310 
NAT COLE 
| JANUARY 24, 1907 
soft glass apparatus, to a far higher temperature than the 
softening point of glass. Calcium heated in this mannei 
is, under suitable conditions, an absorbent of all the known 
gases, with the exception of those of the argon group. 
Provided the initial gas-pressure does not exceed a few 
millimetres of mercury, all the common gases are rapidly 
and completely absorbed by calcium between 7oo° C. and 
800° C., and a vacuum attained through which the electric 
discharge cannot be forced. Arndt (Ber. d. d. Chem. 
Gesell., 1904, Xxxvii., 4733), in an investigation of the 
melting point of calcium, noticed that the calcium vola- 
tilised freely below its melting point when heated in a 
vacuum of «1 mm. mercury, and the vapour reacted 
energetically with the oxygen and nitrogen of the residual 
air, producing a great improvement in the vacuum. He 
did not investigate the behaviour of other gases. 
This behaviour of calcium is all the more surprising be- 
cause in ordinary circumstances it shows a great disinclin- 
ation to react, and may be heated in a tube filled with 
air at atmospheric pressure to a very high temperature 
without causing much absorption. A low initial pressure 
of the gas and volatilisation of the metal are essential in 
using calcium as an absorbent. Barium and strontium 
behave in a manner very analogous to calcium. In the 
case of hydrogen and its compounds, the absorption be- 
comes more complete, and the vacuum improves when the 
calcium is allowed to cool owing to the hydride possessing 
an appreciable tension of dissociation at the high tempera- 
ture. 
The high vacua readily produced by the absorption of 
residual gases by calcium are at least equal to the highest 
attained by any other process. By filling the apparatus 
with mercury after the action of the calcium, and com- 
pressing any residual gas several hundred times into a 
tiny spectrum tube, it was found that the vacuum was still 
so high that the spectrum tube was of high resistance and 
fluoresced brightly under the discharge, showing a faint 
hydrogen spectrum. Since argon is not absorbed, the air 
must be first removed from the apparatus by means of a 
Fleuss pump and by replacement of the last traces with 
some argon-free gas, before the calcium is brought into 
action. The condensed gases evolved from the apparatus 
on heating usually suffice to replace the last of the air 
during the mechanical exhaustion. The calcium, being a 
good conductor of electricity, may be readily heated to the 
required temperature within the sealed glass vessel by 
induction through the walls from an alternating circuit 
outside the vessel. The special feature characterising the 
new method is the rapid and complete absorption by the 
calcium of the gases condensed on the walls, and in the 
electrodes, &c., of the apparatus being exhausted as soon 
as these are expelled by heating. These gases, known 
technically as ‘‘ film gases,’’ consist largely of hydrogen 
and carbon compounds, and cause most of the difficulty 
experienced in practice, for they readily re-condense and 
introduce» a kind of steady vapour pressure within the 
apparatus, greatly increasing the time required for 
exhaustion. 
In the apparatus usually employed for experimental work 
a porcelain tube with an external screw-thread is wound 
with a platinum resistance wire through which a current 
is passed. A porcelain test-tube containing the reagent is 
slipped within this furnace tube, which in turn slips within 
a wider porcelain tube, which again slips within the ex- 
ternal glass tube provided with platinum wires sealed 
through the glass for conveying the heating current. This 
tube is then sealed to the apparatus to be exhausted. 
In one form of apparatus for heating the calcium by 
induction, a calcium disc is bored with central hole 
through which a short bundle of soft iron wires pass. 
Two porcelain crucible lids bored with central holes fit 
over the calcium disc, the ends of the iron wires projecting 
beyond the lids. This arrangement is slipped into a glass 
tube with the axis of the iron core at right angles to the 
length of the tube. A coil of soft iron wire is cut at one 
point and bobbins of wire slipped over the two ends, which 
are then brought opposite and close to the ends of the 
iron core within the glass tube. On exciting the bobbins 
with an alternating current of high periodicity (200 to 400 
periods) a current of the order of a kiloampere is induced 
in the calcium disc, heating it to the required temperature. 
NO. 1943, VOL. 75] 
The phenomena when successive quantities of air are 
| admitted into an apparatus containing heated calcium are 
of special interest, for all but the 1 per cent. of argin is 
rapidly absorbed, and in this way the minimum quay ity 
of argon necessary to carry the discharge and show a 
spectrum has been determined. Below 1/50 mm. argon 
does not conduct; at this pressure the green and orange | 
lines are faintly visible; at 1/25 mm. the reds appear; At | 
1/2 mm. the spectrum tube has a resistance equivalent () 
an alternative air gap of 5 mm., while at 1 mm. pressure 
the tube is still brilliantly fluorescent. With helium, intro- 
duced into the apparatus as a mixture of oxygen with a 
known small quantity of helium, the tube is non-conducting 
to the discharge at pressures below 1/20 mm. of helium 
when every trace of other gases is absent. In presence of 
hydrogen or oxygen one-hundredth part of this amount is 
sufficient to show the D, line of the helium spectrum. The 
conclusion is drawn that the inert monatomic gases in the 
absence of every trace of polyatomic gases show a great 
disinclination to conduct the discharge, and this accounts 
for many isolated facts familiar to workers with high 
vacua. The rapid ‘‘ running out ’’ of spectrum tubes filled 
with inert gases is due, not to the absorption of these 
gases, but to the absorption by the electrodes of the traces 
of hydrogen, &c., always present initially or introduced by 
the electrodes (compare Skinner, Phil. Mag., 1906 [vi-] 
12,481). When this has occurred the pure monatomic gs 
no longer conducts. The fact observed by Lord Blyths- 
wood and H. S. Allen (Phil. Mag., 1905 [vi.] 10,497), that 
an X-ray bulb may be readily exhausted from atmospheric 
pressure of air to a ‘‘ vacuum so good that the tube had 
to be heated to allow the discharge to pass through it,”’ 
by the use of charcoal cooled in liquid air according to 
the method of Sir James Dewar, at first seems inconsistent 
| with the fact that seventeen parts per million of the air, 
consisting of helium and neon, remain unabsorbed, and 
the residual pressure must therefore be about 1/75 mm. 
The explanation is to be found again in the disinclination 
of these monatomic gases to conduct when pure. For 
this reason the electric discharge test of the goodness of a 
vacuum is altogether misleading, for with the inert mon- 
atomic gases pressures within the range of the mercury 
barometer appear to be high vacua. The great power of 
calcium in absorbing every trace of carbon dioxide, 
hydrogen, water vapour, hydrocarbons, &c., derived from 
impurities in the apparatus, and from the lubricating grease 
of stop-cocks, makes it a powerful aid to the methods of 
spectroscopic research. 
Appendix.—‘ Results of Gauging High Vacua by the 
Evaporation Test.’’ By A. J. Berry. 
The degree of high vacua produced by different processes 
may be gauged by the rate of evaporation of liquid air in 
a Dewar vessel exhausted by the process. The same 
globular vessel of about 1 litre capacity, silvered internally, 
was exhausted (1) by the mercury pump; (2) by the use 
of cooled charcoal from atmospheric pressure, using two 
successive quantities of charcoal; (3) by cooled charcoal 
after the air had been first removed by a mechanical pump. 
It was to be expected from the conclusion drawn in the 
preceding paper that the degree of vacuum obtained in the 
second test would be much inferior, tested by the evapor- 
ation method, to that obtained in the third. The expect- 
ation was fully borne out by the experiments. The liquid 
air evaporated at the rate of 898 grams in four days in 
the vessel exhausted by the second method, which was 
rather faster than in the first method, when the vacuum 
was produced by a mercury pump. The vacuum produced 
by the third method was far better, 942 grams evaporating 
in six days, and only 610 grams in four days. 
December 6, 1906.—‘ The Theory of Photographic Pro- 
cesses. Part iii. The Latent Image and its Destruction.’’ 
By S. E. Sheppard and C. E. K. Mees. Communicated 
by Sir William Ramsay, K.C.B., F.R.S. 
The authors consider that ‘‘ developability ’’ is brought 
about by the acceleration of reduction by preliminary treat- 
ment. The essential chemical reaction in development is 
Ag+R”—Ag|met|+R’, 
which normally proceeds to a state of equilibrium. If now 
to this state of equilibrium any cause tending to lower 
