that no exterior application of any nature whatever, or modification of the surface merely, would give any efficacious guarantee of protection against the teredo. Even supposing that one or another of these means would prevent the young teredos from attaching themselves to the wood, yet the constant friction of the water or ice, or any accident, might break the surface of the wood sufficiently to give access to the teredo.
This seems a proper place to mention a practice in general use in Holland for warding off the teredo: this consists in covering wood with a coat-of-mail made of nails. This operation is very costly; for, to really protect wood in this way, it is important that the square heads of the nails join exactly; for insuring the best results, the armored piles are exposed in the open air for some time before being placed in the water, that rust, forming on the surface of the iron, may close up the interstices inevitably remaining between the heads of the nails. But this precaution is not infallible, as the commission examined piles more than once, in the course of its investigation, which had been several years in the water, and whose surface was entirely incrusted with rust more than a centimetre thick, but which were, nevertheless, eaten in the interior by the teredo.
Sluice-gates are frequently covered by sheets of iron, copper, or zinc. It is evident that, so long as such covering remains intact, there is no cause for anxiety on the score of the teredo. Unfortunately, experience has taught us that this protection is not permanent, but is rendered ineffectual by being broken by the force of the water or blocks of ice.
Nature affords sometimes, as we have seen above, a more efficacious protection in covering wood with barnacles or other shell-fish, with the condition that this covering be made before the young teredo attaches itself to the wood. Facts of this sort have led Lehmann to propose the planting on wood of the common mussel (Mytilus edulis).
Impregnation of Wood with Different Substances.—The commission examined in this category the following methods:
1. Sulphate of Copper.—The impregnation of the blocks with this salt was performed at the factory of MM. Van der Elst and Smit, at Amsterdam. Experience proved, even in the first summer (of 1859), that this preparation had absolutely no power against the teredo. Nevertheless, to make sure that the failure of this experiment was not due to insufficient preparation, the commission procured from the establishment of M. Boucherie, at Paris, two pieces of beech-wood covered with its bark, two pieces of beech without bark, and two pieces of pine, all prepared with sulphate of copper. These blocks, when exposed, did not resist the teredo any better than those prepared at Amsterdam. These trials completely confirmed the results obtained by the engineer Noyon (“On the Inefficacy of the Boucherie Process in Sea-Water,” Annals of Bridges and Roads, April, 1859).
2. Sulphate of Protoxide of Iron (Green Vitriol).—The blocks were