THERMO-BEGULATORS. 147 



perature of the bath rises the mercury contained in 

 the cylinder e, under the influence (jf the elevation of 

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

 in temperature proceeds, the expansion of the mercury 

 accompanies it and gradually closes the slanting opening 

 A 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 the tem- 

 perature 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, 

 therefore, 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 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 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 temperatures, regulators containing such fluids 



