THERMO-REG ULATORS. 125 



DOW 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 mercury 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 as cooling begins a gradual contrac- 

 tion of the mercury occurs until there is again an outflow 

 of gas from the opening A, 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 con- 

 stant degree of expansion of the mercury in the tube e. 

 This, in short, is the principle on which thermo-regu- 

 lators 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 



