THE THERMOMETER. 



dilatability of glass in j;he proportion of nearly 20 to 1, and, 

 consequently, the capacity of the bulb and tube will be less 

 enlarged than the volume of the mercury contained in them in 

 the proportion of nearly 1 to 20 ; consequently, for the reason 

 above stated, every elevation of temperature by which the mer- 

 cury and tube would be affected will cause the column of mercury 

 to rise in the tube, and every diminution of temperature will 

 cause it to fall. 



The space through which the mercury will rise in the tube by 

 a given increase of temperature will be greater or less according 

 to the proportion which the tube bears to the capacity of the 

 bulb. The smaller that proportion, the greater will be the 

 elevation of the column produced by a given increase of tem- 

 perature ; for a given increase of temperature will produce a 

 definite increase of volume in the mercury, and this increase 

 of volume will fill a greater space in the tube in proportion to the 

 smallness of the bore compared with the capacity of the bulb. 



16. Such an instrument, without other appendages or prepara- 

 tion, would merely indicate such changes of temperature in a 

 given place as would be sufficient to produce visible changes in 

 the elevation of the column of mercury sustained in the tube. 

 To render it useful for the purposes of science and art, and in 

 domestic economy, various precautions are necessary, which have 

 for their object to render the indications of different thermo- 

 meters comparable with each other, and to supply exact numerical 

 indications of measurement of the changes of temperature. 



For this purpose it is necessary, in the first instance, that the 

 mercury with which the tube is filled shall be perfectly pure and 

 homogeneous. This object is attained by the same means as have 

 been already explained in the case of the barometer.* 



17. In the selection of the tube it is necessary that it be 

 capillary, that is to say, a tube having an extremely small bore, 

 and that the bore should be of uniform magnitude throughout its. 

 entire length. 



The smallness of the bore is essential to the sensibility of the 

 instrument, as already explained ; and its uniformity is necessary 

 in order that the same change of volume of the mercury should 

 correspond to the same length of the column in every part of 

 the tube. 



The uniformity of the bore of the tube may be tested by letting 

 into it a small drop of mercury, sufficient to fill about a third of 

 an inch of the tube. Let this be made to fall gradually through 

 the entire length of the tube, stopping its motion at intervals, and 



* See our Tract on "The Barometer." 

 150 



