CHAPTER VIII.

OF THE MAGNETICAL POWERS, RECEPTIVE AND PERMANENT, OF CAST IRON, BOTH IN SEPARATE BARS OR PLATES,

AND IN VARIOUS COMBINATIONS OF PLATES.

THIS portion of my Magnetical Investigations is here introduced, in order to place the magnetical powers of Cast Iron-a substance of a somewhat intermediate nature betwixt iron and steel-in a distinct position of comparison with reference to those of steel.

So long ago as the year 1832, I commenced. a series of experiments on the magnetical recipient properties of Cast Iron of various qualities, with specimens furnished me by Edward Roscoe, Esq., of Liverpool. These, however, owing to the thickness of the masses employed (being an inch square) were not fitted for decisive results, in regard to the capabilities of cast iron for the magnetic condition. With the view, therefore, of a satisfactory determination of the comparative capacities of cast iron and steel, I recently added another class of experiments with plates of cast

iron constructed of somewhat corresponding dimensions with those of the 7.5 inch steel plates chiefly used in the Investigations described in chapter IV.

As the investigation was one of considerable interest, with reference to the applicability of a material so unexpensive, for the construction of permanent magnets,-I obtained specimens of cast iron in considerable variety, and subjected each kind to the usual tests for the determination of its magnetical properties.

These specimens, with the exception of the series of "Run-steel," from Mr. Lucas's works near Sheffield, were obtained at the Bowling or at the Low-Moor Iron Works, in the parish of Bradford, under the kind assistance of the managers of the respective companies.

It may be convenient, in giving the results of this inquiry, first to describe the several kinds of cast iron employed in the experiments, and then to furnish, in consecutive tables, their respective powers.

1. The first set I have to describe, consisted of six plates of Bowling iron, cast out of grey, or No. 1, pig-iron,--usually considered of the best quality, and of a good description of that kind of metal. It is much less hard, being easily scratched with a file, melts easier, and runs more fluid, than the other kinds of pig

iron. It has a comparatively dark-coloured, coarse-grained fracture. It is smelted from the best ores of the district, which ores are combined with a larger proportion of coke and lime, than in the manufacture of the pig-metal No. 2, or No. 3. The proportions of the materials combined in the furnace of course vary with the relative qualities of the lime-stone and of the iron-stone; but the proportions used in the Bowling Iron Works, in the manufacture of No. 1 pig-iron (taking the average of the seasons of summer and winter), may be considered to be about 7 cwt, of coke and 3 cwt. of lime-stone, to 6 or 7 cwt, of ore. Such is the effect of temperature or of humidity on the process of smelting, however, that the proportions of lime and coke employed in the production of No. 3 iron, in the summer, or in damp mild weather, will generally be adequate for the production of the highest quality, No. 1, in winter.

The six plates of No. 1, cast iron, employed in these experiments, were 7.5 inches long, 0·75 inch broad, 0133 inch thick, and weighed, on an average, 1230 grains.

2. Six plates of white, or No. 3, pig-iron. This is a very hard description of metal, brittle, and of a whitish silvery-grey fracture. None of the plates of this set were scratched with the file. The proportions of lime and coke usually

employed at the Bowling Iron Works, in the production of this quality of metal, taking the average of summer and winter, are 7 cwt. of coke, and rather less than half the quantity of lime-stone, to 7 or 8 cwt. of iron-stone.

The difference of price in the market betwixt cast iron No. 1, and No. 3, is usually about ten shillings per ton.

The dimensions of the plates of this set were nearly the same as those of the preceding one, except as to thickness, these plates being 0.14 inch thick, and of the average weight of 1400 grains.

3. Eight plates of metal, No. 1, from the Low-Moor Works. These were rather shorter than the others, being 7.2 inches in length, and 1188 grains, average weight. These were cast hard, from the "Blow-hole," a furnace heated by air thrown into it by an engine. The iron, it is considered, would have been stronger if it had been melted in an air furnace.

The average quantity of materials used at the Low-Moor Iron Works, for making a ton of pig iron, of the average quality and at the mean of different seasons during last year, is stated to have been 87 cwt. of coal, 25 cwt. of lime-stone, and 76 cwt. of iron-stone. Taken at an average, also, as to season and weather, about 15 cwt. more coal is required to make No. 1, than No. 3, pig iron.

4. Eight plates of "gun-metal," from the Low-Moor Iron Works.-This metal consists of a mixture of different qualities or castings of pig iron, selected of a good description of each kind; the object of the mixture being to obtain an union of toughness and strength. The proportions of this mixture vary according to the size and class of guns-lower numbers of metal being required, for producing the requisite hardness, in very heavy guns. The specimens furnished me were from metal prepared by an extra melting for guns for the navy.

These plates, like the former, were of the length of 7.2 inches; the average weight 1220 grains.

5. Six plates of Bowling iron from picked ore, constituting quality No. 1, pig iron, designed for ultimately being made into spring steel. This, could the result of the furnace be always calculated upon, should be a first-rate quality, being the most expensive in production of any iron made of Bowling ores.

The dimensions of the plates corresponded, except as to thickness, with the other sets of Bowling iron. Average weight 1117 grains.

6. Six plates of "finery iron," from the Bowling Works, of the average weight of 1418 grains. -This description of metal is not used for casting, in any manufacture at Bowling; the process of