Fune 24, 1886} 
NATURE 
183 
The minute structure of the bark is very remarkable. First, 
I project on the screen a microscopic section of the wood of the 
cork tree. It is taken in a horizontal plane, and I ask you to 
notice the diversity of the structure, and especially the presence 
of large tubes or pipes. I next exhibit a section taken in the 
same plane ofthe corky portion of the bark. You see the whole 
substance is made up of minute many-sided cells about 1/750 
of an inch in diameter, and about twice as long, the long way 
of the cells being disposed radially to the trunk. The walls of 
the cells are extremely thin, and yet they are wonderfully im- 
pervious to liquids, Looked at by reflected light, if the speci- 
men be turned, bands of silvery light alternate with bands of 
comparative darkness, showing that the cells are built on end to 
end in regula> order. The vertical section next exhibited shows 
a cross section of the cells looking like a minute honeycomb. 
In some specimens large numbers of crystals are found. These 
could not be distinguished from the detached elementary 
spindle-shaped cells, of which woody fibre is made up, were it 
not for the powerful means of analysis we have in polarised 
light. I need hardly explain to an audience in this Institution 
that light passed through a Nicol prism becomes polarised, 
that is to say, the vibrations of the luminiferous ether are 
all reduced to vibrations in one plane, and, consequently, if 
a second prism be interposed and placed at right angles to the 
first, the light will be unable to get through ; but if we introduce 
between the crossed Nicols a substance capable of turning the 
plane of vibration again, then a certain portion of the light will 
pass. I have now projected on the screen the feeble light 
emerging from the crossed Nicols. I introduce the microscopic 
preparation of cork cells between them, and you see the crystals 
glowing with many-coloured lights on a dark ground. 
Minute though these crystals are, they are very numerous and 
hard, and it is partly to them that is due the extraordinary rapidity 
with which cork blunts the cutting instruments used in shaping 
it. Cork-cutters always have beside them a sharpening-stone, 
on which they are obliged to restore the edges of their knives 
after a very few cuts. 
The cells of the cork are filled with gaseous matter, which is 
very easily extracted, and which has been analysed for me by 
Mr. G. H. Ogston, and proved to be common air. I have here 
a glass tube in which are some pieces of cork which have been 
cut into slices so as to facilitate the escape of the air. I connect 
the tube with an exhausted receiver and project the image on the 
screen ; you see rising from the cork bubbles of air as numerous, 
but much more minute than the bubbles which rise from sparkling 
wines ; much more minute, because the bubbles you see are 
expanded to seven or eight times their volume at atmospheric 
pressure on account of the vacuum existing in the tube. The 
_ air will continue to come off for an hour or more, and from 
__ measurements made by Mr. Ogston I find that the air occluded 
in the cork amounts to about 53 per cent. of its volume. The 
facility with which the air escapes, compared with the imper- 
meability of cork to liquids is very remarkable. 
I throw on the screen the image of a section cut from a cork 
which was kept under a vacuum of about 26 inches for five days 
and nights ; aniline dye was then injected, and yet you see that 
the colour has not more than permeated the outermost fringe of 
cells—those, in fact, which had been broken open by the opera- 
tion of cutting the cork. By keeping cork for a very long time 
in an almost perfect vacuum, and then injecting dye, a slight 
darkening of the general colour of a section of the cork may be 
noticed, but it is very slight indeed. How, then, does the air 
escape so readily when the cork is placed zz vacuo ? 
The answer is, that gases possess the property of diffusion ; 
that is, of passing through porous media of inconceivable fine- 
ness. When two gases, such as hydrogen and air, are separated 
by a porous medium, they immediately begin to pass into each 
other, and the lighter gas passes through more quickly than the 
heavier. 
_ of cork are eminently pervious to gases. 
- Ihave here a glass tube, the upper end of which is closed by 
a thin slice of cork, the lower end dips into a basin of water. 
Some hours ago the tube was filled with hydrogen, which you 
know is about 144 times lighter than air ; consequently, accord- 
ing to 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 
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 disk 
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 suspended 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 im- 
mersed in ammonia and filled with red litmus solution. It has 
been in this condition since February 28, but no diffusion has 
taken place. A disk of wood 6 mm, thick under the same cir- 
cumstances 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 india-rubber, it 
allows ventilation to go on while it keeps out the wet. The 
cell-walls are so strong, notwithstanding their extreme thinness, 
that they appear, when empty, to be able to resist the atmo- 
spheric 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 compression to rise in a manner more 
like the resistance of gases than the resistance of an elastic solid 
such as a spring. Ina spring the pressure increases in propor- 
tion 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. 
But 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 and pressure be then applied, minute bubbles 
will be found to collect on the surface, and their formation will 
go on for many hours. 
On the other hand, if cork be subjected to pressure from all 
sides, such as operates when it is immersed in water under 
pressure, then the cells are supported in all directions, the air in 
them is reduced in volume, and there is no tendency to escape in 
one direction more than another. An india-rubber bag, such as 
this, distended by air, bursts, as you see, if pressed between two 
surfaces, but if an india-rubber cell be placed in a glass tube-and 
subjected to hydraulic pressure, it is merely shrivelled up; the 
strain on its walls is actually reduced. 
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 I 
instituted a series of experiments of this kind. Into a strong iron 
vessel of 54 gallons capacity I introduced a quantity of cork, and 
filled the interstices full of water, carefully getting out all the air. 
I then proceeded to pump in water, until definite pressures up to 
1000 pounds per square inch had been reached, and, at every 100 
pounds, the weight of water pumped in was determined. In this 
way, after many repetitions, I obtained the decrease of volume 
due to any given increase of pressure. The observations have 
been plotted into the form ofa curve, which yousee on the diagram 
on the wall. 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 
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 250 
pounds per square inch. At 1000 pounds per square inch the 
volume was reduced to 44 per cent. ; the yielding then became 
very little, showing that the solid parts of the cells had nearly 
come together, and this corroborates Mr. Ogston’s determina- 
tion that the gaseous part of cork constitutes 53 per cent. of 
its bulk. The engineer, in dealing with a compressible sub- 
stance, requires to know not only the pressure which a given 
