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 thermo- 
regulator may 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 
zine 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. Ifa 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 Murrill’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, 
or Centralb. f. Bakt., 1 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 Anschiitz 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. 1 (scale —15° to + 55°) and No. 2 (scale + 45° to + 105). 
