Solid substances, with the exception of cork, offer equally obstinate resistance to change of bulk; even India rubber, which most people would suppose capable of very considerable change of volume, we find it really very rigid. Metals, when subjected to pressure 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. Not so with cork, its elasticity has not only a very considerable range, but it is very persistent. Thus in the better kind of corks used in bottling champagne and other effervescing wines, you are familiar with the extent to which the corks expand the instant they escape from the bottles. I have measured this expansion and find it to amount to an increase of volume of seventy-five per cent; even after the corks have been kept in a state of compression in the bottles for ten years.[23] 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. This property is common to all solid elastic substances when strained beyond their elastic limits, but with cork the limits are comparatively low.” 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, and for this purpose Mr. Anderson instituted a series of experiments of this kind. Into a strong iron vessel of five and one half gallons’ capacity he introduced a quantity of cork and filled the interstices with water, carefully getting out all the air. He then proceeded to pump in water until definite pressures up to one thousand pounds per square inch had been reached, and at every one hundred pounds the weight of the water pumped in was determined. In this way, after many repetitions, he obtained the decrease of volume due to any given increase of pressure. The observations have been plotted into the form of a curve which is discernible on the accompanying diagram. [23] Showing a permanent set of 12.5 per cent. [Illustration: VOLUME OF CORK] The base line represents a cylinder containing one cubic foot of cork divided by the vertical lines into ten parts; the black horizontal lines, according to the scale on the left-hand side, represent the pressures in pounds per square inch which were necessary to compress the cork to the corresponding volume. Thus to reduce the volume to one half, required a pressure of two hundred and fifty pounds per square inch. At sixteen hundred pounds per square inch the volume was reduced to forty-four per cent, the yielding then becoming very little, showing that the solid parts of the cells had come together and formed a solid, compact mass, thus corroborating Mr. Ogston’s determination that the gaseous part of cork constitutes about fifty-three per cent of its bulk.