THERMOMETERS. 31 



natural brine springs where coal or other fuel is cheap (e.g., in 

 Cheshire), it is usual to boil down the brine until solid salt is 

 left ; but in cold climates where fuel is dear the freezing process 

 is often employed instead to concentrate the brine, being much 

 cheaper. 



Expt. 28. To Construct and Graduate a Thermometer. 

 The ordinary thermometer consists of a piece of very fine glass 

 tube on the end of which a bulb has been blown by softening the 

 glass in a blow-pipe flame and blowing into the tube so as to 

 expand the glass, much as a soap bubble is blown from soap 

 water. To do this successfully requires much more skill than is 

 likely to be possessed by a beginner; besides, the glass tubes 

 ready blown can be purchased very cheaply, with the further 

 addition of being ready filled with quicksilver and sealed up. 



It is a general rule that when things are warmed they expand 

 or occupy more space (vide Chapter XIX.); this is true of the 

 glass bulb itself, which becomes bigger on warming ; but the mer- 

 cury inside expands more rapidly than the glass, so that whilst 

 the bulb becomes absolutely bigger and would hold more, it does 

 not become big enough to hold the enlarged volume of mercury, 

 which therefore ascends in the glass tube (held vertically, bulb 

 downwards) in proportion as it becomes larger in bulk as com- 

 pared with the glass. In short, the quicksilver necessarily runs 

 up the stem as the temperature rises, and retracts or sinks down 

 again as it falls. 



Suppose that you possess a thermometer ready for use but not 

 graduated. It is found by experiment that when pure clean ice 

 is allowed to melt and kept well stirred up with the resulting 

 water, a thermometer placed therein always marks exactly the 

 same point ; this is called the melting-point or ice-point (some- 

 times, but less correctly, the freezing-point, as the temperature 

 at which water freezes to ice is generally, but not always, the 

 same stationary point as that at which ice melts to water). Again, 

 when pure water is boiled in a metal vessel under the average 

 pressure of the atmosphere (Expt. 32), a thermometer wholly 

 placed in the issuing vapour also marks always the same tem- 

 perature, higher than the ice-point; this higher temperature is 

 called the steam-point or boiling-point. 



The thermometer is then graduated by fixing it to a piece of 

 wood, ivory, metal, &c., on which is engraved a scale with marks 

 or degrees on it, so arranged that when the mercury is at the 

 temperature of the boiling-point the top of the column of mercury 

 inside the glass tube is just level with the graduation corresponding 

 with that temperature ; and similarly when the temperature is that 



