Vol. XXIV. No. 12.] 
POPULAR SCIENCE NEWS. 
179 
of hydrogen, or some other substance known to be 
present in the star's atmosphere. If the lines in the 
star spectrum fail to coincide with the lines in 
the spectrum of the Geissler tube, we infer that the 
star is changing its distance from us, and we can 
determine its speed by measuring the amount 
of displacement in the lines of the star spectrum. 
We hasten to say, however, that while the opera- 
tion is easy to describe and understand, it is by 
no means easy to perform; — there is no observation 
in the whole range of practical astronomy more 
delicate. The amount by which the lines in the 
spectrum are shifted is always minute, depending as 
it does on the ratio between the star's velocity 
and that of light. Now light moves so swiftly — 
over 186,000 miles a second — that in comparison 
with this a velocity of even ten or twenty miles 
a second is almost insignificant, and produces a 
displacement of the lines so slight that it can be 
detected only by using a very high "dispersion," 
i. «., by making the spectrum of the star very long 
— and that means making it very faint. A large 
telescope is therefore necessary to collect the 
requisite quantity of light, the spectroscope must 
be powerful, and the most scrupulous precautions 
must be taken to have everything in absolute adjust- 
ment. If the observation is made with the eye, the 
observer must have keen and untiring vision, and 
skill in the most delicate measurement. If, as is 
now beginning to be common, a photographic pro- 
cess is used, the most sensitive plates must be 
employed, with continual vigilance to keep the 
image of the star exactly on the slit of the spectro- 
scope, and to maintain the adjustments of the 
instrument absolutely unchanged, sometimes for 
hours together. 
Dr. Huggins, the father of astronomical spectro- 
scopy, was the first to make an actual observation 
of this kind, in 1868, when he ascertained in this 
way that Sirius was then receding from us afthe 
rate of about nineteen miles a second — a motion 
which appears to have been reversed since then, 
j!robably in consequence of the orbital connection 
of Sirius with its small companion. He also made 
similar observations upon a number of the brighter 
stars ; but it soon became obvious that the means at 
his command would not enable him to attain satis- 
factory accuracy. Since then the Greenwich ob- 
servers have been following up the same line 
of work, but not with very much greater success. 
The published measures have shown discordances 
that are rather disheartening, and indicate that the 
results can be accepted only as provisional appro.\i- 
mations to the truth. 
Very recently, however, the matter has assumed a 
new aspect through the work of Vogel in Germany 
and Pickering in this country. The former has 
applied photography with great success to a spec- 
troscope of the ordinary form, t. e., with slit and 
collimator. The instrument is so constructed that 
it is limited in range to a very small portion of the 
spectrum near the line known as G (where, however, 
there are numerous identified lines, some of which 
are sure to be found in the spectrum of every star) ; 
but within this range the instrument is extremely 
powerful. By the use of very sensitive plates and 
long exposures it is possible to photograph spectra 
that the eye cannot see at all, and Vogel has thus 
been able to secure negatives upon which, in favor- 
able cases, a stellar velocity of a single mile a 
second produces a measurable shift of the lines. 
With this apparatus the " head on" motion of nearly 
sixty stars has already been determined at Potsdam 
with reasonable accuracy. 
Vogel's most interesting results are those re- 
lating to the stars Algol and Spica Virginis. 
The former is a variable star, which acts as 
if it were partially eclipsed at regular intervals 
of 68 8 hours by a dark companion large enough 
to cut off nearly five-sixths of its light. Now 
the observations show that about seventeen hours 
before the minimum, Algol is receding at the 
rate of 24 4 miles a second, while seventeen hours 
after the minimum it is approaching with a speed 
of 28 6 miles. From this it follows that the 
"system" composed of the two stars is approach- 
ing us about 2.1 miles a second, while the bright 
star is inoving in a nearly circular orbit around the 
common center of gravity between it and its dusky 
acolyte with a velocity of 26 5 miles. We have not 
the space to explain how, froin the known data, 
Vogel is able to show that the smaller star must be 
moving more than twice as fast, that the distance 
between the two stars must be about 3,270,000 miles, 
and that their diameters are about 1,075,000 and 
841,000 miles, the smaller star being very nearly 
as large as the sun. These are fair inferences, how- 
ever. It appears further that the larger star has 
a mass about four-ninths that of the sun, the smaller 
one being but half as large ; so that their density 
must be less than one-fourth that of the sun — hardly 
exceeding that of cork. 
The case of Spica is different in that this star is not 
variable ; but it presents the same kind of backward 
and forward motion, with a velocity of 56 5 miles, 
indicating that it is describing an orbit about 
6,000,000 miles in Jiameter in a period of almost 
exactly four days. There is nothing in this case on 
which we can base an estimate of the relative mag- 
nitude of the unseen attendant which is thus waltz- 
ing with its brilliant partner. We can only say that 
the two together constitute a mass about two and 
a half times as heavy as the sun, and that the 
smaller one, although so near the larger that it 
could not possibly be seen by any existing telescope, 
would indicate its presence on the spectrum photo- 
graphs if it were above the third magnitude in 
brightness. It would show itself by causing the 
lines in the spectrum to appear double at the times 
when Spica is moving most swiftly along the line 
of vision. When the great star is advancing 
towards us as the pair whirl around their common 
center of gravity, the smaller one must be receding 
with still greater velocity. 
Just this phenomenon of doubled lines has actually 
been detected at Cambridge (U. S.), in examining 
the spectrum photographs made by the " slitless 
spectroscope" of the Draper Memorial. With this 
instrument it is not possible to compare the spectrum 
of a star with that of a Geissler tube in such a way 
as to determine the star's absolute motion ; but when 
two stars, both reasonably bright, are so near each 
other that the telescope cannot separate their 
images, then, if they have an orbital motion, it will 
reveal itself by a periodical "gemination" of the 
lines in their common spectrum. Miss Maury, a 
niece of Dr. Draper's, who is engaged at Cambridge 
in examining the photographs of star spectra, first 
noticed this doubling of the lines in the spectrum 
of the well-known star Mizar, the one at the bend in 
the "dipper handle." The doubling is found to 
recur at regular intervals of fifty-two days, showing 
that the star is composed of two, which revolve 
around each other at a distance of about 140,000,000 
miles in three and a half months, with a velocity 
1 of nearly a hundred miles a second, the two stars 
] united being fully forty times as heavy as the sun. 
I The interest of the system is increased by the fact 
that a third star, long known, and visible in even a 
small telescope, revolves around the whirling pair 
in a period of centuries not yet determined. 
The star Beta Aurigie presents a similar doubling 
of its lines, but the relative velocity rises to one 
hundred and fifty miles a second, and the system 
closely resembles that of Spica, the distance between 
the components being about 8,000,000 miles, and 
tlie period four days. The two stars of this pair, 
however, are nearly equal in brightness. 
Other stars are under surveillance, and it is 
probable that our knowledge of the stellar motions 
will soon be greatly extended. It is already abund- 
antly evident that the ordinary speed of these 
motions much exceeds anything that we find in the 
motions of the planets around the sun. It makes 
one almost dizzy to think of the swiftness with 
which the stars are flying. 
We have space only for the merest mention of 
the latest achievement of the spectroscope. Mr. 
Keeler, of the Lick Observatory, has just succeeded 
in extending this method of observation to the 
nebulse, and with very interesting results. His 
spectroscope is constructed with a diffraction grating 
instead of prisms, and so far he has worked with 
the eye and not by photography. It appears that 
the nebula; are moving as swiftly as the stars. 
Princeton, N. J., November, 1890. 
SCIENTIFIC BREVITIES. 
A Photometric Balance has been invented by 
M. Lion, who uses for this purpose the dangerous 
explosive substance iodide of nitrogen. If this is 
obtained, he says, by allowing ammonia at 22° to 
act upon iodine, it may be managed without danger 
if kept in the liquid. It yields a supply of nitrogen 
which is proportional to the intensity of the light 
which falls upon it. The evolution of the gas 
begins and ceases instantly with the luminous im- 
pression. 
Plants for Shady Places. — Hardy flowering 
perennial plants that will grow between and under 
trees are subjects for which inquiry is very frequently 
made. One of the oldest and most common plants 
for this purpose, the Vinca, or periwinkle, is one 
of the best — the smaller variety, V. minor, for pref- 
erence. A few common daffodils and winter aconites 
might also be dotted about between the shrubs in 
the autumn ; the early blooming in spring of the 
last-named is very grateful, and they can be de- 
pended upon season after season when once 
planted. 
The Affinity of Metals for Sulphur. — The 
insoluble sulphides can precipitate the solutions 
of the salts of other metals as sulphides of these 
metals. Thus the sulphide of copper gives a pre- 
cipitate of sulphide of silver in a solution of nitrate 
of silver: CuS + 2 Ag N 03 = AgaS + Cu (N 03)2, 
showing that silver has a greater affinity for sulphur 
than copper. In 1837 Anthon published a table 
of the metals arranged according to their decreasing 
affinity for sulphur. Silver is at the head, then 
comes" copper, followed by lead, cadmium, iron, 
nickel, cobalt, and manganese. According to the 
Moniteur Scientifique, E. Schurmann has taken up 
M. Anthon's investigation and carried it further. 
His list enumerates the metals in the following 
order: Palladium, mercury, silver, copper, bis- 
muth, cadmium, antimony, tin, lead, zinc, nickel, 
cobalt, iron, arsenic, thallium, and manganese. 
The salt solution of any metal in this series can be 
decomposed — at least to a great extent — by the sul- 
phide of one of the subsequent metals, but the 
sulphides of the preceding metals cannot be decom- 
posed by that of any subsequent one. The decompo- 
sition is the more complete the further away the 
metals are placed from one another on the list. 
The metals at the head of the series have the great- 
est affinity for sulphur, and the affinity decrea,ses 
insensibly up to manganese, which has but very 
little. The sulphide of manganese is decomposed 
by the salt solution of any of the preceding metals. 
