CORK, ITS MANUFACTURE AND PROPERTIES. 641 



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 com- 

 pressed 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 direc- 

 tion 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 lim- 

 its, 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 vol- 

 ume, 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 occu- 

 pied 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 "per- 

 manent 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 per- 

 manent 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 appreci- 

 able 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 



