CALORIC. 29 



between both their hands, as we do a chocolate 

 mill. In a few minutes the wood takes fire. If 

 the irons of the axle of a coach-wheel be left with- 

 out grease or oil, they will become so hot as to set 

 fire to the wheels ; and accidents of this kind some- 

 times happen. 



It is no uncommon practice in the country, for 

 a blacksmith to hammer a piece of iron till it be- 

 comes red hot, as a substitute for a tinder-box. 

 The heat excited by the boring of a cannon is suf- 

 ficient to cause water to boil. 



Heat is also produced by collision ; when a 

 piece of hardened steel is struck with a flint, some 

 particles of the metal are broken off, and so violent 

 is the heat produced by the stroke, that they are 

 rendered red hot, and melted. If the fragments 

 of steel be caught upon a piece of white paper and 

 examined with a microscope, they will be found to 

 be spherules, and highly polished, showing that they 

 had been fluid. 



No heat seems to follow from the percussion of 

 liquids in soft bodies. 



The instruments for measuring heat by the ex- 

 pansion of bodies are, thermometers for fluids, and 

 pyrometers for solids. 



A thermometer is a hollow tube of glass, her- 

 metically sealed, and blown at one end into the 

 shape of a hollow globe, or bulb. The bulb and 

 part of the tube are filled with mercury, which is 

 the only fluid that expands equally. When we im- 

 merse the bulb of the thermometer in a hot fluid, 

 the mercury expands, and, of course, rises in the 

 tube; but when we plunge it into a cold body, the 

 mercury contracts, and, of course, falls in the tube. 

 The rising of the mercury, therefore, indicates an 

 increase of heat j its falling, a diminution of heat. 



