THE THERMOMETER. 15 



where it is heated throughout its whole length, so as to boil the 

 mercury, the vapour of which drives out all the air and humi- 

 dity, and the balls contain at the end nothing but the metal 

 and its vapour. The open end of the tube, which must not be 

 too hot, is then touched with sealing wax, which is drawn into 

 the tube on melting, and solidifies there on protecting that end 

 of the tube from the heat. That being done, the thermometer 

 is immediately withdrawn from the fire, and being held with 

 the end sealed with wax uppermost, during the cooling the ball 

 by and the portion of the tube below the ball , are filled 

 with mercury. After cooling, the instrument is inclined a little, 

 and by warming the lower ball, a portion of mercury is expelled 

 from it, so that the mercury may afterwards stand at a proper 

 height in the tube when the instrument is cold. The tube is 

 then melted with care by the blow-pipe flame below the ball a, 

 and closed, or hermetically sealed, as in c. The thermometer 

 is in this way properly filled with mercury, and contains no 

 air. 



We have now an instrument in which we can nicely measure 

 and compare any change in the bulk of the included fluid metal. 

 Having previously made sure of the equality of the bore, it is 

 evident that if the mercury swells up and rises two, three, four, 

 or five inches in the tube, it has expanded twice, thrice, four or 

 five times more than if it had risen only one inch in the tube. 

 By placing a graduated scale against the tube, we can, therefore, 

 learn the quantity of expansion by simple inspection. 



In order to have a fixed point on the scale, from which to 

 begin counting the expansion of mercury by heat, we plunge the 

 bulb of the thermometer into melting ice, and put a mark on 

 the stem at the point to which the mercury falls. However 

 frequently we do so with the same instrument, we shall find that 

 the mercury always falls to the same point. This is, therefore, a 

 fixed starting point. We obtain another fixed point by plung- 

 ing the thermometer into boiling water. With certain precau- 

 tions, this point will be found equally fixed on every repetition 

 of the experiment. The most important of these precautions is, 

 that the barometer be observed to stand at 29.8 inches, when 

 the boiling point is taken. It will afterwards be explained that 

 the boiling point of water varies with the atmospheric pressure 

 to which it is subject at the time. 



Thermometers which are properly closed, and contain no air, 



