394 THE PRODUCTION OF HEAT, LIGHT, AND ELECTRICITY 



into two halves by an air-tight partition at c, so that one-half of the plant might be 

 in hydrogen, the other in air, or if the plant was surrounded by cotton-wool at c, one- 

 half could be kept at a higher temperature than the other. At the same time the 

 whole plant, or stem, can be kept in air saturated with moisture. 



To measure the current in the external circuit either a sensitive reflecting 

 galvanometer or a Lippmann's capillary electrometer may be used. The latter 

 instrument consists of a capillary tube containing mercury, whose open end is 

 immersed in dilute sulphuric acid which also fills the tube up to the mercury. On 

 passing a current from the mercury to the sulphuric acid the capillary constant alters 

 and a corresponding movement of the mercury ensues, which when read off by means 

 of a horizontal microscope may enable changes of potential of less than 0-0005 f 

 a volt to be detected by means of a delicate instrument l . Rapid changes in the 

 intensity of the current may be detected and measured by causing the thread of 

 mercury to throw a strong shadow upon a slowly-moving photographic plate 2 . The 

 electrical potential may be determined either by means of a compensator which is 

 adjusted until the mercury regains its original position, or by determining the actual 

 pressure required to drive the mercury back to its original position while the current 

 is still passing. 



SECTION 86. The Influence of External Agencies on the 

 Production of Electricity. 



The existence of externally perceptible electrical currents was discovered 

 by Becquerel 3 on injured plants, and on the uninjured leaves of Dionaea by 

 Burdon-Sanderson and Munk, while Kunkel, Muller-Hetlingen, Haake, and 

 others subsequently extended these observations to a variety of uninjured 

 plants 4 . In fact there does not appear to be a single plant of any size in 

 which differences of potential cannot be detected between points on its 



1 See Hermann, Physiol. Practician, 1898, p. 93; Ostwald, Hand- u. Hilfsbuch f. physiko- 

 chemische Messungen, 1893, p. 247; Hermann and Gildemeister, Pfltiger's Archiv f. Physiologic, 

 1900, Bd. LXXXI, p. 491. 



2 See Langendorff, Physiol. Graphik, 1891, p. 90; Garten, Abhandl. d. math.-physisch. Klasse 

 d. Sachs. Ges. d. Wiss., 1901, Bd. xxvi, and textbooks of animal physiology. 



3 Becquerel, Ann. de chim. et de physique, 1851, 3" se'r., T. xxxi, p. 40; Wartmann, Bot. Ztg., 

 1851, p. 308; Buff, Ann. d. Chem. u. Pharm., 1854, Bd. LXXXIX, p. 76; Heidenhain, Studien d. 

 physiol. Inst. zu Breslau, 1861, Heft i, p. 104; Hermann, Pfliiger's Archiv f. Physiologic, 1871, 

 Bd. IV, p. 155 ; Ranke, Sitzungsb. d. bayrisch. Akad., 1872, p. 181 ; Velten, Bot. Ztg., 1876, p. 273. 



4 Burdon-Sanderson, Proc. of the Royal Soc., 1876-7, Vol. xxv, p. 411 ; Phil. Trans., 1882, 

 Parti; 1888, Vol. CLXXIX, p. 417; Biol. Centralbl., 1882, Bd. II, p. 481; 1889, Bd. ix, p. ij 

 Munk, Die elektrischen u. Bewegungserscheinungen am Blatte \onDionaea, 1876; Kunkel, Pfliiger's 

 Archiv f. Physiol., 1881, Bd. xxv, p. 342 ; Arb. d. bot. Inst. in Wiirzburg, 1878, Bd. II, pp. i, 333 ; 

 Miiller-Hettlingen, Pfliiger's Archiv f. Physiol., 1883, Bd. xxxi, p. 193 ; Haake, Flora, 1892, p. 455 ; 

 B.Klein, Ber. d. bot. Ges., 1898, p. 335; Dubois, Centralbl. f. Physiol., 1899, Bd. xin, p. 699; 

 Waller, Proc. of the Physiol. Soc., 30. Juni, 1900, und 9. Nov., 1901 ; Proc. of the Royal Soc., 1900, 

 Vol. LXVII, p. 129 ; Centralbl. f. Physiol., 1901, Bd. XV, p. 480 ; Tompa, Beihefte z. Bot. Centralbl., 

 1902, Bd. xn, p. 99; Querton, Institut Solvay, Travaux du Laboratoire d. Physiol., 1902, T. V, 

 Fasc. 2, p. 81 ; Bot. Centralbl., 1903, Bd. xcn, p. 145 ; Plowmann, Bot. Centralbl., 1903, Bd. xcm, 

 p. 61. The rather fantastic discussions of R. Keller (Reibungselektrische Untersuch. an pflanzlichert 

 Geschlechtsorganen, 1902) hardly need comment. 



