184 TECHNOLOGY OF BACTERIA. 



used with gas, when great accuracy is required, 

 by employing the valve shown in Fig. 9, which 



was invented by the writer 

 for this purpose several 

 years since. 



The bent tube a is con- 

 nected with the gas supply 

 FJ g- 9- by a piece of rubber tubing. 



The upright arm of this tube is enclosed in a larger 

 tube by having an outlet e, which is connected 

 with the burner. The upper end of this larger 

 tube is closed by means of a piece of sheet-rubber, 

 and when this is depressed by means of the lever c, 

 the flow of gas through the valve is arrested. The 

 lever c has attached to it the armature <7, and is 

 operated by an electro-magnet under the control of 

 the regulating thermometer. To prevent the flame 

 at the burner from being entirely extinguished 

 every time the valve is closed, a small aperture o 

 is made in the upright arm of the bent' tube a. 



2. THE RECOGNITION OF BACTERIA. The 

 breaking down of a culture-fluid, either as the re- 

 sult of inoculation or of accidental contamination, 

 may commonly be recognized by the naked eye. 

 The fluid, previously transparent, may become 

 opalescent or milky in appearance, from the pres- 

 ence of a multitude of bacteria distributed through 

 it ; or we may observe a pellicle upon the surface, 

 while the fluid below remains transparent ; or, 

 if some time has elapsed, the micro-organisms, 



