232 re;ports of investigations and projects. 



known to continue westerly into Guerrero. They are exceedingly prolific in 

 fossil plants, preserved as imprints, casts, molds, and occasionally silicified; 

 and because of this fact, taken together with the important period of origins 

 represented, the possibilities opened up for future work of prime interest and 

 importance are indeed great. Obviously, too, because of the far southerly 

 position, virtually between the western continents, these new florae have espe- 

 cial value with reference to all problems of former plant distribution. 



The extensive collections had to be transported on the backs of burros over 

 the mountains to the nearest railway station, a distance of 120 miles. They 

 include in particular the most recent lepidodendrids yet found, and a fine 

 display of Williamsonia fruits. Hitherto such have only been obtained in 

 any sufficient number and preservation to afiford opportunity for comparative 

 study on the Yorkshire coast and the Gondwanas of India. The laboratory 

 preparation of these unique fossil plants has been completed, and their illus- 

 tration and description in the form of a memoir on the Williamsonia Flora 

 of the Mixteca Alta is nearly complete. 



The successful prosecution of the Mexican work thus briefly outlined has 

 taken up time to such an extent that the immediate preparation of the manu- 

 script of the volume on the Taxonomy of the American Fossil Cycads has 

 not been carried much beyond the stage announced last year. But it is in- 

 tended to resume the work of preparation of this volume as early as possible. 



PHYSICS. 



Barus, Carl, Brown University, Providence, Rhode Island. Grant No. 574. 

 Continuation of study of the properties of condensation nuclei, including 

 ions. (For previous reports see Year Books Nos. 4, 5, and 7.) $500. 



Professor Barus makes use of Thomson's method of measuring the charge 

 of an electron in terms of the velocities of ions and their number. The latter 

 datum, however, is determined from the angular diameter of the coronas of 

 cloudy condensation, produced in a cylindrical fog chamber under given con- 

 ditions of rapid partial exhaustion. By providing the chamber with an elec- 

 trically charged axial aluminum tube within which are sealed tubelets of 

 radium acting through the aluminum walls in virtue of the gamma rays, the 

 fog chamber becomes an electrical condenser whose ionization may be varied. 

 The electric current passing from core to the shell of the cylindrical con- 

 denser may therefore be measured by a sensitive electrometer. If it be as- 

 sumed that negative ions only are caught in the fog chamber used, and if the 

 author's earlier and independent results are employed for stating the nuclea- 

 tion values of the coronas, the following data are typical for the enormous 

 ionizations used: Total number of ions per cubic centimeter, 1,700,000, 

 385,000, 135,000; corresponding value of 10,000 million times the electrical 

 charge, 4.4, 3.6, 3.9 electrostatic units, respectively. 



