438 Mr. Anderson [April 9, 



after having been released from pressures high enough to distort 

 them permanently, yet, while actually under pressure, the volumes 

 may have been considerably altered. As far as I am aware, this 

 point has not been determined experimentally for metals, but it is 

 very easy to show that indiarubber does not change. 



I have here some of this substance which is so very slightly 

 lighter than water, that, as you see, it only just floats in cold water 

 but sinks in hot. If I could put it under considerable pressure while 

 afloat in cold water, then, if its volume became sensibly less, it ought 

 to sink. In the same way if I load a piece of cork and a piece of 

 wood so that they barely float, if their volumes alter, they ought to 

 sink. 



In this strong upright glass tube, I have, at the top, a piece of 

 indiarubber, immediately below it a piece of wood, and below that a 

 cork, the wood and the cork are loaded with metal sinkers to reduce 

 their buoyancy. The tube is full of water and is connected to a 

 force-pump by means of which I can impose a pressure of over 

 1000 lbs. per square inch. The image of the tube is now thrown on 

 the screen and the pressure is being applied. You see at once the 

 cork is beginning to shrink in all directions, and now its volume is 

 so reduced that it is incapable of floating and sinks down to the 

 bottom of the tube. The indiarubber is absolutely unaffected, the wood 

 does contract a little, but not sufficiently to be visible to you or to cause 

 it to sink. I open a stop-cock and relieve the pressure, you see that 

 the cork instantly expands, its buoyancy is restored and it floats again. 

 By alternately applying and taking off the pressure I can produce 

 the familiar effect, so well known in the toy called " the bottle imps." 

 It is this singular pro2)erty which gives to cork its value as a means 

 of closing the mouths of bottles. Its elasticity has not only a very 

 considerable range, but it is very persistent. Thus in the better 

 kind of corks used in bottling cham^^agne and other effervescing 

 wines, you are all 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 /)f volume of 

 75 per cent, even after the corks have been kept in a state of com- 

 pression in the bottles for ten years. If the cork be steeped in hot 

 water, the volume continues to increase till it attains nearly three 

 times that which it occupied in the neck of the bottle. 



When cork is subjected to pressure, cither in one direction, as in 

 this lever press, or from every direction, as when immersed in water 

 under pressure, a certain amount of permanent deformation or 

 " permanent set " takes place very quickly. This property is common 

 to all solid elastic substances when strained beyond their elastic limits, 

 but with cork the limits are comparatively low. You have, no doubt, 

 noticed in chemists' and other shoi)s, that when a cork is too large 

 to fit a bottle, the shop-keeper gives the cork a few sharp bites, or, if 

 he be more refined, he uses a pair of specially contrived j^incers, in 

 either case he squeezes the cork beyond its clastic limits and so makes 



