162 BACTERIOLOGY 



and as cooling begins a gradual contraction of the mercury 

 occurs until there is again an outflow of gas from the opening 

 h, when the temperature again 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, there- 

 fore, a constant degree of expansion of the mercury in the 

 tube e. 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 describe in 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 at which it is to be used. For low tem- 

 peratures, regulators containing such fluids as ether, alcohol, 

 and calcium chloride solution, which expand and contract 

 rapidly and regularly under slight variations in temperature, 

 are commonly employed; whereas for temperatures approach- 

 ing the boiling-point of water mercury is most frequently used. 



Other types of regulators operate on the principle of the 

 unequal expansion of different metals. Thus, if two strips 

 of metal having different coefficients of expansion be fixed 

 together, when expansion occurs, it obviously will not be 

 in a right line, but rather at an angle to such line. Con- 

 sequently if one end of such a composite rod be fixed and 

 the other left free, the higher the temperature the greater 



