1886.] OH New Applications of the Mechanical Properties of Cork. 439 



it permanently smaller. Besides the permanent set, there is a 

 cei'tain amount of what I venture to call sluggish elasticity, that is, 

 cork on being released from pressure, springs back a certain amount 

 at once, but the complete recovery takes an appreciable time. 



While I have been speaking, a piece of fresh cork, loaded so as 

 barely to float, has been inserted into the vertical glass pressure- 

 tube. I apply a slight pressure, you see the cork sinks. I release 

 the pressure, and it rises briskly enough. I now apply a much higher 

 l^ressure for a moment or two, I release it, and the cork will either 

 not rise at all, or will do so very slowly ; its volume has been 

 permanently altered ; it has taken a permanent set. 



In considering the properties of most substances, our search for the 

 cause of these properties is baffled by our imperfect powers and the 

 feeble instruments we possess for investigating molecular structure. 

 With cork, happily, this is not the case ; an examination of its 

 structure is easy, and perfectly explains the cause of its peculiar and 

 valuable properties. 



All plants are built up of minute cells of various forms and 

 dimensions. Their walls or sides are composed chiefly of a substance 

 called cellulose, frequently associated with lignine, or woody matter, 

 and with cork, which last is a nitrogenous substance found in many 

 portions of plants, but is especially developed in the outer bark of 

 exogenous trees, that is, trees belonging to an order, by far the 

 most common in these latitudes, the stems of which grow by 

 the addition of layers of fresh cellulose tissue outside the woody 

 part and inside the bark. Between the bark and the wood is inter- 

 posed a thin fibrous layer, which, in some trees, such as the lime, is 

 very much developed, and supplies the bass matting with which all are 

 familiar. The corky part of the bark, which is outside, is composed 

 of closed cells exclusively, Figs. 1 and 2, so built together that no 

 connection of a tubular nature runs up and down the tree, although 

 horizontal passages radiating towards the woody part of the tree are 

 niunerous. In the woody part of the tree, on the contrary, and 

 in the inner bark, vertical passages or tubes exist, while a con- 

 nection is kept up with the pith of the tree by means of medullary 

 rays. In one species of tree, known as the cork oak, the corky part 

 of the bark is very strongly developed. I project on the screen 

 the magnified image of a horizontal section of the bark of the cork 

 oak ; you see nine or ten bands running parallel to each other, 

 these are the layers of cellulose matter that have been deposited in 

 successive years. I turn the specimen, and you now see the vertical 

 section with the radiating passages clearly marked. 



The difference between the ai'rangement of the cells or tissue forming 

 the woody part of the tree and the bark is easily shown. I have here 

 three metal sockets, supported over a shallow wooden tray. Into 

 them are fitted, first, a cork cut out of the bark in a vertical 

 direction, next, a cork cut in a radial direction, and, lastly, a piece of 

 common yellow pine. By means of my force-pump, I apply a couj)le 



