144 
THE PHARMACEUTICAL JOURNAL AND TRANSACTIONS. 
[August 19,1871. 
most remarkable appearance of all, the moving or flick¬ 
ering stri;e which fill the tube when, after heating it 
considerably, the temperature is quickly lowered. This 
phenomenon was first described by myself in 1863, as it 
is seen in carbonic acid which has been partially liquefied 
by pressure, and afterwards heated a little above 31°. 
It may be observed on a larger scale and to great advan¬ 
tage by heating such liquids as sulphurous acid or ether 
in hermetically-sealed tubes. 
The experiments whose results I am about to describe 
have occupied me for a period of fully ten years; they 
involved the construction of novel forms of apparatus, 
in which the properties of matter might be studied under 
varied conditions of temperature and pressure, such as 
had never been realized before. In my earlier attempts 
I endeavoured, as others had already done, to use the 
expansive force of the mixed gases which are disengaged 
in the electrolysis of water ; and I was able in this way 
to obtain pressures of 150 atmospheres and even more in 
glass tubes; but the method was in many respects de¬ 
fective, and more than one dangerous explosion occurred, 
so that I eventually abandoned it. 
In the apparatus finally adopted, the gas to be com¬ 
pressed is enclosed in a long glass tube, of which the 
greater part of the length, or about 450 millimetres, has 
a capillary bore, and the remainder, about 150 milli¬ 
metres, an internal diameter of 2 millimetres. The free 
capillary end is sealed, while the gas in a pure and dry 
state is passing through; while at the other end the gas 
is confined by a movable column of mercury. The de¬ 
tails of the method by which this is accomplished will 
be found in the Bakerian lecture for 1869, to which I 
must also refer for an account of the process by which 
the original volume of the gas at the freezing-point of 
water and under one atmosphere of pressure was deter¬ 
mined, and also the volumes of the same gas deduced 
from the observed measurements when it was compressed 
at different pressures in the capillary tube. 
A conical protuberance on the capillary part of the 
tube, a little above its junction with the wider part, cor¬ 
responded as nearly as possible with a hollow cone in a 
stout brass flange, the joint being rendered perfectly 
tight by careful packing. The body of the apparatus 
consisted of two cold-drawn copper tubes of great 
strength, to the ends of which four massive brass flanges 
were firmly attached. Two corresponding flanges or 
end pieces, each carrying a fine steel screw packed with 
great care, were bolted on the lower flanges. The suc¬ 
cess of the experiments depended greatly on the packing 
of this screw. It was effected by means of a number of 
leather washers, tightly pressed down and saturated in 
vacuo with melted lard. The apparatus was now filled 
with water; the flanges with the glass tubes, one con¬ 
taining the gas to be examined, the other air or hydrogen 
to act as a manometer or measure of the pressure, were 
bolted down upon the upper flanges of the copper tubes. 
The joints had always leather washers interposed; and 
•when sufficiently tightened, they resisted any pressure 
which could be applied, even for an indefinite time. 
The two copper tubes were connected by a fine hori¬ 
zontal tube, so that the whole of the interior of the ap¬ 
paratus was in free communication. The pressure was 
obtained by screwing one or other of the steel screws 
into the water. I have recently had the apparatus con¬ 
structed of iron and filled with mercury. As mercury 
is much less compressible than water, the same length 
of screw produces a greater pressure on the interior of 
the apparatus, even with a larger cavity. There are 
other advantages in this form of the apparatus which I 
hope will facilitate future research. The objection to it 
is its extreme sensitiveness to changes of temperature, 
so that a variation of x £yth of a degree alters the in¬ 
ternal pressure by several atmospheres. 
In the actual experiments the gas under examination 
does not come into view till it has entered the capillary 
tube, and is exposed to a pressure of thirty or forty 
atmospheres. The limit of the pressure which can be 
obtained has hitherto been the capacity of resistance of 
the glass tubes to bursting. Fine thermometer glass 
tubes of white glass will frequently burst when exposed 
to a pressure of little more than 100 atmospheres; but 
green glass tubes of good quality are much stronger, 
and will easily bear a pressure of 300 atmospheres. One 
of the strongest forms of glass capillary tube for resisting 
internal pressure is obtained by drawing out a thick, 
green glass tube, heated to softening, till it becomes so 
fine as to be flexible. Tubes of this kind can easily be 
drawn out at the blowpipe table, and obtained of very 
uniform bore. I have compressed air in such tubes to 
j-iyth of its ordinary volume without bursting the 
tubes. 
Two rectangular brass cases, closed before and be¬ 
hind with plate-glass, surround, one the manometer, 
and the other the tube containing the gas to be exa¬ 
mined, and allow them to be maintained at any required 
temperature by the flow of a stream of water. The 
manometer was maintained as nearly as possible at 
the temperature of the apartment; the tube con¬ 
taining the gas, on the contrary, was maintained at 
different temperatures, according to the object in 
view. The following observations, published in 1863, 
contain the results of my earliest experiments on this 
subject:—“ On partially liquefying carbonic acid by 
pressure alone, and gradually raising the temperature 
at the same time to 88° Fahr., the surface of demarcation 
between the liquid and gas becomes fainter,. loses its 
curvature, and at last disappears. The space is then 
occupied by a homogeneous fluid, which exhibits when 
the pressure is suddenly diminished or the temperature 
slightly lowered, a peculiar appearance of moving or 
flickering striae throughout its entire mass. At tempe¬ 
ratures above 88° no apparent liquefaction, or separation 
into two distinct forms of matter, could be effected, even 
when a pressure of 300 or 400 atmospheres wms applied. 
Nitrous oxide gave analogous results.” 
The flickering striae referred to can be admirably 
.shown, as I mentioned before, in hermetically-sealed 
tubes of strong glass, partially filled with such liquids 
as sulphurous acid or ether. The liquid must in the 
first instance be heated a few degrees above what I have 
designated the “critical” point. The appearances ex¬ 
hibited by the ascending and descending sheets of mat¬ 
ter of unequal density are most remarkable, but must 
be seen in order to be understood. They only occur 
in this striking form in fluids heated a little above the 
critical point, and are produced by the great changes 
of density which slight variations of pressure or tempe¬ 
rature produce in this case. They are always a clear 
proof that the matter in the tube is homogeneous, and 
that we have not liquid and gas in presence of one 
another. These striae are in short only an extraordinary 
development of the movements seen in ordinary liquids 
and gases when they are heated from below. The ex¬ 
periments to be immediately described will explain their 
great intensity above the critical point. 
When the temperature falls below the critical point, 
the formation of a cloud indicates that we have now 
heterogeneous matter in the tube, fine drops of liquid in 
presence of a gas. We must take care, however, not to 
suppose that a cloud necessarily precedes the formation 
of true liquid. If the pressure be sufficiently great, no 
cloud of any kind will form. 
I now proceed to describe the general results of the 
experiments upon carbonic acid. If a certain volume of 
carbonic acid at the temperature of 13°* 1 and under a 
pressure of one atmosphere be exposed to a gradual!y- 
increasing pressure, its volume will steadily diminish, 
but at a faster rate than according to Boyle’s law, till at 
the pressure of 48 0, 9 atmospheres its volume is reduced 
to about -j^-st of the original volume at one atmosphere. 
Liquefaction now begins and continues with very slight 
augmentation of pressure, the necessity for which I 
