rigid. The latter substance can be compressed indefinitely in one direction while it is left free to move and expand in other directions. When tightly inclosed so that it can not yield, it can not be compressed by any force that can be brought to bear upon it. In the former case all the volume which is lost by the pressure in one direction is regained by the expansion in other directions, and the whole is not changed; in the latter case there is no room for the compensatory expansion; so when India-rubber is stretched, it gains in volume in one direction at the expense of an equivalent loss in other directions.
Metals, when subjected to pressures which exceed their elastic limits, so that they are permanently deformed, as in forging or wire-drawing, remain practically unchanged in volume per unit of weight. Cork behaves in a very different manner. If a cylinder of cork is tightly inclosed in a tube in the same manner as the India-rubber which refused to yield to any force, and pressure is applied to it, it is readily and visibly compressed; and when released it expands back to its original volume. In this case a great change in the volume of the material is easily effected.
When cork is subjected to alternate applications and relaxations of pressure, it coincidently contracts and expands. It is this singular property which gives it its value as a means of closing the mouths of bottles. Its elasticity has not only a very considerable range, but it is very persistent. The extent to which the better class of corks used in bottling the effervescent wines will expand the instant they escape from the bottles, is well known. As measured by Mr. Anderson, this expansion amounts to an increase of seventy-five per cent in the volume, even after the corks have been kept under compression for ten years. If the cork be steeped in hot water, the volume will continue to increase till it becomes nearly three times that which the cork occupied in the neck of the bottle.
When cork is subjected to pressure, either in one direction or from every direction, a certain amount of permanent deformation or "permanent set" takes place very quickly. This property is common to all solid elastic substances when strained beyond their limits of elasticity, but with cork the limits are comparatively low; thus, in chemists' and other shops, when a cork is too large to fit a bottle, the shopkeeper gives it a few sharp bites, or squeezes it with pincers to beyond its elastic limits, and so makes it permanently smaller. Besides the permanent set, there is a certain amount of what might be called sluggish elasticity; that is, cork on being released from pressure, springs back a certain amount at once, but the complete recovery takes an appreciable time.
These peculiar and valuable properties of cork are easily explained after examining its structure. The corky part of bark is composed of closed cells exclusively, and this part is developed to a very unusual degree in the cork-oak. A section of cork, taken in the horizontal
