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CHAPTER XXXVL 



THERMOMETRY. 



.Expansion toy heat. The action of heat causes 

 gaseous, liquid, and solid bodies to expand. Gases 

 expand most, solid bodies least. Yet the expansion of 

 solid bodies by heat is readily shown. A brass ball is 

 taken of such a size that it just passes through a ring of 

 the same metal. The ball is heated, and it is then not 

 possible for it to pass through the ring ; but if the ring 

 also be heated, it may be made to expand sufficiently 

 to permit the heated ball to pass. On cooling, both 

 regain their former size. The expansion of liquids is 

 seen by immersing a flask filled with cold water in an 

 outer vessel of hot water. The neck of the flask is 

 continued into a long capillary tube ; and as the 

 water expands it rises in the tube. Alcohol, or other 

 more volatile liquids, would show the expansion more 

 readily. Similarly, if a flask filled with air, and having 

 a capillary prolongation, be heated, the expansion of 

 air may be shown. All that is necessary is to have 

 a small quantity, an inch or so, of coloured liquid 

 in the capillary tube as an index ; when the air is 

 expanded by heating, the index is pushed up, and 

 when by cooling the air contracts, the index passes 

 down the tube. Water affords a remarkable excep- 

 tion to the general rule of expansion by heat and 

 contraction by cold. If water be cooled it gradually 

 contracts till at a temperature of 4 C. (39 Fahr.) 

 it reaches its condition of maximum density. If the 

 cooling be continued the liquid begins to expand 

 until it becomes frozen, when a sudden considerable 



