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MANUAL OF BACTERIOLOGY. 



ably 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 ofif a lower chamber, which contains mercury 

 and is connected with the upper part by a glass tube. The 

 purpose is to make use of the elastic properties of some volatile 

 fluid, like ether, which floats on the surface 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 

 cutting 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 sup- 

 ports 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 inconvenience will arise 

 from irregularities in the flow of gas from the main supply-pipe. 

 Any incubator will vary a little from such causes. In the 

 experience of the writer, natural gas is of such variable pressure 

 as to be entirely useless. Fluctuations of the temperature 

 within the incubator depend very largely upon the external 

 temperature. Therefore the incubator should, as far as is 



Fig. 25. — Koch Auto- 

 matic Gas-burner. 



