530 
ASTRONOMY: R. G. AITKEN 
centrations are calculated in the usual manner from conductance 
measurements. 
In conclusion it may be noted that the value of E/uc in equation (1) 
when multiplied by F, the value of the faraday, represents the maximum 
work or free-energy change attending the transfer of one equivalent of 
salt from one concentration to the other, and that therefore may be 
calculated from any of the properties related to osmotic pressure, such 
as freezing-point. This free energy-change may also be obtained directly 
from electromotive force measurements of cells without transference of 
the type;Ag, AgCl (solid) + KCl (O.ln), K(Hg). - K(Hg)., AgCl (soHd) 
+ KCl (O.Oln), Ag, investigated by Maclnnes and Parker. 
^ See for instance, Jahn, Zs. physik. Chem. 33, 545 (1900); Tolman and Ferguson, 
/. Amer. Chem. Soc. 34, 232 (1912); Maclnnes and Parker, /. Amer. Chem. Soc. 37, 1445 
(1915). 
2 /. Amer. Chem. Soc. 33, 1454 (1911). 
A STATISTICAL STUDY OF THE VISUAL DOUBLE STARS 
IN THE NORTHERN SKY 
By Robert G. Aitken 
UCK OBSERVATORY. UNIVERSITY OF CALIFORNIA 
Presented to the Academy, August 26, 1915 
The double star survey, initiated in April, 1899,^ has now been com- 
pleted to — 14° Declination for the winter sky and to — 22° Declination 
for the summer sky. This is about as far south as atmospheric con- 
ditions make it desirable to extend the survey at Mount Hamilton. 
The original plan contemplated the examination with one of the 
refracting telescopes of the Lick Observatory of all stars as bright as 
9.0 magnitude in the Bomi Durchmusterung, the identification of all 
previously known double stars, and the cataloguing of any new pairs, 
not exceeding 5 ".00 in angular separation. Limits must be imposed in 
work of this character, and those chosen, while necessarily arbitrary, 
were selected after careful study as being liberal enough for the purpose 
in mind, which was the accumulation of data for a statistical study of 
the number and distribution of the visual double stars. 
The survey has yielded 4300 new double stars, 1328 discovered by 
Prof. W. J. Hussey, the others by the writer. Twenty-eight of these 
have angular separation sHghtly exceeding 5'^00; 3199, or 74.4% of the 
whole number, fall within the limit 2''.00, and 1280 (29.8%), within the 
limit 0''.50. The distance limit adopted has thus been carefully observed, 
but for various reasons many stars fainter than the limit 9.0 were ex- 
