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and other places, Reichert's regulator has proved extremely efficient, this being due in no small degree to its simple construction. The principle of this regulator consists in the mercury expanding by heat and closing the inlet tube, thus diminishing the gas supply. The apparatus is represented in Fig. 18 at about one-fourth of its natural size ; c is the bulb filled with mercury, the thermometer tube widening out above into a cylindrical space which communicates with the inflow tube, A, by a ground air-tight joint; the tube, A, reaches down to the point where the widening of the thermometer begins, and has a fine opening at a. The gas passes off to the burner through the side tube, B. In order to adjust for various temperatures, there is another side tube fitted to the si thermometer stem and filled with mercury, its end being closed by an easily adjustable iron screw, S. The regulating takes place in the following way: The tube, A, is turned until the opening, a, is opposite the tube, B; the screw, S, is set so that the mercury just begins to fill the wide space when the proper temperature is reached ; the mercury is then

Fig. 18.-Reichert's forced upwards by the screw until the

Thermoregulator. flame begins to get smaller. As a result of the closing of the tube, A, the burner is fed only through the opening, a, the size of which can, in some forms of the regulator, be varied by turning the tube, A.

After being in use for some time, a black powdery substance is deposited on the surface of the mercury in consequence of impurities in the coal gas, the sensitiveness of the regulator being diminished. To remove this it is sufficient to take out the inflow tube for a moment and to remove the impurity from the mercury with a brush. The Reichert regulator allows the controlling of all temperatures from 1° above the surrounding temperature almost to the boiling point of mercury.

Fig. 19 represents the improved form of the Reichert regulator, the bulb being omitted. When the temperature

Fig. 19.-Reichert's Improved

Thermoregulator,

Fig. 20.—Muencke's Thermo-
regulator (after Lothar Meyer).

becomes too high, the mercury raises the floating valve, a, b, and thus cuts off part of the gas supply. As in the simpler form there is a by-pass at c.

Muencke's Form of Lothar Meyer's Regulator. — Regulators are often employed in which there is, above the mercury, a liquid which boils at a low temperature, e.g., alcohol or ether. This principle was first proposed by

Lothar Meyer, and has been applied in several ways. A regulator of this kind (by Muencke, Berlin) is shown in the illustration (Fig. 20).

The inflow gas tube is a metal one with a steel end; it passes through an air-tight stuffing box, and can be fixed by means of a screw. It is also provided with a millimetre scale so as to control its position.

This regulator is used in many laboratories, and is perhaps, under certain circumstances, to be preferred. But for most cases the Reichert regulator is quite sufficient.

Roux's Regulator,-Roux's regulator is the one usually employed with the Schribaux thermostat mentioned on page 42. This regulator consists of a steel and a zinc plate soldered together and bent in a U shape. One limb is fixed by a screw, the other remains free. When the temperature rises the two limbs separate, the free limb thus displacing a cone ventilator fitted into a metal box, so that the gas can only enter through a by-pass. If the temperature sinks, the free limb moves back and opens the valve, so that the gas passage becomes free.

Soxhlet's Regulator.–A different regulator from the above must be used for low temperatures. That of Soxhlet, which is shown in Fig. 21, is suitable for use with the Petersen thermostat at 15° C.

Cold water flows down from a constant level cistern through the upper tube on the right. If the temperature of the water in the thermostat is normal all the cold water runs off through the overflow vessel on the left, which is always full;

FIG. 21.but, should the temperature of the thermostat Soxhlet's Ther

moregulator water rise 0·1° above the normal, the ascending" mercury column closes the opening of the syphon, tem and the cold water flows through the horizontal tube on the right into the water of the thermostat, until the normal

r low ratures.

temperature is again reached and the mercury column has sunk so far as to leave the opening of the syphon free again. The bulb on the right contains a few drops of ether or some other liquid of low boiling point.

Koch's Lamp.-For heating the above described thermostats, gas lamps are sometimes used which are provided with an automatically acting apparatus that cuts off the gas supply as soon as the flame is extinguished. Koch's lamp, belonging to this type, works in the following manner: a metal tongue consisting of iron and brass sheet and bent downwards projects into the flame. The lower end is bent round and acts as fulcrum for a lever loaded with a weight. If the flame is extinguished by any cause, the metal tongue cools and approaches the burner; by this motion the lever loses its support and takes up a vertical position, and by so doing turns off the gas.

Thermometers. It is convenient to use thermometers about 10 cm. long with a maximum reading of 25° C. for measuring temperatures in thermostats. They can be fixed in a bored cork which has small grooves down the sides and rests in a wide-mouthed glass vessel. The air in this communicates with that in the thermostat through the grooves on the cork. If the thermometer is fitted up in such a glass, it can be taken out and the temperature read without fear of its changing, as would be the case if the thermometer alone were removed from the warm thermostat. It is not out of place to remember that ordinary thermometers cannot always be depended on, but must be checked before they are used. It is also necessary to do this from time to time, as the thermometers change in course of time. A thermometer is checked by comparison with a standard thermometer, the zero point of which has been exactly determined previously by immersing its bulb in finely pounded ice. A possible error is thus removed.

5.—Sterilising Apparatus. With Dry Heat.-Glass and metal apparatus are sterilised by means of dry heat. This is done by using an oven made of iron, coated with lead and of the shape shown in Fig. 22. It consists of a double-walled chamber of cylindrical form ; the outer wall is coated with asbestos on the inside and is conical at its lower end; a large gas burner projects into the conical part, so that the products of combustion rise between the walls and leave the apparatus by the chimney on the top. The door is also double walled. The burner can be raised or lowered, its position and the proper size of flame being determined by experiment.

There is quite a large number of different designs of these hot air chambers. The principal point is that they must give a temperature of 150° C., and must be durable. A regulator may be used along with them, but it is not necessary in most cases since it is immaterial whether the glass articles are heated to a little more Fig. 22.-Apparatus for Sterilisor a little less than 150° C. On a tion with Dry Heat. the other hand, if the hot air chamber is used for sterilising the gypsum blocks to be referred to later, it is of great importance that these should not be submitted to a higher temperature than 120° C., because they lose water of crystallisation and afterwards crumble on being placed in water The simplest way, however, is to use an iron box, the walls

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