78 BACTERIA IN RELATION TO PLANT DISEASES. 



An excellent water-bath is that known as the Ostwald-Pfeffer. The experi- 

 menter may, however, construct one for himself out of a medium-sized, thick- 

 walled, porcelain-lined iron kettle (fig. 63). This should rest on a ring of heavy 

 strap-iron supported by four stout iron legs. The burner required may be Dr. 

 Friedburg's safety burner (a very inexpensive and good pattern). The thernio- 

 regulator ma)' be a common Reichert if the mercury seal is cleaned from oxide 

 frequently. In such regulators a sharper contact and a longer freedom from obstruc- 

 tion is said to be obtained (Dr. Harris) by putting a drop of olive oil on top of 

 the mercury. A much better instrument is the metal-bar mechanism known as the 

 Roux regulator (fig. 64). This may be procured from the Maison Wiesnegg, in 

 Paris. It should be kept from direct contact with the water and consequent rusting 

 by burying it in a close-fitting glass tube filled with olive oil or glycerin. This 

 tube is then sunk deep into the water and clamped to the wall of the kettle, which 

 should have perpendicular sides. The water is kept in motion by means of a hori- 

 zontal paddle-wheel at the bottom of the kettle. This consists of four light, oblique 

 zinc or copper vanes (nickeled copper is preferable) soldered to a long central rod 

 which fits into a socket, below, and near its upper end passes through a hole or loop 

 in a horizontal metal arm (a foot or less above the kettle), the other end of which is 

 clamped to the upright rod of a solid iron tripod, or fastened to a rod bolted to the 

 table. If compressed air can be had, a stiff cardboard windmill fastened to the upper 

 end of the vertical rod completes the mechanism. The central part of the wind- 

 wheel may be of cork. The vertical rod may be a piece of glass tubing, in which 

 case it is cemented into a socket of the short metal post to which the vanes of the 

 water-wheel are soldered. If a wind- wheel is attached, it is more convenient to have 

 the vertical rod in two parts, fastened by a coupling. The rod, with its water-wheel 

 attachment, may also be turned by some electrical device. The latter is the most 

 convenient method. In fig. 63 the electric motor is not shown. This stands in 

 a small box screwed to the under side of the table at the right. The switch is fastened 

 to the wall above and back of the top of the thermo-regulator. The pulley band 

 is of smooth rounded leather one-eighth inch in diameter. The electric current is 

 passed through an Edison lamp screwed under the table to reduce the velocity of 

 the motion. With the lamp in place and the current cut down to the minimum 

 the number of revolutions per minute is 55, and the temperature of the water is the 

 same in all parts of the bath. The simplest contrivance of all is to make the 

 water-wheel and upright shaft of wood, to be turned by hand. 



In localities where the gas-pressure is exceedingly variable, Paul Mumll's gas- 

 pressure regulator (at the left in fig. 63) will be found useful. This is made by 

 Eberbach & Co., Ann Arbor, Mich, (see Journal of Applied Microscopy, Vol. I, p. 92, 

 orCentralb. f. Bakt, i Abt, Band XXIII, 1898, p. 1056.) The gas-pressure may be 

 somewhat improved by simply passing the gas through a big bottle (see top of 

 thermostat 311 in plate 8). The Ansclmtz normal thermometers, with long stem 

 and scale divided into fifths, are very convenient for determining temperatures 

 (fig. 65). They come in sets of seven, but may also be had separately. The most 

 frequently useful are No. i (scale 15 to +55) and No. 2 (scale + 45 to -\- 105). 



