216 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1933 



number of calcium atoms in all stages of ionization is 10^^. If we 

 make the assumption that the composition of matter in interstellar 

 space is similar to that of the earth, we would have to multiply this 

 result by 100 to get the total number of atoms of all elements. This 

 leads to a density of something like lO"^" g/cm^ 



It is difficult to comprehend such a state of rarification as this, in 

 which there may be an atom or so per 100 cubic centimeters of space. 

 What is not occupied by the atom itself must be " empty ", and just 

 what that means I will leave to the theoretical physicists to explain. 

 I am not sure, however, that it is any easier to comprehend the empty 

 spaces between nuclei and electrons in what we usually regard as 

 dense matter. 



The most important recent advance in the study of interstellar 

 calcium is the investigation of galactic rotation of the interstellar 

 medium published last year by Plaskett and Pearce. They have 

 proved statistically that the effective distance of the calcium column 

 between the star and the observer is one-half of the distance between 

 the star and the observer. This proves that, statistically at least, the 

 density of interstellar calcium is approximately constant in all parts 

 of our Milky- Way system up to distances of 3,000 to 5,000 light-years 

 from the sun. 



I have some doubt as to whether this constancy may be relied upon 

 in the case of every star. There are indications that condensations of 

 the general cloud are actually present. 



So much for the proofs that interstellar space is populated by 

 atoms of calcium and other chemical elements situated so far apart 

 that on the average only one atom is to be found in a cube 2 inches on 

 a face. To see how rare such a gas is compared to our atmosphere, 

 let us recall that at sea level 1 cubic centimeter (about the volume of 

 backgammon players' dice) of air at 0° C. contains 27,000,000,000,- 

 000,000,000 molecules. 



DUST IN INTERSTELLAR SPACE 



Rare gases, however, do not appear to be the sole contents of inter- 

 stellar space. Particles of dust form haze in the earth's atmosphere, 

 as we well know. In the year 1912, for instance, the eruption of the 

 volcano of Mount Katmai in Alaska so filled the atmosphere of the 

 whole Northern Hemisphere with dust that the direct beam of the 

 sun at noon was weakened by about 20 percent. Photographic ex- 

 posures had to be increased in landscape photography because of this 

 volcanic haze. Lord Rayleigh showed in 1871 that when the particles 

 are very small, not much larger in diameter than the wave length of 

 light, they produce a much greater depletion of the shorter wave 

 lengths in the spectrum, such as violet and blue, than they do of 



