New volumes of the Encyclopædia Britannica > Volume 30, K-MOR

282
LIQUID
GASES
having been placed in it, it is heated in an oil- or air-bath
to about 200° C., so as to volatilize the mercury, the
vapour of which is removed by the pump. After the
process has gone on for some time, the pipe leading to
the pump is sealed off, the vessel immediately removed
from the bath, and the small subsidiary part immersed in
some cooling agent such as solid carbonic acid or liquid
air, whereby the mercury vapour is condensed in the small
vessel and a vacuum of enormous tenuity left in the large
one. The final step is to seal off the tube connecting the
two. In this way a Vacuum may be produced having a
vapour pressure of about the hundred-millionth of an
atmosphere at 0° C. If, however, some liquid mercury be
left in the space in which the vacuum is produced, and
the containing part of the vessel be filled with liquid air,
the bright mirror of mercury which is deposited on the
inside wall of the bulb is still more effective than silver in
protecting the chamber from the influx of heat, owing to
the high refractive index, which involves great reflecting
power, and the bad heat-conducting powers of mercury.
Thermal Transparency at Low Temperatures.—The proposition,
once enunciated by Pictet, that at low temperatures all substances
have practically the same thermal transparency, and are equally
ineffective as non-conductors of heat, is based on erroneous
observations. It is true that if the space between the two walls
of a double-walled vessel is packed with substances like carbon,
magnesia, or silica, liquid air placed in the interior will boil off
even more quickly than it will when the space merely contains air
at atmospheric pressure ; but in such cases it is not so much the
carbon, &c., that bring about the transference of heat, as the air
contained in their interstices. If this air be pumped out such
substances are seen to exert a very considerable influence in
stopping the influx of heat, and a vacuum vessel which has the
space between its two walls filled with a non-conducting material
of this kind preserves a liquid gas even better than one in which
that space is simply exhausted of air. In experiments on this
point double-walled glass tubes, as nearly identical in shape and
size as possible, were mounted in sets of three on a common stem
which communicated with an air-pump, so that the degree of
exhaustion in each was equal. In two of each three the space
between the double walls was filled with the powdered material it
was desired to test, the third being left empty and used as the
standard. The time required for a certain quantity of liquid air
to evaporate from the interior of this empty bulb being called 1,
in each of the eight sets of triple tubes, the times required for the
same quantity to boil off from the other pairs of tubes were as
follows :—
f Charcoal . . 5
\ Magnesia . . 2
/ Graphite . . 1 ‘3
\ Alumina . . 3'3
f Calcium carbonate 2-5
\ Calcium fluoride . 1'25
/Phosphorus (amorphous) 1
\ Mercuric iodide . l-5
Other experiments of the same kind made—(a) with similar
vacuum vessels, but with the powders replaced by metallic and
other septa ; and (6) with vacuum vessels having their walls
silvered, yielded the following results
/"Lampblack
\ Silica
/Lampblack
/Lycopodium
/Barium carbonate
\ Calcium phosphate
/Lead oxide
/Bismuth oxide
5
4
4
2-5
1- 3
2- 7
2
6
'(a) Vacuum space empty 1
Three turns silver
paper, bright sur¬
face inside . . 4
Three turns silver
paper, bright sur¬
face outside . 4
" Vacuum space empty 1
Three turns gold
paper, gold outside 4
Some pieces of gold-
leaf put in so as
to make contact
between walls of
l vacuum-tube . 0’3
f (J>) Vacuum space empty,
silvered on inside
surfaces . . 1
Silica in silvered
vacuum space . 1 '1
{Vacuum space empty . 1
Three turns black paper,
black outside . . 3
Three turns black paper,
black inside . . 3
Vacuum space empty
Three turns, not touch¬
ing, of sheet lead
Three turns, not touch¬
ing, of sheet alumi¬
nium
Empty silvered vacuum 1
Charcoal in silvered
vacuum . . . 1 '25
It appears from these experiments that silica, charcoal, lamp¬
black, and oxide of bismuth all increase the heat insulations to
four, five, and six times that of the empty vacuum space. As the
chief communication of heat through an exhausted space is by
molecular bombardment, the fine powders must shorten the free
path of the gaseous molecules, and the slow conduction of heat
through the porous mass must make the conveyance of heat-energy
more difficult than when the gas molecules can impinge upon the
relatively hot outer glass surface, and then directly on the cold one
without interruption. (See Proc. Roy. Inst., vol. xv. pp. 821-826.)
Density of Solids and Coefficients of Expansion at Low Tem¬
peratures.—The facility with which liquid gases, like oxygen or
nitrogen, can be guarded from evaporation by the proper use of
vacuum vessels (now called Dewar vessels), naturally suggests
that the specific gravities of solid bodies can be got by direct
weighing when immersed in such fluids. If the density of the
liquid gas is accurately known, then the loss of weight by fluid
displacement gives the specific gravity compared to water. The
metals and alloys, or substances that can be got in large crystals,
are the easiest to manipulate. If the body is only to be had in
small crystals, then it must be compressed under strong hydraulic
pressure into coherent blocks weigliing about 40 to 50 grammes.
Such an amount of material gives a very accurate density of the
body about the boiling point of air, and a similar density taken
in a suitable liquid at the ordinary temperature enables the
mean coefficient of expansion between +15° C. and -185° C. to
be determined. One of the most interesting results is that the
density of ice at the boiling point of air is not more than O’OS, the
mean coefficient of expansion being therefore 0 •000081. As the
value of the same coefficient between 0° C. and - 27° C. is 0 •000155,
it is clear the rate of contraction is diminished to about one-half
of what it was above the melting point of the ice. This suggests
that by no possible cooling at our command is it likely we could
ever make ice as dense 'as water at 0° C., far less 4°C. In other
words, the volume of ice at the zero of temperature would not be
the minimum volume of the water molecule, though we have
every reason to believe it would be so in the case of the majority
of known substances. Another substance of special interest is solid
carbonic acid. This body has a density of 1'53 at - 78° C. and
1’633 at -185°C., thus giving a mean coefficient of expansion
between these temperatures of 0‘00057. This value is only about
of the coefficient of expansion of the liquid carbonic acid gas
just above its melting point, but it is still much greater at the low
temperature than that of highly expansive solids like sulphur,
which at 40° C. has a value of 0-00019. The following table gives
the densities at the temperature of boiling liquid air (-185° C.)
and at ordinary temperatures (17° C.)> together with the mean
coefficient of expansion between those temperatures, in the case of
a number of hydrated salts and other substances
Table I.
Density
at -185°
C.
Density
at +17°
C.
Mean co¬
efficient of
expansion be¬
tween -185'
C. and
+17° C.
Sulphate of aluminium (18)1
Biborate of soda (10)
Chloride of calcium (6)
Chloride of magnesium (6)
Potash alum (24)
Chrome alum (24)
Carbonate of soda (10)
Phosphate of soda (12)
Hyposulphite of soda (5) .
Ferrocyanide of potassium (3)
Ferricyanide of potassium
Mtro-prusside of sodium (4)
Chloride of ammonium
Oxalic acid (2) .
Oxalate of methyl
Paraffin
Naphthalene
Chloral hydrate
Urea
Iodoform
Iodine
Sulphur
Mercury
Sodium
Graphite (Cumberland)
1-7194
1-7284
1-7187
1-6039
1-6414
1-7842
1 -4926
1-5446
1-7635
1-8988
1-8944
1-7196
1-5757
1-7024
1-5278
0-9770
1-2355
1-9744
1- 3617
4-4459
4-8943
2- 0989
14-382
1- 0056
2- 1302
1-6913
1-6937
1-6775
1-5693
1-6144
1-7669
1-4460
1-5200
1-7290
1-8533
1-8109
1-6803
1-5188
1-6145
1-4260
0-9103
1-1589
1-9151
1- 3190
4-1955
4-6631
2- 0522
0-972
2*0990
0-0000811
o-oooiooo
0-0001191
0-0001072
0-0000813
0-0000478
0-0001563
0-0000787
0-0000969
0-0001195
0-0002244
0-0001138
0-0001820
0-0002643
0-0003482
0-0003567
0-0003200
0-0001482
0-0001579
0-0002930
0-0002510
0-0001152
0-00008812
0-0001810
0-0000733
1 The figures within parentheses refer to the number of molecules
of water of crystallization.
2 - 189° to - 38°-85 C.