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gas supply is thus cut off at the desired temperature. If the temperature o the incubator falls, the mercury contracts, the aperture of the inlet tube is uncovered, the gas supply is renewed and the flame increased. The temperature will then rise until the required point is reached, when the gas supply will again be cut off. A uniform temperature which oscillates within a range of half a degree is thus attained.

Reichert's Thermo-regulator (fig. I6) is another simple and also an earlier form. The stem (S) of the regulator is enlarged above and receives a hollow T-piece (P), the vertical limb of which fits accurately into the enlarged end of the stem, and one end of the P cross-limb receives the inlet gas pipe; the other end is closed. The vertical limb of the T-piece is narrowed down at its lower extremity and opens by a small aperture. Above this terminal aperture is a lateral one of the smallest size. From the enlarged ir end of the stem there passes out a lateral arm (A) which is connected with the outlet Vi pipe to the burner, and lower down another arm (L), which is closed at its outer ex~ § tremity by a screw (R), is also attached. § E The stem and lower arm are filled with E mercury and the bulb of the stem is placed

in the incubator chamber, and gas allowed to pass. When the desired temperature is reached, the mercury in the stem is forced upwards until it closes the aperture of the T-piece, by screwing in the screw (R) of the lower lateral arm (L). here are severa modifications of Reichert's original form. In one of these the screw arrangement in the lower arm is replaced by a piston rod working in a narrow bore of a vertically bent limb of the arm. In another orm, the other end of the cross bar of the T-piece is open and leads through a stopcock to a third arm, which opens into the enlarged upper end of the stem opposite to the outlet arm (A); this modification acts as an adjustable by-pass and replaces the minute aperture in the side of the vertical limb of the T-piece. In Babes' modification the gas supply is cut off, not by the occlusion by the rising mercury of the aperture of the T-piece, but by a Hoating beaded wire-valve. The aperture of the vertical limb of the T-piece (P) is traversed b¥a fine wire which is enlarged at both ends into a bead-like knob. he wire fits loosely in the aperture and not only therefore works easily in it, but allows gas to freely pass. When the lower bead-like knob, however, is raised by the expansion of the mercury, hthe gas supply is cut off by the bead being carried up against the ori ce. Cuccatti's thermo-regulator (fig. 17) is an exceedingly simple and ingenious form. The stem (S) of the regulator is enlarged below into a bulb, while above it divides into a V. The two limbs of the V are of course traversed by a canal and

FIG. 16.-Reichert's Thermo- Regulator.

they are connected above by a tubular

/ ~/ cross bar (C). In the middle of this

/ there is a stopcock situated between the two points where the bar °oins the A . limbs of the V. One end of the cross tube serves as an inlet and the other as an outlet for the gas. The stopcock serves as an adjustable by-pass. About an inch below the point where the two limbs of the V join the stem, thi bcére of the gatter is enla?§§ l, and -f it ea s into a atera arm, con-FIG 17' CuccatuSTherm°' taining a screw (R), similar to the corresponding arm in Reichert's reguin the bulb and stem expands, it rises, and reaching the point when the two limbs of the V meet occludes the orifice to both and thus cuts off the gas supply, except that which is passing through the by-pass of the stopcock. The temperature at which this occlusion will take place can be determined by the screw in the lateral arm. The more this is screwed in, the lower will be the temperature at which the gas becomes cut off, and vice versa. Bunsen's, Kemp's and Muenke's regulators are in reality of the nature of air-thermometers, and act by the expansion and contraction of air, which raises or lowers respectively a column of mercury; this in its turn results in the occlusion or opening of the gas aperture. Such forms, however, are subject to the influence of barometric pressure and an alteration of 0-5 in. of the barometer column may result in the variation of the temperature to as much as 2°. Lothar Meyer's regulator is described in the Berichte of the German Chemical Sombty, 1883, p. 1089. It is essentially a liquid thermometer, the mercury column being raised by the expansion of a liquid of low boiling-point. The liquid replaces the air in Bunsen's and other similar forms. The boiling-point of this liquid must be below the temperature required as constant. The solid forms of thermostats are constructed upon the same principle as the compensation balance of a watch or the compensation pendulum of a clock. This depends upon the fact that the / J S . Regulator. lator. When the mercury co-efficient of expansion is different for different metals. It therefore results that if two bars' of different metals are fastened together along their lengths (fig. IS, Z and ST) with the same rise of temperature one of these will expand or lengthen more than the other. And since both are fastened together and must therefore accommodate themselves within the same linear area, it follows that the compound rod must bend into a curved form, in order that the bar of greater expansion may occupy the surface of greater length, 1'.e. the convex one. Conversely, when the temperature falls, the greater degree of contraction will be in the same bar, and the surface occupied by it will tend to become the concave one. If, then, one end of this compound rod be fixed and the other free, the latter end will describe a backward and forward movement through an arc of a circle, which will correspond with the oscillations of temperature. This movement can be utilized by means of simple mechanical arrangements, to open or close the stopcock of a gas supply pipe. In the construction of this type of thermostat it is obvious that the greater the difference in the co-efficient of expansion of the two metals used, the larger will be the amplitude of the movement obtained. Steel and zinc are two metals which satisfy this condition. fl j C I 2 sp sf 4 l l IE ~ = FIG. 18.-Dr Roux's Thermostat (straight bar); The co-efficient of steel is the lowest of all metals and is comparable in its degree with that of glass. Substances which are not metals, such as vulcanise and porcelain, are sometimes used to replace steel, as the substance of low co-efficient of expansion. <, The bimetallic thermostat most commonly employed is one of the two forms designed by Dr Roux. In one of these forms the compound bar is straight (fig. 18) and in the other it is U~shaped (fig. 19). In the former type the bar itself is enclosed in a tube (T) of metal, the wall of which is perforated; Towards the open end of this tube the gas box or case (C) is fixed. In the U-shape form it is attached to the outer surface (zinc) of one limb of the bar. The gas box is capable of adjustment with respect to its distance from the bar b 1 Y means of a screw (S) and a spiral spring (SP), .. S which moves the box "" -|i outwards or inwards along a rod (R). This adjustment enables the degree of temperature at which it is desired that the gas shall be cut off to be fixed accurately and within a certain more or less extended range. The inlet and the outlet pipe are discon nected from each other in the gas box by means of a piston-like rod (P) slides backwards and forwards in the tubular part (T) of the box from which the outlet pipe emerges. When the valve (V) rests upon the edge of this box, the gas is completely cut off from passing through the outlet pipe, with the exception of that which passes through an in

me FIG 19 Dr Roux s Thermostat (U shaped bar) ||F§ '§ ¢'S€ E - ... ~ C j....... n ~ E; ' I # E E ' / A § § and valve (V), which § § ' Z § E, § #E, I . %l|ll|li exceedingly small aperture (B), serving as a by-pass. This is just large enough to allow sufficient gas to pass to maintain a small flame. The piston-like rod and valve, when free, is kept pressed outwarils by means of a spiral spring. This ensures that the valve shall follow the movements of the compound bar. When this bar bends towards the gas box owing to a fall of temperature, the valve is pushed back away from the orifice and gas in increasing quantity passes through. The temperature of the incubator begins then »to rise, and the zinc bar (Z) expanding more than the steel one (ST), the bar bends outwards and the valve once more cuts off the gas supp y. ° (d) Gas-Pressure Regulators.-The liquid form of thermo-regulators especially work with a greater degree of accuracy if they are combined with some apparatus which controls the variations in gas pressure. There are various forms of these regulators, most of which are figured

and sometimes partially described in the catalogues of various