130 BACTERIOLOGY. 



turned on and the 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 tem- 

 perature of the bath rises the mercury contained in 

 the cylinder e, under the influence of the elevation of 

 temperature, begins to expand, and, as a continuous rise 

 in temperature proceeds, the expansion of the fluid ac- 

 companies 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 open- 

 ing g. This is not sufficient to maintain the highest 

 temperature reached, and as cooling begins a gradual 

 contraction of the mercury occurs until there is again 

 an outflow of gas from the opening h, when again the 

 temperature rises. This contraction and expansion of 

 the mercury in the regulator continues until eventually 

 a point is reached at which its position in the cylinder 

 e allows of the passage of just enough gas from the 

 opening h to maintain a constant temperature; and, 

 therefore, a constant degree of expansion of the mercury 

 in the tube e. This, in short, is the principle on which 

 therm o-regulators are constructed; but it must be borne 

 in mind that a great deal of detail exists in the construc- 

 tion of an accurate instrument. The number of differ- 

 ent forms of this apparatus is comparatively large, and 

 each form has its special merits. 



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

 ulator 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 essential points to 



