constructed several vessels (some of which I now exhibit), having the bottoms formed of pieces of this very incrustation, from to in thickness. I also constructed similar vessels with iron-plate bottoms, and of corresponding thickness. These vessels, being filled with water, were successively exposed to a very great heat, and which was confined exclusively to the bottoms.

The result was, that the theory of over-heating by reason of the incrustation was at once found to be erroneous, the incrustation, instead of a non-conductor, being found to be substantially a good

one.

In comparing the conductibility of iron and incrustation bottoms, I found the difference so insignificant, that I could no longer believe that injury to the metal could possibly result from the latter. I also found that, even when the water was at the full boiling, the heat of the incrusted bottom was not sufficient to injure the hand when touched. [Mr. Williams here took the vessel from the flame over which the water was boiling, and laid his finger to the bottom formed of incrustation. Several other gentlemen did the same, and pronounced it to be at too low a temperature to be injurious.]

To what, then, is attributable the well-known fact of over-heated, and even red-hot plates, and their consequent injury? It is here worthy of remark, that in marine boilers bulging or rupture is never known except when there is a deficiency of water; or, which is equally injurious, when, from the peculiar construction of the flues, the free circulation of the water is impeded.-[Mr. Williams exhibited a model of the boiler of the steam vessel the " Athlone," to show the liability of marine boilers to the injury arising from the want of free circulation of the water, and from which cause SO much over-heating and bulging of the plates occur.]

This over-heating, then, in land boilers, may with certainty be attributable to that other species of deposit which does not assume the form or character of a crystallized body. This, then, is the body which, under certain circumstances, and from its non-conducting character, forms the main obstruction to the transmission of the heat, and its consequent accumulation in the iron until over-heating and redness ensue.

It will be observed, that the deposition from the water, which is invariably some of the salts of lime, may thus be divided into the crystallized and the uncrystallized portions. I have shewn that in the former it is a good conductor; in the latter, however, it is the reverse. Whether this arises from the porous nature of the body itself, or merely from its accumulated mass, is not so easily ascertained. In this uncrystallized state it forms a loose, unconsolidated, earthy, friable compound, being a muriate, carbonate, or sulphate of lime, and merely held in suspension.

The danger, however, commences when this matter (though perfectly innoxious while in this suspended state, and so long as the water is in motion by the act of boiling) is allowed to subside, by

the boiler being at rest. It then becomes a compact and indurated mass, resembling the anhydrous sulphate of lime (commonly called Plaister of Paris), after having been supplied with a due portion of water; in fact, after what is called " setting" has taken place. Subsiding while in the state of powder, it settles and consolidates at the bottom of cylindrical boilers, and the legs or sides of waggon

boilers.

In this state of rest, and when, by the internal heat of the boiler, it has dried and consolidated, it becomes almost impervious to, or at least a very slow absorbent of, water.

Mr. Williams here exhibited a cylindrical and solid body of this apparent concrete, which had been in a state of suspension the preceding day in boiling water, but which, on being allowed to cool, and become thoroughly dried, presented a hard compact body, of the exact shape of the vessel in which it had subsided, and so hard as scarcely to be marked by the nail.

In this state it becomes a mischievous non-conductor, and not unfrequently being found many inches thick, it effectually prevents the necessary access of the water to the plate, when the boiler is again put into action; thus producing the whole chain of events, from an over-heating of the plate to its bulging or rupture.

For the purpose of establishing this fact, I made some vessels, the iron bottoms of which were so inserted, by means of solder, that on the event of the iron becoming over-heated, the solder would melt, and the bottom fall out. The result was as anticipated, and a deposit of but a single inch in thickness, after being allowed to settle and become hard and dry, was found sufficient to cause such an overheating of the plate as to melt the solder.

Under such a state of things, if the plate so exposed to overheating be a sound one, it will bulge with an uniform and regular curve, by force of the internal pressure of the steam; the exact shape or figure of the bulging being the result of accidental circumstances. In this state no injury will be sustained; the plate will remain as strong as before, and as if such bulged shape had been given to it intentionally in the first instance.

Mr. Williams here exhibited a very large iron plate, eight feet in length, illustrative of the bulging, but which had not in any way weakened it.

If, however, the plate be an imperfect one, that is, presenting in its section a laminated and unwelded structure, the bulge will tend more or less towards a crack or rupture, and, becoming necessarily weaker, the two portions, or laminæ, separate; an additional obstruction to the passage of the heat is thus created, and the plate will give way at the weakest spot.

This will be found to be the case with the majority of injured, cracked, or ruptured plates, and which are very erroneously and ignorantly then said "to be burned."

In exhibiting to Mr. Parkes the fact of the good conducting quality of the incrustation, he pointed out an oversight in the experiment; namely, the necessity of having both iron and crystallization in contact, and taking their joint conducting powers as they exist in practice. To meet this case, I had a vessel made with a bottom formed of incrustation, five-eighths thick, to which I added an iron bottom of the same thickness, the bottom being thus 14 inch thick; the result, however, was as before. This great thickness presented no impediment to the passage of the heat, and when the water was at full boiling temperature, this joint iron and incrustation bottom, though exposed to a very strong heat, was so little heated, that on being removed from the flame, the finger was pressed against it without injury. [This was done by Mr. Williams before his auditory.]

In a paper by Mr. Parkes, (see the Inventors' Advocate for July 31st, 1841,) in reference to the late boiler explosion at Gateshead, he observes, "It appears from the evidence given at the inquest, that the internal state of the boiler, as manifested by the quantity of deposit ejected, is quite sufficient to account for its dislocation."

Now, had this matter assumed the form of an incrustation, no danger or over-heating could have ensued. But he further goes on to say, "I have referred to a boiler incrusted, but a similar sudden disengagement of steam, in large volumes and of high elasticity, will result in the disturbance of a covering of mud."

Now, this is quite to the point, for here we have a positive nonconducting body interposed between the water and the plate, the necessary consequence of which must be, overheating and all its train of injuries.

Under such circumstances of deposit, we find that the accidental interposition of any solid body on the boiler has the direct effect of aiding the collection of this deposit around it, and consequently producing a local accumulation of this non-conducting medium. I myself witnessed a rupture of a plate occasioned by the neglect of a workman in leaving a large sledge hammer in the boiler. An accumulation of indurated deposit was found collected at the spot, enveloping the hammer head, and causing the bulging of the plate immediately under the spot.

Now, what inference are we to draw from these facts? and how can we apply them practically to remedy or avoid the danger from such deposits? Either by preventing this dangerous subsidence, by collecting it while in suspension in the boiler, or by a proper attention to its removal before it has been allowed to become dangerous from its quantity and solidity.

Many years back, Mr. Williams observed, his attention had been drawn to the means of collecting this floating matter while the boiler was in action, by the introduction of vessels into the boiler, and which he had tried with the most decided success. In marine

boilers, it was, however, useless and even dangerous, unless the operation could be performed mechanically, and without the attention of human agents.

Mr. Williams illustrated his observations by placing over the flame of a powerful Argand burner one of the vessels, the iron bottom of which was soldered in, and in which a portion of this indurated deposit had been allowed to settle. The result was, that this deposit being a bad conductor, the iron became over-heated, the solder melted, and the water ran through.

When this matter, therefore, (Mr. Williams assumed,) has taken the crystallized state, it may be said to have lost its power of producing mischief to the plate-the reverse, however, when uncrystallized.

But incrustation never adheres, to any dangerous extent or thickness, on those parts which are so exposed to a high temperature that they could be over-heated, or approach to redness. This is an important fact, and opens a new view of the subject, as regards the formation of the deposit. My inquiries lead me to this conclusion: that accumulation, in the crystallized state, takes place by two distinct processes, according to the temperature and character of the plate, and the part of the boiler where such crystallization is formed, and after the following manner:-When the heat is moderate, and the iron not subject to much fluctuation of temperature, or the boiler being set at rest, or the fire being again lighted, as, for instance, on the roofs or parts furthest from the fire, the successive layers or strata are formed over each other, and, for the most part, bearing the character of one uniform density. When, however, the heat is the greatest, as in those parts immediately over the fire, or adjacent to it, and when the iron is subject to greater extremes of expansion and contraction, the incrustation remains attached to the plate but a short time, and arrives at a very moderate thickness. This alternate contraction and expansion so deranges the adhesion, that more or less of moisture or water insinuates itself between the incrustation and the iron plate; and the next time the boiler is set to work, the incrustation is instantly dislodged, and the successive layers or strata are then formed under the preceding ones. Thus nature comes to our aid, and by this process of disunion or disconnection, the plates are freed from the incumbrance, and the steam is generated under such previous layer of incrustation, until, finally, the accumulated mass breaks off and falls to the bottom.

Mr. Williams presented several specimens of these accumulated strata, illustrative of this under formation, the inspection of which manifestly disproved the idea of the additional crystallization having been formed over or on the preceding ones.

E

GALVANOPLASTIK; or the Process of Cohering Copper into Plates, or other given forms, by means of Galvanic Action on Copper Solutions. By Dr. M. H. JACOBI, Privy Councillor to the Emperor of Russia, and Member of the Royal Academy of Sciences of St. Petersburg.*

(Continued from page 498 of vol. vii.)

VII.

We see, from the management, that, however simple the process of obtaining galvanic copper plates may appear, it requires very much attention. We shall also find, by closer inspection, and especially by practical exercise in the art, that many interruptions take place, which it becomes desirable to remove; for, in the first place, as already stated, there is a limit with reference to the saturation of the cupreous solution, the strength of the current, and even in the time in which we can obtain the deposited copper plates. The great tenacity and firmness which the copper attains by slow reduction, is certainly a desirable property, though only of a subordinate consideration where it operates as an impediment to the acceleration of the arts.

If, by some oversight, the precipitated copper should become of a brown red, or be loose, a new layer, even from a saturated solution, will not attain the required hardness and colour. In such cases, the brown surface must be scraped off, or the whole plate be well washed over with diluted nitric acid. The most serious mischief, however, by the hitherto described method, is, that it is too much limited in its operations, both as respects configuration and magnitude of the plates produced; and, in order to obtain a constant and economical action, the zinc, and the original article to be copied, must have placed between them one of the described forms of diaphragms, which ought to keep the fluids separate, and yet keep its pores open so as to permit free circulation. When the impediments in this part of the apparatus are but trifling, they are easily removed; but when considerable, they are attended with many difficulties and inconveniences, and not easily got rid of. According to this first method, it is probable that the galvanoplastik art might have had but a very limited field of operation: at this stage, however, was the beginning of my troubles. Fortunately our success does not depend upon first efforts, nor ought we to consider that the impediments which first present themselves will eventually be of much practical consequence.

In order to comprehend upon what principle the second method depends, it will be necessary that I say a few words on the chemical action of the galvanic currents. It has already been stated, that

* Translated from the German edition.