118 TEXT-BOOK OP BACTERIOLOGY. 



The safety-burner is based on this fact. The flame is between 

 two spiral springs, each of which is composed of two different 

 metals — copper and iron — united in the manner just described. If 

 the flame goes out and the spirals begin to cool, they unroll. But 

 they are placed on a round, movable disc, which they draw to the 

 one or the other side, according to their changes of form. This 

 disc has at one point a catch, by which it holds a heavy lever 

 running parallel with the gas feed-pipes and connected at its other 

 end with a valve in the gas-pipe. As long as the flame burns the 

 valve is open and the lever rests on the disc. If the flame goes 

 out the spirals begin to operate, the disc is turned somewhat round, 

 the lever-catch goes with it, the lever loses its point of support, 

 falls, closes the valve, and shuts off the gas. 



Besides incubators thus regulated, there are a number of others 

 arranged on different principles. 



The most important are those with a membrane regulator, such 

 as the well-known thermostat of d'Arsonval. It is a double-walled 

 copper vessel, in which an equable temperature is likewise main- 

 tained by means of regulating the amount of gas supplied to the 

 flame which warms the water between its walls. Before reaching 

 the flame the gas flows into a small chamber, one side of which 

 consists of a thin sheet of India-rubber. This lies with the other 

 side against the water between the two walls. If the water gets 

 warmer it expands and presses the gutta-percha membrane into 

 the chamber. The gas supply is at once considerably retarded, the 

 flame becomes smaller, the water cools, contracts, etc. 



Lastly must be mentioned a very useful and reliable kind of in- 

 cubator recently constructed by Lautenschlager, of Berlin, in which 

 the regulation is managed by electricity, by means of a contact- 

 thermometer and a magnetic burner. A precise description of this 

 ingenious apparatus would lead too far, however. 



Thus far the ways and means which enable us to obtain a some- 

 what exact view of the vital processes of the bacteria have been 

 considered. 



Clearly as we may and must feel that we have hardly got be- 

 yond the beginnings of a systematic investigation, let us, neverthe- 

 less, gratefully acknowledge that the introduction of new and ex- 

 cellent methods has alreadj'-, and in a very short space of time, 

 revealed to us a rich abundance of previously unknown and alto- 

 gether unexpected facts. 



It may be hoped that the intelligent use of these valuable mieans 

 of investigation will enable us to advance with rejoicing, and to 

 maintain bacteriology in that degree of importance by means of 

 which it has already become a eentre of general interest. 



