CORK, ITS MANUFACTURE AND PROPERTIES. 643 



reason provided with devices for the automatic sharpening of the 

 knives. The cells of the cork are tilled with gaseous matter, which 

 Mr. G. II. Ogston has proved by analysis to be common air, and to 

 exist occluded in the cork, to the amount of about fifty-three per cent 

 of its volume. The facility with which this air escapes when placed 

 in an exhausted receiver, is very remarkable when compared with the 

 impermeability of cork to liquids. It is the coexistence of these two 

 properties that of allowing gases to permeate while completely bar- 

 ring liquids, both of which are easily and clearly demonstrated by 

 suitable experiments that enables cork to be kept in compression 

 under water or in contact with various liquids without the air-cells 

 becoming water-logged ; and it is the same properties that make cork 

 so admirable an article for water-proof wear, such as boot-soles and 

 hats. By virtue of the combination, it is superior to India-rubber, for 

 it allows ventilation to go on while it keeps out the wet. The cell- walls 

 are so strong, notwithstanding their extreme thinness, that they appear 

 when empty to be able to resist the atmospheric pressure, for the vol- 

 ume of the cork does not sensibly diminish, even when all the air has 

 been extracted. Viewed under very high power, cross-stays or 

 struts of fibrous matter may be distinguished traversing the cells, 

 which, no doubt, add to the strength and resistance of the structure. 



We conclude, then, that cork consists practically of an aggregation 

 of minute air-vessels, having very thin, very water-tight, and very 

 strong walls, and hence, if compressed, we may expect the resistance 

 to compression to rise more like the resistance of gases than the resist- 

 ance of an elastic solid such as a spring. In a spring the pressure in- 

 creases in proportion to the distance to which the spring is com- 

 pressed, but with gases, the pressure increases in a much more rapid 

 manner that is, inversely as the volume which the gas is made to oc- 

 cupy. But, from the permeability of cork to air, it is evident that if 

 subjected to pressure in one direction only, it will gradually part with 

 its occluded air by effusion that is by its passage through the porous 

 walls of the cells in which it is contained. 



On the other hand, if cork be subjected to pressure from all sides, 

 such as operates when it is immersed in water under pressure, then 

 the cells are supported in all directions, the air in them is reduced in 

 volume, and there is no tendency to escape in one direction more than 

 another. An India-rubber bag distended by air bursts if pressed be- 

 tween two surfaces, but if an India-rubber cell be placed in a glass 

 tube and subjected to hydraulic pressure, it is merely shriveled up, 

 the strain on its walls is actually reduced. 



To take advantage of the peculiar properties of cork in mechanical 

 applications, it is necessary to determine accurately the law of its re- 

 sistance to compression. For this purpose, Mr. Anderson introduced a 

 quantity of cork into a strong iron vessel of five and a half gallons 

 capacity, and filled the interstices full of water, carefully getting out 



