supports. Results reduced to those of bars 1.00 inch square, and 2 feet 3 inches between

Results reduced to those of bars 1.00 inch square, and 2 feet 3 inches between

In pursuing the experiments, it unfortunately occurred that the stock of No. 2 Coed-Talon metal became exhausted, a circumstance which interrupts the comparisons from below the freezing point to that of melted lead. The No. 3 should have been broken at all the points of temperature, in order to have ascertained the loss of strength sustained upon this iron by the increase of heat. This was, however, not accomplished, and we can now only compare the two qualities, No. 2 and No. 3, at the boiling point of water, and then proceed to the temperature of melted lead. I have already noticed that a considerable failure of the strength took place after heating the No. 2 iron, from 26° to 190°. At 212° we have, in the No. 3, a much greater weight sustained than what is indicated by the No. 2 at 190°; and at 600° there appears in both hot and cold blast the anomaly of increased strength as the temperature is advanced from boiling water to melted lead, arising from the greater strength of the No. 3 iron.

A number of the experiments made on No. 3 iron of different sorts have given extraordinary and not unfrequently unexpected results. Generally speaking it is an iron of an irregular character, and presents less uniformity in its texture than either the first or second qualities; in other respects it is more retentive, and is often used for giving strength and tenacity to the finer metals.

Recurring to the No. 2 iron, it will be observed that the strength continued to diminish as the temperature was increased. Heating the cold blast iron in Experiments 11 and 12 to a perceptibly red colour, we have the breaking weights 663 and 723; whereas, in the hot blast, at nearly the same temperature, the breaking weight is 829.7, being as 693 (the mean) to 829, or in the ratio of 1000 :1289.

From the experiments in Table 1, it appears that a bar of cold blast iron 1 inch square and 2 feet 3 inches between the supports, broke at the ordinary temperature of the atmosphere with 836.9, and in No. 3 cold blast from Table III, the breaking weight is 1137.3. This gives an excess of strength for the No. 3 iron of at least one-fourth.

When the bars were heated to a blood red the utmost care was taken to break them without loss of time. In every instance the deflection was considerable; rather more than 11⁄2 inches was observed on the 2 feet 3 inches bars before they gave way.

Comparative strength and power to resist impact of the Coed-Talon hot and

cold blast irons, at various temperatures.

Transverse Strengths.

Modulus of elasticity in lbs, for a base of 1 inch square.

Temperature. Coed-Talon Cold Blast.

Coed-Talon Hot Biast.

The above summary of results on the strength of the hot and cold blast irons is, with one exception, in favour of the cold blast. On the other hand, the power to resist impact appears, with one exception, also, on the side of the hot blast.

Having prosecuted these inquiries through a considerable range both of time and temperature, and having united my efforts to those of Mr. Hodgkinson on the transverse strain, I shall, before closing this report, give a general summary, with the results of which he has kindly favoured me, from all the irons exprimented upon in this way.

Those results will exhibit in one column the relative and proportionate strength of each iron, and in the other the ratio of the forces to resist impact. Before closing the experiments, it may, however, be proper to state, in addition to the methods described in the preceding inquiry, that of grinding was adopted. For this purpose, an apparatus was made to grind each iron under an equal pressure, in order to ascertain the comparative resistances of different specimens of the same size, as compared with the results from chipping and filing given before. This was done with equal weights, upon VOL. XXV. No. 1.-JANUARY, 1840.

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equal sections, and during equal periods of time; and each piece was carefully weighed, in order to determine which of the irons was most easily reduced. Notwithstanding the care taken to ensure correct results, I was unable to procure data from which any thing satisfactory could be obtained. For instance, in the Coed-Talon, Elsicar, and Milton irons, each specimen (nearly cubical) was reduced, as in the Table below, where Wis a constant weight.

The above results, selected from upwards of fifty experiments, are given, not for the purpose of comparison, but in order to enable others to follow up the experiments with greater success. I am of opinion that something may be done in this way; providing cast-steel cutters are used instead of a grindstone, the interstices of which become filled with metallic particles during the process, as the specimens are reduced; consequently the surface of the stone becomes smoother, and the angular points blunted.

General summary of results, as derived from the experiments on the transverse strength of hot and cold blast iron.

The ultimatum of our inquiries made in this way, stand, therefore, in the ratio of strength, as 1000 for the cold blast, to 1024.8 for the hot blast; leaving the small fractional difference of 24.8 in favour of the hot blast.*

The relative powers to sustain impact are likewise in favour of the hot blast, being in the ratio of 1000 to 1226.3.

For the ratios of the powers of the hot and cold blast irons to resist a transverse strain for an indefinite period of time, and for the resisting powers of the same iron under variable temperatures, I must refer the reader to the results contained in their respective tables.

Engravings in Relief from Copperplates by means of Voltaic Electricity. We lately published (No. 624) M. Jacobi's letter to Mr. Faraday, in * The extraordinary properties of the Devon No. 3 iron in a great measure account for the difference which occurs between the strengths, as also the comparative powers to resist impact.

which he described his attempts to copy in relief engraved copperplates, by means of voltaic electricity. We have since received a communication from Mr. Thomas Spencer, of Liverpool, from which it appears, that that gentleman has for some time been independently engaged on the same subject; and that he has not only succeeded in doing all that M. Jacobi has done, but has successfully overcome those difficulties which arrested the progress of the latter. It is unnecessary here to enter on the question of priority between these gentlemen. To Mr. Spencer much credit is certainly due for having investigated, and successfully carried out, an application of voltaic electricity, the value of which can hardly be questioned. The objects which Mr. Spencer says he proposed to effect, were the following:"To engrave in relief upon a plate of copper-to deposit a voltaic copperplate, having the lines in relief-to obtain a fac-simile of a medal, reverse or obverse, or of a bronze cast-to obtain a voltaic impression from plaster or clay and to multiply the number of already engraved copperplates." The results which he has obtained are very beautiful; and some copies of medals which he has forwarded to us are remarkably sharp and distinct, particularly the letters, which have all the appearance of having been struck by a die.

Without entering into a detail of the steps by which Mr. Spencer brought his process to perfection, many of which are interesting, as showing how slight a cause may modify the result, we shall at once give a description of his process.

Take a plate of copper, such as is used by an engraver; solder a piece of copper wire to the back part of it, and then give it a coat of wax-this is best done by heating the plate as well as the wax-then write or draw the design on the wax with a black lead pencil or a point. The wax must now be cut through with a graver or steel point, taking special care that the copper is thoroughly exposed in every line. The shape of the tool or graver employed must be such that the lines made are not V-shaped, but as nearly as possible with parallel sides. The plate should next be immersed in dilute nitric acid, -say three parts water to one acid: it will at once be seen whether it is strong enough, by the green colour of the solution and the bubbles of nitrous gas evolved from the copper. Let the plate remain in it long enough for the exposed lines to get slightly corroded, so that any minute portions of wax which might remain may be removed. The plate thus prepared is then placed in a trough separated into two divisions by a porous partition of plaster of Paris or earthenware, -the one division being filled with a saturated solution of sulphate of copper, and the other with a saline or acid solution. The plate to be engraved is placed in the divison containing the solution of the sulphate of copper, and a plate of zinc of equal size is placed in the other division. A metalic connexion is then made between the copper and zinc plates, by means of the copper wire soldered to the former; and the voltaic circle is thus completed. The apparatus is then left for some days. As the zinc dissolves, metallic copper is precipitated from the solution of the sulphate on the copper plate, wherever the varnish has been removed by the engraving tool. After the voltaic copper has been deposited in the lines engraved in the wax, the surface of it will be found to be more or less rough, according to the quickness of the action. To remedy this, rub the surface with a piece of smooth fag or pumice-stone with water. Then heat the plate, and wash off the wax ground with spirits of turpentine and a brush. The plate is now ready to be printed from at an ordinary press.

In this process, care must be taken that the surface of the copper in the