TEXT-BOOK OF BACTERIOLOGY. 117 



use of which will shortly be explained. If the ether is volatilized 

 more and more, the quicksilver above the diaphragm mounts 

 higher and higher; first it reaches the glass tube, gradually shuts 

 off the oblique end, and begins to approach the small lateral hole. If 

 now the source of heat be removed i.e., the hand the ether begins 

 to condense again, the quicksilver sinks through the capillary tube, 

 the upper part becomes empty; and the same processes can be 

 repeated ad infinitum. The thermo-regulator is first " set " to a 

 definite temperature for instance, to 37-J- C. We " set " it in a large 

 bath. When this has attained the desired temperature, push down 

 the upper tube so far into the quicksilver above the diaphragm 

 that the oblique opening is completely closed and only the small 

 lateral hole remains open. The apparatus is now ready for use. 

 Place it in the water between the two walls of the incubator, whose 

 temperature is soon communicated to it. The tube is placed in 

 connection with the gas-pipe; the gas which enters the regulator 

 passes off through a small glass tube, which branches off side wise 

 near the top of the main tube, and is conducted to the burner under 

 the incubator; in other words, the burner receives as much gas 

 only as passes through the regulator. 



If the flame is too large and the water becomes too warm, the 

 ether volatilizes, the quicksilver rises and closes the oblique opening 

 more and more, till at length only the small lateral hole remains 

 open, and the entrance, as also the exit, of gas is reduced to its 

 minimum; then the flame becomes smaller, the water cools down, 

 and so on continually. 



The small lateral hole is intended to leave the flame gas enough 

 to keep it alive even when the need for cooling is at its greatest. 

 Yet circumstances might, perhaps, cause the quicksilver to rise so 

 high as to close even the small side hole, or some accident might 

 put out the small flame fed only through it. 



What are the consequences of such an event ? Tl>e water con- 

 tinues to cool, th& quicksilver sinks till the full maximum of gas 

 streams through the regulator. The dangers which ensue need not 

 be enumerated. 



To prevent them from occurring Koch has invented the safety- 

 flame, which in the supposed case would of itself cut off the sup- 

 ply of gas. 



It is well known that different metals behave differently under 

 the influence of heat; that they do not all expand equally; that 

 their coefficients of expansion differ. If we make a band of two 

 strips of different metal, closely united, and tben heat it, it will 

 bend toward the side of the metal which expands least. 



