721 
20 X 7) we take 10 parsecs (1 parsec is 3 x 105 km.), we 
find, with an absorption of 2 magnitudes, 10! for the density of 
the gas-cloud. The mass is independent of the thickness assumed ; 
per cm? diameter it is 2/0,056 weight of air-column on Mount 
Wilson = 25 kg, for an area of 150 square parsecs therefore 
M = 3,5 x 10 kg. As the mass of the sun is 2 x 10% kg, 
the mass of the gas-cloud is equal to about 2 > 10!° sunmasses. 
This can also be found directly, by means of the formula of 
Rayieian for the absorption-coefficient £: 
ae Ate) 
eN 
in which u is the refractive index, A the wave-length, N the number 
of particles (molecules) per cc. If we assume, that the gas-cloud 
consists of hydrogen, (which gives the smallest mass), with an 
ordinary pressure and density therefore u == 1,000143, NM = 2,7 « 10”, 
moet we take 2==5,5 <x 10-5 em, we get k = 2,7 « 10-8-or 
2,7 <x 10-3 for unit of thickness one km., which is equal to 
2,9 > 10 3 magnitudes, whilst a column of 1 em? width per km. 
length has a mass of 8,3 x 10-° kg. The mass of a column of 
1 cm?’ diameter in an absorbing gas-layer is therefore 2,9 « kg., 
if e is the absorption in magnitudes (for 4 = 550). From this we 
find for a mass of gas with an area of 150 square parsecs and 2 
magnitudes absorption 
M=8 X 10° kg. =4 X 10° sunmasses. 
The difference with the former result is due to the difference 
between hydrogen and air. 
According to Kapreyn and van Rayn’) the density of the stars 
in the vicinity of the sun is ‘/,, per cubic parsec, so that in a globe 
with a radius of 2600 parsecs there are 4 x 10° stars. If we take 
their average mass as equal to that of the sun, this one gas-cloud, 
(one third perhaps of all absorbing gas-clouds in that region) only 
140 parsecs distant, contains as much mass as all the stars within a 
globe extending 20 times further. Unless therefore this Taurus-cloud 
is unique for size and density, we may safely conclude that in the 
fixed stars only a small part of the world-substance is condensed. 
§ 2. The assumption, however, that at a distance 140 parsecs there be 
a gascloud of such great mass, leads to a few most remarkable con- 
sequences. The attraction of this mass on our solar system is not 
J.C. KAPTEIJN and P. J. van RHIN, On the distribution of the stars in 
space. Astrophysical Journal 52, 32. 
