8 
SOIL CHEMISTRY: E. W. HILGARD 
that astronomical observers will probably be able to make a definite 
decision as to whether or not any of the existing spiral nebulae have 
developed from bacula or from less regular forms by substantially the 
same method. Till that is decided further discussion would be super- 
fluous. It is possible that there are well developed spiral nebulae in 
which mutual interference has done little or nothing to reduce orbits to 
elhpses of small eccentricity. To such nebulae the analysis here indi- 
cated has no application, nor could their motions be formulated in the 
present state of science.^ 
^Amer. J. Sci., Ser. 4, 5, 102 (1898). 
2 Chapter VI, p. 482, of the edition of 1835. 
3 Amer. J. Sci., Ser. 4, 5, 106-107 (1898). 
* See preface to Worlds in the Making, 1908. 
^ Paris, C. R. Acad. Sci., 158, 1017 (1914); and Astrophys. /., 40, 241 (1914). 
ePo^. ^-r^r., 23, 485 (1915). 
7 A. G. Webster, Dynamics of Particles, etc., 2d ed., 1912, p. 317. 
^ From a photograph taken at the Yerkes Observatory. 
^ Just in time for a reference, I have met with an interesting paper by Mr. E. J. Wil- 
czynski, Astrophys. J., 4, 97, (1896), who pointed out that, if circular orbits are assumed, 
every long streak of nebulous matter must eventually be converted into a spiral as a conse- 
quence of Kepler's third law. The equation of this curve, he says, it would be easy to 
deduce. 
A PECULIAR CLAY FROM NEAR THE CITY OF MEXICO 
By E. W. Hilgard 
UNIVERSITY OF CALIFORNIA 
Read before the Academy, November 17, 1915. Received, November 17. 1915 
In 1912 Dr. George W. Shaw, then connected with the College of 
Agriculture of the University of California, was requested to visit the 
Hacienda Santa Lucia, in the neighborhood of the City of Mexico, in 
order to give advice for the reclamation of certain tracts of land which 
were supposed to be afflicted with ^alkali,' and which had resisted the 
usual methods for rendering them productive. Dr. Shaw brought back 
samples of the soils from these alkali spots, which were as usual de- 
pressed in the middle, but found in them no excessive amounts of car- 
bonate of soda, and that the sulphate (Glaubers salt) was mainly present 
in fractions of one per cent, not enough to injure vegetation. 
In an attempt to leach one sample of its soluble salts he found that on 
pouring water on a few grams placed in a 50 cc. cylinder the substance 
swelled very rapidly, and over-night actually filled the cylinder to the 
top, making a semifluid slush. As I had never seen the like before, I 
undertook to investigate it. 
