Vol.. 7, 1921 
PHYSICS: L. B. LOEB 
5 
idea of the order of the parallax of the nebula by comparing Pease's re- 
sults with those obtained from the present investigation; the correspond- 
ing parallax is about 0".0005. The diameter of the nebula would be about 
100 light-years and the individual points of the nebula would have ab- 
solute magnitudes of +1 and fainter. 
If on the other hand we suppose the dimensions of Messier 33 to be 
comparable with those of the galactic system, its distance would be several 
million light-years; the motions indicated by the photographs would 
then represent velocities of the order of 150,000 to 300,000 km. /sec, 
which, obviously, are extremely improbable. The internal motions in 
the spirals seem now to be well founded, and if time justifies this belief, 
they will accordingly afford a most important argument against the view 
that these nebulae are systems comparable with our galaxy. 
1 van Maanen, A., Mt. Wilson Contr., No. 118, 1916; Astrophys. J., Chicago, III., 
44, 1916 (210-228). 
2 Amer. Astron. Soc. Publ., 3, 1918 (206-207). 
3 Monthly Notices, Lond., 77, 1917 (233-234). 
4 Observatory, Lond., 43, 1920 (255-260). 
THE ATTACHMENT OF ELECTRONS TO NEUTRAL MOLE- 
CULES IN AIR* 
By Leonard B. LobbI 
Ryerson Physical Laboratory, University of Chicago 
Communicated by R. A. Millikan, December 4, 1920 
It is well known that the process of ionization in gases consists of the 
detachment of an electron from the molecules or atoms ionized. It has 
further been proved that in air at atmospheric pressure the carriers of 
negative electricity are neutral molecules 5 ' 6 of the gas carrying an addi- 
tional electron. These are called the normal ions. Now it is of interest 
to determine in what manner the electron liberated by the ionizing process 
attaches itself to a molecule to form an ion. For it is possible that an 
understanding of this process may help us to gain a picture of the surfaces 
of the molecules. 
In air at atmospheric pressure the normal negative ions move with a 
velocity of 2 cm. /sec. in unit electric field while the electrons have a mo- 
bility of about 200 cm. /sec. 3,7 under the same conditions. Such a marked 
difference in the two types of carriers accordingly furnishes us an excellent 
means of investigating the above question as the ensuing analysis will 
show. 
Let us assume that the electrons liberated from the plate P, figure 1, 
by ultraviolet light unite with gas molecules on their first impacts to form 
ions. Now let a second plate, E, connected to an electrometer be placed 
