Lapas attēli

rail (broken to show the structure of the bar) of this mixed iron exhibits the fibrous or toughened tops, in cohesion with a crystalline centre. Mr. Morris Sterling, the patentee, exhibits also other metallic alloys, to show the changes which the molecular composition is capable of undergoing. Mr. Sterling considers the fluidity of Berlin iron to be due to arsenic; that phosphorus will produce the same result when mixed with any iron; that the presence of manganese with cast iron closes the grain, and is an improvement both to it and to steel; and that zinc and tin, mixed with iron, are capable of greatly changing and improving its qualities. By the addition of calamine to common iron, a very superior malleable iron is produced. On the character of these, and other alloys, Mr. Sterling writes as follows, viz:

“ The wrought iron, made either from the toughened cast or by the admixture of calamine, is partirularly useful for tension rods, chain cables, &c. The addition of antimony, and some other metals, to wrought iron in the puddling furnace, gives a hard and crystalline iron, nearly allied to steel in some of its properties, and is adapted, from its hardness and crystalline character, to form the upper part of railway rails, and the outer surface of wheels. When thus united to the iron containing zinc, the best sort of rail results, combining strength, stiffness, and hardness, with anti-laminating properties, and being also cheaper than any other kind of hardened rail or tire.

Compounds of copper, iron, and zinc are found to be much closer in texture, and stronger, than similar compounds of copper and zinc, (the proportion of iron not usually exceeding one and a half per cent.,) and can be advantageously used as substitutes for gun-metal, under all circumstances—for great guns, screws, propellers, mill brasses, and railway bearings. Small additions of tin, and other metals, alter the character of these compounds, and render them extremely manageable as regards hardness and stiffness. The advantages which these compounds possess over gun metal are cheapness and increased strength, being about onefourth cheaper, and one half stronger, and wearing much longer under friction. On many railways the alloys of zinc, copper, tiu, &c., have su. perseded gnn-metal for carriage bearings. An alloy equal in tone to bell metal-cheaper, and at the same time stronger-is made from the alloy of copper, zinc, and iron, a certain proportion of tin being added. The addition of iron seems, under most, if not all circumstances, to alter the texture of metallic alloys, rendering it closer, and the alloys, therefore, more susceptible of a high polish, and less liable to corrosion. Other alloys of iron were exhibited—some showing the extreme closeness of texture; others possessing very great hardness, and suitable for tools, cutling instruments, &c.; others possessing a high degree of sonorousness.

The firmness of grain, and compactness of structure, which characterize various samples of iron contained in the Swedish, Spanish, and Aus. trian department, very strongly resemble the copake iron, made by the Messrs. Pomeroy, of Pittsfield, Massachusetts, which is used by the United States government at the armories. Both, or all these, owe their evenuess of texture not to accident or design; for it is the invariable attribute of charcoal iron. It is wanting, however, in all iron made by coke or coal, which just as invariably possesses a rough, harkley grain, and a crystalline structure. In regard to British iron, it has been suggested that this arises from a minute quantity of impurity in the ore.


there is some truth in this cannot be doubted, since no process can ever render iron which is made from certain ores of a superior class. Yet too much weight must not be given to this as the cause of difference in iron, since chemical analysis barely permits, but does not strengthen, the suggestion. Are we not, then, justified in looking for other explanations, especially when the wondrous changes induced by an altered molecular arrangement of particles are duly considered? Some time ago, a patent was secured in England by a Mr. Heath, for the introduction of a small portion of carburet of manganese into the melting pot with cast-steel; and the result of this is, that steel so melted in contact with manganese will weld either to itself or common iron. Yet the most careful chemical investigations have failed to prove the ex. istence of manganese in steel melted after Mr. Heath's method. Again, pure copper from the refinery is highly crystalline, and incapable of being rolled or hammered into plates, unless it has undergone the mysterious process called "polling;” after which, its crystalline character vanishes, and it may be beaten into thin plates or leaves. Now, in all probability, the conditions which lead to the crystallization of copper also tend to produce those of iron; and hence, instead of sitting with folded hands, under a belief that the bricileness of coal-made iron is irremediable, practical men, in Pennsylvania and other coal-iron producing States, should be on the alert to discover a mode which, like the polling of copper, may answer the end, though incapable of scientific explanation. It is said that splendid fibrous iron is occasionally produced in the forges of England as a work of accident; but in nature there is no such thing as chance.

Few inventions of modern date would so largely repay a discoverer as this; and there is no doubt that the cure for coal-made iron, when found, will prove an extremely simple and easy affair.

Another subject, closely allied to this, deserves notice, to which we have briefly alluded before: Does the substance of iron which has been for a long time exposed to percussions and vibrations undergo any change in the arrangement of its particles, by which it becomes weakened ? A great difference of opinion exists among practical nien with respect to this question. Many curious facts have been elicited, which show that pieces of wronght iron that have been exposed to vibration--such as the axles of railway carriages, the chains of cranes, &c., employed in raising heavy weights—frequently break after long use, and exhibit a peculiar crystalline fracture, and loss of tenacity, which is considered by some engineers to be the result of a gradual change, produced in the internal structure of the metal by the vibrations. In confirmation of this, various facts have been adduced—as, for instance, if a good piece of fibrous iron have the thread of a screw cut upon one end of it by the usual process of tapping, which is always accompanied by much vibratory action, and if the bar be then broken across, it will be found that the tapped part is a good deal more crystalline than the other portion of the bar. Others contend that this peculiar structure is the result of an original fault in the process of manufacture, and deny this effect of vibration altogether; whilst some allege that the crystalline structure can be imparted to fibrous iron in various ways—as by repeatedly heating a bar, red hot, and plun. ging it into cold water, or by coutinually hammering it, when cold, for half an hour or more.

Mr. Brunell, however, thinks the various appearances of the fracture depend much upon the mode in which the iron is broken. The same piece of iron may be made to exhibit a fibrous fracture when broken by a slow, heavy blow, and a crystalline fracture when broken by a sharp, short blow. Temperature alone has also a decided effect upon the fracture: iron broken in a cold state shows a more crystalline fracture than the same iron warmed a litile.

The commissioners appointed to inquire into the application of iron to railway structures examined this question experimentally, in a variety of ways. A bar of cast iron, eight inches square, was placed on supports, about fourteen feet asunder. A heavy ball was suspended by a wire eighteen feet long from the roof, so as to touch the centre of the side of the bar. By drawing this ball out of the vertical position at right angles to the length of the bar, in the manner of a pendulum, to any required distance, and suddenly releasing it, it could be made to strike a horizontal blow upon the bar; the magnitude of which could be regulated at pleasure, either by varying the size of the ball or the distance from which it was released.

Various bars (some of smaller size than the above) were subjected to successions of blows, numbering, in most cases, as many as 4,100; the magnitude of the blow, in each set of experiments, being made greater or smaller, as occasion required. The general result obtained was, that when the blow was powerful enough to bend the bars through one-half of their ultimate deflection, (that is to say, the deflection which corresponds to their fracture by dead pressure,) no bar was able to stand 4,000 of such blows in succession; but all the bars (when sound) resisted the effects of 4,000 blows, each bending them through one third of their ultimate deflection.

Other cast-iron bars, of similar dimensions, were subjected to the action of a revolving cane, driven by a steam engine. By this they were quietly depressed in the centre, and allowed to restore themselves; the process being continued to the extent of a hundred thousand successive periodic depressions for each bar, and at a rate of about four per minute. Another contrivance was tried, by which the whole bar was also, during the depression, thrown into a violent tremor.

The results of these experiments were, that when the depression was equal to one third of the ultimate deflection, the bars were not weakened. This was ascertained by breaking them in the usual manner by sta. tionary loads in the centre. When, however, the depressions produced by the machine were made equal to one-half of the ultimate deflection, the bars were actually broken by less than nine hundred depressions. The result corresponds with and confirms the former.

By other machinery a weight, equal to one-half of the breaking weight, was slowly and continually dragged backwards and forwards from one end to the other of a bar of similar dimensions to the above. * A sound bar was not apparently weakened by ninety six thousand transits of the weight.

It may, on the whole, therefore, be said that, as far as the effects of reiterated flexure are concerned, cast-iron beams should be so proportioned as scarcely to suffer a deflection.

In wrought-iron bars no very perceptible effect was produced by 10,100 successive deflections by means of a revolving cane; each deflection being due to half the weight which, when applied statically, produced a large permanent flexure.

Precious Stones.-Among the minerals employed for personal decora. tion the diamond occupies the first position, both on account of the beauty of the gem itself and from its commercial value. The diamond, like charcoal, is composed of carbon, and, in a chemical point of view, differs from it only in being perfectly free from all traces of the earthy and other impurities with which the other substance, even when most carefully prepared, is to a considerable extent contaminated. This mineral, although principally used in ornamental jewelry, is likewise applicable to many other purposes. In consequence of its extreme hardness, it is now extensively employed for making the pivot holes of the better description of watches; it has also been used in the formation of holes through which very fine metallic wires are made to pass, besides furnishing the only convenient tool which can be employed for cutting glass.

The countries in which this gem has yet been discovered are far from numerous, the only localities in which it has been found being the Indian peninsula, Brazil, the Island of Borneo, and Siberia, on the western side of the Ural mountains. Its geological position appears to be among diluvial gravel and conglomerate rocks, or pudding stone, con. sisting chiefly of rolled flint pebbles and ferruginous sand.

India has, from the most remote ages, been celebrated for the beauty and magnitude of its diamonds, the largest and most valuable of which are obtained from the mines in the provinces of Golconda and Visapoor. The tract of country producing these gems extends from Cape Comorin to Bengal, and lies at the foot of a chain of mountains called the Orixa, which appear to belong to the Trap Rock formation. The diamonds obtained from even the richest localities are rarely procured by directly searching the strata in which they are found, since they are commonly so coated with an earthy crust on the outside as not to be readily distinguishable from the various other substances with which they are associated. For this reason the stony matter is first broken into fragments, and then washed in basins, for the purpose of separating the loose earth; after which the residual gravel is spread out on a level piece of ground, where it is allowed to dry, and where the diamonds are recognised by their sparkling in the sun—thus enabling the miners readily to discrimi. nate between them and the stony matter with which they are associated.

The chief diamond mines of Brazil were discovered in the year 1728. The ground in which they are embedded exactly resembles that of the diamond districts of India, and, besides containing fragments of colored quartz and ferruginous sand, it produces small quantities of gold in connexion with oligist iron ore. This conglomerate or pudding stone, which is seldom of any great thickness, occurs at considerable heights in the mountainous table lands, and is entirely different from all the other mineral productions which are to be found in the vicinity.

The principal mine of this part of the world is that of Mandagra, north of the Rio Janeiro, where the gems are obtained from the sand taken from the bed of the stream, after laying it nearly dry by drawing off the water, during the dry seasons, into large reservoirs prepared for that purpose. The “cascalho," or diamond gravel, which is then removed, is then afterwards formed into little heaps, or mounds, of 15 or

most common.

16 tons each, where it remains until the commencement of the rainy season, when it is carefully washed in large square boxes arranged under large oblong wooden sheds. A negro washer works at each of these boxes, and numerous inspectors are placed at regular distances among the workmen to prevent any abstraction of the diamonds by those who may chance to find them. When a negro finds a diamond he immediately shows it to the inspector, and if its weight amounts to 171 carats, or 70 grains, he receives his liberty.

The diamond is found crystallized in the octahedron form, or in some other immediately derived from it. Its specific gravity varies from 3.4 to 3.6. It is not acted upon by any solvent; but when strongly heated in air, or in oxygen gas, is consumed with the formation of carbonic acid.

The fracture of this mineral is foliated, its laminæ being parallel to the faces of the regular octahedron. When broken, it divides in the direction of these lines; and this property of the gem is extensively taken advantage of by the lapidary when reducing it to the forms best adapted to ornamental purposes.

Diamonds are usually colorless and transparent, but, when colored, are usually of a yellowish tint. Green diamonds are, next to yellow, the

Blue specimens are also occasionally found, and, although they seldom possess much lustre, are in many countries highly valued.

Of all the colored varieties the rose or pink diamond are, however, by far the most esteemed, and sometimes even exceed in value those which are perfectly colorless, although, in general, the most limpid gems will be found to bear the highest price.

The art of cutting and polishing the diamond, although probably known in Asia in remote antiquity, was first introduced into Europe by Louis Bergher, of Bruges, in the year 1456. The object is effected in two different ways, either by taking advantage of the natural laminæ of the gem, and splitting it in directions parallel to the faces of the octahedron, or by sawing it with a very delicate wire covered with diainond powder. By these processes, and more especially by the former, the diamond is so cut away that the weight of the finished gem is rarely more than one-half that of the rough stone from which it is cut; and consequently the weight of the brilliant cut diamond is considered equal in value to that of a similar rough one of twice its weight, exclusive of the cost of labor expended in the workmanship. The weight and value of diamonds are estimated in carats, of which 150 are equal to one ounce troy, or 480 grains.

The difference between the exchangeable value of two diamonds of equal merit is generally estimated in the squares of their weights, so that the value of three diamonds, weighing, respectively, one, two,

and three carats, will be as one, four, and nine.

The average price of rough diamonds is estimated at L2 per carat; and consequently, when cut, the cost of the first carat, exclusive of workmanship, will be £8, which is the price of an uncut diamond of

two carats.

The rapidly increasing value of diamonds in proportion to their weight in carats will be readily seen by a glance at the following tabular state


« iepriekšējāTurpināt »