with raw stone; to this circumstance we ascribe the fact of a given weight of fuel carbonating a greater weight of iron with calcined than with raw limestone. Yet with this important advantage calcined limestone is not extensively used as a flux, probably the readiness with which it absorbs moisture from atmospheric air is an objection. If care is not taken to have it promptly conveyed to the furnace, the water absorbed will scarcely be inferior in weight to that of the carbonic acid in the original stone.

The effects produced on the quality of the metal by change in the atmosphere, are important and constantly recurring. It is known that the metal produced in winter and spring is, with similar volumes of fuel, of a superior quality to that produced in summer and autumn; it is also known that the difference in quality is due to atmospherical changes; but the immediate cause is not so well understood.

We have already stated that the introduction of water into blast furnaces is attended with the most prejudicial effects. At certain seasons of the year the atmosphere contains a considerable quantity of water in the form of vapor, which is forced into the furnace through the tuyeres; at other periods it holds water in the form of rain, which, descending on the materials collected for smelting, saturates them, and is thus conveyed into the throat. of the furnace. With a wet atmosphere, then, water is discharged into the furnace at top with the solid materials, and at the bottom with the blast.

In the dryest seasons, the moisture thus entering the furnace weekly will weigh about 20 tons; but with a prevalence of low wet weather, it will exceed 100 tons, or nearly 20 cwts. of water to each ton of crude iron produced.

The deteriorating effects which such volumes of water have on the quality of the metal is very apparent. With a dry, easterly wind, the quality and make will be greatly superior to that obtained with the wind from any other quarter of the compass. A sudden shifting from east to south-west occasions a deterioration in quality from dark to bright gray, and, if the alteration continues, from gray to mottled and white.

An excess of blast results in a deterioration of quality. For the production of carbonated crude iron, the velocities of ascent of the gaseous, and descent of the solid materials are confined within certain limits. If these velocities are accelerated, by employing additional blast, without a corresponding augmentation of the volume of carbon, the resulting metal will be less carbonated.

CIRCUMSTANCES AFFECTING THE PRODUCE OF CRUDE IRON.

The produce of carbonated crude iron from blast furnaces of a given capacity, and blown with a fixed volume of blast, will

be affected chiefly by the richness of the ore, the quality and quantity of the fuel, and to a minor extent by the quality of the flux and blast. The smelting power of the furnace is limited to the production of a definite quantity of liquid material in a stated time. With the argillaceous ores, the production is at the rate of 28 cwts. per week, for each yard capacity of furnace. This rate of production cannot be exceeded without deteriorating the quality of the crude iron. But with a rich ore, the produce of metal may be augmented, the quantity of cinder being diminished in a similar ratio, so that the gross weight of iron and cinder will not be increased. If an ore yielding 36 per cent. of metal when calcined be employed, 3 tons will be required to produce one ton of metal, but if an ore containing 44 per cent. be employed, 48 cwts. will be sufficient.

The time required for deoxidation and carbonization is 40 hours; if the furnace smelts the lean ores at the rate of 28 cwts. of liquid matter per yard capacity, it must smelt this quantity with the richer ores. But the proportion of metal to cinder, which with the lean ores was as 7 to 18, is as 8 to 17 with the rich ore. The make of metal therefore is augmented in this

ratio.

There is however a limit to this increase of production by using rich ores. The materials must be in the furnace the stated time, and the gross weight of liquid products not exceed 28 cwts. If very rich ores are used, owing to the additional clay and lime necessary as flux, and the large consumption of fuel consequent on their addition, the proportion of metal to cinder is not sensibly increased, and the rate of production remains unaltered.

An exception must be made in the case of the carbonaceous ore so much used in Scotland. This ore yields after calcination from 44 to 64 per cent. of metal; the average is nearly 60 per The matter combined with the metal consists of carbon, oxygen, alumina, silica and lime, the last about 5 per cent. of the weight. In consequence of the presence of this lime, and the small percentage of other earths, the quantity required for flux is usually less than 6 cwts. The carbon is consumed before the blast, when the earthy matrix of the ore uniting with the lime, cinder is formed, but in comparatively small quantities. With argillaceous ores the weight of the cinder will at all times be nearly twice that of the metal, but from the peculiar manner in which the carbon is combined with the metal in the carbonaceous ore, carbonated crude iron is produced with not more than an equal weight of cinder; and instead of 28 cwts. of liquid matter per yard capacity of furnace, 34 cwts. is commonly obtained.

The produce is affected by the qualities of the fuel. A hard, dense, semi-bituminous coal containing a large percentage of carbon, and yielding little ashes, will reduce the largest quantity

of metal. Cokes prepared from coal too bituminous for use in the raw state, will reduce metal in proportion to their density and carbon. The smallest produce is obtained with a weak friable coal, or the coke prepared from such coal. Since the height of the blast furnace has been increased to 45 feet, a blast of 2 to 3 lbs. to the square inch has become general. But with the weakest cokes this pressure of blast is attended with an increased consumption of fuel, if the make and quality of the crude iron is to remain unimpaired.

With coals which expand considerably in coking the produce will be lower with the coke than with the raw coal. The coke consisting chiefly of carbon alloyed with the earthy matters of the coal is qualified for the immediate action of the blast, and undergoes little alteration in its descent to the hearth; but bulk for bulk the coke does not contain so much carbon as coal. Occupying then a greater space, the quantity of other materials in the furnace is diminished, the time of descent being the same, the quantity smelted, and the produce of metal is diminished in the same ratio as the quantity of materials.

The quality of the limestone used as a flux will influence the make of crude iron. If it contains a notable percentage of alumina and silica, its reducing power is diminished by nearly three times the weight of these earths. Their presence in quantity is equal to the disadvantage of a lean ore, fuel and lime being required according to the weight of silica and alumina entering the furnace. The additional materials occupy space in the furnace without contributing to the produce of metal; and for their admittance a proportionate quantity of ore must be withdrawn from the burden. The quantity of cinders in proportion to metal will be increased by the quantity of earths in the limestone, and the lime necessary for their fusion. Hence, the make will be diminished from the lower yield of the liquid products.

The smaller space occupied by caustic lime and its comparative lightness and freedom from moisture, favors the produce of the furnace. Where it is used, an increase of 4 to 5 per cent. is observed in the make. This is accounted for by the reduced consumption of fuel, and lesser space occupied by the caustic, enabling the furnace to carry a richer burden.

The produce is influenced by atmospherical changes in certain seasons of the year. It is well known to smelters, that, with materials of similar quality, and volume of blast unaltered, the make of a furnace is greater in the winter months than at any other season. The reason of this greater make in winter, was, until within the last few years, ascribed to the coldness of the air. On the application of the hot blast invention to the manufacture of iron, this superiority of cold winter over warm summer air was adduced as a reason against using heated air. The greater efficacy of the cold air was by some writers ascribed to

its greater density, which is in proportion to the decrease of temperature. Hence, a given volume of air contains more oxygen in winter than summer. The cause is now generally ascribed to the dryness of the atmosphere in winter as compared with its condition in summer.

Taking for our guidance the make of the 18 furnaces at the Dowlais works, for a period of seven years, we find that the actual increase of make in winter over that in summer, premising that the furnace is working on a similar burden, is between 4 and 5 per cent.; but with this increase there is a marked improvement in the quality of the crude iron obtained.

With the same burden of fuel and other materials the produce and quality will be directly dependent on the volume, and indirectly on the density of the blast. The produce will be in the same ratio as the volume of blast; but the quality, measured by the degree of carbonization, will be in an inverse ratio. The rate of production is more or less rapid according to the volume of blast brought in contact with the carbon; a large volume insuring a rapid combustion of the carbon and fusion of the ores, and vice versa. With an augmentation or reduction of the volume, the velocities of the solid and gaseous columns are augmented and reduced in like manner, and the quality impaired or improved with every variation in the velocity.

With the same volume of blast, the produce, within certain limits, is in an inverse ratio to the quantity of carbon to other materials in the descending column. For the fusion alone of the materials, the consumption of carbon ordinarily is under 20 cwts. to the ton of crude iron. Since the velocity of reduction is dependent on the rapidity of combustion with a minimum proportion of carbon, the maximum produce of inferior iron is attained. On the other hand, an unnecessarily large supply of carbon is followed by a minimum produce.

The increased produce of blast furnaces within the last 70 years is shown in the following statement of the average and highest weekly produce of crude iron from the Dowlais furnaces in the undermentioned years.*

*COMPARATIVE WEEKLY MAKE OF BRITISH AND FOREIGN FURNACES.-Mr.

ART. IV. ON THE MODE OF TESTING BUILDING MATERIALS, AND AN ACCOUNT OF THE MARBLE USED IN THE EXTENSION OF THE UNITED STATES CAPITOL. By PROFESSOR JOSEPH HENRY, Secretary of the Smithonian Institution.*

A COMMISSION was appointed by the President of the United States, in November, 1851, to examine the marbles which were offered for the extension of the United States Capitol, which consisted of General Totten, A. J. Downing, the Commissioner of Patents, the architect, and myself. Another commission was subsequently appointed, in the early part of the year 1854, to repeat and extend some of the experiments,-the members of which were General Totten, Professor Bache, and myself.

A part of the results of the first commission were given in a report to the Secretary of the Interior, and a detailed account of the whole of the investigations of these committees will ultimately be given in full in a report to Congress, and I propose here merely to present some of the facts of general interest, or which may be of importance to those engaged in similar researches.

Although the art of building has been practised from the earliest times, and constant demands have been made, in every age, for the means of determining the best materials, yet the process of ascertaining the strength and durability of stone appears to have received but little definite scientific attention, and the commission, who have never before made this subject a special object of study, have been surprised with unforeseen difficulties at

Blackwell in his Lecture on the iron-making resources of the United Kingdom, delivered before the Society of Arts in 1852, in speaking of the state of the manufacture in this country, has the following remarkable statement."We cannot boast of any exclusive skill in manufacture. In the United States there are now furnaces smelting iron with anthracite coal, and making a weekly produce double or treble that produced by the anthracite furnaces of South Wales."

We are at a loss to conceive Mr. Blackwell's reasons for thus boldly asserting that the British manufacturer was inferior in skill to the American. The average weekly produce of the South Wales anthracite furnace is between 80 and 90 tons; higher makes of 110 to 120 are quite common. From our extensive acquaintance with American works, we can state, that, the capacity of furnace and quality of ores being alike, the produce of the South Wales furnaces is in excess of the American.

We must direct attention to a circumstance of considerable importance in all estimates of produce and quantities. The estimates of the American makes are generally based on the legal ton of 2,240 lbs., but on other occasions their ton is so low as 2,000 lbs. Now the British manufacturer sells at the legal ton of 2,240 lbs.; but in all estimates of production, payments to workmen, &c., the ton of iron is 2,400 lbs.; while that of the coal and ore varies from 2,520 to 2,760 lbs.

From the Proceedings of the American Association for the advancement of Science, held at Providence, R. I., August, 1855. New York: 1856.