He next proceeded to Neuburg and Altona, where he intended to re-embark for England in the steam-vessel which had, during the interval of his continental tour, made a voyage to England, and was again on her way to the Elbe. At the suggestion, however, of Professor Schumacher, the astronomical professor at Copenhagen, but residing at Altona, in whose society he passed a great portion of his time, he accompanied that gentleman to Bremen, in order to make the acquaintance of the venerable Dr. Olbers, who, since his retirement from an extensive medical practice, had entirely devoted his time to the pursuit of his favourite science astronomy; as well as to be introduced to Professor Gauss, of Gottingen, who happened to be at that time carrying on his geodetical operations for the admeasurement of the kingdom of Hanover.
Davy expressed a great desire to see the telescope with which Dr. Olbers had discovered the two planets, Pallas and Vesta, and which to his great surprise turned out to be a very ordinary instrument. His personal intercourse with these two celebrated philosophers appeared to afford him the highest satisfaction; and he spent two days most agreeably in their society.
In his "Salmonia," he gives us some account of his adventures as an angler during this short excursion to Norway and Sweden. "All the Norwegian rivers," says he, "that I tried (and they were in the southern parts) contained salmon. I fished in the Glommen, one of the largest rivers in Europe; in the Mandals, which appeared to me the best fitted for taking salmon; and in the Arendal; but, though I saw salmon rise in these rivers, I never took a fish larger than a sea-trout; of these I always caught many—and even in the fiords, or small inland salt-water bays; but, I think, never any one more than a pound in weight. It is true that I was in Norway in the beginning of July, in exceedingly bright weather, and when there was no night; for even at twelve o'clock the sky was so bright, that I read the smallest print in the columns of a newspaper. I was in Sweden later—in August: I fished in the magnificent Gotha, below that grand fall, Trollhetta, which to see is worth a voyage from England; but I never raised there any fish worth taking. I caught, in this noble stream, a little trout about as long as my hand; and the only fish I got to eat at Trollhetta was bream."
He again embarked, on the 14th of August, on board the Comet steam-vessel, which had ascended the Weser as high as her draught of water would allow, and reached England, after a very boisterous passage, on the 17th of the same month; indeed, the vessel left the mouth of the Weser with a contrary wind, and the pilot was unwilling to put to sea, but Davy insisted on proceeding without delay. During the whole passage he suffered extremely from sea-sickness, and in a letter written to Professor Schumacher, shortly after landing, he remarks that "the sea is a glorious dominion, but a wretched habitation."
On the 9th of June 1825, Sir Humphry read before the Royal Society his third and most elaborate paper upon Copper sheathing, entitled "Farther Researches on the Preservation of Metals by Electro-chemical Means."
In this memoir, he states it to be his belief, that there is nothing in the poisonous nature of the copper to prevent the adhesion of weeds and testaceous animals; for he observes, that they will readily adhere to the poisonous salts of lead which commonly form upon the metal protecting the fore-part of the keel; and even upon copper, provided it be in such a state of chemical combination as to be insoluble. It is then, in his opinion, the solution of the metal—the wear of its surface, by keeping it smooth, which prevents the adhesion of foreign matter. Whenever the copper is unequally worn, deposits will, without doubt, rest in the rough parts, or depressions in the metal, and afford a soil or bed in which sea-weeds can fix their roots, and to which zoophytes and shell-fish can adhere; but there is another cause of foulness on the protected sheathing, arising from the deposit of earthy matter upon the copper, in consequence of its electro-negative condition.
In relation to this subject, Davy has offered some observations upon the effects produced by partial formations of rust, which appear to me to be exceedingly interesting and important.
When copper has been applied to the bottom of a ship for a certain time, he says, a green coating, or rust, consisting of oxide, sub-muriate, and carbonate of copper, forms upon it; not equally throughout, but partially, and which, it is evident, must produce a secondary, partial, and unequal action, since those substances are negative with respect to metallic copper, and will consequently, by producing with it a Voltaic circuit, occasion a more rapid corrosion of those parts still exposed to sea-water: from this cause, sheets are often found perforated with holes in one part, after having been used for five or six years; while in other parts they are comparatively sound.[95] In like manner, the heads of the mixed metal nails, consisting of copper alloyed by a small quantity of tin, which are in common use in the Navy, give rise to oxides that are negative with respect to the copper, so that the latter is often worn into deep and irregular cavities in their vicinity.
A series of very interesting experiments, fully detailed in this memoir, which were instituted for the purpose of ascertaining the extent of the diminution of electrical action in instances of imperfect or irregular conducting surfaces, led him to the general conclusion, that a very small quantity of the imperfect or fluid conductor was sufficient to transmit the electrical power, or to complete the chain. This induced him to try whether copper, if nailed upon wood, and protected merely by zinc or iron on its under surface, or on that next the wood, might not be defended from corrosion: a question of great practical moment with regard to the arrangement of protectors. For this purpose, he covered a piece of wood with small sheets of copper, a nail of zinc of about 1/100 part of the surface having been previously driven into the wood: the copper surface remained perfectly bright in sea-water for many weeks; and when the result was examined, it was found that the zinc had only suffered partial corrosion; that the wood was moist, and that, on the interior of the copper there was a considerable portion of revived zinc, so that the negative electricity, by its operation, provided materials for its future and constant excitement. In several trials of the same kind, iron was used with similar results; and in all these experiments there appeared to be this peculiarity in the appearance of the copper, that unless the protecting metal below was in a large mass, there were no depositions of calcareous or magnesian earths upon the metal; it was clean and bright, but never coated. The copper in these experiments was nailed sometimes upon paper, sometimes upon the mere wood, and sometimes upon linen; and the communication was partially interrupted between the external and internal surfaces by cement; but even one side or junction of a sheet seemed to allow sufficient communication between the moisture on the under surface and the sea-water without, to produce the electrical effect of preservation. This last experiment of Davy is of greater importance than may at first appear, in showing what a small proportion of conducting fluid will complete a circuit, and in thus explaining phenomena, as I shall presently show, which might not otherwise be suspected to have an electrical origin.