[100] In the year 1825, His Majesty George IV. communicated to the Royal Society, through Mr. Peel, his intention to found two gold medals, of the value of fifty guineas each, to be awarded annually by the Council of the Royal Society, in such a manner as shall, by the excitement of competition among men of science, seem best calculated to promote the object for which the Royal Society was instituted.
[101] Numerous are the instances of later date which might be adduced in illustration of the same fact; and it is now generally supposed that it may have been a frequent cause of ships foundering at sea. By oxidation, the volume of the iron at first increases, and then diminishes; in consequence of which the ship leaks, or, to use a technical expression, becomes "bolt sick." When the Salvador del Mundo was docked at Plymouth, in February 1815, the iron fastenings were in such a state of corrosion, that five planks near the bilge dropped into the dock when the water left her.
[102] Page 13 of this Volume.
[103] As far as the principle of Voltaic protection goes, this may be very true; but it must be remembered that the acid generally present upon these occasions is acetic acid, which rises in distillation with water, so that at the boiling temperature it will be carried beyond the sphere of Voltaic influence, and may thus act upon the denuded copper as much as though tin were not present.
[104] There is an excellent example at this time in the London Road leading to the Elephant-and-Castle.
[105] Page 248 of this Volume.
[106] It would appear that Davy latterly preferred zinc to iron, as the protecting metal. In a letter, dated October 1826, addressed to a ship-owner, who had made some enquiries of him upon the subject, he says—"The rust of iron, if a ship is becalmed, seems to promote the adhesion of weeds; I should therefore always prefer pieces of zinc, which may be very much smaller, and which, in the cases I have heard of their being used, have had the best effect."
[107] In the Annals of Philosophy (vol. v.) may be found a paper by Dr. Henry, on the conversion of cast iron pipes into plumbago. This change appears to have been effected by the action of water containing muriate of soda, and the muriates of lime and of magnesia. Cast iron contains a considerable portion of carbon; the change is therefore readily explained on the supposition of the removal of the principal metallic part by these salts. The muriates of lime and magnesia have been observed by Dr. Henry to discharge writing ink from the labels of bottles, to which they had been accidentally applied; and the same ingenious chemist has been baffled in his attempts to restore the legibility of ink upon paper which had been exposed to sea-water. The texture of the paper was not injured, but the iron basis of the ink, as well as the gallic acid, was entirely removed.
[108] His admiration of this work bursts forth in his Salmonia, which he was writing at that time. He styles it "an immortal monument raised by Genius to Valour."
[109] Mr. Children has just communicated to me the following amusing anecdote, which may be adduced in illustration of the delight he took in that sporting dexterity to which he alludes in the above passage. Davy, with a party of friends, had been engaged for several hours in fishing for pike, but very unsuccessfully; our philosopher gave up the sport in despair, but his companions having determined to try some more propitious spots, left him to his contemplations. About an hour afterwards, Mr. Children, on returning to his friend, saw him at a distance seated upon a gate, and apparently lashing the air with his fishing-line. What could be his object? As soon as Mr. Children came sufficiently near to make a signal, Davy, by his gestures, earnestly entreated him to keep away, while he continued his mysterious motions. At length, however, Mr. Children's patience was exhausted, and he walked up to him. "Was ever any thing more provoking!" exclaimed Davy; "if you had only remained quiet another minute I should have caught him—it is most vexatious!" "Caught what?" asked Mr. Children. "A dragon-fly," (Libellula,) answered Davy. "During your absence I have been greatly amused by watching the feeding habits of that insect, and having observed the eagerness with which they snapped up the little 'midges,' I determined to arm my hook with one, and I can assure you I have had no small degree of sport; and had it not been for your unwelcome intrusion, I should most undoubtedly have captured one of them."