From the tying of knots we may pass on to the tying over of bottles, preserves, jars, &c.; the object with which this operation is performed is either to prevent the excess of air or the escape or entrance of moisture; the act itself is so very simple as to require no explanation; but a few words may be said on the choice of material, which should be varied, so as to suit the exigencies of each particular case. When a vessel of spirit is to be tied over, leather is frequently selected—a very erroneous practice, as the vapour of spirit passes readily through that substance, but cannot penetrate bladder, which should be invariably used for the purpose. So effectually is spirit confined by bladder, that when weak spirits are put into bladders or into vessels tied over with bladder, and allowed to remain some time, they are strengthened, as the vapour of the water passes away, that of the spirit being retained. Bladder, or other animal membranes of the same nature, in a moist and flaccid state, are usually selected for tying over preserves and jams, for which they are well adapted; should it be impracticable to obtain them, the waxed paper described at page 165 is a very good substitute. Many persons place a thin piece of oiled paper in the jar resting on the jam, in addition to tying it down; this assists in excluding air and preventing mouldiness, but we have found a piece of very thin paper moistened with white of egg much more efficacious. The thin sheet-lead used for lining the interior of tea-chests, or stout tin-foil, is very advantageously used in tying down vessels containing specimens of natural history preserved in spirits, as they effectually prevent the escape of the latter for a long series of years. The plan usually pursued is to tie the cork over first with a single bladder, then with the metal, and finally with a second piece of bladder, which is afterwards covered with a coat of black paint. [Illustration: _Fig. 20._]