118 BAGTERIOLOGV. 



object the maintenance of a constant pressure of the gas 

 going to the thermo-regulator. There are several in- 

 struments of this form in use, but they do not accom- 

 plish the object for which they are designed. 



The instrument most commonly employed, the appa- 

 ratus of Moitessier (Fig. 29), is based on somewhat the 

 same principles as the large regulators seen at the 

 manufactories of illuminating gas, which act very well 

 when employed on the large scale, as one sees them 

 there ; but which, when applied to the limited and 

 sudden fluctuations seen in the gas coming from an 

 ordinary gas-cock, are practically useless. They are 

 too gross in their construction, and are only seen to act 

 under comparatively great and gradual fluctuations in 

 pressure. If a good form of thermo-regulator is em- 

 ployed there is no necessity for the use of any of the 

 forms of pressure-regulators thus far introduced. 



