CHAPTER XI
WHAT Sh.e.l.lS ARE MADE OF
The body of a sh.e.l.l is made of steel of a fairly strong variety. That is to say, it is stronger than that used for shipbuilding and for bridges and such work: but it is less so than some of the higher grades of steel, such as that used for making wire ropes. Owing to so much of this steel being rolled during the war, "sh.e.l.l quality" has come to be as well known to the general engineer as any of the many varieties which he has been accustomed to since his apprentice days. Many people wondered, at one time, why the cheaper and more easily worked cast iron could not be used for sh.e.l.ls. There was a period when the steel works were quite unable to cope with the demands for steel, yet the iron foundries were crying out for work. This question then arose in many minds, Why not make cast iron sh.e.l.ls? The answer is that cast iron is too weak: it would blow into fragments too soon.
Just think what a sh.e.l.l is and what it has to do. It is a metal case filled with explosive. It is thrown from a gun and is intended to blow itself to pieces on arrival at its destination. It is that self-destruction which carries destruction to all around as well. It is necessary, in order to obtain the best result, that an appreciable time should elapse between the ignition of the explosive and the bursting of the case. The force of the most sudden explosion is not really developed at once, but takes an appreciable time. After ignition, therefore, as the explosion gradually becomes complete, the pressure inside the sh.e.l.l is growing, and too weak a sh.e.l.l would go to pieces before the maximum pressure had been attained. Thus much of the energy of the explosion would simply be liberated into the air instead of being employed in hurling the fragments of sh.e.l.l with enormous force.
That is, of course, not a complete explanation of the whole action of a high-explosive sh.e.l.l, but it indicates generally the reason why a special quality of steel is required in order to get the best results.
Steel having been dealt with in another chapter, we will pa.s.s to the other metals which play important if not essential parts in the production of modern projectiles. So important are several of these that the lack of one or two of them would, under modern conditions, mean certain defeat for a nation.
Let us first of all take copper, of which is made the driving bands of the sh.e.l.ls and which in combination with zinc forms bra.s.s of which noses and other important parts are made.
Its ore is found in many parts of the world, notably in the United States, Chile and Spain. The ores are of several kinds, the simpler ones to deal with being oxides and carbonates of copper, meaning compounds of copper with oxygen and with oxygen and carbon respectively.
It will be remembered that ores of iron are usually of the same nature, namely, oxides and carbonates, and consequently we find that the method of obtaining copper from these ores resembles the methods employed to obtain iron from its ores.
The ore is thrown into a large furnace, like the blast furnaces of the ironworks, and in the heat of the fire the bonds between copper and oxygen are loosened and the superior attractions of the carbon in the fuel entice the oxygen away, leaving the metal comparatively pure.
Unfortunately, however, copper is found most plentifully in combination with sulphur with which it forms what is termed sulphide. This copper sulphide is called by miners "copper pyrites." Another trouble is that mixed with the copper pyrites there is usually more or less of iron pyrites, or sulphate of iron, so that to obtain the copper not only has the sulphur to be got rid of but also the iron. This complicates the operations very much, the ore having to be subjected to repeated roastings and meltings during which the sulphur pa.s.ses off in the form of sulphur dioxide (a material from which sulphuric acid can be obtained), leaving oxygen in its place. Thus the copper sulphide becomes copper oxide, after which the oxygen is carried away by carbon, leaving the relatively pure metal. Moreover, at each operation various substances are thrown into the furnace called fluxes, which do not mingle with the metal but float on the top in the form of slag, and into the slag the iron pa.s.ses, so that finally the copper is obtained alone.
Zinc is another important material for sh.e.l.l-making. Its ores used to be found in great plenty in Silesia, but the chief source of supply is now Australia. It is what is called "zinc blende," and consists of zinc sulphide, or zinc and sulphur in combination. In all these names, it may be interesting to mention, at this point, the termination "ide"
indicates a compound of two substances, so that we can safely conclude that the "ides" consist of the two elements named in their t.i.tles and no others. Thus zinc sulphide is zinc and sulphur and nothing else, iron sulphide is iron and sulphur, copper oxide is copper and oxygen, and so on.
The blende is first roasted in huge furnaces specially built for the purpose. To ensure its being thoroughly treated it has to be "rabbled"
or turned over and over, since otherwise all of it might not be brought into contact with the necessary oxygen. At one time done by men with rakes, it is now generally accomplished by mechanical means.
A description of one such furnace will be of interest. It consists of a long rectangular building of brickwork bound together with steel framework. Inside it is divided up into low chambers, the roof of each forming the floor of the one above.
At intervals along its length mighty shafts of iron pa.s.s up from underneath right through all the floors, emerging finally above the topmost, while along underneath the furnace there runs a shaft the action of which turns the vertical shafts slowly round and round.
Attached to the vertical shafts are long strong arms of iron, one arm to each floor, and upon the arms are placed rabbles, as they are termed, pieces of iron shod sometimes with fireclay, resembling most of any familiar objects a small ploughshare.
As the arms slowly revolve, at the rate of once or twice per minute, the arms are carried round and round and the rabbles plough up and turn over and over the layer of ore lying upon the floor.
There are arms on the top of the furnace, too, sometimes, where the ore is first laid so that it may be dried by the heat escaping from the furnace beneath, an interesting example of economy effected by utilizing heat which would otherwise be wasted.
The whole of the furnace, from end to end and on every floor, is thus swept continually by the rotating arms with their dependent rabbles, and the latter are cunningly shaped so that they not only turn the ore over and over, but gradually pa.s.s it along the different floors or hearths.
It is fed automatically by a mechanical feeder which pushes it on, a small quant.i.ty at a time, to the drying hearth on the top. Then the rabbles take charge of it and gradually pa.s.s it from the area swept by one shaft to that of the next until it has pa.s.sed right along the top and has become thoroughly dried. Arrived there it falls through a hole on to the topmost hearth or floor, along which it travels by the same means but in the contrary direction until it again falls through a hole on to the top floor but one. And so it goes on until at last, fully roasted, it falls from the bottom floor of the furnace into trucks or other provision for carrying it away.
Some kinds of ore require to be heated by means of gas which is generated in a "gas-producer" near by. In others, however, the sulphur in the ore acts as the fuel, and so the furnace, having been once started, can be kept up for long periods without the expenditure of any coal at all. Very little attention is needed by furnaces such as these, so that with no fuel to pay for and very little labour, they are extremely economical.
Owing to the great heat, too, the arms would stand a very good chance of getting melted were they not kept cool by a continual stream of water flowing through the shafts and arms. This furnishes a continual supply of hot water which is sometimes used for other purposes in the works.
The process of roasting, whether carried on in furnaces such as these or not, results in the formation of oxide instead of sulphide; in other words, the sulphur is turned out and oxygen takes its place. The dislodged sulphur then joins up with some more oxygen and forms sulphur dioxide, which can be led away to the sulphuric acid plant and there, by union with water, turned into that extremely valuable substance, sulphuric acid.
We cannot, however, treat zinc oxide as we would iron oxide or copper oxide, for zinc is volatile, and so, instead of acc.u.mulating in the bottom of a blast furnace as the iron and copper do, would pa.s.s off up the chimney.
The oxide is therefore mixed with coal or some other form of carbon and placed in retorts made of fireclay. These retorts are fixed in rows one above the other like the retorts at a gasworks, and hot gases from a gas-producer down below pa.s.s around and among them. To the mouth of each retort is fitted a condenser, also made of fireclay.
Now what happens in the retorts is this: the heat loosens the bonds between the zinc and the oxide, the latter pa.s.sing into union with some carbon from the coal. The zinc at the same time becomes vapour and pa.s.ses into the condenser, the lower temperature of which turns it into a liquid which the workmen remove at intervals in ladles. On being poured into moulds and allowed to solidify this metal is called by the name of "spelter," which bears to zinc the same relation that pig-iron does to the more highly developed forms of iron. Spelter is simply zinc in its crudest form.
Tin, although less important in war than copper and zinc, plays a not unimportant part. It has been found for centuries in Cornwall. The Romans used to trade with the natives of Britain for tin. Although considerable quant.i.ties of it is still obtained from there, the greatest tin-producing country of all at present is the Federated Malay States.
Australia also furnishes ore, as does Bolivia and Nigeria.
In Cornwall the ore occurs as rock in veins or lodes filling up what must once have been fissures in granite rocks. That near the surface has long been taken, so that to-day the mines are very deep and costly to work. Some can only afford to operate when the market price of tin is above a certain limit. Much of the ore from the newer districts--the Malay States, for example--is in small fragments mixed with gravel in beds near the surface. Such is called alluvial or stream tin, since the deposits were undoubtedly put in their present position by streams or rivers. So long as they last these easily accessible alluvial deposits will always be cheaper to work than the deep mines. On the other hand, they may give out, and recent explorations underground seem to indicate that there is still much valuable ore not only of tin but of other metals too, to be obtained from the old mines of Cornwall.
The ore of tin, like so many other ores, is generally oxide. It is first roasted to expel sulphur and a.r.s.enic which are often present as impurities, and then it is melted in a reverberatory furnace such as that described for the manufacture of wrought iron. As usual, the oxygen combines with carbon, the impurities form slag which floats on the top, and the pure metal falls to the bottom of the furnace from whence it can be drawn off.
Mixed with or in the neighbourhood of tin ore there is sometimes found another mineral called wolfram, which plays an extremely important part in modern warfare, so much so that the British and other Governments engaged in the war were at times hard put to it to find enough. Its value resides in the fact that it contains tungsten, an element which has wonderful powers in hardening steel.
It consists of tungsten and oxygen, but is not an oxide since there is also iron in the partnership. This fact is very useful, however, since it enables the particles of wolfram to be picked out from the ma.s.s of other stuff among which they are found by a magnet.
There are some very wonderful machines called magnetic separators, made for this express purpose. In one, with which I am familiar, there is an endless band stretched horizontally upon two rollers. One of the rollers being driven round the belt travels along so that the mineral being fed on to it in a stream is carried along under several magnets. These magnets are very different from the ordinary magnet, inasmuch as they are revolving. We might almost describe them as small magnetized flywheels. As they spin round they pick up slightly the particles of ore which contain iron, but have no effect at all upon those which do not contain iron. They do not actually lift the particles up on to themselves: they just exercise a slight pull upon them, and by virtue of the fact that they are revolving, pull them off the band and throw them to one side. The wheels can be set closer or farther from the belt at will so as to make them act more or less strongly, and thus the most magnetic particles can be separated from those less magnetic, these latter being still kept separate from the wholly non-magnetic particles.
Thus by simple and purely mechanical means are the precious bits of wolfram obtained from the other less valuable or worthless minerals with which they are mixed.
The same method is used with other minerals besides wolfram: it can be applied to all those which exhibit in some small degree the magnetic properties which we usually a.s.sociate with iron.
This sorting out of one mineral from others continually crops up in connection with nearly all the metals except iron. Iron is practically the only one whose ore occurs in vast ma.s.ses which need simply to be dug up and thrown into the furnace. The others, where they occur as rock in veins, have to be crushed to detach what is wanted from what is not wanted, and then the two have to be sorted in some way. Magnetic separation is but one of these ways. Another takes advantage of the fact that we seldom find two things together which have precisely the same specific gravity. Consequently, if we throw the mixture on to a shaking table the heavier particles will behave differently from the lighter ones and the two will separate. The same result can be obtained by throwing the mixture into a stream of water, the water acting differently upon the lighter and upon the heavier particles. Another way which may be mentioned is founded upon the fact that some things can be readily wetted with oil while others throw the oil off and refuse to be wetted by it. If a mixture of these two sorts of thing be stirred violently in a suitable oily liquid the former will be found eventually in the froth, while the latter will sink to the bottom. All these different methods are employed, as they are found necessary in preparing the ores of the various metals to which we have been referring.
Except in the case of alluvial ores which have been broken already by the action of ancient streams of water, nearly all ores (except iron) have to be crushed before the ores can be separated out. Some of this work is done by the very simplest contrivances, showing how in some cases invention has almost come to a stop through the machines having been reduced to their simplest form. A notable instance of this is the stamp mill, in which heavy timbers are lifted up by machinery and then allowed to slide down upon the ore, just like gigantic pestles. More elaborate grinding machines are sometimes used, however, but it is impossible to mention them all here.
The action of sorting out the fragments of ore from the miscellaneous a.s.sortment of crushed rocks is termed "concentrating," and the sorted ores are called "concentrates."
Another metal which has proved itself of immense importance in war is aluminium, and it fittingly comes at the close of the list since it is dealt with in a manner peculiar to itself. Practically all the others are obtained from their ores by means of heat and heat alone. Aluminium is obtained by electricity acting in the process called electrolysis.
It is surprising to learn that aluminium is one of the very commonest things on the face of the earth. Clay and many common rocks are very largely made of it. Clay, to be precise, is a silicate of alumina, a term which is interesting when it is explained. Silica is the name given to oxide of silicon. Sand is mostly silica. Alumina, too, is oxide of aluminium. Silicate of alumina is a combination of the two.
Any clay, therefore, could be used as an ore from which to obtain aluminium, but of course there are certain minerals specially suitable for the purpose, since in them the metal is plentiful and easily extracted.
In another chapter reference is made to the production of caustic soda from a solution of common salt by electrolysis. The same principle, precisely, is used to obtain the metal aluminium from its ore, which is generally an oxide.
Common salt, let me remind you, is sodium and chlorine combined. When you dissolve it in water it becomes ionized, which means that each molecule of salt splits up into two ions one of which is electrically positive and the other electrically negative. Then, when we introduce two electrodes into the solution and connect them to a battery or dynamo, all the positive ions go to one electrode and all the negative ions to the other.
We cannot dissolve aluminium ore in water, but we can in a bath of molten cryolite, and for some reason or other, whether because of the heat or not we cannot say, the ore becomes ionized, the aluminium atoms being one sort and the oxygen atoms the other sort. These ions then sort themselves out, the oxygen ions being taken into combination with the carbon rod which forms the positive electrode, while the metal ions collect upon the negative electrode. Since this latter is a slab of carbon which forms the bottom of the vessel in which the process is carried on, the result is that pure aluminium gradually acc.u.mulates in the bottom of the vessel and can be drawn off from time to time.
Aluminium is always produced in places where electric power can be obtained cheaply, such as near waterfalls.
CHAPTER XII
MEASURING THE VELOCITY OF A Sh.e.l.l