BOTANY: S. C. BROOKS 
569 
light years. These are improbably small distances. If we assume that 
these nebulae are at distances of 1000 light years, which we have reason 
to believe is of the order of more probable distance, the minimum values 
of their masses, in terms of the Sun's mass, lie between 4 and 210. 
13. As explained in the former paper, it is difficult to avoid the con- 
clusion that the so-called ring nebulae are in reality not ring forms in 
space, but ellipsoidal shells. If these forms are rotating about the 
minor axis of figure, which also seems highly probable, it is difficult to 
account for their apparent equilibrium under rotational and gravita- 
tional forces: it would seem that the nebular materials in the polar 
regions — the regions of the extremities of the rotation axes — should be 
depressed toward the central nuclei. Are other forces, possibly includ- 
ing radiation pressure, involved? 
14. Inasmuch as the observer will not in general be situated in the 
planes of the equators of rotation of the nebulae, the observed rota- 
tional velocities are smaller than their true values. On this account 
the actual masses of the observed nebulae should be greater than the 
minimum values assigned above. The effect of radiation pressures 
would likewise make the deduced masses too small. The indications 
are that the nebulae under consideration are capable of developing into 
systems much more massive than is our solar system. 
NEW DETERMINATIONS OF PERMEABILITY 
By S. C. Brooks 
LABORATORY OF PLANT PHYSIOLOGY. HARVARD UNIVERSITY 
Received by the Academy, August 31, 1916 
The purpose of these experiments was to investigate permeability by 
new and independent methods, in order to test as far as possible the 
conflicting views now held by different investigators. For a number 
of reasons the investigation was confined to electrolytes. 
1. A new method of determining permeability was devised, which is in- 
dependent of other methods. This consists in direct measurements of the 
rate of diffusion of dissolved salts through a diaphragm of tissue from 
the thallus of Laminaria Agahrdii (formerly identified as L. saccharina). 
Discs of tissue {E, fig. 1) were placed between two short lengths of glass 
tube {A and B), the ground ends of which were covered with a suitable 
grease (F), and appHed to the tissue in such a manner as to make the 
joint water-tight. One of these cells was closed at the end away from 
the diaphragm by a rubber tube and pinchcock (C and D). The 'lower 
ceir thus formed was filled with sea water or with a salt solution having 
