THE THERMOMETER. 



135 



held over the lamp till the mercury boils. The vapor of the mercury then rising 

 from its surface will fill the unoccupied part of the bulb and tube, and will alto- 

 gether expel the atmospheric air from them, so that the whole bulb and tube 

 will be filled with the mercury and its vapor. The instrument must now be 

 once more inverted into the cistern of mercury, and immediately the mercurial 

 vapor in the tube and bulb will be restored to the liquid form by being removed 

 from the lamp which sustained it in the state of vapor. The atmospheric pres- 

 sure will force mercury into the tube and bulb until both are perfectly filled. 

 The apparatus, therefore, is now filled with pure mercury, free from intermix- 

 ture with any kind of foreign matter, whether in the solid, liquid, or gaseous 

 form. 



Since the indications of the thermometer are made by the rise and fall of the 

 column of mercury in the tube, it follows that, when adapted for use, the in- 

 strument must be only partially filled with mercury. It is evident that at the 

 lowest temperature which the instrument is intended to measure, the surface- 

 of the mercury ought to be above the point where the tube rises from the bulb ; 

 for any contraction of the mercury which would cause the whole of that fluid 

 to enter into the bulb could not be estimated. The whole quantity of mercury 

 in the instrument ought, therefore, to exceed the contents of the bulb when the 

 mercury is at the lowest temperature to which the instrument is intended to be 

 exposed. On the other hand, when the temperature is raised, the expansion 

 of the mercury causing the column in the tube to ascend, it is necessary that 

 the length of the tube should be such that the highest temperature to which it 

 is intended to expose the instrument should be such, that the tube may afford 

 sufficient room for the increase of the column produced by the corresponding 

 expansion. From these observations it will be apparent that the quantity of 

 mercury to be left in the thermometer must depend on the relative magnitudes 

 of the bulb and tube, and on the extremes of temperature which the instrument 

 is intended to. measure. Let us suppose that the range of the instrument shall 

 be confined to a few degrees below and above the temperatures of melting ice 

 and boiling water. If too much mercury be left in the tube, on plunging the 

 instrument in boiling water, the mercury would rise to the top of the tube, and 

 by its expansion overflow if it were open, or burst it if closed. If, on the 

 other hand, too little mercury were left in the instrument, on plunging it in 

 melting ice a contraction of the mercury by the cold would cause it to fall into 

 the bulb, and no indication could be obtained of that part of the contraction of 

 the mercury which took place in the bulb. The law by which the dilatation 

 of mercury is regulated, will determine the length which it is necessary the 

 tube of the thermometer should have, provided the diameter of the tube and the 

 contents of the bulb are known. We shall, however, for the present, suppose 

 that the proper quantity of mercury has been introduced into the apparatus, so 

 that the extremes of heat and cold shall not cause either of the effects to which 

 we have just referred. 



It is now necessary to close the tube at the top by melting the glass with the 

 blowpipe ; but in performing this operation, care must be had to exclude all the 

 air which may remain in the tube above the column of mercury. It is found 

 that if this air were suffered to remain above the mercury in the tube of the 

 thermometer, any accidental agitation of the instrument is liable to cause the 

 bubbles of it to mix with the mercury so as to break the column ; and when this 

 happens, it is extremely difficult to disengage it from the mercury, and cause 

 it to ascend to the top of the tube. 



In closing the top of the tube, the air is excluded by the following process : 

 The bulb of the thermometer is exposed to heat until the mercury has dilated 

 so as to cause the column to rise very near the extremity of the tube. The 



