The iron fastenings used about timber frequently cause its premature decay. Iron, under the influence of moisture becomes rusty, that is, oxygen, either from the air or from the wood itself, unites with the metal, forming an oxide, which, in its turn acts upon the woody fibre, and gradually destroys its tenacity. The iron is further subject to attack from the acid juices of the wood; this effect, however, varies in different woods. Oak contains a smaller proportion of oily or resinous particles than many other kinds of wood; and, in addition to the usual vegetable acid common to most woods, oak contains an acid peculiar to itself, called _gallic_ acid. In teak, on the contrary, the quantity of acid is not only smaller, but the resinous particles are very abundant, and these form a sort of protecting covering to the iron fastenings. Maconochie states, on the authority of the shipping built in India and used in the India trade, that the average duration of an iron-fastened teak ship is thirty years; and that it is a misapplication of expense to use copper fastenings with teak, as the additional advantage gained is not at all commensurate with the additional expense. But it is different with oak; the action of oak on copper is by no means so destructive as on iron, and the reaction of the metal on the wood is not so destructive. The methods which have been from time to time adopted for the preservation of timber are so numerous, that a slight sketch of them would probably fill a good-sized volume. We will name a few of the most successful, and terminate this notice with a description of the method now in practice.