116 BACTERIOLOGY. 



burner lighted and placed under the bath. The gas 

 now streams through the tube a into the cylinder e and 

 out at b to the burner, but as the temperature of the 

 bath rises, the fluid contained in the cylinder e, under 

 the influence of the elevation of temperature begins to 

 expand, and as a continuous rise in temperature pro- 

 ceeds, the expansion of the fluid accompanies it and 

 gradually closes the slanting opening h of tube a. In 

 this way the supply of gas becomes diminished and 

 the rise in temperature of the bath will be less rapid, 

 until finally the opening at h will be closed entirely, 

 when the supply of gas to the burner will now be 

 limited to that passing through the capillary opening 

 g. This is not sufficient to maintain the highest tem- 

 perature reached, and a gradual contraction of the fluid 

 now occurs until there is again an outflow of gas from 

 the opening h, when again the temperature rises. This 

 contraction and expansion of the fluid in the regulator 

 continues until eventually a point is reached at which 

 the position of the fluid in the cylinder e allows of the 

 passage of just enough gas from the opening h to 

 maintain a constant temperature. This, in short, is the 

 principle on which thermo-regulators are constructed, 

 but it must be borne in mind that a great deal of detail 

 exists in the construction of an accurate instrument. 

 The number of different forms of this apparatus is 

 comparatively large, and each form has its special 

 merits. 



The value, that is the delicacy, of the thermo- 

 regulator depends upon a number of factors, all of 

 which it would be useless to introduce into a book of 

 this kind, but in general it may be said that the essen- 

 tial points to be observed in selecting a thermo-regulator 



