444 
PATHOLOGY: E. F. SMITH 
although the constants in this equation were not accurately known, and 
thus to test the quantum relationship (2). The results of HulP indicate 
clearly that the quantum relationship (2) holds for the maximum or 
limiting frequency of the X rays produced at high potentials, but 
although the range of potentials used by Rutherford (13,200 to 175,000 
volts) was sufficient for the accurate determination of the constants of 
(1) or (3), they cannot be determined from his results, either because 
such relationships do not hold for very high potentials, or because the 
X rays had not passed through a sufficient thickness of absorbing material 
to give the limiting values of the absorption coefficients. The X rays 
produced at the potentials used by Rutherford and by Duane are of 
such penetrating power that no special window such as figure 1, was 
necessary. X rays produced at the potentials used in this investi- 
gation (2,500 to 10,000 volts) are of such low penetrating power that 
it was necessary to make the window W as thin as could be made to 
withstand atmospheric pressure, and to use the thinnest sheets of 
absorbing material that could be obtained, and still have them of uni- 
form thickness. 
Summary. — A relationship such as (3) has been known for some time 
to hold approximately for X rays of the ordinary range of penetrating 
power, but the constants have not previously been accurately determined. 
These results indicate that such a relationship holds for very much softer 
X rays, and values of the constants have been determined with con- 
siderable accuracy. These values are in agreement with those which 
hold for the higher potentials, as nearly as the latter have been determined. 
^ W. H. Bragg and W. L. Bragg, X Rays and Crystal Structure, p. 180. W. H. Bragg 
Phil. Mag., March, 1915. 
^Wm. R. Ham, Physic. Rev., January, 1910. 
'Duane and Hunt, Proc. Amer. Phys. Soc, Physic. Rev., August, 1915. 
^Rutherford, Barnes and Richardson, Phil. Mag., September, 1915. 
^Albert W. Hull. Proc. Amer. Phys. Soc, Physic. Rev., January, 1916. 
FURTHER EVIDENCE AS TO THE RELATION BETWEEN 
CROWN GALL AND CANCER 
By Erwin F. Smith 
LABORATORY OF PLANT PATHOLOGY, UNITED STATES DEPARTMENT OF 
AGRICULTURE 
Read before the Academy, April 1 8, 1 9 1 6. Received, June 15,1916 
1. Fundamental concepts. — It is my belief that a diligent study of 
tumors in plants will help to solve the cancer problem. Expressed in 
other words, my text is this: Cancer occurs in many kinds of plants 
in which it passes through an essentially parallel course of development 
