cooling of iron, as is proved by the fact that it occurs alone in electro| iron. A second break, usually one of much longer duration, marks the point at which carbon itself changes from

the dissolved or hardening carbon to the combined carbide-carbon. It follows that, if steel be quickly cooled after the change from 3 to ɑ has taken place but before the carbon has altered its state--that is, before the change indicated by the second break in the curve has been reached-then the iron should be soft, but the carbon, hardening carbon; and as such, the action of a solvent 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 form by stress applied to the metal at temperatures below a dull red heat, provided the stress produces permanent deformation of the iron, but the consideration of this question would demand a lecture to itself. I am anxious to show you an experiment which will help to illustrate the existence of molecular change in iron.

Here is a long bar of steel containing much carbon. In such a variety of steel, the molecular change of the iron itself, and the change in the relations between the carbon and the iron, would occur at nearly the same moment. It is now being heated to redness, but if you will look at this diagram (Fig. 9), you will be prepared for what I want

a'

a

FIG. 9. The bar of steel, a, inch in section and 18 inches long, heated to 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 pointer, c, is added to magnify the motion of the bar. It remains perfectly rigid for a per od varying from 35 to 40 seconds, and then, when the bar has cooled down to very dull redness, it suddenly bends, the pointer falling from 6 to 8 inches to the position c'

you to see in the actual experiment. One end of the redhot bar a will be firmly fixed at b, a weight not sufficient to bend it is slung to the free end, which is lengthened by the addition of a reed, c, to magnify any motion that may take place. Now remember that as the bar will be red-hot it ought to be at its softest, you would think, when it is freshly withdrawn from the furnace and if the weight was ever to have power to bend it, it would be then; but, in spite of the rapidity with which such a thin bar cools down in the air and becomes rigid, points of molecular weakness come when the iron changes from to a, and the carbon passes from hardening carbon to carbide-carbon ; at that moment, at a temperature much below that at which it is withdrawn from the furnace, the bar will begin to bend, as is shown by the dotted lines a', c'. It has been found experimentally that this bend occurs at the point at which, according to Osmond's theory, molecular change takes place. Mr. Coffin takes advantage of this fact to straighten distorted steel axles.2

There is a sentence in the address which has just been delivered before Section G, by Mr. Anderson, which has direct reference to molecular change in iron. He says:

"When, by the agency of heat, molecular motion is raised to a pitch at which incipient fluidity is obtained, the particles of two pieces brought into contact will interpenetrate or diffuse into each other, the two pieces will unite into a homogeneous whole, and we can thus grasp the full meaning of the operation known as welding.''

together and covered with platinum foil, b, so as to exclude the air, and if the junction is heated in the flame of a Bunsen burner, c, the metal will weld, without pressure, so firmly that it is difficult to break it with the fingers, although the steel has not attained a red-heat.

The question now arises, What is the effect of the presence of other metals in steel, of which much has been heard recently? (1) Manganese. Osmond has shown that this metal enables steel to harden very energetically, as is well known. If much of it be present, 12 to 20 per cent., in iron, no break whatever is observed in the curve which represents slow cooling (see line marked “manganese steel" 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 temperature of recalescence is that at which iron ceases to be magnetic. It may be urged that iron cannot therefore be magnetized. Steel containing much manganese cannot be magnetized, and it is therefore fair to assume that the iron present is in the B form Hadfield has given metallurgists wonderful alloys of iron and manganese in proportions varying from 7 to 20 per cent. of manganese. This core of iron round which a current is passing, attracts the sphere of iron, but if nothing is changed. except by replacing the core of iron with a core of Hadfield's steel, it is impossible to make a magnet of it. [Experiment shown.]

66

Prof. Ewing, who has specially worked on this subject. concludes that, no magnetizing force to which the metal is likely to be subjected in any of its practical

applications would produce more than the most infinitesimal degree of magnetization" in this material.

It has been seen that quantities of manganese above 7 per cent. appear to prevent the passage of 5 iron into the a form. In smaller quantities manganese seems merely to retard the conversion, and to bring the two loops of the diagram nearer together.

Time will not permit me to deal with the effect of other elements on steel, 1 will only add that tungsten possesses the same property as manganese, but in a more marked degree. Chromium has exactly the reverse effect, as it enables the change of hard iron to a soft iron to take place at a higher temperature than would otherwise be the case, and this may explain the extreme hardness of chromium steels when hardened in the same way as ordinary steels.

There are a few considerations relative to the actual working of steel with which I can deal but briefly, notwithstanding their industrial importance. The points and b, adopted in the celebrated memoir of Chernoff to which

Trans American Society Mechanical Engineers, ix., 1888, p. 135-
Proc. Roy. Soc., xlv., 1889, pp 18, 445, and 457.

3 Proc. Inst. Civil Engineers, xcii. F'art iii., 1888.

I have referred already, change in position with the degree of carburization of the metal. It is useless to attempt to harden steel by rapid cooling if it has fallen in temperature below the point (in the red) a, and this is the point of "recalescence" at which the carbon combines with the iron to form carbide-carbon: it is called V by Brinell. In highly carburized steel, it corresponds exactly with the point at which Osmond considers that iron, in cooling slowly, passes from the B to the a modification. Now with regard to the point b of Chernoff. If steel be heated to a temperature above a, but below b, it remains fine-grained however slowly it is cooled. If the steel be heated above b, and cooled, it assumes a crystalline granular structure whatever the rate of cooling may be. The size of the crystals, however, increases with the temperature to which the steel has been raised.

Now the crystalline structure, which is unfavourable to the steel from the point of view of its industrial use, may be broken up by the mechanical work of forging the hot

mass; and the investigations of Abel, of Maitland, and of Noble, have shown how important "work" on the metal is. When small masses of hot steel are quenched in oil, they are hardened just as they would be if water were used as a cooling fluid. With large masses, the effect of quenching in oil is different. Such cooling of large hot masses

1 This was well shown in Prof. Akerman's celebrated paper on Hardening Iron and Steel," Journ. Iron and Steel Institute. 1879. Part ii. p. 01.

appears to break up this crystalline structure in a manner analogous to mechanical working. If the mass of metal is very large, such as a propeller shaft, or tube of a large gun, the change in the relations between the carbon and the iron, or true "hardening" produced by such oil treatment is only effected superficially-that is, the hardened layer does not penetrate to any considerable depth, but the innermost parts are cooled more quickly than they otherwise would have been, and the development of the crystals, which would have assumed serious proportions during slow cooling, is arrested. It depends on the size of the quenched mass, whether the tenacity of the metal is or is not increased, but its power of being elongated is considerably augmented. This prevention of crystallization I believe to be the great merit of oil quenching, which, as regards large masses of metal, is certainly not a true hardening process.

There has been much divergence of view as to the relative advantages of work on the metal, and of oilhardening, 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 contains much of its carbon as hardening carbon, and the mass which has necessarily been poured into the mould at a high temperature is crystalline. The effect of annealing 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." 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 that in steel "there is a limited interchange of atoms between molecules under stress, which must be a property common to solids, if, according to Maxwell's conception. solids are made up of configurations in all degrees of molecular stability."

Barus and Strouhal attach but little importance to the change in the relations between the carbon and the iron consider that in hardening steel the "strain once applied during the tempering and annealing of hard steel. They to steel is locked up in the metal in virtue of its

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

viscosity"; tempering is the release of this molecular strain by heat.

Highly carburized steels harden very energetically by very slight modifications in thermal treatment, and it will be evident that a very hard material is unsuitable for industrial use if the conditions of its employment are such as to render it desirable that the material should stretch. To turn to very "mild" steel which does not harden, it is certain that, although wrought iron passes almost insensibly into steel, there can be no question that not merely the structural but the molecular aggregation of even steel containing only per cent. of carbon is profoundly different from that of wrought iron. Formerly, as Sir F. Bramwell pointed out in a lecture delivered at the Royal Institution in 1877,"by the year 1830... from small beginnings in Staffordshire and at Birkenhead sprang a wonderful wrought-iron navy, but steel was a luxury it was made in small portions sold at high prices, as much as a shilling or eighteenpence a pound. It was employed for swords, cutlery, and tools, needles and other purposes where the quantity used was but trifling, and where the importance of the superior material was such as to justify the large expenditure incurred. It was felt in those days that steel was worth paying for because it was trusted; indeed its trustworthiness had passed into a proverb "" as true as steel."

The class of steel which was formerly employed, as I have just indicated, for weapons and tools belonged to the highly carburized, readily-hardening class. It was the "mild steel" containing but little carbon which was destined to replace wrought iron, and when attempts were made to effect the general substitution of steel for iron, fears as to its character and trustworthiness unfortunately soon arose, so that from about the year 1860 until 1877 steel was viewed with suspicion. We can now explain this. Doubts as to the fidelity of steel, even when it was obtained free from entangled cinder, arose from ignorance of the fact that, on either side of a comparatively narrow thermal boundary, the iron in steel can practically exist in two distinct modifications. The steel was true enough, but from the point of view of the special duties to be intrusted to it, its fidelity depended on which modification of iron had to be called to the front. Artificers attempted to forge steel after it had cooled down below the point a of Chernoff, at which recalescence occurs, and they often attempted to work highly carburized steel at temperatures which were not sufficiently low.

Steels may be classified from the point of view of their industrial use according to the amount of carbon they contain, and I have attempted to arrange in this trophy certain typical articles, grouped under certain definite percentages of carbon ranging from to 1 per cent. [This was a trophy 18 feet square, with various typical articles of steel arranged in order according to the amount of carbon they contained. I am greatly indebted to Mr. J. W. Spencer, of Newcastle, who kindly lent me the fine series of specimens of which the "trophy" is built up.] Each class merges into the other, but the members at either end of the series vary very greatly. It would be impossible to make a razor which would cut from boiler plate; and conversely, a boiler made of razor steel would possibly fracture at once if it were superheated and subjected to any sudden pressure of steam. Speaking generally, if the steel contains, in addition to carbon, per cent. of manganese, each class of steel, as at present arranged, would have to be shifted a class backwards towards the left of the trophy.

At the present day, instead of steel being manufactured and used in small quantities, about 4,000,000 tons are annually employed in this country. Let us see how it is used. A steel fleet, the finest fleet in the world, has recently assembled at Spithead. The material of which it was made contained to per cent. of carbon, and

when steel faces are used for the armour plates, the material contains to per cent. of carbon.

It has been pointed out that the crews of the feet a Spithead numbered no less than 21,107 men. This it ha been shown is "a remarkable figure, considering the great economy in men which prevails in a modern navy as com pared with the navy of Nelson's day. A hundred year ago the normal requirements of a fleet were one man a little over four tons, but now, thanks to the part played by steel and hydraulic power, we require but one man to every seventeen tons. Thus it may roughly be said that a aggregate of 20,000 men at the present day correspond to an aggregate of 80,000 men in the days of Nelson. The latest type of battle-ship weighs, fully equipped, abor 10,000 tons, there being about 3400 tons of steel in the hull, apart from her armour, which, with its backing, w weigh a further 2800 tons.1

From the use of steel in the Royal Navy and in the mercantile marine, let us pass on to its most notable us in construction. If the President of the French Republi was justified in appealing, in a recent speech, to the Eife Tower as " a monument of audacity and science," what are we to say of the Forth Bridge, the wonders of which will be described by Mr. Baker on Saturday? By his kindness I am able to place in the position in the trophy justified by the carbon it contains, a plate from the Fort! Bridge, which fell from a height of some 350 feet, and, being of excellent quality, doubled itself on the rockbelow. A single span of the Forth Bridge is nearly as long as two Eiffel Towers turned horizontally and tied together in the middle, and the whole forms a complicated steel structure weighing 15,000 tons, erected without the possibility of any intermediate support, the lace-like fabric of the bridge soaring as high as the top of St. Paul's The steel of which the compression members of the structure are composed contains per cent, of carbot and per cent. of manganese. The parts subjected to extension do not contain more than 6 per cent of carbon.

6.9

Time will not permit me to pass the members of each class in review. I can only refer to very few. Steel for the manufacture of pens contains about per cent. of carbon, and 16 to 18 tons of steel are every week le loose on an unoffending world in the shape of steel

pens.

Steel rails contain from to per cent. of carbon, and, in this class, slight variations in the amount of carbon are of vital importance. An eminent authority, Mr. Sandberg, tells us that in certain climates a variation of

per cent. in the amount of carbon may be very serious The great benefit which has accrued to the country from the substitution of more durable steel rails for the old wrought-iron ones may be gathered from the figures which Mr. Webb, of Crewe, has given me, which show that "the quantity of steel removed from the rails throughout the London and North-Western system by wear and oxidation is about 15 cwt. an hour, or 18 tons a day.

Gun-steel contains toper cent. of carbon, and it may contain per cent. of manganese. It is in relation to gun-steel that oil-hardening becomes very important The oil-tank of the St. Chamond Works (on the Loire is 72 feet deep, and contains 44,000 gallons of oil, which is kept in circulation by rotary pumps, to prevent the o being unduly heated locally when the heated mass of steel is plunged into it.

Now with regard to projectiles. To quote some recent remarks of Lord Armstrong," "the heaviest shot used in the Victory was 68 pounds, while in the Victoria it wil be 1800 pounds; and, while the broadside-fire from the Address by Mr. Baker, Section G, British Association Report, 1

Victory consumed only 325 pounds of powder, that from the Victoria will consume 3000 pounds. The most formidable projectiles belong to the highly carburized class of steel. Shells contain o'8 to 0 94 per cent. of carbon, and, in addition, some of these have o94 to 2 per cent. of chromium. The firm of Holtzer shows, in the Paris Exhibition, a shell which pierced a steel plate 10 inches thick, and was found, nearly 8co yards from the plate, entire and without flaw, its point alone being slightly distorted. Compound armour-plate with steel face, which face contains o'8 per cent. of carbon, is, however, more difficult to pierce than a simple plate of steel.

[A prominent feature in the "trophy," among the class of highly carburized steels which contain over per cent. of carbon, was a fine suspended wire 180 of an inch diameter, of remarkable strength, supporting a weight of 24 cwt., or a load of nearly 160 tons to the square inch. The strength of the same steel undrawn, would not exceed 50 tons to the square inch. A similar wire manufactured by the steel company of Firminy attracted much attention in the Paris Exhibition by supporting a shell weighing 18co lbs., or a load of 158 tons per square inch.]

Lastly, I will refer to the highly carburized steel used for the manufacture of dies. Such a steel should contain 08 to 1 per cent. of carbon, and no manganese. It is usual to water-harden and temper them to a straw colour, and a really good die will strike 40,000 coins of average dimensions without being fractured or deformed; but I am safe in saying that if the steel contained per cent. too much carbon, it would not strike 100 pieces without cracking, and if it contained per cent. too little carbon, it would probably be hopelessly distorted, and its engraved surface destroyed, in the attempt to strike a single coin.

The above examples will be sufficient to show how diverse are the properties which carbon confers upon iron, but as Faraday said, in 1822, "It is not improbable that there may be other bodies besides charcoal capable of giving to iron the properties of steel." The strange thing is that we do not know with any certainty whether, in the absence of carbon, other elements do play the part of that metalloid, in enabling iron to be hardened by rapid cooling. Take the case of chromium, for instance: chromium-carbon steels can, as is well known, be energetically hardened, but Busek1 has recently asserted that the addition of chromium to iron in the absence of carbon does not enable the iron to be hardened by rapid cooling. So far as I can see, it is only by employing the clectrical method of Pepys that a decision can be arrived at as to the hardening properties of elements other than carbon.

A few words must be devoted to the consideration of the colours which, as I said (see ante, p. 11), direct the artist in tempering or reducing the hardness of steel to any determinate standard. The technical treatises usually givenot always accurately, as Reiser 2 has shown-a scale of temperature ranging from 220° to 330°, at which various tints appear, passing from very pale yellow to brown yellow, purples, and blues, to blue tinged with green, and finally to grey. Barus and Strouhal point out that it is possible that the colour of the oxide film may afford an indication of the temper of steel of far greater critical sensitiveness than has hitherto been supposed. It is, however, at present uncertain how far time, temperature, and colour are correlated, but the question is being investigated by Mr. Turner, formerly one of my own students at the School of Mines.

That the colours produced are really due to oxidation was shown by Sir Humphry Davy in 1813, but the nature

of the film has been the subject of much controversy. Barus points out that "the oxygen molecule does not penetrate deeper than a few thousand times its own dimensions,' and that it probably passes through the film by a process allied to liquid diffusion. The permeable depth increases rapidly with the temperature, until at an incipient red heat the film is sufficiently thick to be brittle and liable to rupture, whereupon the present phenomenon ceases, or is repeated in irregular succession.

Looking back over all the facts we have dealt with, it will be evident that two sets of considerations are of special importance: (1) those which belong to the relations of carbon and iron, and (2) those which contemplate molecular change in the iron itself. The first of these has been deliberately subordinated to the second, although it would have been possible to have written much in support of the view that carburized iron is an alloy of carbon and iron, and to have traced with Guthrie the analogies which alloys, in cooling, present to cooling masses of igneous rocks, such as granite, which, as the temperature of the mass falls, throws off "atomically definite" bodies, leaving behind a fluid mass of indefi nite composition, from which the quartz and feldspar solidify before the mica. This view has been developed with much ability in relation to carburized iron by Prof. Howe, of Boston, who even suggests mineralogical names, such as "cementite," "perlite," and "ferrite," for the various associations of carbon and iron.

I am far from wishing to ignore the interest presented by such analogies, but I believe that the possibility of molecular change in the iron itself, which results in its passage into a distinctive form of iron, is at present the more important subject for consideration, not merely in relation to iron, but as regards the wider question of allotropy in metals generally.

Many facts noted in spectroscopic work will have, as Lockyer has shown, indicated the high probability that the molecular structure of a metal like iron is gradually simplified as higher temperatures are employed. These various simplifications may be regarded as allotropic modifications.

The question of molecular change in solid metals urgently demands continued and rigorous investigation. Every chemist knows how much his science has gained, and what important discoveries have been made in it, by the recognition of the fact that the elements act on each other in accordance with the great law of Mendeleeff which states that the properties of the elements are periodic functions of their atomic weights. I firmly believe that it will be shown that the relation between small quantities of elements and the masses in which they are hidden is not at variance with the same law. I have elsewhere tried to show that this may be true, by examining the effect of small quantities of impurity on the tenacity of gold.

3

In the case of iron, it is difficult to say what property of the metal will be most affected by the added matter. Possibly the direct connection with the periodic law will be traced by the effect of a given element in retarding or promoting the passage of ordinary iron to an allotropic state; but "the future of steel" will depend on the care with which we investigate the nature of the influence exerted by various elements on iron, and on the thermal treatment to which it may most suitably be subjected.

Is it not strange that so many researches should have been devoted to the relations between carbon, hydrogen, and oxygen in organic compounds, so few to the relations of iron and carbon, and hardly any to iron in association with other elements? I think that the reason for the comparative neglect of metals as subjects of research arises Bull. U.S. Geo. Survey, No. 35. 1886, p. 51. 2 Phil. Mag., June 1884, p. 462.

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