182 
NA TO RE 
[Fune 24, 1886 
result will be found the same. Extension also is incapable of 
appreciably altering the density of metals. I attach to the 
scales two specimens of iron taken from a bar which had been 
torn asunder by a steady pull. One specimen is cut from the 
portion where it had not been strained, and the other from the 
very point where it had been gradually drawn out and fractured. 
‘The specimens balance, I immerse them, you see the balance is 
not destroyed ; hence the volume of the iron has not been 
changed appreciably by extension. 
But cork behaves in a very different manner. I place this 
cylinder of cork into just such a brass tube as served to restrain 
the india-rubber and apply pressure to it in the same way; you 
see I can readily compress the cork, and when I release it it 
expands back to its original volume: the action is a little 
sluggish on account of the friction of the cork against the sides 
of the tube. In this case, therefore, a very great change in the 
volume of the material has been easily effected. 
But although solids evidently do not change sensibly in bulk, 
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 india-rubber 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 
india-rubber, 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 
india-rubber 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 property 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 
champagne 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 of volume of 75 per cent., even 
even after the corks have been kept in a state of compression 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, either in one direction, as 
in this lever press, or from every direction, as when immersed 
in water under pressure, a certain amount of permanent de- 
formation 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 compara- 
tively low. You have, no doubt, noticed in chemists’ and other 
shops 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 re- 
fined, he uses a pair of specially-contrived pincers; in either 
case he squeezes the cork beyond its elastic limits, and so makes 
it permanently smaller. Besides the permanent set, there is a 
certain 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. apply a slight pressure, you see the cork sinks. 
I release the pressure, and it rises briskly enough. I now apply 
a much higher pressure 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 sub- 
stance found in many portions of plants, but is es) ecially de- 
veloped in the outer bark of exogenous trees, that is, trees 
belonging to an order, by far the most common in these lati- 
tudes, 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 interposed 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, 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 numerous. In the woody part of the tree, on the 
contrary, and in the inner bark, vertical passages or tubes exist, 
while a connection 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 speci-- 
men, and you now see the vertical section with the radiating 
passages clearly marked. 
The difference between the arrangement 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 direc- 
tion, and, lastly, a piece of common yellow pine. By means of 
my force-pump, I apply a couple of atmospheres of hydraulic 
pressure. I project an image of the apparatus on the screen, 
and you see the water has made its way through the wood and 
through the cork cut in the radial direction, while the cork cut 
in the vertical direction is impervious. 
The cork tree, a species of evergreen oak, is indigenous in 
Portugal and along both shores of the Mediterranean. The 
diagram on the wall has been painted from a sketch obligingly 
sent to me by Mr. C. A. Friend, the resident engineer of the 
Seville Waterworks, to whom I am also indebted for this branch 
of a cork tree, these acorns, this axe used in getting the cork, 
and fora description of the habits of the tree, its cultivation, 
and the mode of gathering the harvest. 
The cork oak attains a height of 30 to 40 feet ; it is not culti- 
vated in any way, but grows like trees in a park. The first 
crop is not gathered till the tree is thirty years old, the next nine 
or ten years later ; both these crops yield inferior cork, but at 
the third crop, gathered when the tree is fifty years old, the 
bark has attained full maturity, and after that will yield the 
highest quality of cork every nine or ten years. In the autumn 
of the year, when the bark is in a fit state, that is, for small 
trees, from three-quarters of an inch to one inch thick, and for 
larger ones up to one inch and a half, a horizontal cut is made, 
by means of a light axe like the one I hold in my hand, through 
the bark a few inches above the ground; succeeding cuts are 
made at distances of about a yard, up to the branches, and even 
along some of the large ones, then two or more vertical cuts, 
according to the size of the tree, and the bark is ripped off by 
inserting the wedge-shaped end of the axe-handle. In making 
the cuts great care is taken to avoid wounding the inner bark, 
upon the integrity of which the health of the tree depends ; but 
where this precaution is taken, the gathering of the cork does 
not in any way injure the tree. ’ 
After stripping, the cork is immersed for about an hour in hot 
water, it is dressed with a kind of spokeshave, then laid out flat” 
and weighted in order to take out the curvature; it is then 
stacked in the open air, without protection of any kind, for cork 
does not appear to be susceptible of receiving injury from the 
weather. 
7 a, 
