sists of two gases; about one-fifth being oxygen, and four-fifths azote. According to the present state of chemical knowledge, the oxygen alone is effective in producing heat; and the operation of blowing a furnace may be thus analyzed.
1. The air is forced into the furnace in a condensed state, and, immediately expanding, abstracts heat from the surrounding bodies.
2. Being itself of moderate temperature, it would, even without expansion, still require heat to raise it to the temperature of the hot substances to which it is to be applied.
3. On coming into contact with the ignited substances in the furnace, the oxygen unites with them, parting at the same moment with a large portion of its latent heat, and forming compounds which have less specific heat than their separate constituents. Some of these pass up the chimney in a gaseous state, whilst others remain in the form of melted slags, floating on the surface of the iron, which is fused by the heat thus set at liberty.
4. The effects of the azote are precisely similar to the first and second of those above described; it seems to form no combinations, and contributes nothing, in any stage, to augment the heat.
The plan, therefore, of heating the air before driving it into the furnace, saves obviously the whole of that heat which the fuel must have supplied in raising it from the temperature of the external air up to that of 600° Fahrenheit; thus rendering the fire more intense, and the glassy slags more fusible, and perhaps also more effectually decomposing the iron ore. The same quantity of fuel, applied at once to the furnace, would only prolong the duration of its heat, not augment its intensity.
The circumstance of so large a portion of the air driven into furnaces being not merely useless, but acting really as a cooling, instead of a heating, cause, added to so great a waste of mechanical power in condensing it, amounting, in fact, to four-fifths of the whole, clearly shows the defects of the present method, and the want of some better mode of exciting combustion on a large scale."
The reader will find it interesting to compare this account with the following by Dr. Ure.
Wherever a forced stream of air is employed for combustion, the resulting temperature must evidently be impaired by the coldness of the air injected upon the fuel. The heat developed in combustion is distributed into three portions; one is communicated to the remaining fuel, another is transferred to the azote of the atmosphere, and to the volatile products of combustion, and a third to the iron and fluxes, or other surrounding matter to be afterwards dissipated by wider diffusion. This inevitable dissipation takes place in such a way, that there is a nearly equal temperature over the whole extent of a fire-place, in which an equal degree of combustion exists.
We thus perceive that if the air and the coal be very cold, the portions of heat absorbed by them might be very considerable, and sufficient to prevent the resulting temperature from rising to a proper pitch; but if they were very hot they would absorb less caloric, and would leave more to elevate the common temperature. Let us suppose two furnaces charged with burning fuel, into one of which cold air is blown, and into the other hot air, in the same quantity. In the same time, nearly equal quantities of fuel will be consumed with nearly equal production of heat; but notwithstanding this, there will not be the same degree of heat in the two furnaces, for the one which receives the hot air will be hotter by all the excess of heat in its air above that of the other, since the former air adds to the heat while the latter abstracts from it. Nor are we to imagine that by injecting a little more cold air into the one furnace, we can raise its temperature to that of the other. With more air indeed we should burn more coals in the same time, and we should produce a greater quantity of heat, but this heat being diffused proportionally among more considerable masses of matter, would not produce a greater temperature; we should have a larger space heated, but not a greater intensity of heat in the same space.
Thus, according to the physical principles of the production and distribution of heat, fires fed with hot air should, with the same fuel, rise to a higher pitch of temperature than fires fed with common cold air. This consequence is independent of the masses, being as true for a small stove which burns only an ounce of charcoal in a minute, as for a furnace which burns a hundred weight; but the excess of temperature produced by hot air cannot be the same in small fires as in great; because the waste of heat is usually less the more fuel is burned.
This principle may be rendered still more evident by a numer-
| Cost price of a ton of pig-iron at the Clyde Iron-Works. | |--------------------------------------------------------| | Materials used. | In 1839, with cold air. | In 1833, with hot air. | |-----------------|------------------------|------------------------| | Coal for fusion, at 5s. per ton. | 6 13 | 1 13 3 | 2 0 | 0 10 0 | | for blowing machines, at 1s. 8d. per ton. | 2 0 | 0 3 6 | 0 11 | 0 0 11 | | for the heating apparatus. | ... | ... | 0 8 | 0 0 8 | | Calcined ore, at 12s. per ton. | 1 15 | 1 10 | 1 18 | 1 2 9 | | Limestone, at 7s. | 10 | 0 3 6 | ... | 0 3 6 | | Labour, at 10s. | ... | 0 10 0 | ... | 0 10 0 | | General charges, interests of capital, 6s. | ... | 0 6 0 | ... | 0 6 0 | | Total. | L.3 17 3 | L.2 13 10 |
1 The accurate proportions are, by measure, oxygen 21, azote 79. 2 A similar reasoning may be applied to lamps. An argand burner, whether used for consuming oil or gas, admits almost an unlimited quantity of air. It would deserve inquiry, whether a smaller quantity might not produce greater light; and, possibly, a different supply furnish more heat with the same expenditure of fuel. 3 This mineral is very rich; the average produce of the iron mines of the Glasgow coal basin is 44 per cent. after calcination; in this state it costs from 8s. 6d. to 9s. per ton. The cost of the mineral will, therefore, be very nearly as stated in the table. Since that period the cost has been rendered much lower, and those who have made good arrangements for mineral property, can manufacture iron at a cost of little more than L2 per ton.
At the request of the British Association, Dr. Thomson of Glasgow examined the chemical constitution of hot-blast iron, and he gives the following as the result of his inquiry:
"(1.) The specific gravity of hot-blast iron is greater than that of cold-blast.
"The following are the specific gravities of eight specimens of cold blast iron:
1st. Muirkirk..............6-410 2nd. Ditto..................6-435 3rd. Ditto..................6-493 4th. Ditto..................6-579 5th. Ditto..................6-775 6th. From pyrites..........6-9444 7th. From Carron..........6-9888 8th. Clyde Iron Works.....7-0028
"The specific gravity of the Muirkirk iron is considerably less than that smelted at Carron and the Clyde Iron-Works; the mean of the eight specimens is 6-7034.
"It has been hitherto supposed that the difference between cast-iron and malleable iron consists in the presence of carbon in the former, and its absence from the latter; in other words, that cast iron is a carburet of iron. But in all the specimens of cast iron which we analysed we constantly found several other ingredients besides iron and carbon. Manganese is pretty generally present in minute quantity, though in one specimen it amounted to no less a quantity than 7 per cent.; its average amount is 2 per cent. Silicon is never wanting, though its amount is exceedingly variable, the average quantity is about 11 per cent.; some specimens contained 3½ per cent. of it, while others contain less than a half per cent. Aluminum is very rarely altogether absent, though its amount is more variable than that of silicon. Its average amount is 2 per cent.; sometimes it exceeds 4½ per cent., and sometimes it is not quite ¼ part of the weight of the iron.
"Calcium and magnesium are sometimes present, but very rarely, and the quantity does not much exceed 1½ per cent. In a specimen of cast iron which I got from Mr. Neilson, and which he had smelted from pyrites, there was a trace of copper, showing that the pyrites employed was not quite free from copper; and in a specimen from the Clyde Iron-Works there was a trace of sulphur. The following table exhibits the composition of six different specimens of cast iron, No. 1, analyzed in my laboratory, either by myself or by Mr. John Tennent.
| Specimen | Iron | Copper | Manganese | Sulphur | Carbon | Silica | Aluminium | Calcium | Magnesium | |----------|------|--------|-----------|---------|--------|-------|----------|---------|-----------| | Muirkirk | 90-98 | 0-288 | 0-045 | 7-40 | 0-46 | 0-48 | 0-08 | 0-340 | | Muirkirk | 90-29 | 0-288 | 0-045 | 7-40 | 0-46 | 0-48 | 0-08 | 0-340 | | Muirkirk | 91-38 | 0-288 | 0-045 | 7-40 | 0-46 | 0-48 | 0-08 | 0-340 | | Pyrites | 89-442| 0-626 | 2-458 | 3-600 | 3-086 | 2-458 | 3-855 | | | Carron | 94-010| 0-626 | 2-458 | 3-600 | 3-086 | 2-458 | 3-855 | | | Carron | 90-824| 0-626 | 2-458 | 3-600 | 3-086 | 2-458 | 3-855 | |
"The constant constituents of cold-blast cast-iron, No. 1, are iron, manganese, carbon, silicon, and aluminium. The occasional constituents are copper, sulphur, calcium, and magnesium. These occur so rarely, and in such minute quantity, that we may overlook them altogether.
"The constant constituents occur in the following mean atomic proportions:
- 22 atoms iron, = 77-00 - ½ atom manganese, = 1-75 - 4-36 atoms carbon, = 3-27 - 1 atom silicon, = 1-00 - 1½ aluminium, = 1-40—84-42
"I examined only one specimen of cast-iron, No. 2. It was an old specimen, said to have come from Sweden, but I have no evidence of the correctness of this statement. Its specific gravity was 7-1633 higher than any specimen of cold-blast iron, No. 1. Its constituents were,
Iron, 93-594 Manganese, 0-708 Carbon, 3-080 Silicon, 1-262 Aluminium, 0-732 Sulphur, 0-038—99-414
"The presence of sulphur in this specimen leads to the suspicion that it is not a Swedish specimen; for as the Swedish ore is magnetic iron, and the fuel charcoal, the presence of sulphur in the iron is very unlikely.
"In this specimen, the atoms of iron and manganese are to those of carbon, silicon, and aluminium, in the proportion of 4½ to one, instead of 3½ to one, as in cast-iron No. 1.
"The atoms of carbon, silicon, and aluminium, approach the proportions of 7, 2, and 1, so that in cast-iron, No. 2, judging from one specimen, there is a greater proportion of carbon, compared with the silicon and aluminium, than in cast-iron, No. 1.
"Mr. Tennent analyzed a specimen of hot-blast-iron, No. 2, from Gartsherrie. Its specific gravity was 6-9156, and its constituents,
| Atoms | |-------| | Iron, 90-542 | | Manganese, 2-764 | | Carbon, 3-094 | | Silicon, 0-680 | | Aluminium, 2-894 | | Sulphur, 0-023 |
So that it resembles cast-iron, No. 1, in the proportion of its constituents. The carbon is almost the same as in cold-blast iron, No. 2, but the proportion of aluminium is four times as great, while the silicon is little more than half as much. The atomic ratios are, carbon, 4½; silicon, 0-97; aluminium, 2-28.
"3. Five specimens of hot-blast cast iron, No. 1, were analyzed. Two of these were from Carron, and three from the Clyde Iron-Works, where the hot-blast originally began; and where, of course, it has been longest in use. The specific gravity of these specimens was found to be as follows:
1st. From Clyde Works...........7-0028 2d. From Carron..................7-0721 3d. From Carron..................7-0721 4th. From Clyde Works...........7-1022
Mean................................7-0623
"It appears from this, that the hot-blast increases the specific gravity of cast-iron by about ½ per cent. It approaches nearer the specific gravity of cast iron No. 2, smelted by cold air, than to that of No. 1.
"The following table exhibits the constituents of these four specimens.
| Specimen | Clyde | Carron | Carron | Clyde | Clyde | |----------|-------|--------|--------|-------|-------| | Iron | 97-096| 95-422 | 96-09 | 94-966| 94-343| | Manganese| 0-332 | 0-336 | 0-41 | 0-160 | 0-120 | | Carbon | 2-460 | 2-400 | 2-48 | 1-560 | 1-416 | | Silicon | 0-280 | 1-820 | 1-49 | 1-322 | 0-920 | | Aluminium| 0-385 | 0-488 | 0-26 | 1-374 | 0-399 | | Magnesium| | | | 0-792 | |
The mean of these analyses gives us,
| Atoms | |-------| | Iron, 95-584 or 27-31 | | Manganese, 0-871 or 0-249 | | Carbon, 2-099 or 2-79 | | Silicon, 1-086 or 1-086 | | Aluminium, 0-422 or 0-337 |
Or, in the proportion of 6½ atoms of iron and manganese to 1 atom of carbon, silicon, and aluminium. In the cold-blast cast-iron we have,
In No. 1, 3½ atoms 1 atom In No. 2, 3½ atoms 1 atom In hot-blast, 6½ atoms 1 atom
Thus it appears, that when iron is smelted by the hot-blast its specific gravity is increased, and it contains a greater proportion of iron, and a smaller proportion of carbon, silicon, and aluminium, than when smelted by the cold-blast.
I have been told by Mr. Mushet that the Swedes add sulphur to their iron No. 2. At the request of the British Association, Mr. Eaton Hodgkinson examined the mechanical properties of hot and cold-blast iron. The following are his general results.
### Carron Iron, No. 2
| Property | Cold-Blast | Hot-Blast | Ratio representing Cold Blast by 1000 | |----------------------------------|------------|-----------|-------------------------------------| | Tensile strength in lbs. per sq. in. | 16683 (2) | 13505 (9) | 1000 : 809 | | Compressive ditto | 106875 (3) | 108540 (2)| 1000 : 1020 | | Do. from castings torn asunder | 100691 (4) | 100738 (2)| 1000 : 1001 | | Do. from prisms of various forms | 125403 (13)| 121685 (13)| 1000 : 970 | | Transverse strength from all the experiments | (11) | (13) | 1000 : 991 | | Power to resist impact | (9) | (9) | 1000 : 1005 | | Transverse strength of bars one inch square in lbs. | 476 (3) | 463 (3) | 1000 : 973 | | Ultimate deflection of do. in. | 1-313 (3) | 1-337 (3) | 1000 : 1018 | | Modulus of elasticity in lbs. per square inch | 17270500 (2) | 16085000 (2) | 1000 : 931 | | Specific gravity | 7066 | 7046 | 1000 : 997 |
### Devon Iron, No. 3
| Property | Cold-Blast | Hot-Blast | Ratio representing Cold Blast by 1000 | |----------------------------------|------------|-----------|-------------------------------------| | Tensile strength | 21907 (1) | | | | Compressive ditto | 145455 (4) | | | | Transverse ditto | (5) | (5) | 1000 : 1417 | | Power to resist impact | (4) | (4) | 1000 : 2786 | | Transverse strength of bars one inch square in lbs. | 448 (2) | 537 (2) | 1000 : 1199 | | Ultimate deflection of do. in. | .79 (2) | 1-09 (2) | 1000 : 1380 | | Modulus of elasticity ditto | 22967700 (2)| 22473650 (2)| 1000 : 981 | | Specific gravity | 7295 (4) | 7229 (2) | 1000 : 991 |
### Buffery Iron, No. 1
| Property | Cold-Blast | Hot-Blast | Ratio representing Cold Blast by 1000 | |----------------------------------|------------|-----------|-------------------------------------| | Tensile strength | 17466 (1) | 13434 (1) | 1000 : 769 | | Compressive ditto | 93366 (4) | 86397 (4) | 1000 : 925 | | Transverse ditto | (5) | (5) | 1000 : 931 | | Power to resist impact | (2) | (2) | 1000 : 963 | | Transverse strength of bars one inch square in lbs. | 463 (3) | 436 (3) | 1000 : 942 | | Ultimate deflection of do. in. | 1-55 (3) | 1-64 (3) | 1000 : 1058 | | Modulus of elasticity ditto | 15381200 (2)| 13730500 (2)| 1000 : 893 | | Specific gravity | 7079 | 6998 | 1000 : 999 |
### Coed-Talen Iron, No. 2
| Property | Cold-Blast | Hot-Blast | Ratio representing Cold Blast by 1000 | |----------------------------------|------------|-----------|-------------------------------------| | Tensile strength | 18855 (2) | 16676 (2) | 1000 : 884 | | Compressive ditto | 81770 (4) | 82739 (4) | 1000 : 1012 | | Specific gravity | 6955 (4) | 6968 (3) | 1000 : 1002 |
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Of the three columns of numbers in the table above, the first is the strength or other quality in the cold-blast iron; the second is that in the hot-blast; and the third is the ratio of these quantities.
The results in this table contain nearly the whole information relative to the question of hot and cold-blast-iron that the preceding research affords; and before adverting to them, it may be mentioned, that it is usual for the makers of cast-iron to divide it, when taken from the furnace, into three classes, called Nos. 1, 2, 3, differing from each other in the appearance and qualities of the material. No. 1 contains the softest and richest irons, those which have the largest crystals; No. 3, the hardest and densest irons, those with the least crystals; and No. 2, irons intermediate between the former two descriptions. Beginning with the No. 1 iron, of which we have a specimen from the Buffery Iron-Works, a few miles from Birmingham, we find the cold-blast iron somewhat surpassing the hot-blast in all the following particulars; direct tensile strength, compressive strength, transverse strength, power to resist impact, modulus of elasticity or stiffness, specific gravity; whilst the only numerical advantage possessed by the hot-blast iron is, that it bends a little more than the cold-blast before it breaks.
In the irons of the quality No. 2, the case seems in some degree different; in these the advantages of the rival kinds seem to be more nearly balanced. They are still, however, rather in favour of the cold blast.
Referring to the No. 2 iron, from the Carron Works in Scotland, we find the tensile, compressive, and transverse strengths, together with the modulus of elasticity and specific gravity, all higher in the cold-blast iron than the hot-blast, whilst the ultimate deflection and power of sustaining impact are greater in the hot-blast. The cold-blast iron is the better, but the difference is very small.
In the iron No. 2, from the Coed-Talen Works in North Wales, the tensile strength is greater in the cold-blast than in the hot; but the resistance to compression is higher in the latter than the former, and that is the case with the specific gravity.
So far as my experiments have proceeded, the irons of No. 1 have been deteriorated by the hot-blast; those of No. 2 appear also to have been slightly injured by it; while the irons of No. 3 seem to have benefited by its mollifying powers. The Carron iron No. 3, hot-blast, resists both tension and compression with considerably more energy than that made with the cold-blast; and the No. 3 hot-blast iron from the Devon Works, in Scotland, is one of the strongest cast-irons I have seen, whilst that made with the cold-blast is comparatively weak, though its specific gravity is very high, and higher than in the hot. The extreme hardness of the cold-blast Devon iron above prevented many experiments that would otherwise have been made upon it, no tools being hard enough to form the specimens. The difference of strength in the Devon irons is peculiarly striking.
From the evidence here brought forward, it is rendered exceedingly probable that the introduction of a heated blast into the manufacture of cast-iron has injured the softer irons, whilst it has frequently mollified and improved those of a harder nature; and considering the small deterioration that the irons of the quality No. 2 have sustained, and the apparent benefit to those of No. 3, together with the great saving effected by the heated blast, there seems good reason for the process becoming as general as it has done."