1886.J on NeiD Applications of the Mechanical Properties of Cork. 443 



the law of diffusion, it will get out of the tube through the cork 

 quicker than the air can get in by the same means, and the result must 

 be that a partial vacuum will be formed in the tube, and a column of 

 water will be drawn up. You see that such has been the case, and 

 we have thus proved that the cells of cork are eminently pervious to 

 gases. The pores in the cell-walls appear, however, to be too minute 

 to permit the passage of liquids. 



I closed the end of a glass tube 11 mm. diameter, with a" disc of 

 cork 1*75 mm. thick, cut at right angles to the axis of the tree, I 

 placed a solution of blue litmus inside the tube, and suspended it in 

 a weak solution of sulphuric acid. Had diffusion taken place, both 

 liquids would have assumed a red colour, but after sixteen hours no 

 change whatever could be detected. A like inertness was exhibited 

 when the tube was filled with a solution of copper sulphate and sus- 

 pended in a weak solution of ammonia ; a deep blue colour would 

 have appeared had any intermixture taken place, and the same tube 

 is before you immersed in ammonia and filled with red litmus solu- 

 tion. It has been in this condition since the 28th of February, but 

 no diffusion has taken place. A disc of wood 6 mm. thick under 

 the same circumstances showed, after a couple of hours, by the liquids 

 turning blue, that diffusion was going on actively. It is this property 

 of allowing gases to permeate while completely barring liquids that 

 enables cork to be kept in compression under water or in contact with 

 various liquids without the air-cells becoming water-logged, and that 

 makes cork so admirable an article for waterproof wear, such as boot 

 soles and hats, for, unlike indiarubber, it allows ventilation to go on 

 while it keeps out the wet. The cell-walls are so strong, notwith- 

 standing 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 : these, no doubt, add to the 

 strength and resistance of the structure. 



From what you have seen you will have no difficulty in arriving 

 at the conclusion 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 com- 

 pression to rise in a manner 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. Bat 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. This fact can 

 be readily demonstrated by the lever press which I have used, for if 

 the brass cylinder containing the cork be filled with soap and water 



