M. de Quatrefages placed the four salts he used in his experiments in the following order, according to merit: 1st, corrosive sublimate; 2nd, acetate of lead; 3rd, sulphate of copper; and 4th, nitrate of copper.
In America, white oxide of zinc is used as a marine paint for ships and piles. In the United States Navy Yard at Gosport it is spoken well of, and very frequently employed. It is said to be much superior to white-lead, red-lead, verdigris, or coal-tar, and that timber covered with two coats of white zinc is neither attacked by the worm, nor do barnacles attach to it when immersed in salt water.
We can only find one instance of timber impregnated with water-glass having been tested against this subtle foe. Water-glass is certainly worth a further trial.
The instance we refer to occurred about forty years ago. In 1832, Dr. Lewis Feuchtwanger, of New York, was permitted by the Ordnance Department, under the direction of Commodore Perry, to perform experiments with water-glass on piles in the Brooklyn Navy Yard, and in various docks. The piles in the docks were destroyed by the teredo so fast that they had to be replaced every three years. The experiments proved highly satisfactory: the piles which had been so treated lasting many years, without any indication of being attacked by sea-worms.
The reader is referred to some works on water-glass mentioned below,[22] which are worthy of attentive perusal.
Third. The mechanical processes. They are few in number, and rather expensive.
At Saint Sebastian, in Spain, the piles of the wooden bridge standing in the sea have been guarded against the attacks of sea-worms in the following manner: Each pile is surrounded by a wooden box, and the space between filled up with cement. After six years it was proved that the piles were in a perfect condition, whilst the outer boxes were completely riddled by the worms. A similar method to this was adopted, some years since, to many of the piles in the Herne Bay Pier, which were affected by sea-worms. Several attempts had been made to protect the timber, by saturating it under various processes, with, however, only doubtful success. At last, a wooden casing was formed round each pile, leaving a space of about an inch all round, which was rammed full of lime or cement concrete. That process appeared to be perfectly successful, as the pier-master, who first adopted the method, stated that some of the piles had been so treated for three or four years, and although the worms had commenced their ravages, they appeared to have been checked, and not to have been able to exist when so enclosed.
In 1835, Brunel suggested an easy way of defending piles, which was to give them in the first instance a coat of tar; then powder them with brick-dust, which would render the wood sufficiently hard to receive a coat or two of cement. This is similar to the Dutch method.
Some foreigners use sheet lead nailed on to piles, and wrapped close round with well-tarred rope.
Copper sheathing has often been used for the protection of piling in piers and harbours. The destruction of copper by the action of sea-water is a matter which has long occupied the attention of scientific men, and it appears to be well ascertained that the decay does not result from the bad quality of the copper, for, according to Mr. Wilkinson, no difference could be discovered between the composition of copper that had endured well, and that which had been rapidly destroyed. Copper sheathing was used at Southend, but without success, for although nearly all the piles were covered with it for about 9 feet or 10 feet, the limnoria not only penetrated between the copper and the timber, but the copper had decayed to such an extent as in some cases to be no thicker than the thinnest paper; it was soft, and peeled off the wood very easily, and in two or three years would probably have been entirely destroyed.