THE THERMOMETER. 



part of the reservoir c D, will be filled with mercury after all the 

 air has been expelled. " 



When this has been accomplished, let the tube be removed from 

 the source of heat, and allowed gradually to cool. A file applied 

 at c, where the top of the tube is joined to the superior reservoir, 

 detaches that reservoir from the tube, which remains with the 

 bulb A B completely filled with mercury. 



In this state the instrument would give no indication of change 

 of temperature, no space being left for exhibiting the play of the 

 mercury by dilatation and contraction. 



To obtain space for this, let the bulb A B be exposed to a tem- 

 perature higher than any which the instrument is intended to 

 indicate. The mercury dilating will then overflow, and will con- 

 tinue to overflow until the mercury acquires the extreme 

 temperature to which it is exposed. 



A jet of flame being now directed by a blow-pipe (fig. 3), on 



the end c, it will 

 be hermetically * 

 sealed; after which, 

 '^ being allowed to 

 cool, the mercurial 

 column will subside, 

 the space in the tube 

 above it being a 

 vacuum, since the 

 air is expelled. The 

 column will continue to subside until the mercury assumes that 

 state which corresponds to the temperature of the air surrounding 

 the instrument. 



20. The variation of the height of the mercurial column in such 

 a tube will in all cases correspond with the changes of tempera- 

 ture incidental to the surrounding medium ; but, in order that it 

 may supply a numerical expression and measure of such changes, 

 a scale must be attached to the tube, by which the variations of 

 the column may be indicated, and the divisions or units of such 

 scale must correspond to some known change of temperature. It 

 is evident that such a scale, like all other standards for the 



* An opening of a tube or vessel is said to be hermetically closed or 

 sealed when the material of the tube or vessel itself is fused around it, and 

 the edges when thus soft brought together so as to close the opening, being 

 then allowed to harden by cold as sealing-wax does. The term <4 hermeti- 

 cally " means chemically, the science of alchemy which preceded chemistry 

 being supposed to have been invented by Hermes Trismegistus. So that 

 "hermetically sealed or closed" means to be sealed or closed in the manner 

 adopted in chemical vessels. 

 162 



