24 BACTERIOLOGY 



the opening for the passage of the gas must become coj«» 

 tinually smaller, and this is followed by cooling of the 

 water and sinking of the mercury, so that the aperture 

 transmitting the gas again enlarges, and therewith the 

 flame increases and the temperature ascends, but cannot 

 pass beyond the limit for which the regulator is set. With 

 good management the variations are only very small. 



Meyer's thermo-regulator. — Another thermo-regulator, 

 constructed by Victor Meyer, which is extensively used and 

 can be highly recommended on account of its sensitiveness, 

 consists also of a glass vessel like a test-tube, and which 

 can be closed with a rubber cork. It is furnished with a 

 small side-tube in the upper part, and is divided into 

 two sections by a capillary funnel of glass, the end of 

 which is just above the bottom. The lower division is 

 filled with mercury, the surface of which is only some three 

 cm. distant from the edge of the funnel, and the interspace 

 thus left is occupied by a mixture of alcohol and ether. In 

 the upper part is fitted a glass tube, cut off obliquely below, 

 and passing through the rubber cork; it ends a little 

 above the capillary funnel, and is pierced in one side above 

 the lower opening with a hole the size of a pin's head. 



In order to graduate the regulator it is immersed in 

 a water-bath, the temperature of which is controlled by 

 an accurate thermometer ; even a slight increase of heat 

 volatilises the ether and drives the mercury up, so that it 

 comes to stand above the capillary funnel. If now the 

 water has reached the temperature fixed on, the obliquely- 

 cut glass tube is so far introduced into the mercury that 

 the lower opening is quite covered and only the safety 

 aperture at the side remains pervious. The regulator is 

 so connected with the flame under the incubator that this 

 receives only such a quantity of gas as can traverse the 

 regulator. When the water attains too high a temperature, 



