Abbildungen der Seite
PDF
EPUB

that point is I trust I shall be able to demonstrate with sufficient accuracy for all practical purposes.

We saw that when we changed the recipient from water to air, the latter, being an extremely bad recipient, the stream of heat was interrupted, and accumulation necessarily took place. I shall hereafter be able to shew that the analogy between the stream of heat and the stream of water, through a tube, is sufficiently close to establish the fact I am contending for.

I will now illustrate this further by means of this model boiler and the table of temperatures, now exhibited.-[A small vessel was then shewn filled with water, having two conductor pins projecting one inch into the water, and three inches into the flame. Between these pins, and just between the boiler and the flame, and protected by a shield from both, the bulb of a thermometer was placed.]

Now, when these two bars (Mr. Williams continued) are exposed to the flame, the heat transmitted through them will raise their temperature to such a degree, that if they were sentient beings, they would tell us what they feel. This degree of heat will be indicated by the thermometer, the bulb of which is placed between them.

But I am desirous of establishing this fact, that the degree of heat which they sustain or feel, as mere carriers or transmitters, is very different from that which is transmitted through them; that there are, in fact, two distinct temperatures to be attended to--namely, the one which the bars or conductors may be said to feel, as conductors, the other that which they convey to the recipient, and of which the latter is susceptible.

Mr. Williams then explained a diagram, representing, on an enlarged scale, the model with the conductor pins, by saying the water in the boiler, at starting, was at 40 degrees. In five minutes the heat given out by the conductors to the thermometer was 160, the water being raised to 92. In fifteen minutes the water reached 212, and the conductor thermometer stood at 258.

Here we have two distinct temperatures, and two as distinct measures of heat. The first, which is that felt, as it were, by the conductors, I will call statical heat-that is, the heat due to its status as a conductor, and which, if it could speak, it would say "I feel." The second is that measure of heat which would indicate the current or body of conveyed heat; this I will call dynamic, or power-giving heat. These terms, statical and dynamical heat, I use from want of others more appropriate, but they will sufficiently convey the distinction I am pointing out.

Now, to shew that there are those two temperatures, and which should be separately attended to, I give the following proof:-The bulb, indicating the statical heat, and which also indicates the temperature to which the conductor is raised, resting on the conductor

bar, stands at 270 (the water continuing boiling). To prove that this temperature is affected by the nature of the recipient, I change the recipient water for air. This is done by letting the water suddenly out of the vessel. What follows? The current of heatthe dynamical heat, cannot pass out, and through, as rapidly as it had previously done; it is therefore accumulated in the conductorthe statical heat is instantly increased-and the thermometer rises from 270 to 404, at which it remains.

Thus we see, that so long as the recipient is water, the statical or real sensitive temperature of the metallic conductor is 258; but with air as the recipient, the statical heat is 404.

Now, the statical heat is that which indicates the state of the conductor plates, and decides the whole question of injury to boilers. This is the practical bearing of the question. The temperature of the plate is then dependant, not on the quantity of heat, of dynamical heat passing through it, but on the recipient and the rapidity with which such dynamical heat is absorbed.

We have, therefore, two distinct classes of heat or temperature to attend to, namely, the statical heat-that due to the plate as a carrier, conductor, transmitter, or conveyer of heat; and the dynamical heat-that due to the quantity or current conveyed.

If the dynamical heat or current be taken up, or absorbed by the recipient water, the statical heat will never rise to a degree sufficient to have any injurious effect on the conductor plate. The statical heat or temperature depends on the rate at which the dynamical heat is absorbed by the recipient, and on this depends also the whole question, whether the plates which are the conductors can be injuriously affected or not?

I have now to draw attention to the commonly received notion of the durability of a boiler being influenced by the temperature within the furnace, and the activity with which the fire is urged. We are all familiar with the assertion, that a boiler will not last as long when subjected to active firing, and more rapid evaporation (as is usual under waggon boilers), as with slow combustion and slow evaporation, as is the case in the large Cornish boilers. This position I am prepared to dispute, and it appears to me to have originated in our not attending to the point I have just been enforcing, namely, the temperature of the plates, as conductors, and the causes of an increase in such temperature. Indeed, all the facts disprove this hitherto unquestioned dictum.

But there is one fact, which, if it stood alone, would disprove the allegation—namely, that those parts and plates which are most exposed, and subject to the direct influence of the greatest heat, are the least affected, and never give any indication of injury or deterioration, unless some accidental circumstances may have occurred to prevent access to the water.

Indeed, it has frequently occurred that plates taken from old boilers, and from the very part, above all others, most exposed to the violent action of the intensest heat in the furnace, are selected and put into other and even new boilers; being thus proved to be sound and uninjured.

Again, the plates which exhibit the greatest proofs of deterioration, are found to be those where the heat has been the least. These facts at once settle the question, that heat is not the direct or immediate cause of deterioration, and that we must look to some other source for it.

The whole question of destruction, practically, resolves itself into this, viz., that in land boilers, with quicker evaporation and more active firing, there is a greater average risk from the consequences of neglect in keeping the boiler clean, and the water at its due level; and in marine boilers, from the water being forced from its contact with the plates under the influence of rapid evaporation and ebullition.

It is true, neglect and mismanagement of the interior of boilers must be taken into account, and provided for. But why not endeavour to correct this evil, rather than, by attributing the cause of such evil to a wrong source, have our attention diverted from the true remedy.

Now, as a rapid action of the fuel, and a more intense heat, will give a larger evaporative effect, both as to time and amount of fuel employed, than slow combustion and slow evaporation with large boilers, it becomes a question of great importance, whether we should prefer the latter to the former-that is, the inferior to the superior process; for this would be paying dearly indeed for ordinary attention and cleanliness in the boilers.

What I desire is, to put the question, as regards the scientific and practical application of heat, on the right footing. But if we are to be deterred from giving utterance to our views because they are opposed to old rules and proportions, and because we are gravely told "operative engineers are opposed to such practice," we had better at once proclaim that this is not the age of intellectual and scientific advancement, or ours the country for practical improvement. To this I am not disposed to assent, and trust to be able still further to develope the great advantage of combining science with practice, in all that has to do with the promotion of arts in this country.

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.*

(Concluded from page 74.)

VIII.

In the preceding articles we have spoken of the reduction of copper on copper plates only, and have stated that the application of the galvanoplastik art would be very limited, if the process could not be extended to originals of other metals; to all those metals, for instance, that would not of themselves decompose the copper salts; that is, those which would not dissolve when placed in the cupreous solutions, and precipitate the copper therefrom so as partly to be covered by it. Those metals which sufficiently resist being attacked by the copper solution, are iridium, platinum, gold, silver, quicksilver, arsenic, bismuth, and antimony; and, on the other hand, the copper solution becomes easily reduced by lead, tin, iron, and zinc. It may here be remarked, that pure and bright lead operates very differently to that which is tarnished. In the first condition it

reduces the copper upon itself; but when in the latter condition it scarcely acts upon copper solutions: so that tarnished lead answers almost as well as copper for receiving deposited copper by electric currents. Moreover, although pure lead corrodes a little at the commencement, that action very soon disappears, and in a short time a thin film of copper forms on its surface. By means of this metal, then, we may proceed at once to multiply engraved copper plates without the originals ever entering into the galvanic process. For this purpose a sheet of lead is laid over the original engraved copper plate, and both passed through a strong rolling press; the lead is thus pressed into the engraving, and, when separated, presents a beautiful picture of it in relief. The leaden impression then serves as a form on which a fac-simile of the original engraving can be deposited by the galvanoplastik process. I have seen galvanic copper plates made by these means, which, for sharpness and accuracy throughout, were not surpassed by those deposited on copper plates made by the relief engraving machine.

Pure tin, by the action of cupreous solutions, becomes more corroded than lead, and thus presents an obstacle to its employment where the sharpness and purity of the original picture of the engraving is to be preserved; but answers well enough where those particulars do not interfere with the object of the operator. The dissolution of tin discontinues, however, as soon as, either by che

• Translated from the German edition.

mical or by galvanic means, it becomes covered with a film of copper. Alloys of tin, when this metal is not in excess, with those metals on which cupreous solutions have no action, even including lead, answer very well for patterns on which the copper is to be reduced; the proportions may very easily be determined by experiment. The type-metal, and also the well known fusible metal, are very applicable to this purpose.

It is well known that iron precipitates copper from its solutions until it has gradually and completely decomposed them, and the dissolution of the iron is not stopped by the formation of the loose copper covering that first forms on its surface. When the copper solution is saturated and contains free acid, the metallic copper falls down in the form of a powder; and although the precipitation is slower as the solution of the copper salt is more diluted, yet the metallic copper is so much the firmer cohered, that it very frequently becomes a firm mass, whose shape corresponds to that of the immersed piece of iron. The copper thus reduced by iron is well known by the name of cœmentkupfer, and is distinguished by its purity. In many places, where natural cupreous waters are abundant, this pure copper is obtained in great quantities. In Schmölnitz, in Upper Hungary, from 7,000 to 8,000 pounds of pure copper are obtained annually, by placing pieces of any kind of raw iron in the water naturally impregnated with the metal. I will here just remark, that this kind of chemical reduction as precisely differs from the galvanic reduction, as the chemical dissolution of zinc from its galvanic dissolution, which has already been alluded to in art. 1.

Zinc, as a model for galvanoplastik formations, is a metal that cannot be used, and it appears hitherto that its alloys with other metals are also exceptionable; such, for instance, is the case with brass. This alloy, it is true, by an interworking of galvanic currents, becomes overlaid, in a short time, with a beautiful film of copper; but it is always difficult, and often almost impossible, to loosen it even when grown to considerable thickness. But should we eventually succeed by the application of a strong force, we immediately perceive that the torn off piece of copper is covered over with a coating of brass, as though it were plated with that metal.

As regards the non-metallic bodies, with reference to their practical applicability, charcoal, and indeed in its form of graphite (plumbago), ought to be particularly mentioned. These substances will on no account affect the solutions of cupreous salts, and are capable of being employed to advantage, even better than platinum, in galvanic series, as cathodes in the galvanic decomposition of such solutions. Since, however, these substances present considerable difficulty in giving to them regular artificial forms, we must apply them in another manner, which, as we shall see, can be done to some advantage.

We have already seen that the galvanic action depends, principally, upon the extent of the metallic surfaces brought into a state of activity, and but very little on the thickness of the metal. From

M

« ZurückWeiter »