PHYSICS.-Low voltage discharge in sodium vapor. PAUL D. FOOTE and F. L. MOHLER, Bureau of Standards.

V =

=

The question of separate excitation by electronic impact of the doublet lines of the principal series of thallium was raised by the writers at the New York meeting of the American Physical Society, April 1918. The resonance potential for electrons in thallium vapor was observed to be 1.07 volts. This corresponds on the basis of the quantum relation, hv=eV, to a frequency 8683 or to a wave length 11513 Å, the shorter wavelength member of the first term of the principal doublet series. No indication of resonance at 0.95 volts could be detected corresponding to λ = 13014 Å, the other member of the doublet. The question therefore suggests itself as to whether the line = 13014 is excited at the resonance potential of 1.07 volts. The behavior of thallium in this regard may be very likely similar to that of sodium. Accordingly we might expect the shorter wave length D2 of the sodium spectrum to appear at the resonance potential 2.10 volts and the wave length D, not to appear until the ionization potential of 5.13 volts was attained.

Figure 1 represents the apparatus employed. A is a limecoated Wehnelt cathode, C the anode, and B a glass tube sealed at both ends with glass plates through which the arc discharge was viewed. The object in using such a tube was to permit observation directly upon the arc without the presence of the absorbing unexcited sodium vapor. The tube was electrically

heated to about 300°C. and evacuated to a pressure of about 10-3 cm. Hg as registered by a McLeod gauge. For observing the spectral lines a diffraction grating having 15,000 lines to the inch was employed and the observations were made with an eye piece of such magnification that the two D lines showed distinctly with a rather wide slit opening. This was necessary on account of the low intensity of the light at small voltages.

A visible yellow glow could be detected when the accelerating field across AC was as low as 0.8 volts. Since the resonance potential is 2.1 volts, the initial velocity of a considerable number of the electrons emitted by the hot cathode must have been about 1.3 volts. As the voltage increased the intensity of the discharge rapidly increased. At an applied potential of 5.2 volts the character of the arc changed entirely, the second subordinate series of sodium appearing, thus indicating ionization; and the arc became very brilliant. The applied potential of 5.2

volts is nearly equal to the ionization potential. This fact shows that the initial velocity of the greater portion of the electrons emitted from the cathode could not have been as high as 1.3 volts. At an applied potential of 2 volts, a true potential certainly not greater than 3.3 volts, the two D lines were distinctly visible, and as nearly as could be judged, the ratio of their intensities remained about the same as the applied voltage was decreased to 0.8 volts, when both lines disappeared. Hence our work would indicate that the two D lines appear together at the resonance potential.

This observation is not, however, conclusive evidence that the lines were not separately excited by electronic impact. Wood and Mohler1 have shown that in the excitation of sodium vapor by incident radiation, although it is possible to excite the D lines separately, in general both lines appear since the influence of the surrounding vapor may cause a transfer of energy from the excited line to the other component of the doublet. Possibly a similar influence is present in the above observations.

The fact that the higher frequency line of the thallium doublet determines the value of the energy quantum absorbed, on the other hand, may be analogous to the emission of characteristic X-rays under electronic bombardment. D. L. Webster2 concluded that the K group does not appear until the energy of the impacting electrons is greater than that corresponding to the highest frequency of the K group, namely, Ky. At this voltage all of the lines of the K group appear and the ratio of the intensity of the lines remains the same when the voltage is further increased. Similarly in the case of collision of the atom of thallium, sodium, etc., with electrons of low velocity, both lines of the doublet may always appear simultaneously, as observed directly for sodium, while the line determining the value of the quantum is the line of higher frequency, as observed directly for thallium. However, if the K group is to be looked upon as a series of lines, the above analogy is not justified.

1 Phys. Rev. 11: 70. 1918. 2 Phys. Rev. 7: 599. 1916.

BOTANY.-A sketch of botanical activity in the District of Columbia,-II.22 Bibliography. P. L. RICKER, Bureau of Plant Industry.

ANONYMOUS. Botanical notes. Field & For. 1:5-6. 1875.

Notes on Dentaria heterophylla, Corydalis flava, Viola pedata bicolor, and Poa breviflolia. ANONYMOUS. Autumnal flowering. Field & For. 1: 43–44. 1875.

ASA GRAY BULLETIN, devoted to plant life in field, forest and garden, vols. 5-6, 1897-1898. Edited by C. H. Hicks, Washington, D. C. Vols. 7-8, 18991900, edited by T. A. Williams and C. L. Shear, Takoma Park, D. C.

ASHE, WILLIAM WILLARD. The dichotomous group of Panicum in the eastern
United States. Journ. Elisha Mitchell Soc. 15: 22-62. 1898.
Describes Panicum annulum from the District of Columbia.

BAKER, FRANK P. Native trees of the National Zoological Park. Ann. Rep.
Smiths. Inst. 1890: 65-66. 1891.
Forty-six species.

BARTLETT, HARLEY HARRIS. Euphorbia arundelana, an ally of Euphorbia ipecacuanhae. Rhodora 13: 163-165, f. 1-2. 1911.

BARTLETT, HARLEY HARRIS. On gynodioecism in Plantago lanceolata. Rhodora 13: 199–206, f. 1-3. 1911.

BARTLETT, HARLEY HARRIS. Twelve elementary species of Onagra. Cybele Columb. 1: 37-56, pl. 1-5. 1914.

Oenothera brevicapsula and O. ruderalis from Montgomery County, Maryland.

BESLEY, FRED WILSON. The forests of Prince George County. 40 p. 3 pl. Map. Baltimore, 1913.

Originally published in Maryland Geological Survey report on Prince Georges County, 1911. BLODGETT, FREDERICK HARVEY. A nature study of Maryland plants. Bull. Md. Agr. Coll. 2: 1-22, illust. 1908.

BRAINERD, EZRA. Hybridism in the genus Viola, III.

66-70. 1906.

Rhodora 8: 49-61, pl.

Notes 7 hybrids from the vicinity of the District of Columbia. BRERETON, JOHN A. Florae columbianae prodromus, exhibens enumerationem plantarum quae hactenus explorata sunt; or A prodromus of the Flora Columbiana, exhibiting a list of all the plants which have as yet been collected. Compiled by John A. Brereton. 86 p. Washington, 1830.

This contains the results of the investigations of the Botanical Club of Washington, 1825-1830. BRITTON, NATHANIEL LORD, and BROWN, ADDISON. Illustrated flora of the Northeastern States and Canada. 3 vol. ed. 1, 1896-1898; ed. 2, 1913. Often gives District of Columbia as locality.

BRITTON, NATHANIEL LORD. Manual of the flora of the Northern States and Canada. ed. 3. 1112 p. 1907.

BURGESS, EDWARD SANFORD. Species and variations of biotian asters with discussion of variability in asters. Mem. Torrey Club 13: 1-419. f. 1-108.

1906.

Twenty-four species and 5 varieties recorded from the District of Columbia.

BURROUGHS, JOHN. Spring in Washington. Atl. Monthly, 580-591. 1869. CHICKERING, JOHN WHITE, JR. Erythronium albidum Nutt. Field & For. 1: 8. 1875.

CHICKERING, JOHN WHITE, JR. A season's botanizing. Field & For. 3: 151-155. 1878.

CHICKERING. JOHN WHITE, JR. January flowers. Field & For. 1:72. 1876.

Notes on Draba verna, Stellaria media, Capsella bursa-pastoris, Taraxacum dens-leonis, and Jasminum nudiflorum in bloom first week in January. COVILLE, FREDERICK VERNON. Early botanical activity in the District of Columbia. Records of the Columbia Historical Society 5: 176–194. 1901.

22 Continued from page 498. This bibliography includes all works on the taxonomy of flowering plants and ferns in which have been found definite references to the occurrence of species in or near the District of Columbia. The author will appreciate references to omitted titles.

DEWEY, LYSTER HOXIE.

Agr. 1898: 193–200.

Weeds in cities and towns. Yearbook. U. S. Dept.

1899.

Contains notes on District of Columbia weeds.

DOOLITTLE, M. H. The American mistletoe (Phoradendron flavescens).

& For. 1: 66-67. 1876.

Field

ELLIOT, WILLIAM. The Washington Guide: Containing an account of the District of Columbia, 1822. Florula Columbana by J. A. Brereton, p. 123138, ed. 2, 1826, identical. Additions and corretions, 1830. Ed. 2, "improved" p. 295-310, 1837, revised by Mr. Rich with but few changes. FERNOW, BERNARD EDUARD. Trees of Washington, D. C. Compliments of the Forestry Division. Geo. B. Sudworth, botanist. 18 p. 2 plans, Washington, 1891.

Lists and locates on maps trees on the Department of Agriculture and White House grounds and in Lafayetle Square. Includes 324 species (exclusive of varieties) of which 90 (one doubtful) are said to be indigenous to the District of Columbia. Field and Forest, devoted to general natural history. Bulletin of the PotomacSide Naturalists' Club. Charles R. Dodge, editor. Vols. 1-3. 1876-1878. Flora Columbiana. A catalogue of the plants growing without cultivation in the District of Columbia. By Dr. George Vasey, Prof. J. W. Chickering, Dr. E. Foreman, Prof. Wm. H. Seaman, and Mr. L. F. Ward, committee of Potomac Side Naturalists' Club. Field & For. 1: 83-87, 1876, 2: 13-15; 31-33; 45-46; 61-64; 86-88; 103-105, 1876.

Includes 1083 species.

Addenda made during 1877. Field & For. 3: 145, 164. 1878.
Includes 112 species.

Florula Columbiensis: or a list of plants found in the District of Columbia; arranged according to the Linnaean system under their respective classes and orders, etc., and exhibiting their generally received common names and time of flowering. during the years 1817 and 1818. 14 p. Washington: printed for the Washington Botanical Society by Jacob Gideon, Jun., 1819. FOREMAN, EDWARD. Botany in the D[istrict] of C[olumbia]. Field & For. 1:29-30. 1875.

GALE, LEONARD DUNNELL. On the oaks of the District of Columbia. Proc. Nat. Inst. Wash. n. ser. 1: 67–78, 2 pl. 1855.

Mentions 12 indigenous, 2 introduced species, notes on hybrids, dates of flowering. GOODS, GEORGE BROWN. Diary at the Capital. For. & Stream 6: 115. 1876. Notes spring beauty February 19, and trailing arbutus March 7.

GRAY, ASA. Chrysogonum virginianum dentatum.

Described from High Island.

Bot. Gaz. 7:31. 1882.

GRAY, ASA. Manual of botany, Ed. 6, by Sereno Watson and John Coulter. 760 p. 25 pl.

1889.

Describes Phacelia covillei from Larkspur Island, 5 miles above Washington.

GREENE, EDWARD LEE. Polygonaceous genera. Leaflets 1: 17-50. 1904.

Polygonum coccinea and subsp. asperella.

GREENE, EDWARD LEE.

Toxicodendron compactum.

Segregates of Rhus.

Leaflets 1: 114-144. 1905.

GREENE, EDWARD LEE.

Mutations in Viola.

Leaflets 1: 182–187. 1905.

Leaflets 1: 214-219. 1906.

Viola laetecaerulum and V. aberrana.

GREENE, EDWARD LEE. New species of Viola.

Viola filicetorum and var. parthenica, V. induta, V. emarginata simulata, V. vespertilionia, V. ornthodes, and V. fontana described from the District of Columbia.

GREENE, EDWARD LEE. Violets of the District of Columbia. I. Cybele Columb. 1.7-33. 1914.

Viola grandis, n. sp., described and several species discussed.

GREENE, EDWARD LEE. Remarks on acaulescent violets. Pittonia 3: 139-145. 1896.

Notes on Viola palmata, and V. congener.

GREENE, EDWARD LEE. New or noteworthy species. Pittonia 3: 154-172; 212230. 1897.

Solidago racemosa and Apocynum medium described from the District of Columbia, or vicinity. GREENE, EDWARD LEE. Studies in the Compositae. Pittonia 3: 172-186. 1897; 264 298. 1898.

Antennaria neglecta and A. decipiens described from the District of Columbia.