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a wooden lid, p, provided with an 8 cm. layer of felt. The lid can be opened and shut easily by means of a counterpoise weight hung over a pulley fastened to the wall of the room behind the thermostat.

Each of the spaces 1 to 8 is provided with a tightly fitting glass door, and doors of sheet iron are fitted on each of the four large compartments A, B, C, and D, which are closed tightly by pressing against rubber strips fitted on to the partitions. Four corresponding doors, also shutting tightly, are attached to the wooden case, their inner sides being coated with woollen pads. All these doors are hinged below, and, when opened and resting in a horizontal position on adjustable brackets, may be used as tables.

The space under the thermostat is used as a cupboard.

If the apparatus is working and the regulator set, for example, at 40° C., compartment 1 will be at this temperature while the temperature in 9 will be only a few degrees above 0°. In the intervening compartments the temperature varies between these two extremes. In individual compartments the temperature varies from wall to wall and also from top to bottom.

If the temperature of the room containing the thermostat is somewhat high, as may happen in summer, it will be sometimes difficult to reach a low enough temperature in the cold part of the thermostat. A small iron box (Fig. 15, 10) with an ice holder is then placed in D; the temperature of the whole thermostat is thus lowered and at the same time a specially low temperature compartment is obtained

Schribaux's Thermostat. — A thermostat frequently used is that of Schribaux which is seen in Fig. 16. It consists of a wooden cupboard with a copper floor; the heated gases of the burner pass through brass tubes fixed to the walls. A special regulator, mentioned on page 49,

is usually employed along with it. The temperature variations are greater in this thermostat than in those of Rohrbeck and Panum, as the large doors when opened allow a considerable cooling to take place.

Large Warm Chamber.-In more extensive experiments, in which many cultures are dealt with simultaneously

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at the same temperature, it would, of course, be difficult to find sufficient room for them in an ordinary thermostat. If circumstances allow, a small room can sometimes be made into a thermostat. It is fitted up according to the same principles as an ordinary thermostat, and is, in fact, the same on a larger scale. The Carlsberg laboratory

contains a “warm chamber” of this kind, which is usually kept at 25° C., and which has proved eminently satisfactory. The room is 250 cm. high, 160 cm. long, and 160 cm. broad. Movable shelves are fitted along the walls. The walls and the roof are composed of two layers of hollow stones between which animal charcoal is placed; the door is also double and filled with kieselguhr. The heating is effected by means of warm water which passes from a vessel outside which is heated by a gas flame, circulates through copper pipings along the walls and so returns to the vessel. The piping is about 960 cm. long, 4 cm. in diameter, and is fixed about 55 cm. above the floor. A Reichert regulator is fixed in the wall and is in communication with the room; at another place a thermometer passes from the outside into the room so that the temperature of the latter may be observed from without. There are also two openings in the wall closed with cotton wool which act as ventilators. Through these, such things as tubing may be passed in.

A temperature of 15° C. is often necessary in the analysis of yeast. If a Panum thermostat is not available, a special thermostat for 15° C. must be fitted up. In many cases a cellar might be used instead of the above, although its temperature is of course not particularly constant.

A. Petersen's Thermostat for Low Temperatures.The principle of a thermostat for low temperatures differs from that of the thermostats described where the temperature is above that of its surroundings; in this case the process is reversed. Anton Petersen has constructed such a thermostat for use in the laboratory of the old Carlsberg brewery in Copenhagen. The thermostat consists of a cylindrical double-walled copper box with a loose lid also double walled. The outside is covered with felt, and the space between the walls filled with water. Above on one

side there is an inlet for the water, and on the opposite side an outlet. Near the inlet there are also perforations for regulator and thermometer. In the lid there are, in addition, two openings, one for a thermometer which projects into the interior of the space in which the cultures are placed, the other acting as ventilator. Tap water is used for obtaining the desired temperature, its temperature being generally lower than 15° C. The water passes from the tap through a tube to a small constant-level cistern, and from there through another tube to the regulator (see page 49).

Pfeiffer's Microscope Heating Apparatus.- When it is desired to follow out the development of a micro-organism at a certain constant temperature under the microscope, this can be done by using the small thermostat designed by L. Pfeiffer, which is shown in Fig. 17.

The arrangement consists of a mahogany box, which completely surrounds the stand, and which is almost airtight when closed. Its front wall has a glass window to admit sufficient light for observation; the left and right walls (seen from the observer) have each a well-fitting flap door so that the preparation may be manipulated. In order to make the microscope freely accessible, the side walls are capable of being completely removed along with the halves of the back wall, which is divided down the middle. The whole stands on a thick metal plate with three metal feet. The heating is effected by warming the plate from below by means of a micro-burner, the gas supply of which is controlled by a regulator. Experiments made in the Carlsberg laboratory have shown that the Reichert regulator described below is very suitable for this apparatus. With the side flaps opened or closed the greatest variation of temperature in the apparatus at 25° C. or 32° C. was only 1°. If the apparatus is closed and then the flaps suddenly opened, the temperature falls about 2o and then keeps constant. It is therefore desirable to leave the flaps open when working with temperatures which allow of this. With high temperatures this is impossible.

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Reichert's Regulator.-There are numerous forms of thermoregulators which may be used in combination with the thermostats for higher temperatures described above. In the course of long use in the Carlsberg laboratory

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