. Contributions from the Botanical Laboratory, vol. 11. Botany; Botany. 296 AMERICAN JOURNAL OF BOTANY [Vol. 21, (with the latex in it) by the quinhydrone method (Michaehs, 1926; van Harpen, 1929, 1930- A "type-K" potentiometer and a saturated calomel cell were used in each case. The standards proposed by Clark (1928) were used to convert voltage to pH. Measurements of electrophoretic velocity were made by the microscopic method with a modified Northrup-Kunitz (1925) apparatus after the design used by Mudd, Lucke, McCutcheon, and Strumia (1928). Three radio B batteries, giving approx
. Contributions from the Botanical Laboratory, vol. 11. Botany; Botany. 296 AMERICAN JOURNAL OF BOTANY [Vol. 21, (with the latex in it) by the quinhydrone method (Michaehs, 1926; van Harpen, 1929, 1930- A "type-K" potentiometer and a saturated calomel cell were used in each case. The standards proposed by Clark (1928) were used to convert voltage to pH. Measurements of electrophoretic velocity were made by the microscopic method with a modified Northrup-Kunitz (1925) apparatus after the design used by Mudd, Lucke, McCutcheon, and Strumia (1928). Three radio B batteries, giving approximately 135 volts, were connected at each end of the cell to non-polarizable electrodes of zinc in saturated ZnSO,. The electro- phoresis cell was mounted with oil immersion contact over a Zeiss Wechsel- condensor. A single cell was used throughout. A 28 X Zeiss ocular and a Bausch & Lomb 8 mm. objective combined working distance with sufficient magnification. The apparatus is shown in figure \ Fig. I. The Northrup-Kunitz electrophoresis apparatus. Since velocities are given in ^, the potential drop per cm. must be found. Following Ohm's Law, where E is the potential across the chamber itself; L, the length of the cham- ber- Q, its cross section area; R, the specific resistivity of the fluid filling it; / tiie current; and K, a constant dependent upon shape. For calibration,^ the cell is filled with mercury and placed in series with a dry cell, a rheostat, and a Weston standard ammeter. A potentiometer is connected across the cell by small platinum electrodes led into it. The and current are read 1 Thanks are due to Professor Charles Weyl for suggesting this method of caHbration. »' J .... If X 4 ^IV June, 1934I MOVER — EUPHORBIA 297 simultaneously and the resistivity of the mercury at room temperature is found from tables. Having found K, it is used, in practice, in the equation H = KRI where H is the per cm. R is measured by a Wheatstone Bri
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