Page:Popular Science Monthly Volume 31.djvu/661

Rh 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 barring 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 volume 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 resistance of an elastic solid such as a spring. In a spring the pressure increases in proportion to the distance to which the spring is compressed, but with gases, the pressure increases in a much more rapid manner—that is, inversely as the volume which the gas is made to occupy. 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 between 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 resistance 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