Fig. 165 shows the Wolpert air tester which is a form of this apparatus that has given general satisfaction. The results attained by its use are approximate but sufficiently exact for all practical purposes. The apparatus consists of a graduated glass tube in which fits a rubber piston mounted on a hollow glass rod, through which the sample of air is admitted to the tube. The chemicals used for absorbing the carbonic acid gas are furnished with the instrument but may be replenished without difficulty. Directions for its use are furnished with the tester that may be readily followed after a trial. The results obtained are read directly from the side of the tube. The tester may be obtained from any dealer in chemical or physical apparatus. [Illustration: FIG. 166.--Thermostat regulator and motor-valve attached to a radiator.] =Pneumatic Temperature Regulation.=--Pneumatic temperature regulation is very generally used in large and complicated heating systems, because of its positive action and completeness of heat control. This method of heat regulation utilizes the energy of compressed air, with which to open and close the valves of the radiators. It may be adapted to any mode of heating and can be used with any size of plant, but is particularly suited to extended systems. The radiators, providing heat for any particular space, are under control of separate thermostats, which by means of motor valves admit heat only as required. A motor, operated by compressed air, is attached directly to each radiator valve. Any change in temperature of the room causes the thermostat to correct in the radiator the required amount of heat. With this method of regulation the temperature-controlling element of the thermostat, like that of the electro-thermostatic system, is a sensitive part, which by expanding and contracting with the heat and cold directly controls the heat in any part of the building. The motive power for opening and closing the valves of steam or hot-water radiators or for operating the dampers in a hot-air system is supplied by compressed air. The air supply is furnished by an air compressor which automatically stores air under pressure in a pressure tank, from which is drawn the necessary energy, as occasion demands. The air is conducted to the motors through small pipes which are connected with the regulating elements and also with the motors. The function of the thermostat is to so govern the air which enters the motor as to correct any change in the temperature of the rooms. This it does by opening and closing the valves as occasion demands.