abstracts: botany 553 



This volume will, therefore, serve to meet the requirements of those 

 who wish to have information with regard to the mathematical prin- 

 ciples underlying this projection, which has always been extensively 

 used in America. O. S. A. 



PHYSICS. — Some optical and photoelectrical properties of molybdenite. 

 W. W. CoBLENTz and H. KahlER. Bur. Stand. Sci. Paper 338. 

 Pp. 51. 1919. 



This paper gives data on the transmissivity and the reflectivity of 

 molybdenite; also data upon its change in electrical conductivity, 

 when exposed to thermal radiations of wave-lengths extending from 

 the ultra-violet into the extreme infra-red. The efifect of temperature, 

 humidity, intensity of the exciting light, etc., upon the photoelectrical 

 sensitivity of molybdenite were investigated. 



It was found that: (i) samples of molybdenite obtained from vari- 

 ous localities differ greatly in sensitivity; (2) there are maxima of sen- 

 sitivity in the region of 0.73 ix, 0.85 /x, 1.08 /x, and 1.8 /x; (3) there is no 

 simple law governing the variation in the photoelectric response with 

 variation in intensity of the radiation stimulus; (4) the increase in 

 photoelectric cmrent with increase in intensity of the incident radiation 

 is greatest for infra-red rays. It is greatest for low intensities of the 

 exciting light and it is greatest on the long wave-length side of the 

 maximum; (5) the photoelectric sensitivity increases with decrease in 

 temperature. At 70 °C. the bands at 1.02 fx and 1.8 /x have practically 

 disappeared. On the other hand, at liquid air temperatm-es, the 

 greatest change in electrical conductivity is produced by radiations 

 of wave-lengths between 0.8 fj, and 0.9 ix. 



Unlike selenium, molybdenite appears unique in being photoelec- 

 trically sensitive to infra-red rays, extending to about 3 ix. W. W. C. 



BOTANY. — Flora of the District of Columbia and vicinity. A. S. 



Hitchcock and Paul C. Standley, with the assistance of the 



botanists of Washington. Contr. U. S. Nat. Herb. 21. Pp. 329. 



pis. 42. fig. I. 1919. 



The area included by the Flora is approximately a circle of 15 miles 



radius, with the Capitol as the center. The formal list includes all 



indigenous plants and all introduced ones that have become established. 



All the species admitted to the Ust are based upon specimens in the 



District Flora Herbarium, which has been segregated from the main 



collection of the National Herbarium. Species reported but which are 



