THE former of these acids is obtained by decomposing muriate of soda or sea salt, by means of sulphuric acid. Five parts by weight of strong sulphuric acid are added 'to six of dried sea salt. The sulphuric acid combines with the alkali, and expels the muriatic acid, which is rapidly absorbed, and condensed by water. Nitric acid is obtained by decomposing nitrate of potash, or saltpetre. Three parts of saltpetre, coarsely pow. dered, are put into a retort, to which two parts of strong sulphuric acid are carefully added, which separates nitric acid from the potash, as in the former case it separated muriatic acid from the soda. As the principles of these operations are the same, the same form

A A furnace is constructed capable of containing twenty cylinders, a side view of one, A, in its place, is given in Fig. 1. They are made of cast iron, of a homogeneous texture and uniform thickness, in order to prevent ́unequal expansion and cracks. They are placed in pairs in the furnace, and each pair has its fire-place, as seen in Fig. 2, an end view, and somewhat like the apparatus for making coal gas. Every part of the cylinder should be equally heated, in order that the decomposition of the salt may be simultaneous and the iron be as little as possible injured by the acid. In proportion as the sulphuric acid contains little water, and in proportion as the cylinder is heated, it is less subject to be injured by the acid. The flame should en

velop every part of the cylinder, and should be retained in the archway above it, to give out some of its heat before it flies up the chimney. Each cylinder is closed at both ends by a plate of cast iron entering just within the cylinder, where it meets with a circular rim. Each plate has a handle of cast iron, B, and a small tube, MM, projecting upwards; and being in the upper part, for the purpose at one end of pouring in the sulphuric acid, and conveying off the product at the other. The first cylinder communicates by the curved tube, C, either of glass or earthenware, with the earthen vessel, D, which has three mouths, and again communicates by two other bent tubes, CC, with two other vessels of the same description. All the gas not condensed in the first D, passes into the other vessel, and at the same time the second vessel D receives the gas extracted from the second cylinder, and transmits what it does not condense to another vessel of the same description, which in like manner also receives the gas from the third cylinder, and in this way the process goes on to the last vessel, which receives the gas not condensed in all the others, and, moreover, that which issues from the last cylinder. From this, whatever is not condensed is again transmitted through a second range of bottles, consisting, perhaps, of 20, till the whole is condensed. It is proper to place the first range of bottles in a trough, LL, through which a stream of water flows gently and constantly, cooling the bottles, and getting itself heated. The purest muriatic acid is obtained in the second range of bottles. That which is condensed in the first series always contains a little sulphuric acid, and sometimes sulphate of soda and muriate of iron. All these bottles are to be half filled with water, which will absorb 2-5ths of its weight of muriatic acid. It is neither difficult nor expensive to erect this apparatus, and by its means 130 parts of muriatic acid, of the specific gravity 1.190, may be

obtained from 100 of common salt. Each cylinder receives about 2lbs. of salt, and the end is then luted with clay, the fire is kindled, and the sulphuric acid poured on the salt, in the proportion of 80 to 100 if the acid is concentrated to 66o of Baume's areometer, and 83 to 100 if it is only concentrated to 64°. The fire should be made brisk at first, and be lessened immediately the distillation begins; when this slackens, the heat is increased; afterwards the end is removed, the sulphate of soda taken out, and the process is then repeated. By means of syphons the muriatic acid is drawn into bottles or jars covered with basketing, its strength is 23° of Baume, and in this state it is sent to market.

To make nitric acid it is not customary to use so many cylinders, otherwise the processes are the same. To 100 parts of nitrate of potash, 60 of sulphuric acid, of the specific gravity of 1.845 are added. For making this acid, the tubes which communicate immediately with the cylinders ought to be of glass, that the colour of the acid which passes may be seen; the remainder of the tubes may be of earthenware. Some parts of the plate are yet to be described. E is the door of the fire-place; F the bars; G the cinder-hole; H chimney; K is a piece of cast iron connecting each pair of cylinders in their whole length; L the cooler or trough in which the first series of bottles are placed.

ON THE CORROSION OF COPPER SHEETING.

By Sir Humphrey Davy.

1. THE rapid decay of the copper sheeting of his majesty's ships of war, and the uncertainty of the time of its duration, have long attracted the attention of those persons most concerned in the naval interests of the country. Having had my inquiries directed to this important object by the commissioners of the Navy Board, and a committee of the Royal Society having been appointed to consider

of it, I entered into an experimental investigation of the causes of the action of sea water upon copper. In pursuing this investigation, I have ascertained many facts which I think not unworthy of the Royal Society, as they promise to illustrate some obscure parts of electro chemical science; and likewise seem to offer important practical applications.

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2. It has been generally supposed, that sea water had little or no action on pure copper, and that the rapid decay of the copper on certain ships was owing to its impurity. On trying, however, the action of sea water upon two specimens of copper, sent by John Vivian, Esq. to Mr. Faraday for analysis, I found the specimen which appeared absolutely pure, acted upon even more rapidly than the specimen which contained alloy; and, on pursuing the inquiry with specimens of various kinds of copper which had been collected by the Navy Board, and sent to the Royal Society, and some of which had been considered as remarkable for their durability, and others for their rapid decay, I found that they offered very inconsiderable differences only in their action upon sea water, and consequently the changes they had undergone must have depended upon other causes than the absolute quality of the metal.

3. To enable persons to understand fully the train of these researches, it will be necessary for me to describe the nature of the chemical changes taking place in the constituents of sea water by the agency of copper.

When a piece of polished copper is suffered to remain in sea water, the first effects observed are, a yellow tarnish upon the copper and a cloudiness in the water, which take place in two or three hours: the hue of the cloudiness is at first white; it gradually becomes green. In less than a day bluish-green precipitate appears in the bottom of the vessel, which constantly accumulates, at the same time that the surface of the copper

corrodes, appearing red in the water, and grass-green where it is in contact with air. Gradually carbonate of soda forms upon this grassgreen matter; and these changes continue till the water becomes much less saline.

The green precipitate, when exmined by the action of solution of ammonia and other tests, appears principally to consist of an insoluble compound of copper (which may be considered as a hydrated sub-muriate) and hydrate of magnesia.

According to the views which I developed fourteen years ago, of the nature of the compounds of chlorine, and which are now generally adopted, it is evident that soda and magnesia cannot appear in sea water by the action of a metal, unless in consequence of an absorption or transfer of oxygen. It was, therefore, necessary for these changes, either that water should be decomposed, or oxygen absorbed from the atmosphere. I found that no hydrogen was disengaged, and consequently no water decomposed: necessarily, the oxygen of the air must have been the agent concerned, which was made evident by many experiments.

Copper in sea water deprived of air, by boiling or exhaustion, and exposed in an exhausted receiver or an atmosphere of hydrogen gas, underwent no change; and an absorption in atmospherical air was shown when copper and sea water were exposed to its agency in close vessels.

4. In the Bakerian Lecture for 1806, I have advanced the hypothesis, that chemical and electrical changes may be identical, or dependent upon the same property of matter; and I have further explained and illustrated this hypothesis, in an elementary work on chemistry, published in 1812. Upon this view, which has been adopted by M. Berzelius, and some other philosophers, I have shown that chemical attractions may be exalted, modified, or destroyed, by changes in the electrical states of

bodies; that substances will only combine when they are in different electrical states; and that, by bringing a body naturally positive artificially into a negative state, its usual powers of combination are altogether destroyed; and it was by an application of this principle, that in 1807 I separated the bases of the alkalics from the oxygen with which they are combined, and preserved them for examination; and decomposed other bodies formerly supposed to be simple.

It was in reasoning upon this general hypothesis likewise, that I was led to the discovery which is the subject of this paper.

Copper is a metal only weakly positive in the electro-chemical scale; and, according to my ideas, it could only act upon sea water when in a positive state, and consequently, if it could be rendered slightly negative, the corroding action of sea water upon it would be null and whatever might be the differences of the kinds of copper sheeting and their electrical action upon each other, still every effect of chemical action must be prevented, if the whole surface were rendered negative. But how was this to be effected? I at first thought of using a Voltaic battery, but this could be bardly applicable in practice. I next thought of the contact of zinc, tin, or iron; but I was for

for some time prevented from trying this, by the recollection that the copper in the Voltaic battery, as well as the zinc, is dissolved by the action of diluted nitric acid, and by the fear that too large a mass of oxidable metal would be required to produce decisive re-, sults. After reflecting, however, for some time on the slow and weak action of sea water on copper, and the small difference which must exist between their electrical powers; and knowing that a very feeble chemical action would be destroyed by a very feeble electrical force, I resolved to try s experiments on the subject. I be gan with an extreme case. I rendered sea water slightly acidulous by sulphuric acid, and plunged

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into it a polished piece of copper, to which a piece of tin was soldered equal to about one-twentieth of the surface of the copper. Examined after three days, the copper remained perfectly clean, whilst the tin was rapidly corroded; no blueness appeared in this liquor, though, in a comparative experiment, when copper alone and the same fluid mixture was used, there was a considerable corrosion of the copper, and a distinct blue tint in the liquid.

If one-twentieth part of the surface of tin prevented the action of sea water, rendered slightly acidulous by sulphuric acid, I had no doubt that a inuch smaller quantity would render the action of sea water, which depended only upon the loosely attached oxygen of common air, perfectly null; and on trying a two-hundredth part of tin, I found the effect of its preventing the corrosion of copper perfectly decisive.

5. This general result being obtained, I immediately instituted a number of experiments, in most of which I was assisted by Mr. Farraday, to ascertain all the circumstances connected with the preservation of copper by a more oxidable metal. I found that, whether the tin was placed either in the middle, or at the top, or at the bottom of the sheet of copper, its effects were the same; but, after a week or ten days, it was found that the defensive action of the tin was injured, a coating of sub-muriate having formed, which preserved the tin from the action of the liquid.

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With zinc or iron, whether malleable or cast, no such diminution of effect was produced. The zinc occasioned only a white cloud in the sea water, which speedily sunk to the bottom of the vessel in which the experiment was made, The iron occasioned a deep orange precipitate; but after many weeks, not the smallest portion of copper was found in the water, and so far from its surface being corroded, in many parts

is surface was a regenera

tion of zinc or of iron found upon it.

6. In pursuing these researches, and applying them to every possible form and connexion of sheet copper, the results were of the most satisfactory kind. A piece of zinc as large as a pea, or the point of a small iron nail, were found fully adequate to preserve forty or fifty square inches of copper; and this, wherever it was placed, whether at the top, bottom, or in the middle of the sheet of copper, and whether the copper was straight or bent, or made into coils. And where the connexion between different pieces of copper was completed by wires, or thin filaments of the fortieth or fiftieth of an inch in diameter, the effect was the same; every side, every surface, every particle of the copper remained bright, whilst the iron or the zinc was slowly corroded.

A piece of thick sheet copper, containing on both sides about sixty square inches, was cut in such a manner as to form seven divisions, connected only by the smallest filaments that could be left, and a mass of zinc, of the fifth of an inch in diameter, was soldered to the upper division. The whole was plunged under sea water; the copper remained perfectly polished. The same experiment was made with iron; and now, after a lapse of a month, in both instances, the copper is as bright as when it was first introduced, whilst similar pieces of copper, undefended, in the same sea water, have undergone considerable corrosion, and produced a large quantity of green deposit in the bottom of the vessel.

A piece of iron nail about an inch long was fastened by a piece of copper wire, nearly a foot long, to a mass of sheet copper, containing about forty square inches, and the whole plunged below the surface of sea water; it was found, after a week, that the copper was defended by the iron in the same manner as if it had been in immediate contact.

A piece of copper and a piece of zinc soldered together at one of

their extremities, were made to form an arc in two different vessels: of: sea water; and the two portions of water were connected together by a small mass of tow moistened in the same water: the effect of the preservation of the copper took place in the same manner as if they had been in the same vessel.

As the ocean may be considered, in its relation to the quantity of copper in a ship, as an infinitely extended conductor, I endeavour--ed to ascertain whether this circumstance would influence the results, by placing two very fine copper wires, one undefended, the other defended by a partiele of. zinc, in a very large vesset of sea water, which water might be cons sidered to bear the same relation to se minute a portion of metri as the sea to the metallic sheeting of a ship. The result of this experiment was the same as thrat of all the others, the defended copper underwent no change; the undefended tarnished and deposited a green powder.

Small pieces of zinc were soldered to different parts of a large plate of copper, and the whole plunged in sea water : it was found . that the copper was preserved in the same manner as if a single piece had been used.

A small piece of zinc was fastened to the top of a plate of polished copper, and a piece of iron of a much larger size was soldered to the bottom, and the combination. placed in sea water. Not only was the copper preserved on both sides, in the same manner as in the other experiments, but even the iron; and after a fortnight, both : the polish of the copper and the iron remained unimpaired.

7. I am continuing these researches, and I shall communicate such of them as are connected with new facts to the Royal Society.

The Lords Commissioners of the Admiralty, with their usual zeali for promoting the interests of the navy by the application of science, have given me permission to ascer-tain the practical value of these results by experiments upon ships