96 BACTERIOLOGY. 



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 regu- 

 lator 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 com- 

 paratively large, and form has each 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 essential points to be 

 observed in selecting a thermo-regulator depend in the 

 main upon the temperatures to which it is to be applied. 

 For low temperatures such fluids as ether, alcohol, and 

 calcium chloride solution, which expand and contract 

 rapidly and regularly under slight variations in temper- 

 ature, are commonly employed ; whereas for temperatures 



