HEAT. 303 



Since the magnitude of any body changes with the heat to which it is ex- 

 posed, and since, when subject to the same calorific influence, it alwa\s has 

 the same magnitude, these dilatations and contractions, which are the constant 

 effects of heat, may be taken as the measure of the physical cause which pro- 

 duced them. The changes in magnitude which a body suffers by changes in 

 the heat to which it is exposed, are called changes of temperature ; and the ac- 

 tual state of a body at any moment, determined by a comparison of its magni- 

 tude with the heat to which it is exposed, is called its temperature. At the 

 same temperature the same body always has the same magnitude ; and when its 

 magnitude increases, by being exposed to heat, its temperature is said to rise ; 

 and, on the contrary, when its magnitude is diminished, its temperature is said 

 to fall. The variation of magnitude of any body is therefore taken as a meas- 

 ure of temperature ; but as it would be inconvenient, in practice, to adopt dif- 

 ferent measures of temperature, one body is selected by the dilatation and con- 

 traction of which those of all other bodies are measured, and with this body a 

 thermometer, or measure of temperature, is formed. 



The substance most commonly used for this purpose is a liquid metal called 

 mercury or quicksilver. Let a glass tube of very small bore, and terminating in 

 a spherical bulb, be provided, and let the bulb and a part of the tube be filled 

 with mercury. If the bulb be exposed to any source of heat, the liquid metal 

 contained in it will expand, and, the bulb being no longer sufficiently capacious 

 for it, the column in the tube will be pressed upward to afford room for the in- 

 creased volume of the mercury. On the other hand, if the bulb be exposed to 

 cold the mercury will contract, and the column in the tube will fall. 



If we take another similar instrument, having a bulb of the same magnitude 

 but a smaller tube, the same change of temperature will cause the mercury in 

 the tube to rise through a certain space, and this space will be greater than in 

 the former, in the same proportion as the bore of the tube is smaller, because 

 in this case the actual dilatation of the mercury in both tubes is the same ; but this 

 dilatation will fill a more extensive space in the smaller tube. When the bulb, 

 therefore, has the same magnitude, the thermometer will be more sensible the 

 smaller the tube ; or, in general, the less the magnitude of the tube, com- 

 pared with that of the bulb, the greater will be the sensibility of the instru- 

 ment. 



It is evident, therefore, that the same change of temperature would produce 

 very different effects on these two instruments, and the indications of the one 

 could not be compared with those of the other. To render them comparable, 

 it will be necessary to determine the effects which the same temperature will 

 produce on both. Let the two instruments be immersed in pure snow in a 

 melting state. The mercury will be observed to stop in each at a certain 

 height ; let these heights be marked on the scales attached to the tubes re- 

 spectively. Now it will happen that at whatever time or place the instruments 

 may be immersed in melting snow, the mercury will always fix itself at the 

 points here marked. This, therefore, constitutes one of the fixed points of the 

 thermometer, arid is called the freezing point. Let the two instruments be now 

 immersed in pure water in a boiling state, the height of the barometer being 

 thirty inches at the time of the experiment. The mercury will rise in each to 

 a certain point. Let this point be marked on the scale of each. It will be 

 found that at whatever time or place the instruments are immersed in pure 

 water, when boiling, provided the barometer stand at the same height of thirty 

 inches, the mercury will rise in each to the point thus marked. This, there- 

 fore, forms another fixed point on the thermometric scale, and is called the 

 boiling point. 



The distance between these two points on the two thermometers in ques- 



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