THE THERMOMETER. 



Fig. 4. 



position of its zero, a^d, in fine, the standard temperatures by 

 which alone the indication of different thermometers can be ren- 

 dered comparable, are severally arbitrary. Unanimity, never- 

 theless, has prevailed in the selection of standard temperatures. 

 The temperature at which ice melts, and that at which distilled 

 water boils, when the barometer stands at 29-8 inches, have 

 been adopted in all countries as the two temperatures with refer- 

 ence to which thermometric scales are constructed. 



The bulb and tube, as already described, being filled with pure 

 mercury, and a blank scale being attached to the tube, the instru- 

 ment is immersed successively in melting ice and boiling water, 

 and the points at which the mercurial column stands in each case 

 are marked upon the scale. The former is called the FREEZING- 

 POINT, and the latter the BOILING-POINT. 



The apparatus by which the freezing point is determined is 

 shown in fig. 4. The thermometer is 

 immersed nearly to the level of the 

 mercury in a vessel of pounded ice, 

 which being in a state of fusion, the 

 water proceeding from it discharged 

 through a funnel in the bottom is re- 

 ceived in a vessel placed under it. 



The apparatus for determining the 

 boiling point is shown in fig. 5, where 

 D is the boiler, placed over a charcoal 

 furnace, the whole being shown in sec- 

 tion in fig. 6, p. 145. From the top of 

 the boiler a tube proceeds, open at the 

 top, which is enveloped in another 

 larger one, A, closed at the top, and 

 soldered at the bottom to the top of the 

 boiler. In the external tube A, there 

 are three openings, in one of which, m, 

 the tube of the thermometer T is in- 

 serted. In another, a siphon mercurial 

 gauge G H, and in the third a discharge 

 pipe c B, is inserted. When the water 



boils the steam rises, surrounding the bulb and tube, and 

 descending between the two tubes, issues from the discharge pipe 

 B. If the steam be generated too rapidly in the boiler, it will 

 press on the mercury in the gauge, which will then stand at a 

 higher level, w, in the ascending than in the descending leg. 

 The pressure of the steam will be in that case greater than that of 

 the atmosphere, and the force of the furnace must be moderated 

 until the levels of the mercury in the two legs of the siphon- 

 154 



