THE THERMOMETER. 13 



THE THERMOMETER. 



An instrument for indicating variations in the intensity of 

 heat, or degrees of temperature, by their effect in expanding 

 some body, was invented more than two centuries ago, and 

 has received successive improvements. 



The expansions of solids are too minute to be easily mea- 

 sured, and cannot, therefore, be conveniently applied to mark de- 

 grees of heat. Air and gases, on the other hand, are so much 

 dilated by a slight increase of heat, that they are not calcu- 

 lated for ordinary purposes. The first thermometer constructed, 

 however, that of Sanctorio, was an air one. FIG. 1. FIG. 2. 

 A glass tube, open at one end, with a bulb 

 blown upon the other, (Fig. 1.) was slightly 

 heated, so as to expel a portion of the air from 

 it, and then the open end of the tube was 

 dipped under the surface of a coloured fluid, 

 which was allowed to rise into the tube, as the 

 air cooled and contracted. When heat, the 

 heat of the hand for instance, is applied to the 

 bulb, the air in it is expanded, and depresses 

 the column of coloured fluid in the tube. A 

 useful modification of the air thermometer, for 

 researches of great delicacy, was contrived by Sir John Leslie, 

 under the name of the Differential thermometer. In this in- 

 strument, two close bulbs are connected by a syphon containing 

 a coloured liquid, (Fig. 2.) If both bulbs be equally heated, the 

 air in each is equally expanded, and the liquid between them 

 remains stationary. But if the upper bulb only be heated, then 

 the air in that bulb is expanded, and the column of liquid de- 

 pressed. It is, therefore, the difference of temperature between 

 the two bulbs which is indicated. 



But liquids fortunately are intermediate in their expansions 

 between solids and gases,, and when contained in a glass vessel 

 of a proper form, the changes of bulk which they undergo can 

 be indicated to any degree of precision. 



A hollow glass stem or tube is selected, the calibre or bore of 

 which may be of any convenient size, but must be uniform, or 



