85 
measure at the earth’s distance. Under these conditions a line 
from the sun to the earth will pass through the rift in the rings 
to the planet, and a terrestrial observer, suitably placed, may be 
able to view through the rift a portion of the planet’s surface lit 
up by the sunlight. The effect will be that, of the arc of the 
Cassini division crossing the planet, a small portion will appear 
bright instead of dark, and may almost disappear ; as the albedo 
of Saturn is less than that of the adjacent portions of rings 
A and B, however, it is likely that there will be sufficient con- 
trast to show the phenomenon. e 
There appears to be no record of any previous observation of 
this kind, and it will obviously be one of great delicacy and 
difficulty. As the exact limits of time and place are not abso- 
lutely determinable, it is hoped that the planet will be watched 
for some time before the date given. 
CATALOGUE OF NorTH POLAR STARS.—Prof. Pickering has 
issued a catalogue of 589 stars in the vicinity of the North Pole 
as a separate part, No. 1., of vol. xlviii. of the 4zvnals of the 
Harvard College Observatory. The measures are from enlarge- 
ments made from the central portions of four negatives obtained 
with the 11-inch Draper telescope on November 29, 1887, 
February 23, 24 and March 10, 1897, with exposures of 60, 120, 
120 and IOI minutes respectively. Full details are given of the 
reductions employed, and in consequence of the arrangements 
made at the Astrophotographic Congress of 1900, the positions 
are published in rectangular coordinates, which plan is to be 
adopted in general for future issues. 
THERMAL EXPANSIONS .AT LOW 
TEMPERATORES. 
‘THE apparent specific gravities of boiling liquid oxygen which 
resulted from weighing in the liquid a series of metals and 
other substances were given in a lecture entitled ‘‘New 
Researches on Liquid Air,” printed in the Royal Institution 
Proceedings for 1896. For instance, silver, calc spar, rock 
crystal and iodide of silver gave the respective apparent 
densities 1°1278, 1°1352, 1°1316and 1°1372. Oncorrecting the 
weight of liquid displaced by each substance for contraction to 
— 182°:6—by calculating a Fizeau mean coefficient of expansion 
for the range of temperature employed, on the assumption that 
the parabolic formula might be legitimately extended to low 
temperatures—it was found that the real density of liquid oxygen 
so deduced for all the bodies used was, as a mean, I°137. 
The determination of the densities of substances at the tem- 
perature of the boiling point of oxygen—and hence of their 
mean coefficients of expansion between that temperature and 
ordinary temperatures—opens out a very large field of investiga- 
tion, from which, if a sufficiently large number of observations 
were available, valuable deductions might be drawn. On 
account, however, of the expense and trouble of producing 
quantities of liquid oxygen, its use for this purpose is not likely 
to become general, although, when available, it is the easiest 
body to use in conducting such experiments, especially when the 
vacuum vessel containing it is immersed in a larger vessel con- 
taining the same fluid or well-evaporated air. The ease with which 
liquid air can now be obtained in many laboratories suggests 
that its application to work of this kind would in some cases be 
a convenience, and the present investigation was undertaken 
with the desire of ascertaining what accuracy could be attained, 
and how the method could be applied to inorganic or organic 
substances which occur in the form of fine crystals. 
The use of a mixture of varying composition and density like 
liquid air necessitates a determination of its density with accuracy 
and rapidity before and during the course of the experiments. 
For this purpose, in the experiments about to be detailed, the 
liquid air that had been allowed to evaporate for twenty-four 
hours in advance was used in large silver-coated vacuum vessels 
of some 3 litres capacity. In order to ascertain the density of 
the liquid, a polished silver ball, which had been weighed once 
for all in liquid oxygen, was weighed in the sample of liquid 
air, and from the relative weights thus found the density of the 
liquid air could be approximately determined, assuming that of 
liquid oxygen to be 1°137.2 To prevent any disturbing ebulli- 
1 “* Coefficients of the Cubical Expansion of Ice, Hydrated Salts, Solid 
Carbonic Acid, and other Substances at Low Temperatures.” By Prof. 
James Dewar, F.R.S. Abridged from a paper read before the Royal 
Society on May 1. 
2 As the correction due to the contraction of the silver ball between the 
temperature of boiling oxygen and that of the air sample is small, it may be 
neglected. 
NO. 1699, vor. 66] 
NATURE 
[May 22, 1902 
tion in the liquid-air flask in which the weighings took place, 
and to reduce the rate of its evaporation to a minimum during 
the course of an experiment, the substance to be used was 
previously cooled in a supplementary vessel containing liquid 
air and then transferred to the large flask. To avoid as far as 
possible the formation of cracks in the bodies during the process 
of immersion in the liquid air, it was found advisable to cool 
them slowly in the air of the vacuum flask first, and then to 
lower them into the liquid. 
In this way, with proper care and attention, results were 
obtained comparable in accuracy with the density taken in liquid 
oxygen. Substances like solid carbonic acid and ice were 
weighed in the cool, gaseous air of the vacuum vessel, and their 
weights subsequently corrected for buoyancy. The temperature 
of the densest and lightest samples of liquid air was ascertained 
by the hydrogen thermometer, and that of the others deduced 
by graphic interpolation. As the entire range of temperature 
through which the bodies were cooled amounted to about 200°, 
a degree or two up or down has no real influence on the results ; 
the extreme range of temperature in the air samples was from 
83°'8 to 86°"1 Abs. 
When the body to be examined was a salt, it was employed 
in the form of a compressed block. One experiment was, 
however, made in a section of a large crystal of chrome alum. 
The salt, previously reduced to a fine powder, was moistened 
with water and compressed in a cylindrical steel mould under 
great hydraulic pressure. During compression the saturated 
salt solution drained away, and finally a cylindrical block of 
some 50 grammes of the salt was obtained free from porosity 
and hard enough to allow its surface to be polished. In this 
form salts and other materials similarly treated are especially 
adapted for accurate specific gravity determinations. After 
such treatment it was found that all the mechanically attached 
water was got rid of in the case of hydrated salts, and also in 
such as did not combine with water. In order to get cylindrical 
blocks of the salts showing no porosity, the presence of water, 
or rather the saturated salt solution, was found to be essential 
during the application of pressure. In the same way it was 
found to be an advantage in compressing such a substance as 
solid carbonic acid to moisten it with a fluid like ether before 
applying the hydraulic pressure. 
Recalling the work of Playfair and Joule,! which originated in 
a suggestion of Dalton’s that the volume of a hydrated salt in 
solution was simply the volume of the water of crystallisation, 
ice and some hydrated salts were selected, as well as some other 
bodies the coefficients of expansion of which they had determined. 
Substances of special interest were included in the list, like 
mercury, sulphur, iodine and solid carbonic acid, the latter 
being particularly important as an example of a solidified gas. 
In the further conduct of an experiment, the observations 
made on a substance were three, namely, (az) the weight in 
grammes of the substance and suspending platinum wire, either 
in air of about 17° C. temperature or in the gaseous air in the 
flask containing the liquid air; (6) the weight in grammes of 
the body and wire when immersed in the liquid air ; and (c) the” 
weight in grammes of the suspending platinum wire in ordinary 
(17°) air. 
: In the case of substances of less density than liquid air, a 
polished copper ball weighing about 38 grammes was used as @ 
sinker. 
Two experiments were made on compressed cylinders of solid 
carbonic acid. In the first of these the carbonic acid was com- 
pressed dry, in the second, after a few drops of ether were 
added. The specific gravities of solid mercury, iodine and 
sulphur were also determined in liquid air. The iodine was in 
the form of a compressed cylinder, but the sulphur was a piece 
of a crystalline mass of native origin. 
The specific gravity of the actual portion of the substance 
weighed in the liquid air was, with one or two exceptions, 
determined also at the temperature of the laboratory, about 
17°C. From the two sets of observations, the value of the mean 
coefficient of cubical expansion between 17° C. and the tem- 
perature of liquid air was calculated. 
In calculating coefficients of expansion, various forms may be 
given to the formula employed, and correspondingly different 
results may be obtained from the same set of observations. For 
short ranges of temperature these results are practically identical, 
but this no longer holds for a range of temperature such as we 
1 ‘Researches on Atomic Volume and Specific Gravity ’ (Chem. Soc. 
Journ., vol. i,, 121). 
