140 



ELEMENTARY GENERAL SCIENCE 



CHAP. 



and a fine scratch made with a three-cornered file on the tube 

 at the level of the mercury. 



Marking the Boiling Point. The apparatus used fordoing 

 this will bs readily understood by examining Fig. 59. Water is 

 contained in the cylinder E, which is of a much smaller diameter 

 towards the top, where it is open in the way the drawing shows. 

 Over this smaller open cylinder is placed a larger on,e, which 

 rests on the larger cylinder containing the water, and is provided 



FIG. 58. Marking the Freezing 

 Point. 



FIG. 59. Marking the Boiling Point. 



with three openings one at the top A through which to pass the 

 thermometer, one at the side C to allow superfluous steam to 

 escape, and one for the introduction of a bent tube containing 

 mercury, for indicating whether the pressure inside the apparatus 

 is the same as that of the surrounding air, and which is known as 

 a manometer. 



The apparatus is arranged as shown in the left figure (Fig. f> ( .), 

 and the water in E boiled. Steam is generated, and circulates as 

 shown by the arrows. The thermometer is exposed to the .steam 



