154 



LABORATORY EXERCISES IN BACTERIOLOGY. 



lation of the heat obtained from electricity or from steam or hot -water pipes; the 

 general principle of all depending upon the expansion or contraction of volume of some 

 solid or liquid upon the application of heat or loss of surrounding temperature. A 

 gas thermostat, as the cheap, and efficient form shown in figure 45, may be described 

 as illustrative. The tube A, a large test-tube, is connected by perforated rubber 

 stoppers, and the co-shaped tube, B, with the glass cylinder, C, the arm of B projecting 

 some distance into C. This cylinder, C, is closed above by a rubber stopper having 

 double perforation. One of the tubes, the feed tube, extending through this stopper 

 reaches into the arm of the tube B, projecting through the 

 stopper in the bottom of C. Its lower end is ground to a bevel, 

 and there is a tiny hole bored through its wall a little distance 

 above the beveled end. The other, the exhaust tube, extends 

 but a short distance into the cylinder. The test-tube, A, is filled 

 with a mixture of alcohol and ether or any other fluid having a 

 large coefficient of expansion by heat, the mixture extending 

 well into the co-tube. The dependent curve of the latter tube 

 contains mercury, w r hose level joins the fluid in the other arm and 

 serves to inclose it and prevent its evaporation. The beveled 

 end of the supply tube reaches to the free surface of the mer- 

 cury in the co-tube. The large tube (A) containing the ex- 

 pansile fluid is now inserted into the water-bath of the incu- 

 bator, the rest of the apparatus projecting into the outer 

 atmosphere. The supply tube is connected by means of a 

 rubber tube with the gas-pipe of the laboratory, and a tube leads 

 from the exhaust tube to the burner beneath the incubator. 

 As the water of the bath rises in temperature its warmth 

 causes the fluid in A to expand ; this in turn forces the level of 

 the mercury in the co-tube to rise higher and higher about the 

 beveled end of the supply tube, thus narrowing more and more 

 its orifice over the mercury. To offset entire closure of the 

 opening over the mercury, otherwise completely cutting off the 

 gas-supply and extinguishing the flame beneath the incubator, 

 the tiny hole in this tube has been provided. As the orifice 

 of the supply tube becomes less and less by the elevation of 

 the mercury column, the gas passing to the flame is diminished 

 and the flame lowered. Gradually the temperature of the water 

 in the bath decreases and with this the fluid in A diminishes in 

 volume and allows the level of the mercury to fall away from 

 the beveled end of the supply. Again a greater volume of gas 

 passes to the burner, the flame rises, and the temperature of the water in the bath 

 increases. By a careful adjustment, if the pressure of the gas be even, the tempera- 

 ture of the bath, and consequently of the incubator chamber, may be maintained 

 indefinitely within a variation of less than one degree. For delicacy of the appa- 

 ratus the volume of the expansile fluid in A should be large in proportion to the 

 volume of mercury which it is made to lift in the co-tube. 



One of the most annoying interferences with the maintenance of an even tem- 

 perature arises from inequalities in the pressure of the gas in the general supply pipes. 

 To obviate this the pipe connected with the incubator should be connected as directly 

 as possible with the main (near the meter) and not attached to a series having many 



FIG. 45. THERMO- 

 STAT. 



A. Tube containing 

 expansile liquid. 



B. co-shaped tube 

 containing in shad- 

 ed part mercury. 



C. Cylinder serv- 

 ing as gas cham- 

 ber. D. Feed tube. 

 E. Escape tube. 



