9 o 



MANUAL OF BACTERIOLOGY. 



Fir,. 25. 



causes expansion of the mercury, which rises, and, little by 

 little, cuts off the inflow of gas through this tube. The 

 flow is never completely cut off, as there is a capillary open- 

 ing in the tube considerably above any point to which the 

 mercury could possibly rise, which will always allow the 

 flow of a small quantity of gas (Fig. 24, b). This diagram 

 also shows a modification of the simple form of regulator, 

 in the shape of a partition which divides off a lower cham- 

 ber, which contains mercury and is connected with the upper 



part by a glass tube. The pur- 

 pose is to make use of the elastic 

 properties of some volatile fluid, 

 like ether, which floats on the sur- 

 face of the mercury at a. The 

 gas coming from the gas-regulator 

 passes to a Bunsen burner, which 

 stands underneath the incubator. 

 This burner should have some 

 kind of automatic device for cut- 

 ting off the flow of gas in case it 

 becomes accidentally extinguished 

 by a sudden draught of air or 

 from any other cause. The auto- 

 matic burner invented by Koch 

 is an ingenious, simple and effec- 

 tive device. A bar of metal 

 stands above the flame; by its 



expansion, through a system of levers, it supports a weight ; 

 the weight controls a gas-cock. While the flame is burning 

 the expansion of the metal holds the weight horizontally; if 

 the flame becomes extinguished, the metal contracts, the 

 weight falls, and cuts off the flow of gas. Some incon- 

 venience will arise from irregularities in the flow of gas 



Koch Automatic Gas-burner. 



